Shaving The Head... How Far?

[25GTV]

Hey guys got a mill at work so i think im gonna shave the head in my r34 na. Its stock appart from an exhaust and CAI. Thinking i will take off 1mm. Assuming from calaulations this will provide 11:1CR. Apparently the engine is 10:1cr standard?

Any info much appreciated

Cheers

[SKiT_R31]

11:1 sounds like a good number - no sure what the stock compression is on the neo's, but 10:1 sounds about right, is this on stock management? I dont know how the neo would react, but you should also fit a thinner head gasket. Also, providing your not a greenie, fit some colder range plugs, say some 7's, it will help cool the combustion chamber so you have less detonation so you wont lose ignition timing. Also, higher compression engines can be a bit thirstier, so it could be happier with some higher fuel pressure down the track (depends on how far you go). Are you going to fit lumpier cams? Cams cancel out higher compression, for win-win trade off with a shave.

ta

[25GTV]

Yeh thanks for the info, Ive worked at performance workshops before but mainly with turbo setups or carby NA's. I plan to see how the stock ecu runs with a safc for a bit of extra fuel and if i run good fuel hopefully it wont ping...? Might just get and intake cam aswell, but not EX as they are all aimed at turbo performance.

[SKiT_R31]

Yeh thanks for the info, Ive worked at performance workshops before but mainly with turbo setups or carby NA's. I plan to see how the stock ecu runs with a safc for a bit of extra fuel and if i run good fuel hopefully it wont ping...? Might just get and intake cam aswell, but not EX as they are all aimed at turbo performance.

if your going to get cams, i would definately reccoment getting both, combined with adjustible cam gears. Higher compression generates more heat, so to lower cylender temps, you will want to get the exhaust out just as quick as your getting intake air in, otherwise you will not only lose some of the gain from the intake cam, but you will lose power from having to lower ignition timing.

Also, as strange as it may sound, a turbo ecu and injectors are a good option for the N/A route, not only is the power and air flow more matched with mappings, the fuel mixtures are better (so you wouldnt need an safc unless you wanted fine tuning) but more importantly, n/a ecu's have alot of ign timing down low, wheras turbo ecu's have all their timing uptop (i know you would think it would be the other way around). But on a high compression motor, the pinging is going to be in the low revs, and with your only ignition timing adjustment being the base timing, to stop pinging down low, you will lose power up top. Also the DET ecu's have a much better rev limiter .

I've got 255deg cams with 8.3mm lift on my 30de, with twin throttlebody's (so it can breath) it makes peak torque at 5000rpm and peak power at 6000rpm, which then tapers off until limiter (at the time it was restriction) - if you ask me thats perfect, i wouldnt go any bigger cams for an rb25.

So aswell as that, i would reccomend (if your after performance) letting it breath some more, im sure you could figure somthing out with the 2 into 1 thing the 25de's have, give it 2 throttles or somthing, but if the engine cant breath up top, you will lose out with your cams and higher compression - if the cylinder isnt at atmospheric pressure by the time the intake valve closes, working the same way a boosted car works but in reverse (vacuum instead of posetive pressure) the engine will have a lower dynamic compression ratio. So once again with the compromises, you have higher compression down low, its able to breath, might have higher timing depending on the ecu you use, under load in low revs is where the detonation will be, so lowering the timing to compensate for that will lsoe even more power uptop if you have a lower dynamic compression because the engine cant breath enough. (but once again, cams help alot with this, with overlap lowering compression at low rpm)

keen to see how it goes - good luck

cheers

Edited February 7, 2009 by SKiT_R31