CB378 high compression *HELP*

irk miller

You've been mostly-dead all day.
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GS850, stock bore for a 378 swap, is 69mm. That's all I see wrong. Unless we're running aftermarket overbore pistons.
 

MrMister

Been Around the Block
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So long as you're not pinging, high compression would be a good thing. Harder to kick over, sure, but a stronger engine is something for which people usually pay extra. :p
Thanks. My next question would have been is it something to worry about’?.
So did a hot test and interestingly enough both side read 185 .


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MrMister

Been Around the Block
DTT BOTM WINNER
Sonrier: Agreed. Higher CR makes for better street performance as long as the fuel octane level is high enough.

At least you have a real number now to confirm that the compression is quite high

Let's assume you would like to reduce cranking pressure from 195 (nominally 13.2:1) to say 180psi or 12.2:1, that would require an increase in clearance volume from 15.5cc to 16.8cc which is an increase of 1.3cc and with a 70mm bore that amounts to around 14thou (0.014") spacer or a gasket .013-.015" thicker than you have now.

Somebody should check those calculation though.
Thanks for the help. I guess like Sonreir said, if it’s not going to blow up the engine, it’s all gravy.
I would have been more concerned if it was 200+


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MiniatureNinja

Coast to Coast
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13.2:1 compression isn't something to be scared of - just be sure you get a smooth complete burn of the fuel using the proper octane of fuel and you should be good.
my hot compression is 185~psi as well..
 

crazypj

Split personality, I fake being smart
If a 378 is done properly the cold cranking pressure will be at least 210psi, up around 40 from stock.
Mine run around 220psi cold cranking pressure. If you have more than that it may be the issue? If you fit a semi-race cam it will lower the cranking pressure quite a bit but you lose low end torque
Usually, peoples kick start technique is pretty crap (think 100cc dirt bike 'slap kick') so they don't know how to kick-start a built motor.
You have to be just past compression on the correct cylinder or you run into compression on the other one before crank is turning quick enough to push through.
With a 180 crank you get two firing impulses quite close together then a pretty big gap (rotationally speaking, 540 degrees) until it happens again. If your in the wrong place it will give you problems, particularly if your much less than 160lbs. You also have to 'push' through' the swing instead of stamping on kick-start for only 2"~3". It probably is closer to starting a big twin but at least you won't snap your leg in two if you get it wrong
 

crazypj

Split personality, I fake being smart
The modified 360 has a very good combustion chamber and doesn't really need even 93 octane fuel.
I've used 87 without issues (but not at very low rpm slogging around, idle about 1200 and ride around 6~10,000rpm))
Couldn't feel any difference with 89 or 93 although E10 does mean some carb adjustments are needed plus a slightly larger secondary main jet (I now modify carbs with a 115 secondary instead of 110)
 

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