Compression On Boosted Engine

[stevechumo]

I'm wondering what is the lowest & safe compression in psi for a boosted engine, especially for the 1UZ. And at what boosting psi that you have for that compression. Or at what compression you're currently boosting your ride, even though you think it's not safe.

 

[cribbj]

Steve did you mean to say what's the "highest" safe compression for boosting?

The 2nd dyno on my motor was when it still had around 10.5 to 10.7:1 compression, stock cams, 240 psi cranking compression. We lifted both heads at 15-18 psi supercharger boost. That was on 93 octane pump gas, and we'd just started tuning the M/W injection system when it blew.

 

[4000GT]

Im running stock compression on an early motor and 14psi and once at 16psi for 490WHP.

 

[-Nemesis-]

Bit off topic Steve, but have you got a build thread on your SC? Been looking for it with no luck yet.

 

[stevechumo]

I'm really sorry for not making myself clear. Felt asleep. I'm asking for the cylinder compression pressure in psi, not the compression in ratio. The example is if the cylinder pressure ranges from 135 - 190 psi, how low are you willing to boost the engine?

Nemesis,
The car has been running great at 6 psi after the build. I just put in 460cc injectors and waiting to crank to at least 12 psi. I'll update my other thread when I have the final number.

 

[Zuffen]

My last compression check showed 145psi on all cylinders. This is on 8.5"1CR.

Boosting to 21.5psi with twin screw supercharger.

Haven't had anything nasty happen yet but engine only has 700klm on it.

 

[stevechumo]

My last compression check showed 145psi on all cylinders. This is on 8.5"1CR.

Boosting to 21.5psi with twin screw supercharger.

Haven't had anything nasty happen yet but engine only has 700klm on it.

Wow! That's rough. So do you think 175 - 185 psi on 9:5:1 CR should be safe?

 

[XR8tt]

Cams can bleed off compression.. Unless timing events are similar ?
Its very hard to compare.. unless you go by what other builds have done..
Again it's mainly in tune.. Especially around / just before peak torque at higher boost levels..
MLS head gaskets help when heads lift, hopefully they seal again ..
Usually a problem with std head bolts stretching...

 

[turbotalon1g]

on my dsm i had 120 125 130 135, both below the 125 service limit and 10% difference.

I said F it and boosted teh hell out of it, 30psi on a small turbo to net me 410whp 458tq on a dynojet aka viagra

 

[stevechumo]

Did you boost a lot after that and did it last?

 

[XR8tt]

30 PSI on small turbo.. Would possibly be pumping hot air and way outside its flow map..
It's a little like asking how long is a piece of string??
As there are SOoo many variables..
What one person may say is a small turbo etc ??
Could be quite different to some one else..