Rb26 And Higher Compression

[Sidwysracr]

I been thinking about running higher compression ratio. Note this isn't a debate thread.

I would like to get up to 10:1 CR (I can settle with less down to 9:1 min) and eventually roll with gt2860-5s.. maybe -9s.. making around 500 hp, goal is to make a super responsive machine.

What I have is stock rb26 block and heads. I already have 86.5 pistons w/ 16 cc dome volume, manufacture states a 8.5 CR for these pistons.

I used a quick compression ratio calculator, assuming the rb26 heads are 70 cc. I calculated with a 1 mm HG that 2mm would be shaven off. However, with no idea about piston deck clearance height that 2 mm may be more or less.

The ultimate problem rests on the valve clearance.. and if I were to do a 10.25 lift, how much would I have left =/

Anyone have experience with increasing CR in a rb26 and boosting decent power?

[T04GTR]

stock 26 deck height is 0.5mm above the deck

a few i have heard of are custom cp's.

a fresh decked head will come in around 64cc. (just measured one at work)

[bradsm87]

High comp is an awesome idea. I'm tossing up between an R33 GTR or an S15 for my next car. Either way, i'll be getting custom 9.5:1 pistons. If I get a GTR, i'll run -9 turbos and Tomei 260 cams with a Nistune, fuel pressure reg and stock AFM's for an easy, cheap HELLA RESPONSIVE 320rwkw.

Shaving the head or head gasket thickness is NOT the way to go about it though. It will stuff up the cam timing for sure. You're bringing the cam sprockets closer to the crank which makes them turn slightly.

Edited July 12, 2010 by bradsm87

[TiTAN]

i built my engine with 9.3:1 arias pistons, if you are building an RB30DET they also make 9.8:1 pistons.

[R31Nismoid]

-5s are far to big for 500hp FYI - -9s would be what you want.

[DAS KAMU]

-5s are far to big for 500hp FYI - -9s would be what you want.

i have them and there fine

[rob82]

The biggest problem with th RB26's is the massive chamber volume meaning you need to run massive domes on the pistons to get any sort of compression increase which then puts more stress on the rings.

If response is your biggest motivator I would be aiming for around 9:1 comp and look at getting VCAM if the budget will stretch.

[R31Nismoid]

i have them and there fine

Based on what? The fact the car runs?

Thing is - it could be better

500hp using -5's means you'll have added lag/less throttle response/transient response/come on boost later/part throttle and so on.

The -9's for 500hp blow the -5's away in every single way unfortunately as that is what they are made for.

The -5s around 650-700hp... So why use 650hp worth of turbo for 500hp when you can get 500hp out of a more reponsive choice?

There is no logic in your comment as i see it as the car with -9s vs -5s for 500hp - car with -9s will be faster every single time (all things being even).

The OP also says he wants super responsive, so again going the larger turbo for no reason, is silly.

[Sidwysracr]

^ I been thinking about it a lot actually. I been stuck. The reason why -5 stays in the fight... is because it has more top end. And at 500 hp it is more efficient, 77% whereas the -9's at maximum is 74%. This means the the air is less hot when it's exit's the turbo. Or ultimately a cooler air charge. And for a higher compression motor every little bit helps. And from reading around the difference of 500 rpm of response can be made up else where.

That's why it's still a card on the table.

Other news:

Also reading up... 70 cc is wrong for the volume of the chamber..it's more close to 63cc.. and if the deck clearance is 0.5 above... then my calculation says that only 0.7 mm needs to be shaven to get a 10:1 ratio

But ultimately I need to know if anyone who has experience with this CR. How much of the upper rpms can be played with since ignition timing is shortened. And using if any effects using a 260 cam duration with 10.25 lifts.

Thanks.

Edited July 12, 2010 by Sidwysracr

[DAS KAMU]

Based on what? The fact the car runs?

Thing is - it could be better

500hp using -5's means you'll have added lag/less throttle response/transient response/come on boost later/part throttle and so on.

The -9's for 500hp blow the -5's away in every single way unfortunately as that is what they are made for.

The -5s around 650-700hp... So why use 650hp worth of turbo for 500hp when you can get 500hp out of a more reponsive choice?

There is no logic in your comment as i see it as the car with -9s vs -5s for 500hp - car with -9s will be faster every single time (all things being even).

The OP also says he wants super responsive, so again going the larger turbo for no reason, is silly.

have you driven a car with them?

when ur in qld ill let you have a drive

[GTScotT]

Sorry to say, yet I feel 500HP from -9s is really pushing the limits. 500HP = 375KW, assuming your would lose 50KW via the drivetrain thats still 325RWKW. 50KW loss is abit rich in itself.

I know the talk, I also know the numbers.. I dont personally have faith in -9s for that sort of power on a stock RB. The RB just is not efficient enough in its given form to do that.

When talking about a modified motor with better than standard VE and flow, I believe you could benefit from the 'laggier' turbos.

The term 'dynamic' comes into effect, and while more boost at lower RPM may be acheived, it is not rocket science that pressure and flow are not one in the same.

Dont want to start a turbo war here, yet food for thought.

[R31Nismoid]

have you driven a car with them?

when ur in qld ill let you have a drive

Yep, and a car with -9's, many of the variants.

Either way mate 650-700hp worth of turbo for only 500hp is a silly choice.

From any RPM the -9's are on sooner. Given you have the same power - it's pretty easy to workout which car will be faster mate... The one that is more responsive (as i said, given all things equal and just the turbos changed)

Sorry to say, yet I feel 500HP from -9s is really pushing the limits. 500HP = 375KW, assuming your would lose 50KW via the drivetrain thats still 325RWKW. 50KW loss is abit rich in itself.

I believe the figure is more like 60-70kw loss that seems to come from the depths of these pages. Which would pretty much be spot on (if all the people are right )

The OP is also talking about a built motor, that again would easily make 320rwkw out of -9's like everyone else does

I agree -5's on a built motor would be good - however ONLY if you are chasing 600hp+, and not the 500hp stated here.

[lachlanw]

put a neo head on it and use the vct

[R31Nismoid]

^ indeed, this is another option.

Imagine bringing -9's on even earlier... ooo ye

[zebra]

-9's with WMI injection and 2 Bar of boost

[R31Nismoid]

I think E85 would be very interesting. Seeing as though its usually around 30-40rwkw gain...

370rwkw through turbos that are coming on hard by 3300pm would be rather sweet

[bradsm87]

put a neo head on it and use the vct

That would be sweeet but aren't the intake AND exhaust flanges different?

[PM-R33]

Yep.

[R31Nismoid]

That would be sweeet but aren't the intake AND exhaust flanges different?

Who said money was an issue?

[lachlanw]

oh yeah forgot bout that .

just said it on the "smaller chamber" side of things

if it was goin single turb there is no shortage of manifolds . and nothing a plazmaman on the intake wont fix .

can someone say if a neo head has a realistic chamber size for a 26 ? as in does it achieve a higher comp ratio in a way that is beneficial ?