Comp Ratio Question

[humz]

ok done some research in comp ratios confused teh hell out of me, plus i dont know of the figures needed for my engine. so could someone tell me what comp ratio i would get if i used a 1.5mm head gasket on an rb25det r33 which has 1:standard pistons 2: 0.02" over sized pistons?

both with using the standard head/ valves ect (for convience use std values even tho heads gona get ported)

would this gasket be too big? comp ratio too low?

application is street, 400-430rwhp, pump fuel (ultimate)

what are pros and cons of a slighly lower and slightly higher comp ratio than std?

Edited November 13, 2006 by humz

[MR331307]

benefits of higher comp will be

Quicker boost (reduced lag)

More hp can be made with less boost

Bad stuff

Less timing

can cause pinging more (up to tuner)

have to maintain real high octane fuel(due to static comp Rise)

and what comp pistons do you have

[r33_racer]

if the pistons give a std cr...then with that gasket you would be around 8.6:1 roughly. A 1.6mm hg gives a 8.5:1CR.

[humz]

MR331307- well im actually tryna buy your arais pistons lol i msged ya just now but no reply as yet either that or some other piston on the forums will definately be bought soon.

r33_racer- excatly what i wntd to know, thanx!

is a 8.5:1CR quiet low, or within normal cr range?

what would be teh limits of CR? both uper and lower limits? obviously it depends on many factors, but lets stick with safe, street power.

[Abo Bob]

8.5:1 is what RB26's run. RB25's run 9:1.

It wont be that if you go 20 thou over though. More like 8.6 I think.

For your power level the higher the better but it's safer to stay low. What turbo are you using?

I'm running 8.75:1 and getting 260 odd rwkw.

Generally you need to increase boost by between 0.1 and 0.2 bar for every 0.1:1 drop in CR to maintain the same power.

[blind_elk]

C/R will INCREASE if you bore for, and fit, bigger pistons, because you are increasing the swept volume. But I doubt the increase will be significant (more than 0.1) for a 20 thou O/S.

By fitting a thicker gasket, you are effectively enlarging the combustion chamber, so the C/R will drop.

[humz]

im using a big daddy ITS T04e.

engine (being bulilt now) is a 25/30

if you use say a 87mm head gasket on std bore is this bad? not a huge differance! whats the mm of the std rb25 pistons?

[humz]

ahh i think i found size of std bore- 86.5mm

so thats a 1/2mm diff, seems like bugger all!

[r33_racer]

actually isnt std bore size 86mm i believe 86.5mm is first oversize(20 thou over).

Im pretty sure i veryneared (verniered )measure the rb30 bore before we got it opened up for the new CPs and it was spot on 86mm. As 87mm is final oversize i think.

[Abo Bob]

Wait. When did this engine become a 25/30?

86mm is standard bore size for RB25, RB26 and RB30. I think you can take them out to 88 without sleeving but don't quote me on that.

You use the right size gasket for the bores you have.

[discopotato03]

Personally I'd rather keep the static CR up around 9:1 because mega boost pressure is not everything . IMO thicker head gaskets are a poor way to decrease CR particularly if the engine is being built up and you have some say in piston type . Lifting the head higher off the piston crown will increase the chamber volume but its not ideal around the quench or squish zones so may not be any more detonation resistant . The higher 9:1 with the 100 oct Shell pump fuel should work out well enough . Having low CR and high boost would be ok if the engine lived in a boosted environment all the time but they dont , and they do feel like a soggy breathless boat anchor off boost because of the low dynamic compression pressure .

High boost pressure does not always guarantee high airflow , far better to remove where possible the resistance to flow to get a greater weight or mass of air into the chambers . Doing this with less boost also means lower fuel pressure requirement/less energy to recover from exhaust gass (meaning less resistance to flow through turbine/housing) / ignition timing optimised around burning efficiency rather than detonation supression .

My 2 cents spent for today , cheers A .

[Abo Bob]

Gibson gtr = 9.4:1

[The Alchemist]

I increased my compression ratio to 8.8:1 on my GTR rebuild. I run it on our piss water 95 Octane here in NZ with no problems on 18psi, with tuning to suit of course. It makes 285 RWkw on a dyno dynamics 2WD dyno.

Lag is not in my vocabulary. Comes up on to boost like a bloody rocket ship and I contribute the higher comp ratio for this when compared to other GTR's I have driven.

Mike