MTFCA Forum

Anyone interested in finding out what the compression of a new 8:1 Prus head is in PSI?

If you didn't see the multiple times I used bluing to check for interference with the pistons - run through the playlist.

I believe there are two videos where the pistons hit the head (Z head was worse "bang" just to be fair).

https://youtu.be/nkCxxObvptA
.

I wonder how many other Prus head users have encountered clearance problems ? Its strange that I haven't seen or read any other posts to that effect on the Forum. Since there are replacement pistons made by more than one manufacturer and different head gaskets available, both New manufacture and New Old Stock, as well as USED, I can see where there could be enough clearance issues to cause a problem; its just odd this comes up so rarely, if ever, before. The other clearance problem is the one that John mentioned in the post after mine; a block having been previously decked. since the block in question was not a N.O.S. block, there's no telling whether, or how many times it might have been decked.

I have not had any fit issues with Prus heads. I have not tried one on a block decked 1/8"
I have had interference issues with every Z head. Thats from inconsistent combustion chamber castings.

Based on an online compression ratio vs PSI calculator you should get around 115 PSI for an 8:1 ratio
75 PSI is more likely to result from a 5 to 1 ratio

Speaking of pistons hitting the head, I had that problem fitting .080" OS pistons to a Ricardo head. Rather than machine the head, I turned down the top edges of the pistons. Solved the problem.

I'll second John K's observations. Z heads do vary in combustion chamber casting. I always fit them dry with no gasket first, and hand crank them to see if there are any issues to be worked around. The Prus heads I have seen have been trouble free, but I have had to put my own "Made in Canada" and Ford script on mine.

Allan from down under.

WillsT wrote: ↑

Mon Mar 18, 2024 3:13 pm

Based on an online compression ratio vs PSI calculator you should get around 115 PSI for an 8:1 ratio
75 PSI is more likely to result from a 5 to 1 ratio

IMG_0695.png

I thought I read that this head is not actually 8:1 as advertised. Anyone know for sure?

Compression ratio is a mathematical equation and not a measured pressure. Simples form, swept volume + chamber volume divided by chamber volume. There are other factors such as gasket volume and pistobpn dome/dish. Compression ratio is not measured by cylinder pressure.

Andy

AndyClary wrote: ↑

Mon Mar 18, 2024 10:55 pm

Compression ratio is a mathematical equation and not a measured pressure. Simples form, swept volume + chamber volume divided by chamber volume. There are other factors such as gasket volume and pistobpn dome/dish. Compression ratio is not measured by cylinder pressure.

Andy

Agreed. I believe I read somewhere that a local chapter measured the actual displacement and determined it was not 8:1. It would be interesting to know what the cylinder pressure is before and after installing a Prus head (or a Z head).

Jim11787 wrote: ↑

Mon Mar 18, 2024 9:40 pm

WillsT wrote: ↑

Mon Mar 18, 2024 3:13 pm

Based on an online compression ratio vs PSI calculator you should get around 115 PSI for an 8:1 ratio
75 PSI is more likely to result from a 5 to 1 ratio

IMG_0695.png

I thought I read that this head is not actually 8:1 as advertised. Anyone know for sure?

I have CC'd various heads. The Prus head is a little more than the Z. Comparable. A little over 5:1.(I don't have the #'s with me now)
Don't let the 8:1 claim scare you away from it.

speedytinc wrote: ↑

Tue Mar 19, 2024 8:18 am

I have CC'd various heads. The Prus head is a little more than the Z. Comparable. A little over 5:1.(I don't have the #'s with me now)
Don't let the 8:1 claim scare you away from it.

Same here.. Replaced a Z head on my 27 touring with the new Prus a few years ago.

While on the bench, filled a chamber of each with fluid and measured.

The "Z" head held just at 250ml, the Prus head held just tad over 200ml, done with a measuring cup.

As one ml = one cubic centimeter, results were 250cc for the "Z" and 210cc for the Prus.

After the head swap, the touring gained top end, and smoother running due to the exact CNC machined size for each chamber for each cylinder. Much better performance over the "Z" IMO.

Tim, you will be hard pressed to find any people to agree with your opinion of the Prus head.

