Z head, or high compression piston's?
Let's say you had a T engine all ready for reasembly, and you were trying to determine wether to go with the Z head or the high comp. pistons. Let's forget about cost, ease of installation, stock appearance, etc.
Just looking at the performance and reliability aspect, which way would you guys go, and why?
Thanks in advance for all input.
Well, if you're engine is all ready to re-assemble, you already made the choice! You need to bring the pistons in when you have it bored. That being said - I've had both domed pistons and "special" heads. I vote for flat pistons and a Z-head. Runner up would be a Reeder head, then domed pistons and a stock head, last choice would be stock pistons and stock head. The domed pistons just seem to me to be backwards from optimal- meaning it should be dished to focus the force, not domed to deflect it off the top. They do work though.
With regular pistons you can swap heads, eg, stock, Ricardo, Z, Reeder, Sherman etc. No flexibility with domed pistons.
I have an engine with domed pistons and another one with a "Z" head. I will never again use domed pistons, because they just don't do it as well as a "Z" head. It's the combustion chamber design in the "Z" head that makes the difference, as Tim alluded to above.
Fordially, Keith Gumbinger
I have said this before, so listen up . . . . The Special heads with flat top pistons have shapes that cause better breathing because they direct the flow of gasses to and from the valves over on one side on a flat head engine.
Domed pistons work well with overhead valve cross-flow heads not flat heads because they distribute the gasses equally around the head and piston and when the exhaust valve opens they must all turn around and try to get out before it closes again. They bump into each other and you don't get as much bang for your buck. They work well on the power stroke but are not a good selection when it comes to evacuating the hot combustion gasses.
On a relatively low compression flat head engine it is better to have the combusion shape in the cylinder head than in the piston. Diesel and two stroke engines and perhaps direct injection gasoline engines are a different matter.
I have attached a photograph of some trick Model T pistons that do have the combusion chamber shape shape in the piston and not in the head. These are for a RAJO BBR overhead valve cross-flow engine cylinder head and the pistons are ramped to aid in the gasses getting to the spark plugs which are set rather low in the head. Most likely this was because most common folks used to use stock pistons in the olden days and so it was O K to have the spark plug close to the deck of the block. The high castle shapes are to raise compression yet not hit the valves. Yes those are period correct A K Miller tubular connecting rods.
Is there any undue stress or premature wear by using the domed pistons since they "deflect" the gases and charge off to the sides of the piston?
So most people are unhappy with whatever gain the domed pistons provide?
"Most people" might be unhappy with domed pistons, but I'm not. I built a "new" engine for my '15 Touring just before the Richmond trip last July. I wanted it to appear stock on the outside, so I used an original Ford low head milled 1/8" with domed pistons. The previous engine has flat-top pistons and a Reeder 6:1 aluminum head. All other components were the same: Stipe 250 cam, Anderson timer with Ford coils, Simmons straight-through carb on an early aluminum manifold, etc. I have put 3,500 miles on the new engine since last July. There is one long hill I drive up from time to time, and I can now go up and over it without downshifting. I could not before, so apparently the new engine has more torque than the previous one. I have heard several folks bad-mouth domed pistons, but I am quite happy with them.
My fordor came with a set in it when I bought it. I did not know they were in there. It's a great setup for "the cheap way" to get more power. I found them when I tried to put a Lizard Head on it - surprise! Domed pistons = you can't change the head to anything but stock.
Deflecting is just my idea of what must be happening in there, I can only imagine that the flame hits the top and rolls off somewhat and uses the rings to push the piston down. It works, but I prefer the flat ones and a good aftermarket head.
I never got that engine to move the car past 35 mph, not even downhill. I can't blame the pistons completely, as the engine was rebuilt in 1976 and was probably used a few times before I got to it. It ran great, just not too quick. I don't know what cam was in it (still have not taken the engine apart) or if it was worn out. One of these days I'll pull it apart and measure.
Are all the high comp. heads made of aluminum?
Do they have the Ford script and "Made In USA" on them?
The heads claim a 6:1 comp. ratio?
What ratio do the domed pistons achieve?
I guess I'm still undecided on which way to go. I'm not worried about not being able to do over 35mph. I was not trying to do this as "cheaply" as I could, but I did not want to spend money that was'nt needed.
