Most folks seem to agree that installing a Z or comparable hi-compression head is the one best thing you can do to increase performance. The second thing mentioned seems to be the Stipe Cam.

There's a really nice article on increasing the compression ratio and it's benefits in the latest issue of "Skinned Knuckles" mag.

I sure learned some stuff on this subject. This info is quoted from about a 1935 "Motor Service Repair manual". Included was a chart showing average compression ratios from 1925 to 1935 increased from about 4 to 1 to about 6 to 1. This about parallels the change involved in changing from a stock to a Z head. The reason given for the ability to increase the compression ratio was the availability of higher octane fuels.

Benefits listed:

Power and acceleration are improved.

Better gas mileage

Engine runs cooler, which...

Improves lubrication, and

lessens mechanical wear, and

lenthens life of parts, including valves, bearings, wrist pins, and cylinder walls.

Bearing pressures are lowered

If these claims are all true, it sounds like a win, win, win, win to me. Any thoughts?

Fred, any data or verifiable information concerning which high compression head does the best job. I had one on my 23 Roadster & it wasn't a Z head but it made a great differance & gave me no problems. I see high compression heads from the vendors & wondered is there anything special or differant about the Z head.
Edward R. Levy

I wonder if providing improved power and acceleration might reduce the other benefits? It seems EVERY car I've had that was modified to increase horsepower was somewhat reluctant to go slow. There's a tendency to make use of the extra zip!

It is true. Thermal efficiency rises with increases in compression ratio.

The diesel engine gets its high thermal efficiency from its very high compression ratio - allowable in its design because fuel is added after the compression stroke.

In a gasoline engine fuel is included in the compression stroke - limiting compression ratio.
The side-valve engine is further limited due to the size of the combustion chamber.

Today's gasolines of very high anti-knock quality compared to those of the T era should easily allow 7:1 CR in a T engine, provided those heads are aluminum or have other features that promote heat rejection to the coolant.

Fastest way to cool off that glowing exhaust manifold? Replace that 4:1 cylinder head with something like a Z head!

Seth

Ken, every car with an engine I have modified to increase horsepower goes faster and also runs slower before "bucking" in high gear - almost down to it's idling rpm. One benefit to this is being able to turn corners without shifting down into low. This is of course on level ground... I'd be interested in comparing notes with you off forum, did you do the work on those cars?

You won't find this in the SK 1935 article: It is my understanding that CR in a flahead or side valve engine is limited due to the distance from the spark to the furthest point in the chamber. That's why the Hemi was such a breakthrough: it puts the spark near the center. Lots of modern heads have 4 valves to enable putting the spark in the exact center, allowing higher CR on same fuel.

Somewhere I have a chart that shows higher CR increases power at all rpm, including idle. The exhaust just doesn't sound quite as hollow. Oh, well.

You can get all of the benefits listed in the article, but not all at the same time. If you use the extra power, you won't get better mpg, as was pointed out in the article.

I don't visualize how higher CR will make cooler running. It gets more power from the same amount of fuel, but how does that relate to heating of the engine? In fact, higher compression results in higher temperature due to compression, regardless of fuel.

The big scare with the Z and other raised CR heads on a T is the expected extra strain on the crankshaft. Is that not true? The article seems to refute it, but maybe more by ignoring the question than answering it with technical explanation.

www.SkinnedKnuckles.net

rdr

The Z head is the Ricardo design that has most of the compressed mixture over the valves, and just a trough over the piston. My aluminum Reeder head is the plain T style chamber, but higher CR. I had no complaints at all when running it. More power yet when running the Reeder would have been nice. The Z has slightly higher CR, IIRC.
rdr

Over the winter I added a Z-head and a Stipe 280 cam to my '24 touring. Before I did this, my top speed was 37 MPH DOWNHILL. Now, I recently clocked at 49.9 (not quite 50!) MPH on my GPS unit, and can take almost any "reasonable" hill in high gear. I must say as a disclaimer, that I did not enjoy my T at that speed, and do not make a habit of it!

However, I would certainly check my bearings before adding these things.

If you need to drive in modern traffic, these modifications (along with Rocky Mountain brakes) are very helpful.

Bob

Forgot to mention, the better cooling is because the Z head has a larger water capacity.......

Check to see what kind of pistons you have. You can raise compression in two different ways. One way is to raise the top of the piston higher and the other way is to decrease the size of the combustion chamber by making it smaller.

If you already have high compression pistons, you can't use a high compression head because the two parts will make contact.

Choose one or the other but not both.

High-top pistons push the gasses equally all around but do not aim them at the exhaust ports. Some high compression cylinder heads have shapes molded into them which assist with the flow of the hot gasses and thusly improve breathing by aiming the hot gasses at the exhaust valves.

