ShopTalk Forums | Aircooled VW Tech & Restoration Help

Hey everyone.

Ive been reading through a lot of posts and write ups about forced induction and there is one thing I just cant get my head around..

How is it when building an N/A motor people say to keep your compression ratio below the 9-9.5:1 for a safe street engine to avoid detonation and over heating; but most of you guys are running not much lower compression ratio, but with huge amounts a boost added on, and pulling massive HP numbers without detonation or overheating?!

I was just looking over a build with a 1915 running 8.5:1 with 17lbs boost dyno'd just over 200HP and running cool as a cucumber

Surely a 8.5:1 compression motor getting 17lbs shoved up it would have higher cylinder pressures than an N/A running 9.5:1 and be much more prone to heat and detonation??

What am i missing??

Thanks so much

that motor probable had an 'Intercooler' /charge cooler of some sort, that helps with destination plus good fuel at those higher boost levels

Your not missing anything, and this is my very humble understanding. I would say a lot of guys over 14psi of boost are also running EFI giving great control of timing and fueling.

Intake temp plays a large part in the detonation game. So Intercoolers and water injection help there.

Last game changer would be e85

I’m not a VW expert by any stretch but I worked on motorcycles for a bit before changing employment. All that I think VW guys run lower compression to help combat heat but really it’s an ignition and fueling game. I think you could run 9:1 on a turbo motor and be ok. With the above items used.

I agree. Tune is everything. Low intake temps helps too, but mine still gets up there if keep on it. When it's 97 degrees out it doesn't take much to bump up into the 130 temp range, but I've seen no troubles so far. Also you have to keep in mind, with a turbo, those high compression numbers are only there during boost. On the street you really can't boost that long.
Also 9:1 really isn't that much compression on an N/A engine either. Plenty of guys run more than that. I think it mostly depends on the cam. More overlap bleeds the compression off.

buguy wrote: ↑Thu May 28, 2020 5:18 pm I think it mostly depends on the cam. More overlap bleeds the compression off.

^^ That!

Blown engines also have a better mixture I think.
The rest is just pure magic

Yeah! It's just cuz we're cool and stuff!

The effective compression ratio for an engine with a static compression of 8.5:1 and 17psi of boost is 18.33:1. What does that mean- well, you can brag to that NA guy that you're running 18.33:1 in you bug!

You've also gotta remember that a boosted car is usually running at low compression levels most of the time.