going with custom pistons. What's the best compression ratio and why?

[zandrew]

Going use custom piston set and the CR is basically anything from 8.5:1-9.5:1 and more or less with shave or thicker hg. Part of me likes higher CR for better spool but its a trade off for top end. I was wanting to see some examples and impressions for both ends.

thanks

[Seerlah]

How much power are you chasing? With my current engine I placed in stock pistons from the AEB that have a higher 9.5:1 CR versus my original 9.3:1 pistons (cheapest route available at the time for 20 mm wristpins). If I had the money to spare, I would have sourced AMU pistons that have a 9:1 CR. Depending on your power goals, my personal opinion is it isn't that big of a deal. And with that being said, I plan on going 9:1 83mm stroker pistons on my next engine.

[catbed]

You'll want 8.5-9:1 for a FI application for best torque and drivability all around.

[spindoctor]

Going use custom piston set and the CR is basically anything from 8.5:1-9.5:1 and more or less with shave or thicker hg. Part of me likes higher CR for better spool but its a trade off for top end. I was wanting to see some examples and impressions for both ends.

thanks

Off boost, high comp would be better but you could still get away with the top end by running meth injection :)

[zandrew]

I have seen guys run extremely high CR before without the issue of knock. I am talking 12.5:1 while running SC62 sized snail in a daily driver making 400+whp. It was actually tuned for more whp but for daily use it just spun the tires. It was in Honda motor and not a VW but this is still has me wondering why not. He specifically wanted higher CR for the off boost characteristics. I know meth works but too me it seems like a band aid that can be fixed with a proper design and tune. I always somewhat laugh about meth since it was actually a feature on the first mass produced turbocharged motor back in 1962. Think about that, the first mass produced turbocharged motor was an all aluminum V8 215ci that ran 10.25:1 CR and used meth to prevent knock. You would think in 50 years with all the advances in tuning and monitoring the engines vitals you could run the same CR without the aid of meth, which I am sure you can.

I was leaning toward 9.0:1-9.25:1 though.

Does anyone make a piston that has enough clearance to make the motor a non interference?

[redline380]

No. These are interference engines anyway you put it. As for compression I run 8.5 and won't be doing it again. definitely go over 9. but it depends on what fuel you want to run. If you will be running e85, I don't see why 9.5 would be too high, I think it would be great. But for 93 I'd say 9.25 should do

[zandrew]

I am also on the fence wether to go 2.0 or run the 1.9. Ultimately I would trade some torque for a lil more RPM range and from my prior expierences it seems like the smaller cranks spin better. I was thinking about punching the motor out with 83mm pistons and running a mild set of cams like the 210's from Eurospec or maybe even just their single intake. I really don't think going to 2.0 is necessary to achieve what I am trying to accomplish but at the same time I am the type that will always wonder what if? I also have a GT3076R that I am contemplating as well but for now I am sticking with the GT2871R.

What is the safe limit of the stock block as far as RPM's go (not hp)?

Is there any consenus on which is a better upgrade as to cams or valves?

WHen I build this block I would like to have provisions in place for say 500awhp in case I want to do something really stupid one day.

[juxsa]

depends on what you want. If you want more bottom end torque then go with the higher CR pistons... if higher RPM HP is your goal than go with the lower CR pistons. If you have the the big port heads I'd be tempted to run maybe a 9:1 CR. The 8.5:1 would give you more top end but these motors already lack torque and if this is a DD it would suck on the street.
If you have the small port heads I would just stick with the stock CR which I think is 9.5:1... or at least that is what I think it is for the 2001+ 1.8T

[catbed]

Higher CR will not give you more bottom end. At WOT a 8.5:1 CR engine will out-spool and produce more torque than an identical engine with 9.5:1.

[ddillenger]

8-8.5:1 If you're going for serious power.

[juxsa]

Higher CR will not give you more bottom end. At WOT a 8.5:1 CR engine will out-spool and produce more torque than an identical engine with 9.5:1.

I would be really interested in seeing where you got that info because that is the first time I have ever heard that before

[catbed]

I would be really interested in seeing where you got that info because that is the first time I have ever heard that before

Its the nature of a FI engine. Look up dynamic compression ratio.