Most people will find the 100 to 116 psi measurements, as they depend on what the measurement were with the original stock head.

The Model T engine will run, without much power, with measurement as low as 35 to 40 psi.

Kevin buys those heads in a fairly large group, and half are sold to people on a waiting list.

The rest go fast, and no one brings them back, if they did, he would return their money.

Kevin built the engine in my 1917 T Truck about 20 years ago. I took it back about two years ago for him to repair a water leak. I bought a Prus Head, but have never installed it, as the truck runs fine without it, and still has the quietest running engine that I have ever seen or heard in 40 years of my working with Model Ts.

Kevin invited me to ride with him on the MTFCI 2022 Ohio Tour. He was driving a 1916 that went on the last Cross Country Tour. The engine has a few Model A parts and a Prus head. The car also has disk rear brakes, a Ruckstell, and perhaps a few other improvements. The temperature was over 70 degrees and quite often we were doing 50 mph, but usually 40 to 45 mph. The radiator never boiled over and Ruckstell was only used in Low about three times, and only when there was a slow T going up a long hill in front of him.

Perhaps, I’m a little biased, but the actual ratio has no bearing on the fact that the head is a great improvement to the stock Model T engine for any year vehicle.

I have not installed my head, as the engine works very well, as it is now, and I never drive it on the road or need higher speeds.

I have CC'd various heads. The Prus head is a little more than the Z. Comparable. A little over 5:1.(I don't have the #'s with me now)
Don't let the 8:1 claim scare you away from it.
[/quote]

Thanks. Have you measured a stock head for comparison to the Prus as far as compression?

Jim11787 wrote: ↑

Tue Mar 19, 2024 4:05 pm

Thanks. Have you measured a stock head for comparison to the Prus as far as compression?

Here is link from Tulsa Model T Club info on cc of various Model T cylinder heads.


https://mtfctulsa.com/Tech/head_design.htm

Jim11787 wrote: ↑

Tue Mar 19, 2024 4:05 pm

I have CC'd various heads. The Prus head is a little more than the Z. Comparable. A little over 5:1.(I don't have the #'s with me now)
Don't let the 8:1 claim scare you away from it.

Thanks. Have you measured a stock head for comparison to the Prus as far as compression?
[/quote]

Heads I have CC'd
Stock head - 302 = 3.97-1
low head 294 = 4.07-1
low head -1/8" 260 = 4.59-1
Low head with pop ups 95# 6.46-1
ricardo - 250 = 4.77-1
Z head 223 - 5.4-1
Prus - 220 = 5.38-1
sherman super fire - 203 = 6-1
hi head with hi dome pistons - 70# 4.75-1
lizard (un detailed) 160 = 8.1-1

results will vary due to previous machining.

There is more to compression pressure than compression ratio and atmospheric pressure. Such things as camshaft profile and a thermodynamic action called heat of compression. A few years ago, Glen Chaffin explained that cam lifts greater than .26 inch affects valve overlap and it's effects.

My car has 62 psi compression with 3.8 compression ratio and with std camshaft. The atmospheric pressure is about 14.4 psi at 500 feet above sea level. The heat of compression raises the pressure from 55 psi to 62 psi.

For those inclined: raise the compression ratio by a power factor of 1.10. Example. (3.8)^1.1 = 4.34
Hence 4.34 x 14.4 = 62.5 psi.

Art Mirtes

John,
Is the Ricardo head that you cc'ed a Waukesha Ricardo head or the Ricardo Nov 13 1923 head with the combustion chamber that looks like the
Prus head combustion chamber?
Thanks.
Craig.

This thread certainly wandered. As to the original post, that is the first I’d heard of having to massage a Prus head but hey things happen.
I’m glad Art was able to find numbers to fit his theory but by his formula my speedster motor must have 10 to 1 compression (which it doesn’t). There are just too many variables to directly relate compression ratio to cylinder pressure. I have right about 7 1/2 to 1 compression and 135 pounds cranking pressure.

Andy

AndyClary wrote: ↑

Wed Mar 20, 2024 11:29 pm

There are just too many variables to directly relate compression ratio to cylinder pressure. I have right about 7 1/2 to 1 compression and 135 pounds cranking pressure.