Send me an email if you have some time to spare. I can help you get the best bang for your buck.
Steve -- There is a bunch of information on the various heads, cams, etc., on the Tulsa Model T club's website.
Click on the "Technical" link at the left of their home page.
The Z heads have Ford and Made in USA on them, but they are only available in high head conficuration. The earlier Reeder heads had the same on them, but later (and current) ones do not. Ralph decided not to pay Ford's increased royalty, so he deleted the name. The Reeder heads are available in high or low style.
The reason I went with domed pistons and a stock head is because I wanted this engine to appear totally stock from the outside. It will go into a 1915 Coupelet I'll soon be restoring, and I want the engine to look "correct". I wasn't trying to do it the cheapest way either; I had a spare Z head in stock at the time I built the engine. It was the authentic appearance that swayed me toward the domed pistons rather than an aftermarket head.
I don't know what the final compression ratio is with my domed pistons and 1/8" milled low head. There was a lengthy thread here a year or so ago about it, but I don't think we ever came to any agreement on the CR. I do know that this engine runs at least as well as, if not better than, the previous one with the Reeder head, with other components being the same.
I was thinking that domed pistons did not give as high a compression ratio as a Z head, but I don't remember what the ratio was.
Steve, If you aren't worried about going over 35mph, then a well-balanced bone-stock engine will be just fine for you. A well-built precision balanced engine will be almost dead smooth from the time you shift into high gear all the way to maximum throttle. If you want the best bang for your buck, just make sure all the machining and assembly is done right and everything is balanced. Proper precision balancing of moving parts is the very first place to start if you are looking to increase engine performance. I would look into having your crankshaft, flywheel, magnets, triple gears, trans drums, connecting rods, and pistons precision balanced (make sure the revolving parts are dynamically balanced on a computerized balancing machine, not static balanced with mandrels on "knife edges" or an old grinding wheel balancing fixture). Reciprocating parts, triple gears, and magnets should be equalized to at least 1/2 gram. A precision balanced engine will run smoother than stock. That smoothness translates into not only better power and performance, but also reduced stress on some of the internal parts.
Some Model T engines are smoother and more powerful than others because manufacturing tolerances of individual parts were not always maintained very well. Some of these manufacturing variances translate into imbalance of one sort or another. There are a dozen areas where a good amount of imbalance can occur on a T engine that can be easily remedied. The cost of having this done should be about the same as you would pay for a new high performance head and special pistons. A well-balanced engine is the very best foundation for any future improvements (if you even find them necessary). Adding a high performance head to an engine that has already been precision balanced will be more fruitful because more of the extra power that the head creates will be transmitted to the rear wheels instead of being consumed inside the engine and transmission by imbalances in rotating and reciprocating weights. Even more important, is that the well balanced internal parts will now also be better equipped to handle the extra power that is created by adding a high performance head or cam.
Adam, aside from changing that "35" to 40 or 45, I agree with you 100%. Great minds think alike? It takes power to make something shake, power that could have been put to the ground instead.
From memory the domed pistons ( depending on which manufacturer ) increase the compression to somewhere in the 5 - 5.5 range.
I am open to correction on this.
Having said that , my 13 runabout (light car ) runs a set of egge domed pistons and a standard 13 head - shightly machine, but not much , engined balanced using 1/100 th gram scales and l cant say that l have any trouble with speed or (now) smoothness , except for the fact that at 50mph its a long time to stop !!<grin>
Domed pistons make almost twice the improvement with a low head, vs. a high head.
IIRC, low head volume is 13 cc, and high head is 22 cc. Domed pistons subtract the same amount from the volume, regardless of head, of course.
Adam, your discussion re a dynamically balanced engine is right on the mark. I had the same done to my 10 this last year and it has all the power, speed and smoothness I want. And I am a stickler for a smooth running engine. It has a 250 Stipe cam, alum flat top pistons with the later style rods and hard valve seats. All else is bone stock including the 5 ball carburetor and low head without the Made in USA.
IMHO, balance everything and put the engine together. Run it first. Then if you want more power add a high compression head. You just might surprise yourself and keep the stock set up after all.