[sweets4style]

You guys must be joking. Where do some of you get your info from.

Compression ratio going up is giving you more power at any powerband level. Its simple. All that volume is compressed into a smaller space. When the bang goes off it in turn explodes that much more outward. That is just in lamens terms. High CR will net you more power with any turbo on and off boost. You will have more torque, faster spool up, and more area under the power curve all else being held constant. There is just one caveat. When you run a higher CR you end up hitting a wall quicker of how much boost you can run on 93. You can compensate by running e85, meth injection with 93 or e85, or race gas.

Smaller crank spinning better??? What the hell are we talking about. The cranks are the same size. The offset of the crank journal for the rod is just slightly higher. We are talking MM here. Your worried purely about rod to stroke ratio and the ratio is perfectly fine for even high revving motors on out particular engines with out particular stroker kits. No matter how you cut it more displacement will net you more power across teh entire RPM rang, spool up quicker, and overall more area under the curve. Sound like a familiar statement? More displacement doesnt net you a potential wall with boost though. Its impartial in that respect.

Your block isnt your rpm limiter. Its your valvetrain. While there is no static number to not cross typically you cant rev higher then 8500 (id prefer to say 8000) on our stock hydraulic lifters without going to a solid lifter design. I also would rev past 7500 on the stock valvetrain. Sodium filled valves are rather brittle with age and weigh quite a bit to comparable options. Really easy to float a valve even with upgraded springs. Not a definitive means of expression here but good rules of thumb.

Remember there isnt a better than this or that idea mentality. Everything is a compromise between driveability, where power happens, and durability. You can only compromise and pull from each column not exponentially add to each column with new shiny parts.

[catbed]

You guys must be joking. Where do some of you get your info from.

Compression ratio going up is giving you more power at any powerband level. Its simple. All that volume is compressed into a smaller space. When the bang goes off it in turn explodes that much more outward. That is just in lamens terms. High CR will net you more power with any turbo on and off boost. You will have more torque, faster spool up, and more area under the power curve all else being held constant. There is just one caveat. When you run a higher CR you end up hitting a wall quicker of how much boost you can run on 93. You can compensate by running e85, meth injection with 93 or e85, or race gas.

LMAO. This is NOT TRUE.

You are right about the block/valvetrain though.

[juxsa]

Its the nature of a FI engine. Look up dynamic compression ratio.

You should read about the nature of static CR as well. What sweet4style said is correct

[catbed]

You should read about the nature of static CR as well. What sweet4style said is correct

He is correct... For naturally aspirated engines. Are our engines naturally aspirated?

NA engines are VE limited, where ours are knock limited.

So if what he is saying is correct, then why did audi drop the CR from 9.5 to 9 for the TT225? Surely audi has some idea what's going on in the combustion chamber.

[juxsa]

He is correct... For naturally aspirated engines. Are our engines naturally aspirated?

NA engines are VE limited, where ours are knock limited.

So if what he is saying is correct, then why did audi drop the CR from 9.5 to 9 for the TT225? Surely audi has some idea what's going on in the combustion chamber.

Because with a lower CR you can run more boost without worrying about detonation. why are current forced induction DI engines running 10.5:1 CR or higher and produce torque at a lower RPM than our 1.8T could ever dream?
The answer is because their CR is crazy high

[catbed]

Because with a lower CR you can run more boost without worrying about detonation. why are current forced induction DI engines running 10.5:1 CR or higher and produce torque at a lower RPM than our 1.8T could ever dream?
The answer is because their CR is crazy high

Exactly.

Direct injection is a WHOLE different ball game. You can't compare a DI to a port injection enginr

[juxsa]

Exactly.

Direct injection is a WHOLE different ball game. You can't compare a DI to a port injection enginr

the principle is the same just like the principle is the same with a forced induction motor as compared to a N/A motor... all turbo motors are N/A motors until they start building boost. I am done with this. I am not trying to be a dick but you can go ahead and continue to believe what you want, but don't go telling people wrong information.

Again to the OP, the advantage of low CR is that you can run more boost and have more top end but at the expense of low end torque because it takes longer to spool the turbo. A high CR will have more bottom end and will be able to spool the turbo faster but at the expense of top end due to having run lower boost because of detonation.

