My bbc has just shy of 11:1 compression, and I'm wanting to know if 91-93 octane will be ok to run? And if so what do you recommend the timing to be at? It's a 454 block with 781 heads. Or am I forced to run av gas/ race fuel


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Ok thank you


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Comp cam xtreme marine 288h-12


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Parker oil.. just after you cross the bridge

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Race Fuel - How much a gallon and what is the octane?  Never seen it or bought it before.

Av gas is 100 octane and the vp race gas is 110. Not sure on price but you're probably looking at around $10 a gallon.

That would be great if you could mix a gallon or 2 of race has to 93 octane instead of 100% race fuel


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Not sure yet on timing, still putting the motor together at the moment, scrambling to get it in the boat for the 4th


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From what I was told by a very well known motor builder " the Bostick"
10 3/4 :1 max on pump gas mix in a gallon or 2 of 110 and you should be fine


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Let's say I run av gas, what degree of timing would you guys recommend that would be safe?


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From what I was told by a very well known motor builder " the Bostick"
10 3/4 :1 max on pump gas mix in a gallon or 2 of 110 and you should be fine


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(Almost lupo... I've said 10.25)

Depending on cam specs, cylinder head, port velocity, quench, timing curve (or lack thereof) and a couple other little details... 10.25:1 is about as far as I'd go on pump and know it to be safe in a raw water boat application. I've fudged as much as 10.4:1 on iron heads (and 11.2:1 on aluminum) But considering the cam had quite a bit of overlap, it helped bleed off a little dynamic compression. If you do not, or not sure about having a good quench (.040-.050) area... Deburred piston domes and combustion chamber... And a cam that would help out on the dynamic compression... Then anything over 9.75:1- 10.0:1 can be risky. Also, the boat will play a little role as well... Heavy ass 21' daycruiser is going to create much more low speed load than a lighter 18' when getting up on plane... Therefore what you can get away with will vary. I'm sure someone will chime in saying they've ran 94:1 compression on 79 octane all day long for months of continuous use at 10k rpm (blah blah blah)... But as far as SAFE and not knowing the nitty gritty of the build... As a generic rule: if you're over 10:1... Gonna want to at least "spike your fuel" with something of a hotrod or av gas fuel to kick the octane up a bit. Timing is all gonna depend on what the motor likes with the fuel (or blend of fuels) used... I'd start at 32* to be safe and keep adding a couple degrees at a time until the plugs read good on the ground strap... I like the line to be right at the bend.


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Based on the numbers that you provided:

4.280 Bore
4.250 Stroke
.000 Deck Height
9.700 Gasket Volume
20.0cc Piston Dome
116.0cc Combustion Chamber
6.385 Rod Length

I come up with 10.48:1 Compression Ratio

I wouldn’t be the least bit surprised if the actual measured compression ratio is lower.  Without CCing the heads, we can not be certain that they are truly 116.0cc.

Using http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=406&sb=0 and the above data, I calculate your dynamic compression ratio to be:

3.046 Effective Stroke with the intake valve closing at 74-degrees ABDC

7.79:1 Dynamic Compression Ratio

3.110 Effective Stroke with the intake valve closing at 72-degrees ABDC (Advanced 2 degrees)

7.94:1 Dynamic Compression Ratio

3.172 Effective Stroke with the intake valve closing at 70-degrees ABDC (Advanced 4 degrees)

8.08:1 Dynamic Compression Ratio