My humble opinion is that we would get more complete combustion from spark closer to the pistons. After 100 years I'm sure somebody smarter has already built the perfect design.
Is there a chart anywhere comparing the efficiency of each head design currently available for our Ts?
My current head is not aluminum and has a "Z" next to the water outlet. I'm guessing that this a stock original head, right?
My understanding is not that of an expert in any way, but here's my take on it. Ignition of the fuel mixture is not instantaneous. It begins at the location of the spark and the flame propagation, or flame path, continues to expand from there, reaching maximum combustion pressure when ignition is complete. Therefore, if the ignition source occurs in the smaller cavity, just above the valves, the flame path will propagate outwards, towards the larger volume over the piston, where it can continue to expand, as it pushes the piston down. However, if the flame begins over the piston, the flame path will propagate from the large volume over the piston, towards the smaller volume over the valves, where it can no longer expand, causing the combustion pressure to sort of "double back" towards the piston.
In other words, if you want max. combustion pressure over the piston, you have to begin the process near the valves.
Are you sure it is not an aluminum head painted black? Test it with a magnet.
The aluminum "Z" heads are very efficient. The spark plug is located where it is most effective, not a random choice. You will pick up 4 - 5 HP by installing an aluminum aftermarket "Z" cylinder head. That is a big change.
Chris, Yes, many original heads are marked with a letter showing which mold they came from. The modern high compression Z-heads are aluminum.
Charlie Yapp tried to market a cast iron high compression head for Model T's with the spark plugs in the squish area over the pistons - it wasn't any success, many buyers of the "Leaping Lizard" head had problems with the sensitivity to knocking it gave. It has been proven more efficient to concentrate the combustion gasses and the spark plug over the valves in a flat head engine.
The fastest flat head T's has Sherman Superfire heads that are made by Ben Serar nowadays. They have a combustion chamber that resembles the 1930's and 40's Ford V8 design and they have small combustion chambers for a high compression, over 8:1, that might be too much for the babbitt in a splash oiled original engine? (Has anyone tried one of Ben's heads on a std oiled engine for some distance by now?)
The other current options are the previously mentioned Z head and the Prus head that is slightly more expensive but better made - and gives about 2hp more than the Z due to better squish area even though both have about 5:1 in compression and are quite babbitt friendly - though if the rods are starting to be a little loose by the time you switch heads, you'll certainly hear that with higher compression. A std head is about 3.8:1 and a common mid teens low head might be 4:1 - only really early low heads has 4.2:1 according to Larry Young's excellent technical comparisons at the Tulsa Model T club site.
Ford cast letters in the top of the cylinder heads for i.d. of molding tooling. They used the whole alphabet too I think
'Z' letter in Ford iron high head.
Thank You for the great info. Always thought the centerfire him he heads were very efficient.
I just ordered an aluminum Z from Lang's.
Him he? Could it be Hemi head you're after?
They're effective - but they have overhead valves or OHV - so the spark plug is still near the valves, just like on the side valve engines we discuss above. Side valve engines can never be as effective as OHV engines, but they had their place as a reliable low cost design back when the gas didn't allow higher compression than a flathead can have anyway.
And the Briggs & Stratton lawnmower engines are still flatheads
Over head valves with a hemispheric combustion chamber or Hemi head:
And here's the most effective flat head design, with a squish area over most of the piston and the combustion concentrated over the valves with the spark plug right there:
(Message edited by Roger K on July 05, 2016)
Ahh, yes. I said "Hemi" but SIRI had her own ideas what to write.
I see now that the close proximity to valves may have been more influential in the center plug design of the Hemi rather than locating the spark near the center of the piston.
I'm surprised that flatheads don't have irregular cylinder wear patterns with the offset combustion chambers. It also seems like it would put a diagonal load on the wristpin.
Oh well, I've ordered an aluminium Z head and will be happy with it.
Steve, at Lang's, told me about the Lizard head problems and hot spots on some other aluminum heads that are out there. He agreed that the alum Z head is the best option for just adding a bit of torque to get an otherwise stock Model T up hills.
The Z head will be fine, I think. Haven't heard about "hot spot" problems on other heads - anyone who can elaborate?