[catbed]

the principle is the same just like the principle is the same with a forced induction motor as compared to a N/A motor... all turbo motors are N/A motors until they start building boost. I am done with this. I am not trying to be a dick but you can go ahead and continue to believe what you want, but don't go telling people wrong information.

Again to the OP, the advantage of low CR is that you can run more boost and have more top end but at the expense of low end torque because it takes longer to spool the turbo. A high CR will have more bottom end and will be able to spool the turbo faster but at the expense of top end due to having run lower boost because of detonation.

Ok, we agree to disagree. You say turbo motors are N/A before the turbo spools. You are right, but even on my laggy as fuck 60 trim, I make positive pressure before 2.5k RPM. That is considered "down-low" and you will have knock and less than ideal spark advance with higher CR.

OP, the correct info is use 8.5:1 to 9:1 CR pistons for a turbo engine. Any more and you're losing out on power because of timing pull due to knock.

[Seerlah]

I was always taught rule of thumb is to run lower CR if you want to run more boost and higher CR for better off boost driveability. And with the CR you are looking at, you are really limited to what is offered for our vehicle. Highest on the market would probably be 10.5:1 (usually 9.5:1 is listed but there are higher CR pistons out there) and lowest being 8.5:1. I just considered 9:1 to be the compromise between the 2. Just an opinion.

[zandrew]

There is significant difference between DI and port injection. Direct injection allows you to run more timing (thus more torque) down low with higher CR since DI actually keeps combustion chambers cooler by directing the much cooler fuel directly into the chamber. Dynamic compression ratio means alot but that is beyond the scope of this thread. It takes in a lot more factors like valve timing and so on. However i have never seen a higher dynamic CR on the same motor with the same valve timing when comparing a higher static CR to a lower static CR. YOU WILL NOT GET MORE TORQUE AT LOWER RPM's WITH LOWER CR THEN WITH HIGHER CR.

Sweet4style you obviously are misinformed about what I am concerned about and what I am not. The cranks are different weight. The mean piston speed for a larger crank per revolution is higher then with a smaller crank. Also force equals MASS * acceleration so by reducing weights (like forged rods and pistons) you reduce the force that is also exerted on the bearings and so forth. Now to tie all this together most think that higher rod stroke ratios is what you want for RPM's. It is IF we are talking NA, however we obviously are not. In a forced induction motor you need lower rod stroke ratio. A lower rod stroke ratio increases the speed of the piston at TDC opposed to a longer rod stroke ratio that slows it down. A longer rod stroke ratio increases your chance of detonation. Also the safe average mean piston speed is typically 4200 fpm on motors 1.8 and up (this is why those shitty D16 motors with short rod stroke ratio seem to hold up so well to turbo builds). This also depends on type of material used and quality of the build. You can spin them alot faster but these are numbers given too me by a company that all they do is build hi performance motors and they consider these safe speeds for your average builder. Obviously newer motors are different but a great deal of engineering went into them to achive this and the VW 1.8 was not one of them.

For a 86.4mm stroke you are doing 4250 fpm at 7500 RPM which is more then safe with forged rods.

A 92.8mm stroke your are doing 4566 FPM which honestly should also be just fine.

AT 8000 RPM you are still at less FPM with the 1.8 crank opposed to the 2.0 crank.

The idea would be to balance between the mean piston speed and rod/stroke ratio. However it would seem that you "should" be able to run more compression on the 2.0 setup opposed to the 1.9 since it ultimately has a lower rod stroke ratio and would be less prone to detontantion. However you are also increasing the rod angle which is a lot harder on the crank, bearings, and rod itself. Meaning you best have your $h!t right.

You are also generalzing that displacement is everything when it comes to making power. Its not. I would take a motor that can spin safely to 10,000 RPM that is a 1.6 opposed to a 2.0 that can not go past 7500 if I want to make hp. My point of stating that I like a 1.9 with cams and valves is that if I want to spin it to a higher RPM I can do so with out a significantly higher quality build (like balanced internals etc). I guess in fairness I genralised as well since there are other things that can be done to aid a longer stroke like lightened flywheel, pulley, etc but this is taking in that both setups are the same.


Still I prefer real world examples.

