MTFCA Forum

I've been looking, without success, for the compression boost from the domed pistons offered by the vendors. From stock 4 to 1 to ???

Anyone know or have a feel for it?
Walt

I have a note on my compression/displacement list. No idea where it came from. Like you I was seeking the same info.
"Reported 70# compression. 4.75-1" This must be for a stock hi head.
I have a set in an early motor(low head) Produces 95# compression.

when I had the head off my speedster I calculated it to be 5:1. Mine is a High head. I have the calculation notes around here somewhere.

I think your better with a high compression head to be honest.

I agree with Ronald; There is no question that an improvement in "performance" is gained by a boost in C.R., and the aftermarket aluminum heads provide that. The shape of the combustion chamber with the aluminum HC heads helps as well by creating better turbulence over the stock set-up. It's also a much easier modification, and easy to reverse if you chose. I've tried the Z head, the Prus head and the Reeder head and the Prus is my first choice due to the machining tolerances, the gasket surface area between the cylinders and the shape of the combustion chamber.

Since you can’t currently get a high compression low head, high compression pistons are a viable option for early cars. Domed pistons with a low head run quite well.

Andy

AndyClary wrote: ↑

Sun Feb 05, 2023 1:11 am

Since you can’t currently get a high compression low head, high compression pistons are a viable option for early cars. Domed pistons with a low head run quite well.

Andy

Yes they do.

My query came from basic curiosity. Years ago I added a Fronty T head to my speedster, when using flat top pistons simply changing the head was a noted improvement. When the crank broke (no great surprise) I went with a shortened A crank (1/4" more stroke) and domed pistons. That combination really woke up the engine but it was changing two things at once rather than one at a time so I'm still a bit curious about how much improvement was from the 1/4" stroke vs the domed pistons. Does it matter? Well it isn't world peace but it would be nice for a buyer of the domed pistons to know what to expect.

For a flathead motor - improved combustion chamber design is significant as well as a higher CR. Then again I've ridden in a stock(ish) T with domed pistons and low head and it was a very good runner.

Walt

FWIW https://www.mtfca.com/discus/messages/5 ... 1225240312
By Frank Harris on Sunday, November 02, 2008
A domed piston pushes the exhaust gasses equally around in the chamber but everything has to change direction when the exhaust valve opens up. Scavenging at high speed is faulty. The combustion chamber shape is determined by the piston. The power pushes the piston down very well but exhaust scavenging is not efficient.
On the other hand a flat topped piston in a head with a formed combustion chamber is designed to breath better and will beat the domed piston hands down with like cam shafts and valves. Just my two cents with an 18.6 second 1/4 mile at 71 miles an hour does it for me with flat topped pistons turning 48.6 horsepower at the rear wheels on a dyno with 150 foot pounds of torque.
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Other opinions
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If the engine is apart now - you're better off with stock pistons and a Z head. I've had both, domes left me wanting.

If I was building a flathead engine it would have standard pistons and a Prus head. I know of several near me that are real happy.

If you run the numbers… adding the A crank with its extra 1/4” of stroke to a 4:1 compression engine will only bump the compression ratio to 4.25:1. Unfortunately, at these low compression ratios it requires a lot of volume displacement to increase compression ratio. No doubt the high dome piston displaces a noticeable amount of volume, but likely doesn’t raise the compression much beyond 5:1, even with the A crank.

The higher the compression, the smaller the change in volume needed to adjust compression ratio. Eg, at 7:1 you would only need .010” of extra stroke to increase the ratio to 7.25:1