9:1 compression piston (Pros and Cons)

[ekanddsm]

I have a 91 TSi AWD that I bought from a friend and I was wonderin.

What kinds of problems do 9:1 compression pistons bring to a turbo'd engine?

Detonation, boost spikes, etc., etc., etc.?

I bought the Talon in non-running condition for cheap! and the engine would not rotate.
I removed the engine and found the wrist pin had come out of the #1 piston and the rod bolts were loose. The engine had 160k miles on the odo. but the wall look great. rod journal on #1 is in excellent shape.
So here comes some new pistons. Maybe?

 

[herostar]

I haven't heard of many problems, but 95% of dsmers who run aftermarket pistons go 8.5:1 or 8.8:1. With 9.0:1 it will be a little more difficult to tune at higher boost levels.

 

[Mirage2LTurbo]

I still have yet to be explained why it's harder to tune on a higher c/r motor. I'm running 9:1, it runs fine. In retrospect, I should have built a 8:1 or 8.5:1 c/r, but I thought I'd be driving it on the street more than I am.. it's mostly a track car.

 

[n3gativerr0r]

9:1 is going to give you nominally better spool times, better low/mid range, and better gas mileage if that matters to you. You will not be able to run as much boost which will give you slightly lower peak power numbers.

Lower compression will allow you to run more boost and ultimately make more power. For a street car I would go with 8.5-9.0. For a track car Id go with 8.0 or even 7.5 with some N20.

There is a guy here running a 10.0 comp 2.4 Liter making some pretty good numbers on pump gas. So higher compression is doable, tuning isnt necessarily harder.. you just cant run as much boost.

 

[highboosted98]

If there forged pistons should as JE`s the should be able too handle 600+hp of whatever u wanna throw at it (tuned accordingly) As far as cons go, The higher compression will be harder too tune. You should decide what turbo your gonna use first then you`ll know what kind aftermaket mods u`ll need.

 

[leakyfaucet]

As far as cons go, The higher compression will be harder too tune.

Reading comprehension, anyone?

 

[diambo4life]

The higher compression will be harder too tune.

It will not be "harder to tune" than a lower compression car. You'll just have a smaller margin of error.

 

[DSMJim]

It will not be "harder to tune" than a lower compression car. You'll just have a smaller margin of error.

However the difference in the safty margin purely going from 8.5 to 9.0 is very small in its self. Your tuning and overall HP and boost numbers will likely be quite the same. A forged 9:1 piston will take more abuse than a cast 8.5:1 piston will so really your margin of error is larger vs. a stock piston and will be roughly the same as a forged 8.5:1 piston.

Lets put it this way, my buddy was working on his setup on the dyno. Wiseco 9:1 pistons Pauter rods (in a Miata motor but much the same as a 4g63 really..) and he was making around 355whp and he sprays nitrous on the car and the air fuel hits 17:1 on a spike, the motor makes this horible noise, he lets off the gas then pinns it again, and it goes down to 14:1 at about 8000rpm making 460whp shaking like a mofo. Nothing happened the motor was fine. He had to make some fuel pressure adjustments and sprayed again after that and the car was fine. If he had a cast piston in the car it would have been all over. The nitrous made such a huge spike in power on his car in 3rd gear he car woudl go sideways on the track with drag radials just to give you an idea what kind of power jump the motor was dealing with on the dyno at 14:1 air fuel ratio...

For the easiest tuning lower compression is better as everybody has said, however as long as your not trying to run 25psi on pump gas or anything else we wouldn't normally do you will be just fine the same as normal tuning your car.

 

[noreaga0221]

9:1 is going to give you nominally better spool times, better low/mid range, and better gas mileage if that matters to you. You will not be able to run as much boost which will give you slightly lower peak power numbers.

Lower compression will allow you to run more boost and ultimately make more power. For a street car I would go with 8.5-9.0. For a track car Id go with 8.0 or even 7.5 with some N20.

Higher compression is bad for guys trying to make 500hp on pump.. If i had a race car, i would run like 11:1 CR and this is why. At the track you only run alcohol, straight c16, whatever race gas you like, fukc pump gas. With such good gas, no matter what boost you run, as long as your timing and AFR are in check then the car will hold together. Same boost on the same turbo with same tuning, the higher compression motor will make more power.

Raising the CR is like spraying or running more timing or boost, it all raises cylinder pressures to make more power, that pressure (=heat) is what causes preignition. Remember octane is the measure of gasolines resistance to preignition. So run race gas

That 7.5 CR will be a pig off boost but you could run some crazy boost on pump gas. But once some good octane fuel is being used, power that could be made isnt because of the lower compression ratio.

 

[bfdahl]

I have a 91 tsi awd that I bought from a friend and I was wonderin.