[Seerlah]

Another general rule of thumb is to not increase bore on this engine to 83mm unless going stroker. It could be speculation, but 82.5mm is what you would want to stop at if staying stock stroke. Reason being is that you place more stress on the piston rings with them being larger in diameter. The longer stroke alleviates this tension, with the added bonus of more displacement. On the upward motion of the piston, the rod angle basically would not be ideal with that large of a bore/piston with the stock crank. All in theory though.

[catbed]

There is significant difference between DI and port injection. Direct injection allows you to run more timing (thus more torque) down low with higher CR since DI actually keeps combustion chambers cooler by directing the much cooler fuel directly into the chamber. Dynamic compression ratio means alot but that is beyond the scope of this thread. It takes in a lot more factors like valve timing and so on. However i have never seen a higher dynamic CR on the same motor with the same valve timing when comparing a higher static CR to a lower static CR. YOU WILL NOT GET MORE TORQUE AT LOWER RPM's WITH LOWER CR THEN WITH HIGHER CR.

So why did you start this thread if you think you already know the answer. What you are saying holds true for N/A like I said before. NOT FI.

Quoted from another thread on a different forum:

"Dynamic compression ratio, as opposed to mechanical compression ratio. sqrt((boost+14.7)/14.7) * CR = effective compression ratio. Dropping CR allows a greater effective CR (therefore greater oxygen with which to combine with fuel). Say you drop from 9:1 CR to 8:1, which allows an increase in boost from 15 psi to 18 psi without detonation onset and all else being fairly equal. You've lost 3-4% in power potential as a result of the decrease in mechanical compression ratio, but you've gained 10% in absolute potential, for a net increase of 6-7%. Could be wrong, but I doubt most people could tell the difference in throttle response in off-boost situations between 8:1 and 9:1. There is a point where a lower CR will not be enjoyable in off-boost conditions, but I don't think 8:1 or 8.5:1 is it."

[catbed]

Another general rule of thumb is to not increase bore on this engine to 83mm unless going stroker. It could be speculation, but 82.5mm is what you would want to stop at if staying stock stroke. Reason being is that you place more stress on the piston rings with them being larger in diameter. The longer stroke alleviates this tension, with the added bonus of more displacement. On the upward motion of the piston, the rod angle basically would not be ideal with that large of a bore/piston with the stock crank. All in theory though.

Larger diameter does not mean more stress. Quite the opposite in fact. More surface area for the same amount of force equals less force per unit area. I'm not sure how you think it relates to bore size with stock crank though?

Also, the rod angle will NOT change if you only change pistons. The wrist pin location will stay the same, as well as rod length.

[redline380]

does no one ever read my posts? forget all the number bullshit, dynamic this and that, whatever. the simple fact is lower compression will net you less torque low down, but allow you to run more boost on worse gas. high compression will net you more torque off boost, but is less stable in high boost and poor octane fuel. boost is obviously good, but if you cant run any kind of timing while in boost to keep knock down than whats the point? high compression is good but if you cant run high boost while in high compression whats the point? the fact is you need something in between, and my explanation is below, as quoted from above. all of the high tech number bullshit is fine, but you cant change any of it. you get TWO numbers to deal with. bore and compression ratio. choose wisely.

No. These are interference engines anyway you put it. As for compression I run 8.5 and won't be doing it again. definitely go over 9. but it depends on what fuel you want to run. If you will be running e85, I don't see why 9.5 would be too high, I think it would be great. But for 93 I'd say 9.25 should do

[arorem]

A friend of mine is running 12.2:1 in his speed6. Not really related to this thread though since the MZR is DI, and he's running E85. I just figured I'd share.

[catbed]

does no one ever read my posts? forget all the number bullshit, dynamic this and that, whatever. the simple fact is lower compression will net you less torque low down, but allow you to run more boost on worse gas. high compression will net you more torque off boost, but is less stable in high boost and poor octane fuel. boost is obviously good, but if you cant run any kind of timing while in boost to keep knock down than whats the point? high compression is good but if you cant run high boost while in high compression whats the point? the fact is you need something in between, and my explanation is below, as quoted from above. all of the high tech number bullshit is fine, but you cant change any of it. you get TWO numbers to deal with. bore and compression ratio. choose wisely.

