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Discussion Starter #1
The variables are staggering, BUT, what are peoples thoughts on this?,,, Say a BBC, with 13.5-1 compression and it likes 37* best,,, If we go to 16-1, or to 11-1 what timing changes would be "likely'',,, lets say though there was no dome on the piston to complicate matters....What say you all?
 

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Long Live The King
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I’m going to go out on a limb here. If there were no dome and you were at 13.5:1 for starters, the timing wouldn’t be 37* to start with. If you bumped it 3 pts it would want less timing. 4* less +\-
 

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If the chambers are good and the only thing changed, the small chamber will want less timing, the big chamber more timing.
 

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Discussion Starter #9
If the chambers are good and the only thing changed, the small chamber will want less timing, the big chamber more timing.
Agreed.. as with bore size.. but hypothetically this is just compression only. realising that of course it is impossible to do that without alteration..... perhaps a stroker engine might come the closest?
 

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The things you have to think about is volume/ flame travel and density of the mixture, all things equal as far as afr/mixture the smaller higher compression chamber will be "hotter" (because it's compressed more) than a bigger lower compression chamber. What only a few people have done is achieve a very high VE with a lower compression engine and use the bigger chamber to pack additional air/fuel into the engine to make the engine "act" bigger and require less timing than with a lower VE.

Also a lower rpm engine will require the intake valve to be closed sooner so you don't push the mixture back into the port (at lower rpm's you have more time to do everything). At higher rpm's you need to close the intake valve later because you start to run out of time to fill the cyl. and trap closer to 100% so the compression is what you think it is.
 

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Discussion Starter #12
Ok thanks,,,,,depending on the cam,,in what way pls? and are you referring to say Pontiac vs chev vs Ford
Especially getting down in the pump gas range and big cams. ;),, coz that is where I am with one engine
 

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My last alloy headed Cleveland liked no more than 28* on dyno. Cast liked 36. So depending on head. Just dynoed a oval chevy and 34* was its happiest!!
 

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Discussion Starter #14
Wow 28 to 36, that is quite some difference. Oval chevy iron or alloy heads? assuming iron ,, but you know!! The engine that provoked the thought for this thread is on a run stand at present, has alloy heads and is sitting at 35* locked out
 

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Discussion Starter #16
That seems a lot for a Big block with iron heads, but they want what they want, I have had small blocks with iron heads that liked more,,, but not big blocks,,, although I must admit I am a bit of a novice with Big blocks
 

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That seems a lot for a Big block with iron heads, but they want what they want, I have had small blocks with iron heads that liked more,,, but not big blocks,,, although I must admit I am a bit of a novice with Big blocks
Same here. I haven’t a clue what big blocks need, but my tiny engines, the aluminum headed ones, at 23 degree, wanted more timing then the iron ones. Not by a bunch, but up to 53 degrees, total.
 

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DamCoonass
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Same here. I haven’t a clue what big blocks need, but my tiny engines, the aluminum headed ones, at 23 degree, wanted more timing then the iron ones. Not by a bunch, but up to 53 degrees, total.
Same here but the last 2 high compression alum headed sbc’s I’ve done don’t like a lot of timing.

39 on my personal 357 is all it likes as a best balance.
More & it et’s a little quicker but mph falls off

I woulda thought they’d be in the mid 40’s minimum
 

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Same here but the last 2 high compression alum headed sbc’s I’ve done don’t like a lot of timing.

39 on my personal 357 is all it likes as a best balance.
More & it et’s a little quicker but mph falls off

I woulda thought they’d be in the mid 40’s minimum
Well, high compression on a 289” with a Brodix spec head is a non issue. No matter what, there ain’t a lot of compression. So, on an engine that has some compression, it may be less. It’s weird, but every engine, on a sbc, seems to just want whatever that particular engine wants. Some built almost identical, want a degree or two different.
 
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