Domed Piston Compression?

Discuss all things Model T related.
Forum rules
If you need help logging in, or have question about how something works, use the Support forum located here Support Forum
Complete set of Forum Rules Forum Rules
User avatar

Topic author
walber
Posts: 213
Joined: Sun Jan 06, 2019 12:55 pm
First Name: Walt
Last Name: Berdan
* REQUIRED* Type and Year of Model Ts owned: '18 Speedster had 25 touring and 26 coupe
Location: Bellevue, WA
MTFCA Number: 16421

Domed Piston Compression?

Post by walber » Fri Feb 03, 2023 7:01 pm

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


speedytinc
Posts: 3840
Joined: Fri Feb 12, 2021 12:24 pm
First Name: john
Last Name: karvaly
* REQUIRED* Type and Year of Model Ts owned: 14/15 wide track roadster. 23 touring, 27 roadster pickup, 20ish rajo touring
Location: orange, ca
MTFCA Number: 14383
Board Member Since: 2020

Re: Domed Piston Compression?

Post by speedytinc » Sat Feb 04, 2023 11:50 am

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.


tselliott
Posts: 291
Joined: Mon Jan 14, 2019 11:28 am
First Name: Scott
Last Name: Elliott
* REQUIRED* Type and Year of Model Ts owned: 1923 Mercury speedster replica, 1923 speedster
Location: Tigard, OR

Re: Domed Piston Compression?

Post by tselliott » Sat Feb 04, 2023 12:45 pm

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.


Model T Ron
Posts: 322
Joined: Tue Jun 08, 2021 12:31 am
First Name: Ronald
Last Name: Bakow
* REQUIRED* Type and Year of Model Ts owned: 1915 Model T Touring
Location: Troutman, NC
MTFCI Number: 25350

Re: Domed Piston Compression?

Post by Model T Ron » Sat Feb 04, 2023 11:42 pm

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


Bruce Compton
Posts: 576
Joined: Thu Jan 10, 2019 11:23 am
First Name: Bruce
Last Name: Compton
* REQUIRED* Type and Year of Model Ts owned: 1921 Coupe, 1925 Coupe
Location: Kemptville

Re: Domed Piston Compression?

Post by Bruce Compton » Sun Feb 05, 2023 12:21 am

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.

User avatar

AndyClary
Posts: 605
Joined: Sun Jan 06, 2019 11:15 am
First Name: Andrew
Last Name: Clary
* REQUIRED* Type and Year of Model Ts owned: 1914 Runabout 1926 Coupe. Mercury Speedster #1249
Location: Usa
MTFCA Number: 24057

Re: Domed Piston Compression?

Post by AndyClary » 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


speedytinc
Posts: 3840
Joined: Fri Feb 12, 2021 12:24 pm
First Name: john
Last Name: karvaly
* REQUIRED* Type and Year of Model Ts owned: 14/15 wide track roadster. 23 touring, 27 roadster pickup, 20ish rajo touring
Location: orange, ca
MTFCA Number: 14383
Board Member Since: 2020

Re: Domed Piston Compression?

Post by speedytinc » Sun Feb 05, 2023 12:16 pm

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.

User avatar

Topic author
walber
Posts: 213
Joined: Sun Jan 06, 2019 12:55 pm
First Name: Walt
Last Name: Berdan
* REQUIRED* Type and Year of Model Ts owned: '18 Speedster had 25 touring and 26 coupe
Location: Bellevue, WA
MTFCA Number: 16421

Re: Domed Piston Compression?

Post by walber » Sun Feb 05, 2023 12:20 pm

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

User avatar

TRDxB2
Posts: 5409
Joined: Tue Jan 08, 2019 4:56 pm
First Name: Frank
Last Name: Brandi
* REQUIRED* Type and Year of Model Ts owned: Speedster (1919 w 1926)
Location: Moline IL
Board Member Since: 2018

Re: Domed Piston Compression?

Post by TRDxB2 » Sun Feb 05, 2023 1:05 pm

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.
--
--
Other opinions
https://www.mtfca.com/discus/messages/3 ... 1369036739
https://mtfca.com/phpBB3/viewtopic.php?t=1015
viewtopic.php?t=17165
The past is a great place and I don't want to erase it or to regret it, but I don't want to be its prisoner either.
Mick Jagger


Gen3AntiqueAuto
Posts: 390
Joined: Mon Aug 22, 2022 12:36 pm
First Name: Tim
Last Name: Foye
* REQUIRED* Type and Year of Model Ts owned: 27 Fordor, 15 speedster (2), 23 touring, 26 fordor, 25 TT
Location: Middleborough MA
MTFCA Number: 292
MTFCA Life Member: YES
MTFCI Number: 106
Board Member Since: 1999
Contact:

Re: Domed Piston Compression?

Post by Gen3AntiqueAuto » Sun Feb 05, 2023 1:26 pm

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.
Gen III Antique Auto - we do Model T Ford Restorations

You can have everything in life you want, if you will just help other people get what they want -Zig Z.
https://www.youtube.com/channel/UCeROBg ... pB-KImprjw

User avatar

Topic author
walber
Posts: 213
Joined: Sun Jan 06, 2019 12:55 pm
First Name: Walt
Last Name: Berdan
* REQUIRED* Type and Year of Model Ts owned: '18 Speedster had 25 touring and 26 coupe
Location: Bellevue, WA
MTFCA Number: 16421

Re: Domed Piston Compression?

Post by walber » Sun Feb 05, 2023 4:34 pm

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.


Kevin Pharis
Posts: 1354
Joined: Thu Jan 10, 2019 3:54 pm
First Name: Kevin
Last Name: Pharis
Location: Sacramento CA

Re: Domed Piston Compression?

Post by Kevin Pharis » Mon Feb 06, 2023 12:33 am

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

Post Reply Previous topicNext topic