It's not the diagonal load (gas pressure inside a chamber is uniform, so there won't be any odd load on the pistons) but the uneven heat from the exhaust close to one side of the cylinder that may be the cause for a need for a larger play between the cylinder and the piston on flat head engines than on OHV engines. The piston producers writes 2.5 thousands play would be enough and that may be enough on a modern OHV, but in a Model T you'll want at least 0.004" play to avoid scoring when running in the engine after a rebuild.
I have a feeling the centrally located spark plug has the advantage because the flame front has a shorter distance to travel to reach all the combustible mixture, and thus it reaches max pressure earlier. As there is no squish area in a hemi, one depends on the shape and location of the intake ports to create swirl and/or tumble in the incoming charge. A hemi's biggest advantage is the space it creates for larger valves.
Here's some comparison photos.
Stock Ford high iron head (right).
Sir Harry Ricardo's design, with compression squish design. (1920's) Iron. Waukesha-Ricardo. (middle)
'Z' head, high compression, alum. (left)
Note the 'Z' head has a flame path groove, but takes away some chamber.
Latest alum high compression, the P head, Prus, have this one now in place of an alum 'Z' and like it much better. Has a ramp flame path. Smoother engine, more power.
Now I'm searching for a Prus.
Is that from an Asian manufacturer?
Kevin Prus is from Canfield, Ohio
Lang's sells his heads, though they are misadvertised as a 8:1 head in the sales material, it's 5:1 in reality. Usually the alu version is $400 and a similarly shaped cast iron version is $500.
I just put a Prus head on my 27 Roadster. It runs and pulls hills real good. Compression is 83,85,86, 91 psi. Very good quality. PK
That looks like a very clean install. Did you paint your manifolds?
I wish I could put a bowl fuel filter on my 1924.
The Prus has a different name on Lang's but that's what I tried to order before Steve steered me to the Z. He has 2 cars running Z heads smoothly but had pre-ignition when they tried a Prus, due to a sharp edge that had not been smoothed at the factory.
I smoothened the edges myself somewhat with sand paper on my Prus - but they weren't sharp to begin with and I haven't had any problem whatsoever with preignitions.
Without a head gasket, the pistons just lightly contacted the edges of the head where the squish area started on every cylinder. Some light file work cleared it.
Chris, I'm using a Anderson ANCO one piece manifold. I used hi temp paint on it. After 700 miles this spring, it's turning grey and coming off. I need a better paint. PK
I've never heard of skipping the head gasket. Tell me more.
Only to check clearance before assembly. Look up Model T Tips, Mike Bender. Great video series on T motor builds. One covers aluminum head install. PK
There have been many variations in the plug location for Model T heads. The most common location is over the valves. Sometimes they are offset toward the intake valve. I have seen many variations with the Sherman - over the piston, over the exhaust, centered between the valves and dual plug.
Apparently, The Sherman heads had many variations in compression ratio too. An old post by Dan McEachern mentioned 159 and 138 cc heads. I have one with 214 cc and measured the one on the Pietenpol engine as 172cc. The current repro Sherman is 125 cc according to Les Schubert. That's a lot of different heads, but some of the variation may be due to measurement error. If you want to measure a head accurately, you should use a graduated syringe and plexiglas plate - http://www.mtfctulsa.com/Tech/images/HeadCCing.jpg
Using your wife's measuring cup is about like using a yard stick to measure bearing clearances.
Speaking of Sherman heads. Does any one know how to get hold of the guy that makes them? The ad in Vintage Ford gives only an email address which bounced.
This has turned into a good thread about cylinder heads. If Kevin Prus makes a run of cast iron heads machined for stock spark plugs, I'll buy one!
Mark, i was also interested in the cast iron ones, but since it was only available for the 14mm plugs decided to try the aluminum one. Very happy with the quality and performance so far. BTW, i was told that there is plenty of material to work with if you wanted to re-tap the threads for the stock plugs, but it seems to me that's something not everyone is willing to tackle on a new head without the proper tools.
I emailed Kevin a while back asking if he could have the machine work done to convert one of his iron heads to standard plugs (I don't have the equipment), but never got an answer back. When he comes on the forum again announcing a new run of iron heads, I'll ask him again.