7424 heads and CR

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Veedubster
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7424 heads and CR

Post by Veedubster » Tue Aug 22, 2017 10:39 pm

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.

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raygreenwood
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Re: 7424 heads and CR

Post by raygreenwood » Wed Aug 23, 2017 3:46 pm

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
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Re: 7424 heads and CR

Post by Veedubster » Wed Aug 23, 2017 11:19 pm

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.

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Piledriver
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Re: 7424 heads and CR

Post by Piledriver » Thu Aug 24, 2017 2:00 am

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.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

Veedubster
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Re: 7424 heads and CR

Post by Veedubster » Sat Aug 26, 2017 6:07 am

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.

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Piledriver
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Re: 7424 heads and CR

Post by Piledriver » Sat Aug 26, 2017 7:54 pm

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.
I, for one, regularly embrace our new robot overlords, as I am the guy fixing the robots...

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Clatter
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Re: 7424 heads and CR

Post by Clatter » Wed Aug 30, 2017 5:41 pm

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..
"Oh, You don't need to do all that... The valve seats are just going to fall out of it anyway!"
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Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

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Clatter
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Re: 7424 heads and CR

Post by Clatter » Wed Aug 30, 2017 5:43 pm

PS. My 7424 HAM heads had the step.
"Oh, You don't need to do all that... The valve seats are just going to fall out of it anyway!"
- Doug Ellsworth

Beginners' how-to Type 4 build thread ---> viewtopic.php?f=1&t=145853

Veedubster
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Re: 7424 heads and CR

Post by Veedubster » Thu Aug 31, 2017 6:56 am

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.

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