It is NOT that simple with FI engines.

People keep throwing around the term off-boost. You are rarely off boost at WOT with a streetable turbo, and you will NOT notice a difference between CR at part throttle.

[redline380]

It is NOT that simple with FI engines.

im not saying its simple. im saying you dont have a choice. what are you going to do, order custom crank and piston to get a good rod angle? like i said, for all intensive purposes, you have two numbers to choose from. bore and compression ratio. you can research the fuck out of pistons, do math till theres numbers popping out of your ass, but at the end of the day you will be (for a 1.8/9) running a 86.4mm stroke with a 144mm rod length. those wont change. and all other numbers rely on those variables. so like i said, you got TWO numbers, no matter how you chalk it up. no one ever said the vw 1.8 was designed to be a race engine because it wasnt

[ddillenger]

All things being equal, a lower compression forced induction engine can run more timing. More timing, more power. Catbed was right on point when he said we're knock limited.

[Seerlah]

Larger diameter does not mean more stress. Quite the opposite in fact. More surface area for the same amount of force equals less force per unit area. I'm not sure how you think it relates to bore size with stock crank though?

Also, the rod angle will NOT change if you only change pistons. The wrist pin location will stay the same, as well as rod length.

I should have worded it way differently. Typed too fast not going with what I wanted to say (I know the rod top wristpin end does not change position ). But the theory was what you are negating. Once again, just a theory. For better words, more like a tilt of the piston that would be there if not for the rings (piston to cylinder clearance). Theory is as you increase the bore you in fact place more tension on the rings. Your statement would hold true if it were on a straight upward motion with equal force applied to the sides. But that is not the case.

Not trying to start a debate at all. Just stating what I was taught .

[catbed]

I should have worded it way differently. Typed too fast not going with what I wanted to say (I know the rod top wristpin end does not change position ). But the theory was what you are negating. Once again, just a theory. For better words, more like a tilt of the piston that would be there if not for the rings (piston to cylinder clearance). Theory is as you increase the bore you in fact place more tension on the rings. Your statement would hold true if it were on a straight upward motion with equal force applied to the sides. But that is not the case.

Not trying to start a debate at all. Just stating what I was taught .

Not trying to either, just confused as to your statement. But, all cleared up now.

[ZimbutheMonkey]

Look, not to stir things up here, but Catbed, I think some of your reasoning is faulty. While a lower compression ratio is ultimately going to lower the knock threshold and allow for more power, the advantage doesn't materialize until the lowered knock threshold of a say, 8:1 compression ratio exceeds that of a higher one of say 9.5:1.

So say that all things being equal, you can run 15 degrees advance at 10 PSI from 3000 RPM to 7500 RPM on an 8:1 and a 9.5:1 compression engine, the high compression engine will ultimately make more power. Yes, there is slightly more space in the cylinder on the low compression engine owing to the piston dishing, but that's not what's making your power on the low compression engine, as you said, it's the increase in detonation resistance.

However, lets up the boost to 30 PSI and all of a sudden I need to start dropping the timing to 5 degrees advance for the 9.5:1 or even dropping the boost to 20 PSI vs leaving it at 30 PSI and at 15 deg advance for the 8:1, now the low compression engine starts to make more power.


What am I trying to get at, your comment about the low compression engine outperforming the high compression one doesn't hold true until you start running up against the knock limitations of a higher compression ratio, which can be pretty high actually. With water/meth I can hold around 18-20 degrees advance at 30 PSI on my 5556 and that's on an AEB CR of 9.5:1. So really, you're probably looking at north of the 250-300 WHP range before you start seeing the advantage of a low CR engine and by then you're probably north of 5000 RPM in your powerband.

Easiest way to look at it is this way: hold VE at a fixed 100%, assume infinite amounts of boost and infinite detonation resistance, which makes more power at a given boost level 5:1 compression or 10:1? But as you rightly pointed out, it doesn't work that way, sooner or later detonation will rear it's ugly head and you'll have to pull boost or timing. You just had it a little off on when it sets in, it doesn't occur as soon as you hit positive pressure, it occurs gradually and at a certain cylinder pressure threshold. It's also probably the case (and I'm guessing, but it makes intuitive sense) that detonation likelihood occurs logarithmicaly as boost increases, not on a linear scale.