Andy

I am no cam design expert… but my old Sherman head measured at 7.25:1 static compression, but made 190 psi cranking (dynamic) compression! Pretty sure that the cam profile can have an effect on dynamic compression measurements…

Craig Leach wrote: ↑

Wed Mar 20, 2024 11:28 pm

John,
Is the Ricardo head that you cc'ed a Waukesha Ricardo head or the Ricardo Nov 13 1923 head with the combustion chamber that looks like the
Prus head combustion chamber?
Thanks.
Craig.

I believe it was the crow foot combustion chamber.

I knew I would get beat up for my posting. I base my numbers on well established physics principles. Andy, if you raise the 7.5 to the 1.1 power then multiplied by 14.7 you will get about 135 psi. Which is what you claim to have.

3.8 ^ 1.1 = 4.34
7.5 ^ 1.1 = 9.17
5.0 ^ 1.1 = 5.87

Checking the engine compression,when hot, will yield higher compression. This is because not as much heat generated by the compression process escapes to the surrounding surfaces. Under ideal conditions the n value becomes 1.3 for air.

Andy, it is comments like yours that drive competent members off this site. Scott Conger is a prime example of one driven off.

Art Mirtes

Ok Art, don’t worry, our disagreement won’t drive me off the forum.

To the original post, has anyone else had a piston to head interference with a Prus head?

Andy

Andy:
regarding your question ...has anyone else had problems with Prus head ... i have only installed 2 prus cyl. heads ... the machined chambers are uniform and NO INTERFERANCE was experienced ...i have installed 7 Z heads ... there was interference in ALL 7 ... i believe this occured due to warpage in the casting and when the gasket surface was machined the center 2 chambers were shallow ... resulting in the #2 and #3 pistons hitting the cyl. head ...Gene French

So here's the first test fit of the prus head. After this I pulled a Z head out of the pile and that hit in more places than this one did. I then changed the rods. I believe this engine was decked mercilessly, however - it didn't raise the compression much! As you could see in the video I posted above, it's a 5-ish to one compression ratio. I did this test cold, I can repeat it hot later after the engine has a couple miles on it if there is a demand for the information.

No clue where Kevin got 8:1 but I had a sneaky feeling it wasn't when I saw the ten foot deep valves live here caves in it. Anyway - here's the first test fit:

https://www.youtube.com/watch?v=IpAibLBmGUA

Gen3AntiqueAuto wrote: ↑

Fri Mar 22, 2024 12:27 pm

So here's the first test fit of the prus head. After this I pulled a Z head out of the pile and that hit in more places than this one did. I then changed the rods. I believe this engine was decked mercilessly, however - it didn't raise the compression much! As you could see in the video I posted above, it's a 5-ish to one compression ratio. I did this test cold, I can repeat it hot later after the engine has a couple miles on it if there is a demand for the information.

No clue where Kevin got 8:1 but I had a sneaky feeling it wasn't when I saw the ten foot deep valves live here caves in it. Anyway - here's the first test fit:

https://www.youtube.com/watch?v=IpAibLBmGUA

Tim, its no secret, and its been posted here many times previously, that forum members do not believe Kevin Prus' head produces anywhere near 8 to 1, but something must be right, because most users, in addition to liking the accurate machining of the Prus head, most users report a difference (increase) in power over a Z head (if they had run one previously) to compare. With the scarcity of original high compression heads available, you, I, and every other hobbyist should just be grateful someone like Kevin, makes one available for us. If you want, and must have 8 to 1, find a Sherman head made of unobtainium.

I don't have any issues with Kevin Prus or his amazing cylinder heads, but I have to wonder why he keeps claiming that 8:1 compression ratio when everyone should know it's not. I wonder just how many potential customers shy away from buying these great heads only because of the advertised C.R. , being afraid of breaking their crankshaft. I've talked to Kevin at Hershey and complimented him on his product, having owned two of them after running a Z head and having it blow head gaskets.

