Fast Burn Head

[ozzy2]

I know this topic has been done befor but ill throw a question i have a MAC 45 with two heads both have the same volum but they are of diffrent shapes one is a shalow bowl the other is deeper every thing else is the same both make great power but one detonates and the other is perfict any idears why.

Ozzy

 

[boaterbill]

I know this topic has been done befor but ill throw a question i have a MAC 45 with two heads both have the same volum but they are of diffrent shapes one is a shalow bowl the other is deeper every thing else is the same both make great power but one detonates and the other is perfict any idears why.
Ozzy

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One is for low nitro, the other one for high nitro content, I forget which is which, Andy stopped doing that quite a while ago. If the one you are using with the nitro you're using , it's working, leave it alone.

Bill

 

[Terry Keeley]

I know this topic has been done befor but ill throw a question i have a MAC 45 with two heads both have the same volum but they are of diffrent shapes one is a shalow bowl the other is deeper every thing else is the same both make great power but one detonates and the other is perfict any idears why.
Ozzy

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Which one detonates? Are you sure they're both the same volume, how are you measuring them?

 

[gil sonsino]

I have two heads for my Mac 67 and both are the same volume but with different head clearances.Make sure if you are using high nitro to use the higher h.c to avoid detonation.Take a look to the portion that enters the sleeve it must be shorter for the high nitro application using a caliper to get this number!

Gill

 

[ozzy2]

Ok ill try to clarafy this these heads were made by me both heads are of the same volium mesured with a buret at .34 cc and this is acurat this engine is running on 50% H/C .008

The shallow bowl has less squish area this one detonates

The deeper bowl has more squish area this one dosent

Ozzy

 

[Bob Kensill]

I have found the opposite to be true,higher head clearance for lower nitro,and less head clearance for higher nitro.

 

[izitbrokeyet?]

I'm with Bob on the low clearance/ high nitro and deck high for low nitro thing

 

[DaveMarles]

Paul, The head with the narrower squish band/wider bowl will have a lower maximum squish velocity, and if too low, this will detonate easily. I just ran some some figures through an MSV program on a hypothetical .45 engine and to give you an example: a head with 50% squish area at 28000 rpm will decrease its MSV by approx 1/3rd if the squish area is reduced to 40%. The only way to compensate for that is by increasing the rpm a lot, or decreasing the head clearance, which I should think is impossible or by reducing the CR by a big amount. So maybe the best idea is to junk the narrow squishband head.

Dave

 

[ozzy2]

Thanks Dave

That sounds like a resonabl answer to problem i was thinking on the same line and wanted to see if any body else had similer thoughts.

Looks like i will move back to a larger sqish area and keep testing.

 

[gil sonsino]

Man wonder age is starting to blurr my mind hehe!

Sorry for my wrong input but logging after a hurry day that's what happens!

Well just have checked up my both boats and obviously that's what we have here:

take a look to the pic and see in the left the head I use on my heat rigger that is set at .0016 head clearance burning 40% nitro and on the right the head used on my saw rigger set from .0013 to .0015 that burns 65% nitro and depends on other factor of course like pipe ,prop,weather etc.. so now we have the right data for your question.

Both head have the same squish band but you can see a little bit higher volume on the righ one with the help of the shade. The small hole in it was originally made by CMD to identify the difference.

Thanks

Gill