high compression pistons?

[tstkl]

Ive been thinking, and i know all the advantages of running higher compression pistons, and i also know some of the disadvantages... but i was wondering, if you have a really efficent really big intercooler, whats keeping you from running a high compression (im not talking 12:1 or something, more along the lines of 9.5:1)? i know the evo 8's use 8.8, and the 2g's use 8.5, so why hasnt anyone run anything higher... a good intercooler would prevent knock, so what are the limits of the stock engine... im guessing that if your running 30 psi at a 12:1 ratio, even if the intake charge was cold enof to survive premature ignition, the pressure would break something, so does anyone know what the limits on our block are?

 

[Cyclone_90]

Ive been thinking, and i know all the advantages of running higher compression pistons, and i also know some of the disadvantages... but i was wondering, if you have a really efficent really big intercooler, whats keeping you from running a high compression (im not talking 12:1 or something, more along the lines of 9.5:1)? i know the evo 8's use 8.8, and the 2g's use 8.5, so why hasnt anyone run anything higher... a good intercooler would prevent knock, so what are the limits of the stock engine... im guessing that if your running 30 psi at a 12:1 ratio, even if the intake charge was cold enof to survive premature ignition, the pressure would break something, so does anyone know what the limits on our block are?

I don't know the limit of the block but what you are saying is correct. If you can cool that incoming air really good you can run 9:5s, hell a few people on here are running that kind of compression but i would not go past that unless you are going to be drag only. I am running 8:3 wesico's in my car

 

[DSMSpyder99]

i wouldnt go past 9:1

 

[94awdcoupe]

im building my 2.3 stroker with 7.5:1

 

[1fstSS]

buddy has 9:1 on stock sidemount in his eclipse had no problems at all.

 

[tstkl]

now to find a dealer with 9.5:1 pistons, altho i guess i could wait til i bore it out to .2 or .4 and get custom je or whatever that company is that makes really good pistons... (price... EEK)

 

[Syndicate13]

My question is why? Higher compression requires higher octane and you may find that you won't be able to get away on just pump gas anymore. The difference in spool will be negligible on a turbo that's big enough to count anyway. Lower compression can always be made up for with more boost. Hell the fastest car semi-locally is 7.7:1, old numbers were 468hp, 450tq and 10.89@129 all on 93 octane and water/alky injection.

 

[tstkl]

so your telling me that forcing the exaust gas out with a pressure of 1.7/7.8 times as much force wouldnt create a shorter spool time? have you tried this? or are you guessing?

 

[Syndicate13]

so your telling me that forcing the exaust gas out with a pressure of 1.7/7.8 times as much force wouldnt create a shorter spool time? have you tried this? or are you guessing?

No, I'm saying the differences would be negligible at that point. You want faster spool? Get more displacement. What are you going to do with this car? Strictly drag racer? Then get some high compression pistons and boost the hell out of the motor on C16 and run some good times. Is it gonna be a daily driver? The do the same and try to get by on pump gas and have fun picking metal pieces out of your oil pan. Don't do something stupid just trying to be different.

 

[tstkl]

damnit...thats a good point......does anyone know the numbers for an evo? (octane recomendation from factory...) cuz i will probably end up running those pistons, because you can find them used for cheap... those are the most realistic for me,.. and they are 8.8 CR,... 9.5 doesnt seem too much higher than that tho.....

 

[sonicnofadz]

People usually don't go that high with there compression. At 9.5:1 its going to be very difficult to tune without destroying the engine. I'm not saying it is impossible, but I am saying that your chances of a disaster are higher (smaller tuning window) at this compression ratio.

 

[94Jettameowpsst]

This has nothing to do with talons, but Brian Kirchberger has/had the worlds fastest/quickest VW jetta. His motor is built now but his record pass in his turbo vr6 was like a 9.6 on his stock bottom end. Btw if you dont know, the pistons in mk3 vw's are cast and the rods and crank are forged stock and the stock compression is 10:1. I dont understand how he was able to do this, run 28+psi and not break anything. Read up on it, you'll see he made over 20 passes on his stock block. Why hasnt anyone incorporated this into a talon
Edit: Btw, the car was a streetable almost dd

 

[Syndicate13]

This has nothing to do with talons, but Brian Kirchberger has/had the worlds fastest/quickest VW jetta. His motor is built now but his record pass in his turbo vr6 was like a 9.6 on his stock bottom end. Btw if you dont know, the pistons in mk3 vw's are cast and the rods and crank are forged stock and the stock compression is 10:1. I dont understand how he was able to do this, run 28+psi and not break anything. Read up on it, you'll see he made over 20 passes on his stock block. Why hasnt anyone incorporated this into a talon
Edit: Btw, the car was a streetable almost dd

But were those runs on pump gas? Almost streetable? Does the car retain AC and power steering? Is it full weight, meaning not a stripped shell with a driver's seat and steering wheel as it's only amenities.

Ocatne recommendation for every DSM as well as the Evo is 91+.

