7424 heads and CR

[Veedubster]

Hi all, I purchased some new parts from someone who had brought them for a project which didn't go ahead. I'd really like some advice before I go any further (or spend any more!). I'm building a 2 litre to go in a bus. The heads are new Hoffman 7424 heads flycut(?) to 49cc (so no lip) and was told this would be 8:5:1 compression?? The cam is a Raby 9590 and was told this combo might be good... but run hot for a bus? The engine is fuel injected with tuned length headers.
Any advice appreciated.

[raygreenwood]

Normally....8.5:1 compression in a bus...I would say is going to run too hot. However....the 9590 from what Jake noted is designed like the former 9550 to run cooler head temps. Also a tuned length header will also run cooler head temps.

So between those two parts...if you also keep your max ignition advance down right at the 27 BTDC range....and are careful with fuel tuning...you might very well be ok.
This is also assuming minimal deck height with that combo...like .035" to .040". Ray

[Veedubster]

Thanks for the reply, if I run 98 high octane fuel that should help a bit too. If I wanted to lower the compression I'd have to add a metal gasket between the case and cylinders? Sorry for the questions, just want to be sure I'm on the right track.

[Piledriver]

Making the deck larger to lower compression frequently has the opposite of the desired effects...

The engine has stock EFI/ignition or a modern programmable setup?

You can control things ~infinitely better with the latter.

Are you in Europe? (European 98 (research) octane premium is about equivalent to 92 octane in US (R+M/100) IIRC)

I have always wondered about the supposed "better gas" in Europe, esp back in the day when you could buy ARCO(?) 102 octane unleaded at the pump here, which would be ~120 octane the way its calculated in Europe.
Sadly MBTE and friends club baby seals or something.

[Veedubster]

Hi, thanks for the input, I'm based in Australia and our 'premium' fuel is 98 octane. I'm running a Haltech ECU with crank and cam sensors so will have quite a degree of control. I'm thinking I should invest in a good head temp gauge to monitor whats going on.

[Piledriver]

Google indicates Oz apparently uses RON octane rating like Europe, so your 98 would be equivalent to US ~92 (R+M/100) octane or so... Making US, UK and Oz pump premium fuels ~identical in actual octane rating.

I can still locally get 100 octane (R+M/100) "racing gas" (for boats/no ethanol or MBTE) for about $6/gallon.
Mixed into E85 that would make for decent racing fuel.

[Clatter]

Ugh, can't find it in my notes,
But,
IIRC, you want to shoot for 8:1 with that combo, not 8.5.

Hence, the talk here of racing fuel..

[Clatter]

PS. My 7424 HAM heads had the step.

[Veedubster]

Thanks Clutter, need to get my head around compression ratios I guess!! And then work out the best way to modify what I've got.