cheekychimp
Well-known member
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?
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?