Hi comp pistons

BGTV8

I have 14.7:1 pistons in my race car engine (Alloy 5-litre V8) that runs quite happily on E85. That said, I would not run this engine on unleaded (even 100RON unleaded).

The origin of this thread talked about bumping static compression ratio in a VQ37 which is 11:1 from the factory and I would not go about 12:1 on pump gas UNLESS I was running 100RON fuel AND I also had a knock sensor to tell the ECU when to pull timing.

My previous race engine incaranation ran 12.25:1 on AVGAS 100/110RON fuel (actually called Leaded Race Fuel here in Oz, but getting very difficult to get and quite expensive at $4/litre).

Remember also that US octane numbers are quoted based on MON numbers whereas we in Oz use RON numbers (wikepedia can tell you the difference).

An earlier post talked about upping CR to improve power which is not quite right - increased CR increases torque available with each power stroke (and torque increase is roughly linear with increased CR - so going from 11:1 to 12:1 should be excepted to increase torque by 1/11th or therabouts). Power is a product of torque times rev's, so at the same rpm, a 9% increase in torque will generate a 9% increase in power.

Torque is all about an engines ability to accelerate a car, power is about an engines ability to overcome resistance (eg: to do work). Torque establishes how quickly a vehicle will reach a given speed, power determines (ultimately) how fast it will go (at high speed, the determinant is the amount of drag generated by the body and frontal area).

Gear ratio's also influence these factors.

I would be quite reluctant to increase the CR on a road car without understanding a whole lot more about the Nissan ECU capabilities.

The other point is that the VQ37 engine has an oil-pump that will not survive sustained engine speed above 7500/7800 rpm and it will certainly not survive for long at 8500rpm - albeit that the rest of the engine should be able to tolerate those rotational speeds and more.

The Nissan VQ GT3 and GT4-spec engines all run up to 9000rpm, but they all run NISMO oil pumps and upgraded rod and big-end fasteners to do so.

I personally would not contemplate increased CR pistons without a significant investment in co-requisite mods.

The VQ series heads do not require a lot of modification to flow enough air to support 500+hp at 9000+rpm, but this would require cams, pistons (and careful attention to valve clearance), rods, a full fastener kit, an induction kit (individual TB) and decent exhaust.

Pistons alone are likely to be a WoFTaM.

RB