MTFCA Forum

thoughts and opinions which would make more horsepower, flat top pistons with a z head or domed pistons with stock head

I can't see much point in fitting a Z head to raise the compression and then going to flat top pistons, other than you might avoid clearance problems with the way Z head combustion chambers vary.

Allan from down under.

I tried different things on a dyno, there is a lot of variables here you are asking, the cheapest bang for the buck is High compression pistons, you can get more power other ways but cost you more.

Having everything in the engine and transmission properly balanced is the very first step before adding any horsepower. A good dynamic balancing of the flywheel with magnets is the most important part. Next item to add would be a 280 cam at around $450. Next item to add would be a Prus cylinder head at about $425. A new Scat brand crankshaft at around $1650 will eliminate the possibility of a broken crankshaft that is more likely when you create more horsepower… The reproduction aluminum stock style T pistons are the way to go because they work with any cylinder head.

It depends on your application Bob. If you have an early car no one is currently making a high compression low head, Pop up pistons with a low head is a good running combination. If you have a running car that doesn’t need the low head look, a high compression head is instant power.

Andy

Allan wrote: ↑

Wed Apr 03, 2024 7:02 am

I can't see much point in fitting a Z head to raise the compression and then going to flat top pistons, other than you might avoid clearance problems with the way Z head combustion chambers vary.

Allan from down under.

Allan, Everything that I've read about both the Z and the Prus heads are that they MUST be used with stock flat top pistons to avoid clearance problems. No where have I read that either head can be used with high compression pistons, unless a person mills almost ALL of the high compression dome off a domed piston, leaving only the small part that extends into the main combustion chamber.

Joe Bell wrote: ↑

Wed Apr 03, 2024 7:22 am

I tried different things on a dyno, there is a lot of variables here you are asking, the cheapest bang for the buck is High compression pistons, you can get more power other ways but cost you more.

Joe, What is the maximum that can be milled off a low head when used with domed pistons? I have a set of Snyder's high compression pistons and 17-19 low head and want to mill the head for maximum compression with these pistons. I know .100-.125 is possible with a high head. I, even know a forum member who has milled his high head .150, but I've never seen a figure for a low head.

robertw413 wrote: ↑

Wed Apr 03, 2024 2:39 am

thoughts and opinions which would make more horsepower, flat top pistons with a z head or domed pistons with stock head

Everything I've read about the Z and Prus heads say they can't be used with high compression domed pistons.

Terry- trying to mill a stock head to raise the compression is false economy- you can't mill enough off to make a noticeable difference before you ruin the head. What Andy stated above is true- domed pistons with a low head is a great combination and looks totally stock. The domed pistons are not that much more than stock height. None of the aftermarket hi compression heads can be used with domed pistons- all those heads are low over the piston area in order to promote turbulence in the chamber.

Dan McEachern wrote: ↑

Wed Apr 03, 2024 1:07 pm

Terry- trying to mill a stock head to raise the compression is false economy- you can't mill enough off to make a noticeable difference before you ruin the head. What Andy stated above is true- domed pistons with a low head is a great combination and looks totally stock. The domed pistons are not that much more than stock height. None of the aftermarket hi compression heads can be used with domed pistons- all those heads are low over the piston area in order to promote turbulence in the chamber.

Dan, I always value your input. I knew that flat top pistons must be used with any aftermarket high compression head. One possible exception might be that if you milled and shaped the top of a domed piston to allow a domed piston to clear the chamber of a Ricardo head; that might be the only exception, but even then, a lot of work and careful measuring would be involved for very little gain.

I know that it would take a lot of milling of a head's surface to produce a significant increase in compression with stock flat top pistons, but with domed pistons, I figured any amount of compression achieved by milling the head would be of benefit to the total compression increase added to the domed piston increase, but I stand to be corrected.

Also, I have a Z head, but know of their history of not sealing well and leaking. The Z head I have was bought used from a reputable vendor and he said the head was not known to leak on the engine it was pulled from. No explanation was given as to why it was pulled. As insurance against leakage, would it be possible to weld up the head around the water passages that are known to leak and then mill the head just enough to make the surface true. I hate to throw away the head just because it MIGHT leak, or might not.

