350 Wide Case CR question

[ericpritchard]

I am building a wide case 350 for racing. Can anyone help by suggesting a suitable compression ratio to aim at?
My NC 250 race engines are built with a CR of around 10.25:1 and I measured the current CR on the 350 motor at 11.90:1 which maybe too high. Before I start machining the piston (or adding gaskets/spacers to the cylinder base), does anyone have any comments?
Your input would be much appreciated
Regards
Eric Pritchard

[DewCatTea-Bob]

" I am building a wide case 350 for racing. Can anyone help by suggesting a suitable compression ratio to aim at?
does anyone have any comments? "

____ Sorry I have no idea about that for racing with whatever fuels for such these-days.
__ I've had 13.5:1 -(calculated) in a n-c 350, and obtaining fuel with a high enough octane was always an issue. _ So I'd suggest to be sure to try staying under 12:1.
However, I suggest that you be sure that all CRs are compared by the same CR method, either 'actual' (with calibrated measuring devices), OR 'calculated', CRs.


Hopeful-Cheers,
-Bob

[ericpritchard]

Thanks for the responce, Bob. Appreciated.
I use the "calibrated measuring" method for calculating the CR's and we have access to 110 octane leaded fuel at the majority of race tracks in the US. Do you think keeping the CR at 11.9:1 will be a problem for the bottom end? (I'm using a Carillo rod). Are there any other downsides?
Regards
Eric Pritchard

[DewCatTea-Bob]

" we have access to 110 octane leaded fuel at the majority of race tracks in the US. "

____ 110-octane was not quite good enough for my 13.5-CR. _ Might be okay for 12:1 though.


" Do you think keeping the CR at 11.9:1 will be a problem for the bottom end? "

____ I sure can't think of any reason for why a high-CR would much matter to the bottom-end. _ (Perhaps Eldert knows something of such [?].)


" Are there any other downsides? "

____ Compared to std.CRs, the cyl.head will get hotter and thus be a little tougher on your motor-oil. _ So be sure to use a heavier oil (40 - 50W) which will carry-away more heat.

____ BTW, what piston do you have that it gives you a "calculated" 11.9:1 CR ?


Fun-Cheers,
-Bob

[Eldert]

Hi Eric

i would aim for 11 to 1 . there is almost no HP gain i you go higher.
things only get more critical with octane ratings , ignition settings and jetting
and it looks like the engine will rev better in the upper range with sligtly lower c.r.

i would lower the c.r. by machining the piston crown , by adding spacers under the barrel the squishband wont work
as it should

the bottom end should be no problem , i build 500 Ducatis al the way up to 13 to 1 with no problems
but i lowered the c.r.and the engine runs better

Eldert

[ericpritchard]

Many thanks for the info. Appreciated.
Regards
Eric
P.S. I'm using a Venolia piston described as 11:1