The high compression head is the best solution. The higher lift cam improves breathing. To stop bucking, simply run on D C rather than the self adjusting A C magneto and put the spark where it belongs. That way the car will think it has a distributor.

You can go around a corner in high gear on D C battery by simply retarding the spark as you come into the corner. Then apply the accelerator and advance the spark slowley just as much as it will take it as you feather off on the gas.

Higher CR makes it cooler running because less fuel is consumed to do the same amount of work.

A "stocker" that gets 20 mpg @ 40 mph is burning some 12 pounds of gasoline per hour - some 215,000 BTU/hr.

Switch to a Z head and it gets 25 mpg @ 40 mph, burning 20% less fuel - some 40,000 BTU/hr not being rejected as heat to the cooling and exhaust systems.

Open the throttle all the way and use that extra power - then the rules have changed and it will heat up!

A gentleman that I met a few years back by the name of John Owad in southern PA said that the first thing he usually does when he gets a T is to shave 1/8" off the head to bump up the compression. Not .020" or .030", but 0.125"! That sounds a little scary when thinking about valve to head clearances. Or is he right on the ball?

Also, aren't factory T cranks pretty spindly? I'd be afraid of breaking mine! Should this be a concern?

One of our club members got 5 miles on his new high compression head before the rods started knocking. This change is not recommended for tired T engines.

Took .090 off high head a few years back. Don't know about the top end, but it definitely pulled hills better.

The head on my speedster has .157 shaved off of it. That was about all that can be taken off and still have the water passages on the ends of the head still match up.

Paul

In theory raising compression is great. But Ron Kelly running 1928-31 Model A Ford engines on a dyno found that many other variables impacted the performance and in some cases more than just increasing the CR. See his article reprinted from Secrets of Speed at: http://www.amuffler.com/dyno/dyno1.htm The T engine with a Z or Ricardo head has a similar profile to a Model A with a high compression Brumfield or Police "B" head.

Hap Tucker 1915 Model T Touring cut off and made into a pickup truck and 1907 Model S Runabout, Sumter SC

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quote:

Ken, every car with an engine I have modified to increase horsepower goes faster and also runs slower before "bucking" in high gear - almost down to it's idling rpm. One benefit to this is being able to turn corners without shifting down into low. This is of course on level ground... I'd be interested in comparing notes with you off forum, did you do the work on those cars?

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Ken,

Your tongue in cheek comment reminds me of the teenager explaining to his parents that he is replacing the 2-barrel carburetor and intake with a 4-barrel setup to get better gas mileage. Right!

LOL... Yeah, that's about it. I remember explaining to my parents that I needed a 312 T-Bird engine in my '54 Ford because the old straight six through a rod. And... that the "three on a tree" was inefficient compared to "four on the floor". Man, that was a good running car for a sophomore in high school. ;)

Seth, I'm giving the other subject some thought.

Ralph Zajicek did an outstanding job in designing and producing the "Z" head for the Model T. Another outstanding job was done in Elmer Layne in producing the KC Warford transmission. Glenn Chaffin has done an outstanding job in reproducing and making available the Ruckstell axle and its components. We owe a lot of thanks to these people and others who have done so much to help us enjoy our Model T.

Smaller primaries to save fuel! My dad didn't fall for that one either.

It must be in my blood. It seems that whenever I take a stock head off and run it over to the machine shop to have Dave mill it flat, I end up asking him to take off .100" "to save weight". Now, I can use the excuse that I'm improving cooling efficiency.

A higher compression ratio increases the heat upon compression but the increase is overcome by the overall reduction in heat from the change in combustion as more of the energy of the fuel is used for power and less for heat. Consequently, the engine with higher compression will run cooler as compared to lower compression at the same speed and load.

Larry Brumfield

Umm, doesn't the power come from thermal expansion, due to heat?

Grasping with the concept...
rdr

Larry is exactly right.

At the same speed, the engine sees the same load, and produces exactly the same power.

In the high-compression engine, less fuel is burned to do this. Less heat in, same work out, means less heat is rejected.

The negative work of the compression stroke is recovered as positive work in the expansion stroke.

In the HC engine, because the throttle is "less open", the pressure in the cylinders is less at the beginning of the compression stroke, so even though the CR may have been raised 50%, this negative work of compression is increased very little.

The concept is impossible to grasp if you are comparing the engines at wide-open-throttle.

It is much easier to grasp when the engines are compared at same-speed and same-power conditions.

As I said before - open the throttles and the HC engine with more power is going to heat up.

Seth

Don't know about the theories - theories often are less important than reality. In my real world experience the addition of an aluminum head to each of my Model Ts did nothing but add power. Both ran cool before and after the aluminum heads were added.