Rb26 Compression Ratio

[Shaun]

Gday,

im having my RB26 engine assembled at the moment, and im at the point of purchasing a head gasket. The engine builder recommended puchasing a 1.5mm gasket that will give me a compression ratio of about 8.5:1. I was, however, hoping to go a bit higher in order to maintain off boost drivability, but as im no engine builder, i should probably take his advice.

Im just wondering what comp ratios people have in their built RB26's and what i should aim for.

My engine has forged pistons, prepped rods, all the usual oil control stuff and a gt35/40 turbo with a 1.06 turbine housing. The car is a street car.

The reason why im wondering is that previously the engine was built with a 1.0mm head gasket that gave me 9.0:1 compression, but boost was uncontrollable with a smaller turbine housing (0.86 i think). This resulted in detonation and head gasket failure on the dyno. I then went and bought the bigger turbine housing, but im now wondering if it'll be unresponsive off boost with the lower comp ratio.

Thanks for any advice,

Shaun.

[Piggaz]

I didnt build my motor but my comp ratio is about 8.9: 1 running twin HKS GTR RS's with a 2.8 litre stroker. No pinging problems at all.. Highest i see on the power fc is about 14, bottom end is great. Wouldnt have it any other way.

[Adriano]

No offense intended, but if the engine builder is recommending a thick headgasket to get an appropriate comp ratio, he shouldnt be building rb26's. I wouldnt have any problem running the higher comp ratio(8.8-9:1) as long as you give it nice tight piston to head clearance to improve combustion efficiency and reduce detonation.

[Abo Bob]

A 1mm head gasket should give 8.5:1 CR as that is stock. I would repeat that if I was you.

I had much discussion and head scratching about CR in my RB25 and ended up with 8.75:1 but if you are building an engine I can't see why you would use the headgasket to determine the CR. Use the pistons, block and head machining so you maintain proper quench and squish zones.

I love the off boost response (not too much different from the 9:1 standard on RB25) and wouldn't wouldn't want to go much lower due to piggish response or much higher due to harder to tune and got to keep a closer eye on things.

[T04GTR]

all depends on what your "high boost" setting will be.

i had a 1.5mm one in my dr30 (rb26) it made 328rwkw on 1.2 bar. going 1mm oversise and using a 1.5mm gasket will give standard comp.

using a standard gasket thickness with a overbore will raise it.

[Shaun]

Yeah, its 20 thou over size and i had to have the head machined to within an inch of its life due to damage sustained in the last failure. Hence, gasket thickness is the thing that will vary my comp ratio this time.

Is standard RB26 comp ratio 8.5:1 is it? I thought it was higher for some reason.

Shaun.

[discopotato03]

With you combination of parts there is always this significant point to think about .

Nissan designed that engine to have parallel turbochargers because it is a convienent way the group the front and rear cylinders and take advantage of the pulse effect - this is the way to have high exhaust gas flow and acceptable turbine response . Also having two integral wastegates means they have plenty of bypass area and the whole shooting match is compact - along the line of the engine for ease of packaging in a Skyline .

When you go to a normally single scroll single turbocharger your tossing all the exhaust putts back into one area so you lose the multi passage effect and this will cost you turbine response . Its your call but if it were me I'd stick to either known parallel twins or a properly set up twin scroll single turbocharger .

Given a choice I'd go a little higher than the factory 8.5:1 CR and juggle this with the specific turbo characteristics and the dynamic or effective compression ratio with better than standard camshafts .

The things that will have the most say about how it drives just round the burbs will be turbine response/exhaust restriction/effective CR - and a tuner that knows their stuff ..

Your call .