Also, a higher CR will also have the effect of increasing the exhaust gas velocity as the piston raises to expel the exhaust gas from the cylinder. That said I don't know if it's negligible or not.

[ZimbutheMonkey]

BTW, has anyone had a look at using these pistons in a 1.8T?



http://www.spaturbo.com.br/loja//com...mart/Itemid,1/

SPA makes them and I find it to be an interesting design. Not so much for efficiency's sake, but for detonation resistance. Reason being is that my understanding of detonation is that it commonly occurs at the edge of the flame front closest to the cylinder wall. Basically, the gases there heat up and ignite before the flame front has a chance to reach that area.

Now, that said, if you decrease the compression in that region by giving it a concave area, would it not stand to reason that you would lessen the compression and hence the possibility of autoignition. Also, as SPA claims, it does increase swirl and the homogeneity of the air/fuel mix and therefore lessens the possibility of detonation (part of why the FSI and TFSI engines can run high compression and run lean at cruise).

***EDIT*** I looked at the website and they're actually designed to run with longer rods to give a 1.9L displacement boost. So there's that whole rod ratio thing that was being discussed earlier.

[redline380]

Catbed was right on point when he said we're knock limited.

not trying to sound like an ass, but duh! all engines are knock limitied. (lets not get techinal on this, you know i mean internal combustion, reciprocating engines) its why back in the days, the easiest way to increase power was to increase displacement. the gas was shit back then, as well as the ignition systems. we have the same limiting factors in todays engines, just to a different degree. if we werent knock limited, compression ratios would be ∞:1 and gas would be 0 octane controlling the knock now, that is a whole other story and many factors play into it.

BTW, has anyone had a look at using these pistons in a 1.8T?

havent seen them. interesting design, but i feel they are a bit gimmicky. im sure they spent a long time desinging them carefully, but whats the point when you can accomplish the same thing by running better fuel? as i stated a couple times, i really think the sweet spot for a 1.8 is at 9.25 compression and e85. run the timing sky high, crank the boost to whatever, and hang on.

[catbed]

Look, not to stir things up here,

So say that all things being equal, you can run 15 degrees advance at 10 PSI from 3000 RPM to 7500 RPM on an 8:1 and a 9.5:1 compression engine, the high compression engine will ultimately make more power. Yes, there is slightly more space in the cylinder on the low compression engine owing to the piston dishing, but that's not what's making your power on the low compression engine, as you said, it's the increase in detonation resistance.

However, lets up the boost to 30 PSI and all of a sudden I need to start dropping the timing to 5 degrees advance for the 9.5:1 or even dropping the boost to 20 PSI vs leaving it at 30 PSI and at 15 deg advance for the 8:1, now the low compression engine starts to make more power.

Yes, but who is running a built motor at only 10psi? If someone is building a motor to make power, then they will be riding the knock threshold, right? You can give examples of situations where what I said is untrue, but I'm talking real world situations here. Sure you can build a high CR engine and run low boost, but why would you do that?

What am I trying to get at, your comment about the low compression engine outperforming the high compression one doesn't hold true until you start running up against the knock limitations of a higher compression ratio, which can be pretty high actually. With water/meth I can hold around 18-20 degrees advance at 30 PSI on my 5556 and that's on an AEB CR of 9.5:1. So really, you're probably looking at north of the 250-300 WHP range before you start seeing the advantage of a low CR engine and by then you're probably north of 5000 RPM in your powerband.

I'm talking strictly 93 pump gas, as 90% of people aren't running meth. I agree that meth will add knock resistance. And like I said, odds are if you are building an engine, you'll be riding the knock threshold.

[zandrew]

redline- Gas was shit when? Remember they went to unleaded and leaded gas could handle loads of timing. This is why you can not achieve the same level of hp with pre unleaded motors with unleaded gas. Too give you an idea airplane prop motors from WWII ran 130 octane in todays equivalent where as todays run 100 octane. Manufacturers have learned to work around this though.

I don't have the option for E85.

catbed- You can run any set of numbers any amount you want. Real world examples give you better models to work from. Still its a good idea to have guidelines to build within.

