Compression Vs Cam Duration

[B18cEG]

Hi guys i just need some help here

I have a b18c2 and am installing the following

forged longer stroke crank (maybe keeping stock)
forged h-beams
forged raised comp pistons: but this is the problem, not sure how high i can go for a daily driver/weekend racer

Full valvetrain upgrade with the following cam profile
IN.12.00@ 304@1mm EX.11.50@ 304@1mm

Thanks

[dsp26]

add ARP head bolts to your list while your at it, CR increase usually leads to a chemical transformation of the stock bolts into chewing gum :p

you should really limit to 12:1 CR for daily driver due to availability of fuel. I would get my car tuned to the extreme on 96oct and run 98oct after only because there seems to be a growing shortage on 98oct.

but overall your question is a tad tricky because you realise that your static CR could be 11:1 but with those seemingly huge cams your dynamic CR would only be 9:1...

[B18cEG]

i was wondering if 12.5:1-13:1 would be alright, in perth 98octane is plenty bro so thats alright for now, but any input perhaps from experience would be much appreciated

[Limbo]

i haven't seen a prob with 98 octane in syd?
But yeah that's a pretty high compression you might need a tune to adjust for the extra compression, otherwise you might get detonation.

[B18cEG]

oh sorry guys i meant those will be the main components upgraded, yes there will be arp head studs and all that and also a hondata s300. all necessary parts will be upgraded

[grumpy rooster]

That compression and cam should be fine if tuned correctly. In broad brush terms the bigger the cam, the more static compression you can run. Assuming that all your clearances (ie valve to piston etc) are fine I can't see it creating a huge mess on the road.

[ZeForce]

I disagree, see below....


Ideally you want the combustion space to be quite small and confined to a single isolated volume so the air/fuel mixture can burn quickly, this requires the static compression ratio to be fairly high. A larger combustion space means the air/fuel mixture is spread over a greater area and takes a longer period of time to burn. Having a 'fast' burn means the engine will not require as much ignition advance in order to burn the entire mixture before the piston reaches BDC. Reducing the amount of ignition timing advance will reduce chances of detonation and can be done without sarcificing power. Another disadvantage of having a larger volume combustion space is that even though the overall air/fuel ratio is satisfactory there may be areas within the combustion chamber where the air/fuel mixture is much leaner and may lead to detonation. A smaller combustion volume ensures the air/fuel mixture is more homogenously distributed throughout the combustion chamber.

If the engine is built and tuned correctly there shouldnt be any issues running compression that high on pump fuel

[grumpy rooster]

I agree with ZeForce. I had a crappy designed 8valve SOHC engine with 12:1 comp on 98 fuel. Cam was 298deg advertised duration. I was running quads though.

If I can run 12:1 with a crap design engine you most certainly should be able to run ATLEAST the same with a far superior engine design.

[B18cEG]

ok thanks for the input guys, i just need to decide now, should i go 12:1 12.5:1 or 13:1 i want it to be pretty "gnarley" (never used that word before) but yeah. thanks, also anyone udes cams this aggresive before?

[ginganggooly]

I'm running around 12.2:1 -from memory- on pump fuel, it's not had any issues at all.

[Mitchman]

I have 11.5:1 comp pistons but had the head shaved down so now its at 12.3:1 and im running stock headers atm! once my new headers are on i wouldnt mind even bumping it up a lil more lol go 12.0:1 and just shave it down a bit :p