9:5:1 + pistons in a RB25det.....

[meet07]

I have to rebuild my bottom end again. Long story that I really don't want to get into but it seems that alot of guys bump the compression up a bit on turbo build these days. I mean have seen anything fro hondas to Subarus running upwards of 10+ compression ratio(of coarse on race gas) Would it be a good idea to get 9:5:1 compression pistons on street driven RB25 with a litte drag racing and possible meth instal in the future??

[Carl H]

as long as you have a good tuner it should be fine.

[meet07]

What about the valve to piston clearances? Or If I decided to upgrade to a bigger cam in the future?

[Darius]

Call up the piston manufacturer and ask what they recommend. They have seen enough of these engines that they should know details like that by now.

And if you are bumping the compression up, I'd definitely plan to run on E85 or race gas. Pump gas won't get you very far with that kind of compression due to knock.

[OutToWinPAHC]

I wish E85 were more common for these reasons. Even at 8.5:1 I would crank the boost and advance the timing to suit

[Darius]

You're right on. And like Carl said, a good tuner will get as much safe power from whatever setup is put in front of him regardless of fuel, boost, or compression.

[huafist]

Call up the piston manufacturer and ask what they recommend. They have seen enough of these engines that they should know details like that by now.

And if you are bumping the compression up, I'd definitely plan to run on E85 or race gas. Pump gas won't get you very far with that kind of compression due to knock.

Is there some issue with the RB combustion chamber design that causes this? I ask because I regularly have ran 9:1 and higher compression on many different makes of turbo cars in the past. As long as quench is correct, and the piston dish/combustion chamber designs promote good turbulence and don't have hot spots, I wouldn't think 9.5:1 would be unachievable. Granted, it's a lot more sensitive to the tune, but it's doable.

Of course, this is academic, as some engines respond better to higher compression/lower boost, and others vice versa - I don't know enough about the RB platform to know which way its biased.

[Darius]

I don't think 9.5:1 is out of the question at all. The RB combustion chamber is very efficient for its era and on par by today's standards (4 valve/cylinder, squish pads, interference valve setting, etc.)

If I was to do it all over again, I'd probably do a 10:1 compression RB26/30 on E85.

[Carl H]

i'd love to do 9:1 cr on my motor...its somewhere around 8.3:1 so my timing map is obscene.

[ST240]

My biggest pet peave with a low cr like 8.3:1 is the off boost performance. On boost is great tho .

i'd love to do 9:1 cr on my motor...its somewhere around 8.3:1 so my timing map is obscene.

Care to elaborate?

[RRRRB]

mines 7.9:1 ish im happy with my off boost performance i dont think its that bad, nor much different than it was before with 8.5

[Carl H]

i mean off boost its got loads of torque for sure, and surprising amounts of pep for being low CR.

@st240 just run lots of timing, it would probably scare most people who saw it but its what the motor likes.

[ST240]

How much are you running up top?

[Carl H]

~25* at 14psi.
started low and worked my way up on the dyno car did not like 15* and egts were silly high.

[Bucketcoupe]

I don't think 9.5:1 is out of the question at all. The RB combustion chamber is very efficient for its era and on par by today's standards (4 valve/cylinder, squish pads, interference valve setting, etc.)

If I was to do it all over again, I'd probably do a 10:1 compression RB26/30 on E85.

Do you think the head needs to be touched at all to run 9.5:1 pistons on the rb20?