Also riding the knock threshhold for what reason? Pushing a motor til it knocks and then back it off a degree or 2 will not yield the most power (though sometimes it can). This is a major misconception that a lot people get suckered into believing. I actually believed this myself and tuned a 4AGTE in AE86 to peak 218rwhp. Sold the car and the next owner had it dyno tuned. It made peak 222rwhp at the same exact psi on the same exact setup. However he pulled 8 degrees of timing and gained 40 lb ft at 3500 RPM.


seerlah - You need to look at the anatomy of piston. The rings are basically springs that push outwards on the walls to keep the compression and exhaust gasses on one side. The skirt of the piston is actually what rides the cylinder wall. When you measure the piston diameter and compare it too the spec you do so at 90 degree from the centerline of wrist pin. Not the crown. Too give you a specific example I have 82mm forged piston and the skirt is 81.92mm (for a 82mm bore). The area above and between the rings is 81.15mm. The area above the oil ring is actually narrower then the skirt. Also the width of the top 2 compresion rings are 3.35mm and the groove depth for them is 3.75mm. Most people think the piston is the same diameter top to bottom

[redline380]

redline- Gas was shit when? Remember they went to unleaded and leaded gas could handle loads of timing. This is why you can not achieve the same level of hp with pre unleaded motors with unleaded gas. Too give you an idea airplane prop motors from WWII ran 130 octane in todays equivalent where as todays run 100 octane. Manufacturers have learned to work around this though.

I don't have the option for E85.

i was talking wayyy back, like 1900-1920ish like when gas was getting popular. even with lead, the gas was pretty crappy. and lead is a whole other subject. how awesome would leaded gas be? yeah it kills cats, but like any of us actually run cats. and 02 sensors? id have no problem replacing them in exchange for leaded gas. as for no e85 option, thats a shame. it really is. its some great stuff. in that case, id still stick with 9.25 compression, youd just have to kill the timing a bit up top with 93. its not like your car would be slow or anything

The skirt of the piston is actually what rides the cylinder wall.

i hate to be a stickler and i know you know better but i like to clear up information that can be miscontrued by other users. at no point in time does any part of the piston, be it skirt or ring, contact the cylinder wall. there at least should always be oil in between two moving parts. if they come in contact, they will seize which is kind of a bad thing

[catbed]

catbed- You can run any set of numbers any amount you want. Real world examples give you better models to work from. Still its a good idea to have guidelines to build within.

Also riding the knock threshhold for what reason? Pushing a motor til it knocks and then back it off a degree or 2 will not yield the most power (though sometimes it can). This is a major misconception that a lot people get suckered into believing. I actually believed this myself and tuned a 4AGTE in AE86 to peak 218rwhp. Sold the car and the next owner had it dyno tuned. It made peak 222rwhp at the same exact psi on the same exact setup. However he pulled 8 degrees of timing and gained 40 lb ft at 3500 RPM.

I AM talking real world examples. Where do I throw arbitrary numbers out?

When tuning a car, you shoot for MBT. In the 1.8t engine, MBT is knock limited.

And you are saying that your old car gained 40 lb-ft due to pulling 8 deg of timing? That doesn't make a lick of sense. And were the dyno runs on the same dyno, same ambient conditions, and back to back pulls? Didn't think so.

[ZimbutheMonkey]

Yes, but who is running a built motor at only 10psi?

I do, every time my engine is below about 4000-4200 RPM. in fact, every motor out there runs only 10 PSI right until it hits 11 PSI. The point I'm trying to make here is that the OP is asking about overall driving characteristics of an engine. Let's face it, the bulk of a small displacement boosted engine is spent at part throttle and in less than full boost conditions.

Until you hit the that threshold where a high compression engine starts to detonate, the high compression engine will yield better torque and power delivery as opposed to it's low compression counterpart whenever it's not riding that knock threshold.

Myself, I live at 3500 ft elevation and I'll tell you, a 1.8l engine off boost can hardly get out of it's own way. That little extra bit of power I get from the stock 9.5:1 compression is well worth it when I'm taking off from a light or when I get stuck in a position where I need to accelerate and downshifting to 6500 RPM (like that weird spot in 3rd around 75-80 km/hr) isn't practical.