My 25 came to me with a Low Head installed on a 25 motor. In the name of "correctness" I'm considering installing a High Head. As I understand it, the Low Head gives the motor a slightly higher compression ratio then that with the High Head. So, the question is, will there be a "noticeable" power loss, or will the power loss be barely delectable?
I believe that most of the "high" in later head is due to larger water jackets. I'd say your power loss would indeed be barely detectable, all other factors (such as milling) being equal.
Thanks R.V.Anderson, that was my understanding as well, that he reasoning for the changeover was to offer additional cooling capacity. The combustion chambers are different as well. I had heard that there was a difference in compression, looks like it's not enough to be concerned about.
You will need longer bolts
The low head will be easy to sell!!!
The low head gives you 2 more horsepower. I think I would keep it in place.
The pre - 1913 low heads have a smaller combustion chamber that, combined with the better pre - 1913 camshaft, gave an extra 2 HP.
The low head compression dropped in 1913 to something like 4.75:1, same as all high heads.
The high head will cool the engine better as the water jackets are larger.
Royce, I do not believe I have ever seen anything definitive on the c/r of the early non-water pump engines.For instance, I had a ball of rust, dug out of the ground early head that had the holes for the priming cups.Tiny combustion chambers.What do you suppose that was? Also, risking wrath, still got your XR7-GTE?I'd like to know what happened to mine.
Jim,There is one of those heads on our 14 that uses the side bowl priming cups! Bud.
Interesting,Bud.my rusty head had the plugs in it.Is there a picture of what the cups look like, or could you post one?
Sorry i'm picture dumb.They look like normal primers but they dump from the side.I also have set screws in the head as i run with Champion X H 14 priming cup spark plugs. Bud in Wheeler,Mi.
I have 2 good low heads for sale if anyone is interested. Check the classifieds.
Jim yes I still have three Mercuty Cougars. One GT-E 427, one 428CJ XR-7 and a 390 XR7-G.
Take a high head for the extra cooling and shave off some metal.
High head was done for two reasons lower compression for poor quality of gas during late teens and add little more cooling capacity there is approx 3/16" differance in the chambers on the two head styles I have
Low head givestart 2.5 Hp difference on the same motor
If you have a good radiator, the low head would cool just fine, but it has more power.
Thanks everyone for the input. I think I'll stay with the Low Head for now. The motor cools great so that's not an issue. As for power, I need all I can get being it's a closed car.
Again, there is a lot of disinformation here, particularly from Bob Middlefield. Ford lowered the compression ratio of the low head on March 14, 1911. Combined with the revised camshaft introduced about serial number 95,000 in December 1912, this reduced advertised horsepower to 20.
Compression was lowered a second time - still the low head - on July 17, 1914.
The high cylinder head was introduced May 17, 1917 with the notation "Redesigned — High Head — Changes ASAP, Use Up Old Stock". My particular 1917 torpedo runabout was built in late June 1917 with a low compression low head.
Interestingly, the high head had its combustion chamber reduced, this raising compression on September 25, 1918.
Compression was raised again on May 18, 1926. So there is a "high performance" High head that was produced for only one month in 1926, because compression was lowered again on June 22, 1926.
See the record of changes here:
And here's Larry Young's compression comparison sheet between different heads made for the Tulsa Model T club: http://mtfctulsa.com/Tech/head_design.htm
Don't know if the 1911-17 stock low head Larry had access to was the pre or post 1914 style, though the difference is likely barely noticeable.
For a real power boost you need a Z or Prus head with 5:1 compression and a squish area above the piston that gives a more effective combustion.
I have a Reeder low head on my '15 - it made a remarkable difference compared to the original low head.
I don't think there is a whole lot of difference in power between a low head and a high head if they have the same sized combustion chamber.
The smallest combustion chamber allowed on Montana 500 cars is 270 CC's. A typical low head just cleaned up, or a high head milled about .080" is usually right around 270 CC's. The top cars are about evenly split between low head use and high head use. I've used both and won with both and honestly don't know which is better.
I think the notion that high heads cool better is largely nonsense. The capacity of the cooling system has little to do with the ability to cool. It might take a few seconds longer for a high head to come up to temperature, but after that there would be little difference.
So, other then monetary value, the difference between a High Head and Low Head is for all intents and purposes....the same.
One of my Winter projects will involve pulling the head to check things out and also time the valves according to piston height. At that point if I have found a nice High head I'll have it cleaned up and install it when I button things up.
How do I know If I have a low head or high head?
Bill, here's a good thread that explains the differences.
Bill - by looking at it, the low heads are more rounded in shape and are, well, lower
To be sure, study the water outlet bolts - if they were longer, they would hit a head bolt on a high head, while they would go free on a low head. It's even easier with a loose head - there are ridges around the spark plugs in a low head combustion chamber you won't find in a high head.
Here's pictures from earlier threads:
Thank you that was very informative.
Measurements: Bolt holes on a low head are 21/8"; on a high head they're 25/8".