Instead of going the P or Z head route I've been considering improving my Coupe's performance old-school/Montana 500 style and milling the head down. It's a 26 Coupe so high head. I'm thinking 0.10" would probably be a safe number to have shaved off by a local machine shop. What kind of performance boost can I expect from this? The stock compression of a high head is what normally, 3.8:1?

I have had them milled about .120,and you get some more low end power.lf you overdo it,the head twists out of shape when you tighten the headbolts and head gaskets blow and or leak.

I had one milled 0.060" and had to pretend to imagine to notice a difference. We stopped at .060 as the area around the water passages was getting thin. I went to a Z head and am quite happy with the results and some guys say that the Prus head is even better. Apparently some stock high heads can be milled to 0.125" and some "performance" increase is evident if you call going from 20HP to maybe 21.5HP an increase in performance. Good luck : Bruce

Matt, when milling heads, .010" is a clean up pass to make a new sealing surface. Cutting .100" - .125" will get you in the ball park of starting to see a difference. You also have to consider the head deck thickness. They are not all the same, so something else to consider.

You may need to use thick washers to space the bolts--or you may not. You'll have to measure the depth of the holes. My experience shows stock head bolts tend to be on the short side of my liking, but you will have to measure the hole depth either way. Don't assume this too will be ok just because someone said it worked on theirs.

As far as compression, If I really want to know the true compression, I never rely on published numbers. The only true way to know is to CC the head before and after milling and do the calculations.

My opinion, leave the guess work at home and buy a new Z or Prus head.

Here is the chart from an article the Tulsa Club did years ago.



(Message edited by Chad_Marchees on June 05, 2017)

(Message edited by Chad_Marchees on June 05, 2017)

Matthew, Put your money in a new head... The Z has a very different combustion chamber shape that is much more efficient plus has more coolant capacity. I also like the alum just for the weight difference.

I bet you could find someone like Me that wants to buy a Prus head but already has a Z that would make you a good deal.

It's the #1 best thing for increasing the performance of a T just by putting on a new head.

The Model T club of Tulsa has done the testing and made a great article on the net: http://mtfctulsa.com/Tech/head_design.htm

Milling a high head 0.125" increases compression from 3.8:1 to 4.2:1 or like what the early low heads from 1909-10 had. Unfortunately the peak power increase is theoretically only 1.2 hp, or from 20 to 21.2.

The accessory heads gives much more from 5:1 since they have an improved shapes of the combustion chambers with squish areas over the pistons. Without the squish 5:1 would theoretically only give about 23.2 hp, while actual peak performance has been measured as 26 hp for the Z and 28 hp for the Prus.

What dimension can a person measure to determine if and/or how much his "original" head has already been cut?

You can expect a nice seal surface for you new gasket, but not worth the effort even with a deep cut in the .125 range. Do you need more performance or just want it. My 26 coupe would run fine on flat ground but dragged up hills. A high compression head made the hills much more enjoyable. A cut down head had made no real noticeable difference.

Andy

Matthew, as the other fellows have said the combustion chamber shape is the real secret to better power. I've attached a photo of a new Prus head and a really high compression Sherman Spitfire head. Look at the squish area, shallow flat area, shape. Compare them to your stock head chamber. Higher compression is good but combustion chamber shape is huge.

My 27 Coupe is bored .060 over and the head milled .060. It will move right on down the road. More zip than my 24 touring .030 over with a stock head.

Save the $'s it would cost to have a high head milled and invest it in a Z or Prus head.

I have a Bridgeport mill and have Milled several high heads and it made no real difference. Running high compression (Domed) pistons and a low milled head makes a noticeable difference.....still not quite as good compared to a Z or Prus head. Without a water pump, the low head runs warmer compared to aluminum heads.
Chamber head design in both Z & Prus heads provide better performance over stock heads.

JMO....based on what "I" have experienced.

As far as Montana 500 stuff goes, the head has little to do with it. Typically we mill about .080" off with a high head and just enough to clean up a low head.

Most of the horsepower gains are made elsewhere.

I agree that the best bang for your buck is a Z head or the like for easy bolt on power.

Bruce, I love your "I had to pretend to imagine" any difference in performance. It reminded me of my childhood cycling days when removing the mudguards made such a difference to the speed I could attain.

Allan from down under.

Thanks for all the responses guys. I was just thinking of going the milling down route to keep the motor stock, but with just a slight bit more "oomph" that would cause no harm. I've been looking at the Cast Iron Prus heads and really like them, and it would definitely save me here in the Catskills with the big hills we have. In the same breath I've been playing with the idea of some day running the Montana 500. Nothing says I can't mill the head down .10, AND get a Prus head right? ;) Easy enough to swap out! When it comes time to pull the motor down I'll probably also add a Scat crank and Stipes 280 cam.

I know of a particular speedster that was running a very hot roof back in the 80's that had been machined all the way to the waterjackets. Eventually the space between the enlarged valves cracked but for a time it was a fast car. While not being a flathead it does show that it is possible to machine all the way and still run a head.

I've milled a few (low) Ford heads .125" and used those with pop-up pistons. That makes a big seat-of-the-pants difference for way less than the cost of a Z or P head. In fact, I couldn't tell any difference between that combo and a Z head.