What kinds of problems do 9:1 compression pistons bring to a turbo'd engine?

Detonation, Boost spikes, etc., etc., etc.?

I bought the talon in non-running condition for cheap! and the engine would not rotate.
I removed the engine and found the wrist pin had come out of the #1 piston and the rod bolts were loose. The engine had 160k miles on the odo. but the wall look great. rod journal on #1 is in excellent shape.
So here comes some new pistons. Maybe?

Keep in mind that most of these guys that say that they have no probs running on 9.1 have better gas than we do, that in mind ive talked to guys from our neck of the woods all the way up to seattle and the cars are very sensetive on 91 oct. with 9 to 1s. 8.5 forged is a better way to go. Especaily if you resurface the head or block, your compression with be slightly higher than you think.

Definetly have the big ends of you con rods checked by a good machine shop, and either re condition them and install ARP rod bolts or get eagles or scats. Doing all that work and the expence of parts will be for not if something is bent or streched in the bottom end IMO.

 

[1fast97gsx]

I think 9:1 is the way to go. I had 8.5:1 last year and my new pistons are wiseco 9:1. On pump gas you have to run slightly less boost unless you are one hell of a tuner, but who has a forged engine and races on pump gas? On race gas, as stated above, high boost and high compression owns. As far as just daily driving and mid level boosting, high compression is also better as it will help with spool up and give better throttle response. I don't really think there are any negative side effects of running 9:1 unless you are trying to run crazy boost numbers on pump gas. ( unless of course alcohol injection is used )

 

[anconover]

im the opposite. Id rather have a lower compression. Because i like to run pump gas on the street, and at the track id like to run high boost and spray. That will take care of the lack of off boost power. Extreme did 8's with a 7:5:1 CR, and showdown does 10.1's with a 7:0:1 CR i believe. So theyre is nothing wrong with running a lower compression if you plan to spray.

Now i wouldnt run 7:0:1 on a street car of coarse, but my preference would be 8:2:1 or so. Of coarse its pretty dependant on the setup, a turbo that poops out at 25psi, wont be any fun on a low CR.

 

[DSMu4ia]

im the opposite. Id rather have a lower compression. Because i like to run pump gas on the street, and at the track id like to run high boost and spray. That will take care of the lack of off boost power. Extreme did 8's with a 7:5:1 CR, and showdown does 10.1's with a 7:0:1 CR i believe. So theyre is nothing wrong with running a lower compression if you plan to spray.

Now i wouldnt run 7:0:1 on a street car of coarse, but my preference would be 8:2:1 or so. Of coarse its pretty dependant on the setup, a turbo that poops out at 25psi, wont be any fun on a low CR.

...and there are guys going 9's on 10:1.

9:1 is fine, my car feels peppier without boost compared to most DSM's I've been in.

No tuning issues here either.

 

[ekanddsm]

...and there are guys going 9's on 10:1.

9:1 is fine, my car feels peppier without boost compared to most DSM's I've been in.

No tuning issues here either.

I am a new DSM owner as I can't wait to drive my car. But at the same time I am curious as to the numbers. I like asking forums car tech questions. All butt kissing aside will the 14b be able to boost 20 psi, my ultimate goal is 400-450 ponies on pump gas. I dont want to build a car i cant drive to work. But I want something that will walk away from my civic(daily driver).

So can 400 ponies be easliy be made with that compression ratio, and a 20g?

 

[peregrine]

So can 400 ponies be easliy be made with that compression ratio, and a 20g?

that power could be made easily with a STOCK motor and a 20g. so to answer your question, yes.

 

[DSMJim]

that power could be made easily with a STOCK motor and a 20g. so to answer your question, yes.

400whp is not "easy" on a 20G with a stock motor. You can do it however your running a lot of boost on pump gas to do it. On race gas, yes thats easy. 26psi of boost will do wonders on 117octane gas.

 

[anconover]

50trim, stock motor, cams and an intake manifold (probly not necessary) will make between 400-450whp on pump pretty easily.

 

[peregrine]

400whp is not "easy" on a 20G with a stock motor. You can do it however your running a lot of boost on pump gas to do it. On race gas, yes thats easy. 26psi of boost will do wonders on 117octane gas.

i would never try to make 400 horse without race gas so i should have made that clearer.

