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Compression Ratios and NA vs turbocharged cars!

cheekychimp

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Apr 19, 2004
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Since being back in the UK I have taken the GTi out for a few runs and I have to say that around town, at low rpms and uphill, it's a real pleasure to drive and seeing as it only produces about 145 hp stock, I imagine that must be due to the higher compression ratio. I believe these cars came stock with a 9.5:1 or 9.8:1 CR whilst the later model GTis and AMG Galants came with a 10.4:1 CR that netted about 20 hp more.

Now I know people have been using 2G pistons for years and it is not uncommon to see 9.0:1 CR pistons in builds but what about going higher?

I believe Tom Noonen did a 10:1 CR build on an EVO III 16G setup a few years back that was quite successful and tuned using an AEM EMS and I think I have heard of people using 10.5:1 CRs on turbocharged 4G63s.

In years to come if I keep the GTi, I think I will definitely fit higher compression pistons but what about for a turbocharged daily driver? Is it a practical method of engine building? It seems to me that there are obvious advantages to a build like this. Clearly you would require forged pistons rather than straight NA ones but as long as you tuned for knock and didn't expect to run as high a boost pressure as with the 7.8:1 or 8.5:1 pistons it seems you are just starting from a higher baseline.

I imagine there is a theshhold of how high a CR you can obtain and I also presume that when you get to a certain CR the amount of boost you can run safely becomes negligible so that you meet a point of diminishing returns where lower CR and more boost yields better results. I am sure also that in the maximum horsepower stakes you can make more power off boost than off the CR.

But for a 300-320 hp car running a small 16G how high could I go with a CR if it was tuned correctly and how much boost could I run?

Last question is will putting NA equivalent CR pistons in a turbocharged car make it just as responsive off idle as the NA car or does gearing, cam profile, and the extra length of the intake tract have much more to do with it?
 

thecman02

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Nov 3, 2007
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Kalamazoo,MI
You only have about 1/4 the equation. Boost + CR + Timing Advance = Required Octane.

Most people that get away with CR's over 9.0:1 have really great tuners. Didn't Tom Noonen get like 400whp on 93 with a 16g. He had the best possible scenario for getting horsepower and not detonating the crap out of the engine.

With a small 16g, a great intercooler, a great tune, and 10:1 CR I could see you running about 20psi with 93 octane. I think it might get a little sketchy with how much timing advance you could run. If you have too low timing advance exhaust valves start burning up and piston tops get brutal hot. If you ran race gas then you could max out that turbo and have it beg for mercy. A buddy of mine in town is going 10:1 but runs e85 all the time.

Most of this is just theorycrafting of course, and I think some people on here actually have first hand experience. All I know is with higher octane you can get away with higher compression, more boost, and more timing.

 
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broxma

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Nov 16, 2009
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San Antonio Tx
The English guys at English Racing were running(Still are??) an aluminum rod high CR Evo at 10.5-1. If you check on EvoM, search for the fastest stock turbo Evo. They run homeblend E-90 and a crazy built motor, with a stock Evo 9 turbo and exhaust manifold. Last I heard, they were laying down 522 at the wheels....stock Turbo....stock manifold. At some point they were talking about running 11-1 but I am not sure if they did or not.

When I rebuild the bottom end of 379, I'm going 9.5:1 but will run on E-85.

/brox
 

4thStroke

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Oct 22, 2007
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Luke's motor wasn't anything uncommon for a built motor. Aside from the aluminum rods, it was a pretty basic build. He ended up losing the timing belt or something and is now back to his stock turbo setup, but now with a 2.4 and not so high strung. I can't keep track of how many times he has changed the setup on his Evo.

Here's another high compression aluminum rod motor they built. The car was scrapped, but the motor is going into a FD /ubbthreads/images/graemlins/devil.gif
click

It must have been determined a while back that 9:1 was the magic number that satisfied most people. It must be the happy medium between running the most timing and boost while still getting away with 91oct.

If you have access to 93oct, I'd still take the route of building a 9:1 and running more boost.

I've had a 16G seize up on me before, and the NA power was nothing in comparison that you speak of.
 

cheekychimp

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Spence, I didn't mean to suggest that the NA power of the GTi was comparable to the VR4 because I know it isn't once boost comes on, it is just that off boost response. I can climb inclines in third and fourth in the GTi that I know would make the VR4 bog below about 2500 rpms but the lightened flywheel on the VR4 might have something to do with that.

I also loosely figured timing advance as being part of the whole 'tuning' part of the equation but I had forgotten about octane. I think in Hong Kong as in the UK I would be fine as our lowest octane is 95 and super unleaded is 98. The Type R is I believe at least 11.0:1 and runs fine on regular.

Tom Noonen did make 400 whp off an EVO III 16G and also claimed to get over 30 miles per gallon on the AEM EMS tune. I guess there is an element of risk with a build like this but it does sound as if my setup would certainly be a good candidate for a high compression ratio motor. Although people run 22-23 psi on the EVO III 16G, I think I'd be very happy with 18 psi on a Small 16G to be honest.

It's good to hear the 9:1 CR is reckoned to be a good compromise because that is what I have in the 2.3 and that motor will need 25-30 psi. I'd like to try a 10:1 build in the 2.0 litre later. I guess we'll see.
 
