MTFCA Forum

My '26 speedster came with a low head. I have a high head in my stash. It looks like the high head has larger water jackets (better cooling?). What would your suggestions be on which would be best and why?

Thanks!

Open your wallet for a new Prus head.

Petrah Phyre wrote: ↑

Fri Feb 06, 2026 6:36 pm

My '26 speedster came with a low head. I have a high head in my stash. It looks like the high head has larger water jackets (better cooling?). What would your suggestions be on which would be best and why?

Thanks!

To answer your question, it kind of depends on what radiator you're using. The low head water outlet lines up well with a brass radiator and high head lines up better with a later, high radiator, (all assuming you're also using the correct water outlet for each). Since you have a speedster, any combination could be possible. I would tend to use the head that fits up well with whatever radiator you have.

The high head does have a larger water capacity, but the low head has a slightly high compression ratio (i.e. smaller combustion chamber).

ModelTWoods wrote: ↑

Sat Feb 07, 2026 8:22 pm

The high head does have a larger water capacity, but the low head has a slightly high compression ratio (i.e. smaller combustion chamber).

This is what I saw listed on TBay. Didn't know if it was true or not. May be why the guy who originally put it together used the low head. Is this why the 22hp vs. 20hp? The low head is already painted, so I might just use it. In South Dakota, so it doesn't usually get too warm. If I find I need the extra cooling, it's not too hard to change out. Not like I'm going to win any competitions for 'ALL ORIGINAL' with a speedster anyway.

Petrah Phyre wrote: ↑

Sat Feb 07, 2026 10:11 pm

ModelTWoods wrote: ↑

Sat Feb 07, 2026 8:22 pm

The high head does have a larger water capacity, but the low head has a slightly high compression ratio (i.e. smaller combustion chamber).

This is what I saw listed on TBay. Didn't know if it was true or not. May be why the guy who originally put it together used the low head. Is this why the 22hp vs. 20hp? The low head is already painted, so I might just use it. In South Dakota, so it doesn't usually get too warm. If I find I need the extra cooling, it's not too hard to change out. Not like I'm going to win any competitions for 'ALL ORIGINAL' with a speedster anyway.

The horsepower difference may have had something to do with the head change, but also with a cam profile change, which I have read, also occurred during Model T production. If you really want to go, Zoom Zoom with your speedster, and the engine is presently, basically stock and your on a budget, Buy a reground, or better yet, a new .280 lift cam, better carburation (like a Stromberg OF or similar), or change to a Model A intake, exhaust and carburetor.