Hi Robert,
I have heard someone say they had a low head that was milled enough that the high compression pistons hit the head with no gasket
so they didn’t use that head & they where happy with the low head they did use with those
pistons. I have noticed that high compression piston are not all created equal. Some MFGs look to be higher than others but I’m not sure which is which. I would do a clay impression on the Z head before I had it milled to clean up the surface to find out just how much you have to work with. If the Z head is badly pitted around the water openings you might try Devcon to fill & surface.
Craig.

Terry, I was thinking "flat top pistons" were other specials like the domed ones. Reference to the standard pistons with raised flat tops didn't cross my tiny mind. Duh!

Allan from down under.

I'm thinking that there is some confusion between "domed" stock pistons with a flat top that protrudes well above the block at TDC, and "high compression" pistons that have a pronounce dome (no distinct machined taper). I've never seen real "flat top" pistons in a T block, but someone would have to have a special need for them , maybe with some of the overhead conversions

Generally model T flat top pistons are considered what ford made regardless of if they are ford or aluminum reproductions, flat on top & angled
up to the flat. Domed or high compression pistons are round like half a soft ball sticking out of the block. Some A crank conversions are done with 292cu.in. pistons and depending on how the rods are done will be from flat with the block to sticking out a 1/4" but are completely flat on top.
Craig.

Bruce, I have seen true flat top pistons, with no intrusion into the combustion chamber. They were from a Dodge Phoenix V8. They were all that we had back in the 1960's. I still have a non generator block which was bored to 3 and 7/8" to take them.

Allan from down under.

Bruce Compton wrote: ↑

Wed Apr 03, 2024 7:07 pm

I'm thinking that there is some confusion between "domed" stock pistons with a flat top that protrudes well above the block at TDC, and "high compression" pistons that have a pronounce dome (no distinct machined taper). I've never seen real "flat top" pistons in a T block, but someone would have to have a special need for them , maybe with some of the overhead conversions

Bruce, In a Floyd Clymer book, 'Model T Ford Restoration Handbook', that has been in print since the 1960's, by Leslie R. Henry, it is stated that .060 Ford 312 Y block pistons, original Ford part number B6A-6505-A, (3.75" diameter) can be used but wrist pin bushings must be made to reduce the wrist pin size down to T wrist pin size. Of course this was information from the 1960's when reproduction parts for T's were scarce. Now days, with the current availability of reproduction parts, using pistons that you have to make wrist pin bushings for, doesn't make sense.

Terry. We have several people with Z heads. I have one of the very first available and have not heard of any leaking issues? Where was the leaking that you talk about? I know the lands between the cylinders is very narrow but unless the casting has some pitting should not be an issue more than any other head. Did you mean leaking from the inlet or outlet fitting? If you have a Z head already I'd put that on and decide if you've your happy? No loss in trying..

Allan and Terry. I agree that several other types of flat top pistons can be made to function in a T block, but if they fit flush with the top of the block with no protrusion into the combustion chamber the resulting C.R will be awfully low if using a stock head, or even a modern H.C. head.

Bruce, that's exactly what I had in mind in my first response.

Allan from down under.

SurfCityGene wrote: ↑

Thu Apr 04, 2024 12:19 am

Terry. We have several people with Z heads. I have one of the very first available and have not heard of any leaking issues? Where was the leaking that you talk about? I know the lands between the cylinders is very narrow but unless the casting has some pitting should not be an issue more than any other head. Did you mean leaking from the inlet or outlet fitting? If you have a Z head already I'd put that on and decide if you've your happy? No loss in trying..

Gene, Since I haven't used the Z head, I'm just repeating what I've read on the forum from people who have used them. I would hope there wouldn't be any leakage. The leakage I have read about, I think, occurred between cylinder bores.

Yes Terry, I had a Z head that blew the gasket at the narrow surface between the cylinders. An old "stock-car" mechanic had the fix.......using the same used gasket he cleaned the leaking section of the gasket and applied some soft solder to both sides of the gasket especially over the rolled over seam (fire ring) . He sanded the patch to be close to the original thickness, installed the head and I drove the car for 4 years with no problems.