1968 SBO tuning

[fleming442]

*turbo
Superchargers are spinny, whistly (or whiny ) things..... 😉

 

[Stupidquestionspro]

Either works for me if I win the lottery. Until then guess I’m stuck fine tuning and hoping for the best.

So can anybody do magic and tell me how to get my compression ratio without tearing the motor apart since I don’t know the type of pistons exactly and i don’t know the type of head gaskets and I’m fairly certain the heads haven’t been shaved or the block decked but not positive I do know it was bored .30

Yea current tune is absolutely terrible. So from what I gather I need to change my advance bushing to 18 the lowest I have and then basically advance engine base timing as much as possible until it’s either too hard on the starter or pings?

Also seems my vacuum advance can goes all the way to 16 at 12 inches vacuum. So I have to consider that as well.

All sound corrext


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[olds307 and 403]

You can run 40 base mechanical especially with a 10 degree vacuum can, probably more. It depends what rpm you want to rev it to. The Olds V8 can tolerate a lot of part throttle timing, 40 is the most I have tried for base and mechanical. Figure 64 to 68cc combustion chambers, most untouched are 68 to 70cc. I assume your heads were milled during the rebuild. Mine #6 heads had a clean up mill and are around 64 to 65cc. Figure .041" thick head gaskets with a 4.230" gasket bore, 4.087" cylinder bore with a 24cc piston dish and possibly .040" pistons to deck, which is what makes those pistons so bad, not just the huge dish. Also a 3.385" stroke. Here is the calculator I use https://uempistons.com/p-27-compression-ratio-calculator.html. With the above specs, I get 7.865 to 1 compression. I don't see you above 8 to 1, unless the block was 0 decked. Even with the .011 head gaskets and you would have to measure piston to deck, you would still be below 8.5 to 1.

 

[Stupidquestionspro]

Wow you made my life so easy and so sad all at the same time [emoji32] thanks for the help yea I’ll have to work on the timing and report back once I have time.


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[fleming442]

Tune what ya got as best ya can, and it'll be a fine cruiser.

 

[Stupidquestionspro]

Question what curb idle should I be shooting for or is that determined by correct timing and other changes? And where should my vacuum be considering my low compression motor [emoji24]

Also my MSD lowest advance stop is 18 so where can I get lower if I have crazy initial timing? As well I see everywhere it says 34/38 total for best performance is that correct or will the SBO low compression go into the 40s for performance or what? And how do I limit my vacuum advance now that I know I was wrong and it can actually get all the way to 16?

Thanks


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[olds307 and 403]

Yes, let your vacuum advance get to 16. Start with 22 base and 18 mechanical and go from there. My 8 to 1 350 liked it. Like I said if you plan on running up past 4000 rpm, too much timing can cause high speed detonation. You will be gaining some low speed kick with the extra 6 degrees of vacuum advance, should give around 56 total part throttle, I ran 58 with more vacuum advance and less mechanical, around 48 at idle. I could only pull 18" of vacuum with the 204/214 cam, stock peanut cam could pull over 20". I set my idle around 1000 rpm in park and use manifold, not a ported source for the vacuum advance.

 

[Stupidquestionspro]

Thanks you are just an absolute wealth of information. I’m very grateful


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[Stupidquestionspro]

Since I have to have such high initial timing would it be worth my while to just lock out the mechanical advance and leave the vacuum advance alone?


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[Stupidquestionspro]

So I took the carb off while I was doing other things and looked at the primaries and secondaries

primary transfer slot certainly doesn’t look like a square so I’m trying to find the adjustment for that.

as I thought the only adjustment listed in the manual on the base of the carb actually corresponds to the closed position of the secondaries

seems the secondaries have a transfer slot as well but see below

As per the tuning instructions from summit I adjusted the secondaries and wouldn’t you know it no transfer slot showing any ideas on why not or if these transfer slots are supposed to be square as well.

Thanks


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