Cleveland 408 with high compression
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  1. #1
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    werner.staubli is offline

    Cleveland 408 with high compression

    Hi guys

    My name is Werner Staubli from Johannesburg, South Africa. I have a Ford 1946 Jailbar with a Cleveland 408. We are running a 13:1 compression ratio, hence we need to run 102 octane for ultimate performance. The problem is that its hard to come by, damn expensive and not practical when travelling long(ish) distances, so the timing has been retarted and is now running off 95 octane.

    We obviously had issues with pre ignition as a result, and managed to tune it now that I cannot hear pre ignition anymore. After speaking to other mechanics, their view is that we should reduce the compression to avoid pre ignition and get the most potential from the engine.

    What are your thoughts and what (best) methods are there to achieve this ?

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    Official RnS Addict mygasser's Avatar
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    at 13:1 it does need reducing to use it a road vehicle with normal pump fuel. it probably has 'pop up' pistons which if swapped for dished or at least flat top would be one way to lower it. next what heads are fitted? replacing them with bigger chamber versions would be another way. the cheapest way would be shims between the heads and block. as for losing performance with the lower compression it shouldn't be any worse than retarding the timing as you have done now and the retarded timing will make it run hot too, never a good thing.
    good luck, neil.
    need a job done on your project? i may be able to help.

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    Quote Originally Posted by mygasser View Post
    at 13:1 it does need reducing to use it a road vehicle with normal pump fuel. it probably has 'pop up' pistons which if swapped for dished or at least flat top would be one way to lower it. next what heads are fitted? replacing them with bigger chamber versions would be another way. the cheapest way would be shims between the heads and block. as for losing performance with the lower compression it shouldn't be any worse than retarding the timing as you have done now and the retarded timing will make it run hot too, never a good thing.
    good luck, neil.
    As Neil said shimming is the least invasive but remember to shim the manifold too, a cc calc on the current set up will let you decide shim size and then a quick calc on this thickness will give you the required thickness for the other shims on the intake.

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    Test or Kill is offline
    If the engine has 'quench' heads then you should have built it to use that to your advantage. I build quench into my stuff and max power is made between 20-25 degrees BTDC total advance which brings other benefits. TBH, whatever now, if you can't access high octane fuel the only realistic solution is to fit a good, modern water injection system. That stuff really works and I'm keeping my options open on that front for the day our SUL becomes unavailable or too expensive. HTH
    Cheers, Martin
    Maybe I'm just sick in the head.

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    My name is Martin 30psi's Avatar
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    I'd question also what cam you also have and its suitability. You've described the static comp ratio, but what about the dynamic comp ratio after taking in to account intake valve lag?

    With a wider duration cam or narrower lobe seperation angle you can drop the dynamic compression. But of course this depends how severe the cam is already and whether it suits 13:1 or say 10:1.
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    You need pistons that are 20thou down the hole on TDC if you are running a zero deck height at present, ....please dont tell me you are running cast iron heads?,.... you can go up roughly one point with ally heads but realistically 10:5 is enough for any street engine on pump swill.
    It can be bled off through cam duration also..

    68roadrunner

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    Can you source race fuel where you are? - buy highest octane available and mix with pump gas - worth a try if you're not doing many miles?

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    yup , quick easy fix ,water injection... or swap it over to run on lpg ,which has a much higher octane .
    Bicester scramble ,Medusa enjoyed the trip.https://www.youtube.com/watch?v=Q-7I7-nA19U

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