 

[Black94TSI]

A 200 horsepower, 7.8:1 compression motor will make about 210 horsepower at 9:1. Each full point in compression is worth about 4% in total power. I don't belive the compression helps spool your turbo BS, when I still had a honda I went from a stock B16 at 10.4:1 compression to SRP 8:1 compression pistons, there was zero difference in when boost came on. It felt a little bit sloppier on the low end, but I look at it this way. Do I want to have 10 more horsepower from idle-4000 so that I can short shift through that intersection quicker, or do I want to have the ability to make 100 more WHP on pump gas for when i'm going WOT and want to be accelerating fast? If you want to spool the turbo quicker, spend a couple hundred more bucks on a 4G64 crank and build a 2.3. If you have 94/93 octane I would probably build it at 9:1 compression (shit, look at evo's, they run like 19 psi stock on 9:1 compression, and mitsu wouldn't put out an engine that was on the edge of detonation stock otherwise they'd have soo many returns with blown engines) and if you only get 91 i'd stick with 8.5:1.

 

[Mirage2LTurbo]

A 200 horsepower, 7.8:1 compression motor will make about 210 horsepower at 9:1. Each full point in compression is worth about 4% in total power. I don't belive the compression helps spool your turbo BS, when I still had a honda I went from a stock B16 at 10.4:1 compression to SRP 8:1 compression pistons, there was zero difference in when boost came on. It felt a little bit sloppier on the low end, but I look at it this way. Do I want to have 10 more horsepower from idle-4000 so that I can short shift through that intersection quicker, or do I want to have the ability to make 100 more WHP on pump gas for when i'm going WOT and want to be accelerating fast? If you want to spool the turbo quicker, spend a couple hundred more bucks on a 4G64 crank and build a 2.3. If you have 94/93 octane I would probably build it at 9:1 compression (shit, look at evo's, they run like 19 psi stock on 9:1 compression, and mitsu wouldn't put out an engine that was on the edge of detonation stock otherwise they'd have soo many returns with blown engines) and if you only get 91 i'd stick with 8.5:1.

I'm in the same boat. A higher c/r just makes more offboost power, which gets you through the lower rpm's faster, it doesn't spool the turbo any sooner. If anything, a higher c/r spools slower because is goes through the rpm's faster - kinda like being in 1st gear versus 3rd gear.

 

[Neo863369]

hello Im new here i own a 9? Talon TSI fwd 5 speed. last year when i was building a Beretta GTZ motor, 2.3 DOHC, i called Pruven Performance in Milford CT, they are DSM experts featured in SCC. I was going to build it up for turbo so i called to ask what cpr i should run. to make a long story short they said they build all their motors at 9:1 cpr. They have DSM's in the 10's and an EVO VIII in the 11's.





Todd

 

[Aaron91RS]

High compression on a turbo motor is so F$%#ing stupid for 99% of people here.
I wish I could lower mine to from 8.5 down to 8:1 which is near ideal for turbo cars.
As it is I have to back the timing way off to run 20psi without knock. Who cares that it has better off idle torque. Don't race at 2000rpm's, do you?. I want my old 1g motor back. 22psi with stock timing on pump gas.
You don't see domestic's putting 15psi blowers on their stock 10:1 compression v8's do you?? Want to know why? It's not cause they don't know what they are doing. It's cause its stupid, it's a bi*** to not knock with any amount of timing and it blows shit up and you gain nothing vs a lower compression higher boost engine.

Anyway all you smart guys know better, because this stupid topic comes up way too much. Go tune that 9.5:1 engine for 25psi. Let me know how long you can afford to fill up everyday on c16 or when you blow it up.

 

[DSMu4ia]

I'm in the same boat. A higher c/r just makes more offboost power, which gets you through the lower rpm's faster, it doesn't spool the turbo any sooner. If anything, a higher c/r spools slower because is goes through the rpm's faster - kinda like being in 1st gear versus 3rd gear.

HAHAHA you are the funnay!

There is a LLLOOOTTT more to this than just picking a compression ratio piston and thinking this is the end all decision of your build. Things such as ANY head milling, deck work, improper cam degree'ing, combustion chamber volume/quench, deck heigth, etc. There are TONS AND TONS of factors. Don't just pick a number and stick to it, work with what you have.

 

[Mirage2LTurbo]

HAHAHA you are the funnay!

There is a LLLOOOTTT more to this than just picking a compression ratio piston and thinking this is the end all decision of your build. Things such as ANY head milling, deck work, improper cam degree'ing, combustion chamber volume/quench, deck heigth, etc. There are TONS AND TONS of factors. Don't just pick a number and stick to it, work with what you have.

I was trying to keep it simple

 

[sunflashx]

Here's my experience with 9:1 compression.

9:1, 25 psi, 91 octane, no problem.

9:1, 28 psi, 91 octane, now you need to have a way of adjusting timing.

On my old 8.5:1 95 piston big rod motor it didn't seem real partial to running over 23 psi, so who knows... Different motor, different turbo, different cams, different intake manifold, so comparing the two isn't apples to apples.