Squish band clearance?

[jaso]

Just made a new decompressed head for my CMB .90 . My question is what is the minimum squish clearance you can run in this size motor and is the only limiting factor the slop of the crank bearing, big end and gudgeon. By this I mean if you go below say 12 thou, are you risking the piston contacting the head under running conditions or is there another thing that comes into play?

I'm fairly confident that the head is relieved enough to run down to about 11thou of squish and still not being too highly compressed for plugs or start detonating.

Anyone ?? :blink:

 

[Anders_Martinelle]

I`m trying to not go under 10 thou on a 90 engine or any engine at all.

You have not a clue how mutch things are moving around in a engine so just to be on the safe side keep it at least 10 thou...

Anders

 

[jaso]

Thanks Anders, I've put it together and checked the clearance, at the moment it's at 11 thou. the next size shim I have is 3.5thou so that would take it up to 14and1/2 thou. do you think that is getting a bit high or should I try it at 11?

 

[Anders_Martinelle]

It depends on the bowl size you have.

11 thou is ok but if the bowl is small you can try 14.

I never go over 14 thou.

I always try and adjust the CR with the bowl size and keep the distance the same.

Anders

 

[nitrotoys]

My .80 gold head is 12 thousands.

and it runs and works good. i wouldnt go any lower than 11.

Tom

 

[boaterbill]

I run .008 in my 3.5 Nova Rossi, 7.5 Nelsons, 7.5 Macks, 67 Macks with no problems.

Bill

 

[jaso]

My standard head was set at 15 thou from the factory and it seemed to be on the verge of detonation. I put a shim in it to see what would happen and put a tank of fuel trough it with a std 1667. Seem ok plug wise, then I put a 1667 I've been working on, on and that was it, plug destroyed. So I made another head botton to play with and I was a bit unsure as to the minimum squish I could have with out it causing problems or worst case the piston kissing the head. I've learn't that raising the head only makes the detonation worse due to the heat issue with the flame front bla bla bla.

I've also learn't through a bit of research that the lower the squish the more power and acceleration. Sounds good to me. I can machine another head, just need the time if need be. The head I made has a larger bowl. I'd rather have the squish set right and work from there. Am I nuts or does this sound right? :blink:

 

[Guest]

Sounds good to me. I've also found with higher clearance and castor oil will cause more detonation. I run 100% synthetic, 18-20% per gl. on 60% nitro.

Bill

 

[boaterbill]

Forgot to log in above.

Bill