MTFCA Forum

Anybody know what the compression should be with high compression pistons and a high head?

May help to give a ball park figure, I just checked one with HC pistons but a low head and come up with 75PSI.

My torpedo with HC pistons reads out at 90# on all but one, the other one was 88#

That would depend on the specific head and pistons used. Other factors include the overall condition of the engine and how much overbore, if any, it has and whether the deck has been resurfaced or not. A performance camshaft might lower readings at cranking speeds compared to a stock cam. Cranking speed is another factor. The height of the valve heads above the block deck when they're seated can affect compression pressure, especially in smaller combustion chambers. Compression pressures will be lower at high altitudes and higher at or near sea level. 90 psi or anywhere near it is pretty good for a Model T. That's nearly double the stock reading, I believe.

What Pat said, and how fresh is the engine will be a factor also....
I would say anywhere from 60psi to 80psi is good, as long as all 4 are close to the same reading.
Be sure to do a dry test and a wet test with a little oil in the plug hole to see if the rings are sealing, if the dry reading is low.

Yes correct, having been a mechanic for years I am aware of all "it depends on" I was just looking for a ball park figure thanks to all.

I think the stock pressure for most Ts is around 45-50 lbs if everything is in good shape. If you can get that up to 70-75 psi on a tight engine, you're probably doing pretty good. I belive that using a well-designed high compression head with improved chamber design will give better results than using high compression pistons with a stock head.

I had high compression pistons with my roof 8 valve head and a re ground cam, it pumped 125 psi and it had the throttle snap of a modern sprint car. I don't want to get that drastic these days, its funny I never broke a crank or threw a rod.

BarrettR wrote: ↑

Mon Nov 01, 2021 11:35 pm

Anybody know what the compression should be with high compression pistons and a high head?

I thought I had that answer in my records, I do not. I have wondered this exact thing. I got 95# From a .100 milled low head with some necessary trimming to clear. The performance boost did not meet my expectations. I agree, all things equal, The better combustion chamber design of even a stock Recardo head is an improvement without a compression raise. Which would more encourage me away from HD pistons & a stock head. Although not a fan of aluminum heads, I would recommend the PRUS head.(Addressed the corrosion issue) Have a T here so equipped. Very impressive.(5.38-1, 220CC's #79) I have built a few HD motors & notice definite performance improvements. What WE need is someone to take an actual compression reading in a fresh HD piston motor for an actual #. I dont currently have access. Guesses are not useful.
Hope this helps some.

P.S. A Montana legal (stock) T motor reads #60

Hi Roger,
I know this thread has gone on for awhile. I just ran a compression test on my fire truck it has HC pistons with a high head has about 2500 miles on it. Had 70+ lbs. on all cylinders cold. ( or as cold as it gets in Arizona this time of year)
Craig.

Thank you that's very helpful

Tim left out one bit of information about the 90 psi on his 1911 torpedo, it was stroked by .25 inches and bored by .060 in. The displacement is 194 cu. In. With stock bore and stroke it would have about 80 psi. Still very good.

My stock 23.has 62 psi. on all 4 cylinders. New rings but not bored 6000 miles ago

Art Mirtes