Combustion chamber volume

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Model T Ford Forum: Forum 2015: Combustion chamber volume
Top of pagePrevious messageNext messageBottom of page Link to this message  By Dennis Plank - Three Way, TN on Monday, August 10, 2015 - 05:22 pm:

I want to tap into some of the vast knowledge on this forum.

On a flat head engine, higher compression does not necessarily mean more power. There has to be a balance between the volume of charge in the cylinder and the compression ratio. If the volume of the combustion chamber gets too small as a result of trying to raise the compression ratio, the result is that the fuel charge in the cylinder is less than optimal for a good, strong fire. Does anyone know what the ideal volume of the combustion chamber should be to attain this balance? I am wondering if any of the cam designers have looked at this when developing their cam profiles. I would think that would be a factor to consider.

Also wondering if anyone knows what the valve spring length should be with the valve closed. This is affected by how deep the seats are cut.

I am very interested in any information you might have. Thanks. Dennis


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Monday, August 10, 2015 - 06:15 pm:

The problem when flat head engines gets too high compression is the flow - the room around the valves gets so cramped it stops up the flow even with max lift.
Max / optimum compression ratio may be 9:1?, but then you'll need an oil pump and a beefier crank than a standard T.
Max compression ratio for a reliable splash lubed A or T is often said you be around 6:1.

The Tulsa Model T club's investigation in head alternatives years ago found the std high head measures 294cc, that corresponds to 3.8:1 in compression - much too low for an effective use of the gas, but that's what they had to do back in the 20's to avoid knocking since pump gas was mixed with kerosene.

A stock early teens low head measures 278 cc, that's 4:1.

Today there are a few available accessory heads; the Z and the Prus heads measures both about 210cc, that's about 5:1 - even though the Prus is misadvertised to be a 8:1 head. So don't worry about trying the Prus on a std T - it'll power it up, even more than the Z, since it has more squish area over the pistons - another important feature for flatheads that was discovered by Harry Ricardo in England.

I wanted more compression so I shaved off 0.100" from my Prus head to get 185cc and 5.8:1. The pistons also needed some shaving at the squish surface to avoid unwanted contact. I'm very satisfied with the hill climbing ability of my car even with 3:1 in the rear end.

There is an even racier head available from Ben Serar, it's the Sherman Superfire with 125cc combustion chambers and 8.8:1 or about as much as you may ever want in a flathead? But I would prefer a SCAT or at least a model A crank under such a head..

Haven't got data on the spring length, usually T's are tolerant, rpm's are low and valve springs are soft


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