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 Post subject: Compression ratio
PostPosted: Tue Mar 13, 2018 9:04 am 
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Around the block

Joined: Thu Dec 05, 2013 12:32 pm
Posts: 82
Country: Austria
Bikes owned: GT750
Hi, I need your expertise to figure out correct cylinder head skimming.

I have raised the top of my exhaust port from 40mm to 33mm. Since compression ratio is calculated from top of exhaust port, I need to skim the head to maintain same compression rate or even increase it.

I measured the volume of what I believe to be a stock M/A/B cylinder head at 30cc. My calculations give a compression ratio of ca. 5,3 for a stock M/A/B cylinder with 40mm exhaust port. I assumed a deck height of 0,6mm and a head gasket of 1mm thickness.

How much would I need to skim the original cylinder head to keep the same compression ratio of 5,3 with a reduced exhaust port upper edge of 33mm?
In my calculations I would need to reduce the combustion chamber volume from 30cc to 22,5cc, which means skimming off close to 2mm from the head.

Is my assumption correct? Can someone confirm the volume of an original cylinder head?


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 Post subject: Re: Compression ratio
PostPosted: Tue Mar 13, 2018 9:42 am 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5941
Location: Illinois
Stock heads vary a bit but are around 30-32cc and deck height is usually around 0.5 - 0.6mm. Don't forget the piston crown when you do the CR calculation.

On my current motor, I decked the barrel 0.5mm to bring deck to zero, and I usually do that to any motor. Then I had another 1.0mm off the head and it gives cranking pressure of 165psi. That was with exhaust ports raised to only 37.3 mm - a tiny raise from stock.

With your >TR spec barrels and mild pipes, 2mm sounds about right. It raises CR to somewhere close to 6.0:1 (13.0:1) - still lower than stock. 3mm would get you to around 7.1:1 (15.8:1) which is higher than stock 6.9:1(10.4) and too much for street gas.

At 3mm off, the combustion chamber diameter is smaller than stock and if you machine in a squish band, that would lower the CR a bit. I have not alculated how wide the squish band would be, but it's not very wide.

The ideal answer is proper squish heads and to do that you can weld up a head and re-machine it to a new shape or have the head machined to take inserts and that way you can swap inserts to raise or lower the CR.


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 Post subject: Re: Compression ratio
PostPosted: Tue Mar 13, 2018 9:57 am 
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Around the block

Joined: Thu Dec 05, 2013 12:32 pm
Posts: 82
Country: Austria
Bikes owned: GT750
I struggle to get the math done for the compression ratio of a stock engine.
AFAIK it calculates: total volume above piston at exhaust port just closed divided by volume above piston at TDC
using 40mm exhaust top edge, 0.6mm deck height, 1mm head gasket and 30cc head volume it gives me like 5,3:1. I am ignoring the piston crown which reduces head volume by few cc potentially, but does not bring compression up to above 6.
What am I doing wrong?


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 Post subject: Re: Compression ratio
PostPosted: Tue Mar 13, 2018 10:39 pm 
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Moto GP
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Joined: Mon Nov 03, 2008 10:47 am
Posts: 5941
Location: Illinois
The piston crown volume is more than you might imagine. But, a few people have tried to calculate stock compression and had problems with the mathematics of it.

Swept volume above TDC is 154cc plus trapped volume which is closer to 28cc net divided by 28=6.9

At 33m down, that swept volume is only 127cc, so (127+28)/28=5.5 or so.

TR750 had something like 21cc heads, so at say 1mm per 4cc, that means it has to have closer to 22mm taken off, but 1.5mm is a good place to start.


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