Forged pistons and high compression ratio?

**spetz** · 24-09-2011 06:37 PM

Hi guys,

So the general rule is that the maximum CR for a street car using street fuel should be about 11:1.
How come forged pistons can have higher compression ratio? And would it not cause detonation?
The only thing I can think of is that cast pistons may crack under pressure where forged won't, but what happens to tuning when running 12.5:1 on pump fuel and forgies?

Also, at what stage are forged pistons worth it?
Is it only when the rev limit is raised for higher rpm/power?

**GSi_PSi** · 25-09-2011 12:31 PM

Forged Pistons are mainly desired for turbo charged builds but they can work equally well for an allmotor build. Ive seen engines with 12+ compression running on our 98 RON run okay, but
due to the E85 (100RON) fuel available that is much more desirable as it gets the most out of the motor. Usually you would not want a car with 12:5+ compression as a daily car, more so a track/drag orientated car.
Running with a crappy or no tune+ low octane fuel will cause a high comp (even 11+ comp) motor to ping and eventually detonate. So provided you have a good tune, you can get away with that sort of compression.

**spetz** · 25-09-2011 12:57 PM

Thank you for the reply.
Can you tell me though why you wouldn't want such high compression for a street car?
And why forged pistons can have higher compression safely?
Also isn't the E85 fuel bad overall?

**GSi_PSi** · 25-09-2011 11:42 PM

Personally, i just dont feel a higher compression motor is safe in 40+ degree heat australian weather as the thermal aspects of a high compression motor is more suited to cooler climates. And if your can is mainly built to run on
E85 fuel and your in an area which does not supply could be a bit of a hassle sourcing it out, although recently there are more and more E85 stations being available.
I would assume forged pistons are more tolerant in regards to pinging and knocking.
The majority E85 fuel we have isnt the true race E85 and it burns faster than normal 98RON

**Glocker** · 26-09-2011 03:37 AM

If you are running hi-comp like that on the street you shouldn't even bother with E85, just go with water injection to reduce detonation possibilities. Tuning for E85 adds less benefits to NA motors compared to turbo ones. One of the biggest reason is that NA motors can only make more power faster it revs, but turbos can make more power at the same revs but with just more air crammed in.

**joe.teg** · 25-10-2011 10:24 AM

im building a 12:1 comp b18 bottom end for my dc2 and i am planning to run it on 98RON fuel, obviously tuned (not sure who atm) and itll be driven everyday. As for water injection, i think its a stupid idea. E85 would be a more viable option to go for before water injection as E85 is not as extensive and allows for many different tuning options ie: two fuel maps etc etc. True, the gains for NA cars are nowhere near the gains for turbo cars when upgrading to E85 but the "saftey" is there.

**Bludger** · 27-10-2011 10:08 AM

Originally Posted by joe.teg

im building a 12:1 comp b18 bottom end for my dc2 and i am planning to run it on 98RON fuel, obviously tuned (not sure who atm) and itll be driven everyday. As for water injection, i think its a stupid idea. E85 would be a more viable option to go for before water injection as E85 is not as extensive and allows for many different tuning options ie: two fuel maps etc etc. True, the gains for NA cars are nowhere near the gains for turbo cars when upgrading to E85 but the "saftey" is there.

We got 12.7:1 & head shave, daily driver, 98 pump fuel, b18.

motor is wonderful.