 

[sonicnofadz]

This has nothing to do with talons, but Brian Kirchberger has/had the worlds fastest/quickest VW jetta. His motor is built now but his record pass in his turbo vr6 was like a 9.6 on his stock bottom end. Btw if you dont know, the pistons in mk3 vw's are cast and the rods and crank are forged stock and the stock compression is 10:1. I dont understand how he was able to do this, run 28+psi and not break anything. Read up on it, you'll see he made over 20 passes on his stock block. Why hasnt anyone incorporated this into a talon
Edit: Btw, the car was a streetable almost dd

First of all who the hell wants to drive a VW jetta anyway, its butt ugly (joking) And second of all I'm sure if we stripped a first gen down to get the curb weight down to like 2300-2500 lbs, and boosted the stock block to around 500 hp (which as been done) it could make around a 10 sec quarter mile. This is all in theory though

 

[Warmachine91vr4]

my buddy ran 9.5-1 in one of his projects he is a spoiled rich boy, i knew from day one he was not going to get many pulls out of it, he was running a holeset ,ems ,dual 990S etc etc.., to make a long one short he filled the cell up with VP ultimate 4=> an oxygenated race bike fuel and jet a <an OK batch it was 154 octane +OR- 5 points. ran 60/40,turned the duty cycle of the alky injection to 100 percent, pulled out burned 3 gallons warming up and messing around the car pulled very hard i would say it would have been good for mid to high 9's @ a hair over two bar, we wanted to open it up, we re filled the cell and trailered the car to local deserted road about 14 miles of straight its amazing. he started his first wot pull he hit about i guess 130 or 140 he heard a loud bang bang boom i heard in in the chase car about 20 car lenghts back, and fire started rolling out of his hood, he slammed the brakes on the car stopped fast i love brembo brakes i got out and i put the fire out it took about 15 min,i always carry an extinguisher, by this time he was on the ground thanking god that he didn't roll it, i yelled at him to come over and look at this, the head was blown off the block one stud held it on, i have never seen anything like it in my life. it could have been caused by a number of things i bet if he was running 7.5-1 it wouldn't have never done that. compression is h/p the more you have the more you get but boost can compensate for low compression w/o these effects. and race fuel is very very expensive the vp is almost 20 a gallon, and she drank it like water. i run it on the race weekend's at the track in my zx10r its bad ass shit

 

[Lunch_Box]

Swordfish on here is running 10:1 compression in his 2.4L and he drive his car daily.

 

[94Jettameowpsst]

Regardless of how much the car weighed or what he had done as far as weight reduction, its the fact that he ran 28+psi boost on a stock bottom end w/ 10:1 pistons. Idk if he ran that on pump or not, im assuming it was on race gas but even so, i dont see why that cant be incorporated into a dsm. All mk3 vw's (idk about the mk1, 2, or 4) were rated at 10:1 compression with the exception of the tdi and i think the early 16v passats (9.8:1). The gas motors are all rated with regular gas (87 octane) with no problem. One other thing i noticed: What yields a higher torque number than a hp rating? Almost every vw and/or audi i have seen has had almost the same or higher torque # than hp.

 

[tstkl]

yea, your right, high compression will give better torque numbers, thats why im looking into getting higher compression,... clears out the exaust gas faster and sucks in the intake charger faster... that makes for a higher VE which is what torque is... so its pretty obvious that the torque will see gains... thats why whenever you loosen up the exaust or free up the intake you can see torque gains....and why tuning the air fuel ratio doesnt do as much for torque as it does for hp....

 

[94Jettameowpsst]

That sounds very correct but why on a honda, acura tsx actually, i think that motor has pretty high compression but it makes like 200hp and less torque than it takes for me to bolt on my valve cover. I dont understand, does it have to do something with the intake manifold design or the header/ manifold design too?

 

[Syndicate13]

Here is a take on it from Dave Mertz, one of the creators of DSMlink.

I think the title of the thread should really be "10:1 compression and turbo". I think it's generally a bad idea unless you are going to limit your boost to lower than stock. High compression, moderate boost and pump gas = detonation problems. High compression, high boost and race gas = cylinder sealing problems.

By raising the compression ratio that much from stock, you will probably need to retard timing signficantly in most areas.

The highest-power drag DSMs are running compression ratios in the 7s:1, from what I have seen.

If you want better off-boost performance with a big turbo, increase displacement, not compression ratio.

DSMLink will probably allow you to retard timing enough (as much as -14 degrees on each slider in the current PC app version) to eliminate the knock, but it probably won't make much power that way.

The archives contain at least one thread of an application with something like 9.5:1 and knock problems on pump gas. The owner wanted to use more timing advance, but the engine configuration just doesn't seem to support it. Even 9.5:1 is too much compression in my opinion.

Dave

 

[tstkl]

thats because the displacement is so low and that 200 hp is made at like a million rpms.... i doubt there will ever be a honda with more ft. lbs. than hp... thats just something they arent interested in... s2000-240hp-160ftlbs... we dont drive hondas, so it doesnt really matter...

as for the bigger displacement point of view... yes displacement will definatly help more than compression,... but from what ive read the stock walls get to as small as .8, which means boring it out .4 would reduce the walls on some places on the block... so high compression with race gas, like he said will have seal problems, or in this case it will just blow up your engine as well... im not really willing to pay the 3k+ it would take for a rebuild just to run like 50 ml more displacement... imho boring out an engine thats 2L or less is pointless unless it can be bored out 1.5 or more over...()...

 

[Syndicate13]

thats because the displacement is so low and that 200 hp is made at like a million rpms.... i doubt there will ever be a honda with more ft. lbs. than hp... thats just something they arent interested in... s2000-240hp-160ftlbs... we dont drive hondas, so it doesnt really matter...

as for the bigger displacement point of view... yes displacement will definatly help more than compression,... but from what ive read the stock walls get to as small as .8, which means boring it out .4 would reduce the walls on some places on the block... so high compression with race gas, like he said will have seal problems, or in this case it will just blow up your engine as well... im not really willing to pay the 3k+ it would take for a rebuild just to run like 50 ml more displacement... imho boring out an engine thats 2L or less is pointless unless it can be bored out 1.5 or more over...()...

You do know you can stroke our motors, right? Or go big block style with a 2.4 bottom end.