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4thStroke

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Sorry of what I said came off wrong, it didn't come across as you were comparing the GTi to the VR4 in that way.

But, what I was trying to say was that for when my turbo motor was stuck in NA mode for the drive home, it was certainly a chore to climb hills. Throttle response was horrible, and it just didn't have any pep at all. With it being a 9:1 motor, with exhaust, intake, the works, it seemed as if those mods did absolutely nothing to help the NA 2.0 breathe, it was a dog.

A turbo car needs to work around the turbo, an NA car needs to focus more on volumetric efficiency than anything.

I've heard it said once before, and although it may not be the full story, it may be worth thinking about. I always thought it was interesting when people turbocharged the high displacement domestics, the results usually didn't pan out like we would expect. Someone told me it was because the high compression ratio of the OE motor wouldn't allow for more boost and timing to take real advantage of the turbo.
 

SouthCaliVR4

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Jul 31, 2010
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North county San Diego
^^^ Had you removed the frozen turbo & run an N/A mani or header with the intake bypassing the turbo as well, you're power would have been much better but with a dead turbo in there you had major restrictions in both the exhaust & intake.
 

4thStroke

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Ah, that didn't even really cross my mind.

I should just delete my previous posts /ubbthreads/images/graemlins/blush.gif
 

Rausch

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Dec 21, 2004
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Cleveland, OH
^That and cam profiles would also play a big role. Boosted cars don't like much overlap. There are scavenging advantages that come with overlap that are more or less killed by the huge restriction in flow (turbo while not under boost). Granted that has more effect at higher revs (N/A).
 

RedTwo

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Evo III runs 9.0:1 with the 6 bolt? rotating assembly but they also have a tendency to explode - mostly due to defective parts located between the steering wheel and the front seat. /ubbthreads/images/graemlins/devil.gif

The turbo Hondas (integras in particular), as in those that turbocharge the stock engine run 9.2 - 10.4:1 compression ratios. I don't know how much boost they expect to run but I doubt it's near or over 1 bar. /ubbthreads/images/graemlins/dunno.gif
 

Don't know if it helps but I've been running 4g63 N/As on boost, always kept under 8 pounds, Think that was 9.1:1 CR. Run 98 in a galant here in NZ. Also an eclipse in Mexico but I'm not sure what gas I was using there /ubbthreads/images/graemlins/rofl.gif /ubbthreads/images/graemlins/dunno.gif They went pretty hard /ubbthreads/images/graemlins/grin.gif
 

cheekychimp

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Apr 19, 2004
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East Sussex, U.K.
Quoting RedTwo:
Evo III runs 9.0:1 with the 6 bolt? rotating assembly but they also have a tendency to explode - mostly due to defective parts located between the steering wheel and the front seat. /ubbthreads/images/graemlins/devil.gif

The turbo Hondas (integras in particular), as in those that turbocharge the stock engine run 9.2 - 10.4:1 compression ratios. I don't know how much boost they expect to run but I doubt it's near or over 1 bar. /ubbthreads/images/graemlins/dunno.gif



I've actually heard that about the EVO III pistons before, but is it an issue with tuning? I get the feeling that idiots who have no idea that they are driving a higher compression ratio just turn up the boost and laugh ... until it all ends in tears.

I also think for a street car that 1.0 bar is fine so if there is a safety margin to play with, I'd rather take advantage of that with a higher CR than more boost.

Justin, can you expand upon "went pretty hard?"

Was it a better driving experience than running a lower CR with more boost? I'm getting a lot of conflicting views on this. Some say go for it whilst others are saying "Nooooooooooo!"
 
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Wish I had first hand experience with evos, they are so expensive I would rather buy a tidy skyline /ubbthreads/images/graemlins/blush.gif

Quote:
Justin, can you expand upon "went pretty hard?"



I mean loss of traction through 1st, 2nd and 3rd /ubbthreads/images/graemlins/grin.gif But they were FWD. I like tinkering with front wheel drive versions as they're light, and parts are cheap/easily obtained.

As for driving experience, Maybe just me but I thought they felt a bit more responsive off boost. I know due to the piston design knock can be dangerous for an NA

I know heaps of people who run Turbo+NA, funnily enough friends who have melted pistons etc all had factory turbo engines. I asked a lot of people about turboing an NA motor, I got a lot of mixed feelings on the subject, a few just said "you don't have a clue mate /ubbthreads/images/graemlins/uhh.gif" Turns out most of them were or had been doing it themselves and just didn't want to admit it. After asking a lot of people the consensus was it's ok as long as the boost is kept factory. Honestly the people who do turbo NA engines (like me) tend to have a lower budget or like to experiment. In theory it's not ideal, but in practice it seems to work.

Tell you a funny story I heard about a mate's tinkering. He had a SOHC 4g63 in his L200 truck, changed the head for a Dohc but origional bottom end, then added a turbo. Aparently the sohc pistons in a dohc give you something like 13:1 CR and then he boosted it /ubbthreads/images/graemlins/rofl.gif don't know what happened though /ubbthreads/images/graemlins/laugh.gif Don't know if it's true but that's what I heard from a class mate.
 
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