Thanks for most everyones kind comments. When everybody is doing the compression calculation no one seems to minus the volume that is not figured in by the piston height above the block in the calculation. I know the product is great and we just run another 50. So if any of you are on the fence jump back down on the ground and order one since they have been selling faster than we can have them made. I'll have them at Luray and Richmond if they stay on the shelf that long. Im here to help our hobby thrive and offer quality products.

We’ll, as it turns out the clearance issues were due to a sketchy block and not the Prus head. That’s good news as no other problems had been reported. Thanks for the quality product Kevin. As I haven’t had one of these heads in my possession I won’t comment on the compression. I will say that with an 8:1 Sherman head premium gas was essential.

Andy

Racer_22 wrote: ↑

Fri Mar 22, 2024 7:56 pm

Thanks for most everyones kind comments. When everybody is doing the compression calculation no one seems to minus the volume that is not figured in by the piston height above the block in the calculation. I know the product is great and we just run another 50. So if any of you are on the fence jump back down on the ground and order one since they have been selling faster than we can have them made. I'll have them at Luray and Richmond if they stay on the shelf that long. Im here to help our hobby thrive and offer quality products.

I do take into account the piston pop up, CC's & compression reading comparisons.

This is a superior quality head. I recommend & install them for customers. The hobby is better off having a better choice than Z heads.
Still wish they were available in cast iron, but I know the story.
I have talked a few people off the ledge who were concerned with an 8:1 compression ratio.
No matter, this head really wakes a T motor up.

I have not driven it yet, a performance increase was the motivation for buying this. Sounds like it might happen!

https://www.youtube.com/watch?v=IpAibLBmGUA

ModelTWoods wrote: ↑

Fri Mar 22, 2024 2:07 pm

Gen3AntiqueAuto wrote: ↑

Fri Mar 22, 2024 12:27 pm

So here's the first test fit of the prus head. After this I pulled a Z head out of the pile and that hit in more places than this one did. I then changed the rods. I believe this engine was decked mercilessly, however - it didn't raise the compression much! As you could see in the video I posted above, it's a 5-ish to one compression ratio. I did this test cold, I can repeat it hot later after the engine has a couple miles on it if there is a demand for the information.

No clue where Kevin got 8:1 but I had a sneaky feeling it wasn't when I saw the ten foot deep valves live here caves in it. Anyway - here's the first test fit:

https://www.youtube.com/watch?v=IpAibLBmGUA

Tim, its no secret, and its been posted here many times previously, that forum members do not believe Kevin Prus' head produces anywhere near 8 to 1, but something must be right, because most users, in addition to liking the accurate machining of the Prus head, most users report a difference (increase) in power over a Z head (if they had run one previously) to compare. With the scarcity of original high compression heads available, you, I, and every other hobbyist should just be grateful someone like Kevin, makes one available for us. If you want, and must have 8 to 1, find a Sherman head made of unobtainium.

Clearance between the top of piston and the underside of the combustion chamber to generate quench (create turbulence) should be around 0.038 to 0.060. On a flat motor, quench can become problematic, because you need space around the valves to generate effective laminar flow both underneath and around the valve heads (increase cc volume). The only place where quench can really be attained is over the top of the piston. So ideally, you want to get the top of the piston as close as you can to the underside of the cylinder head. In the case of Mr. Pruss's design, it appears to be about the thickness of a compressed head gasket.

As you tested the head with no head gasket in place, there should be 100% contact because the head gasket is going to add in the thickness to generate that space required. Watching your blueing test showed that the machine inside of the combustion chamber above the piston seems acceptable, with a relatively complete contact area.

Put a head gasket on, grab some plasti-gauge, torque down and check. That should give you the answer you are seeking.

Gen3AntiqueAuto wrote: ↑

Fri Mar 22, 2024 12:27 pm

So here's the first test fit of the prus head. After this I pulled a Z head out of the pile and that hit in more places than this one did. I then changed the rods. I believe this engine was decked mercilessly, however - it didn't raise the compression much! As you could see in the video I posted above, it's a 5-ish to one compression ratio. I did this test cold, I can repeat it hot later after the engine has a couple miles on it if there is a demand for the information.

No clue where Kevin got 8:1 but I had a sneaky feeling it wasn't when I saw the ten foot deep valves live here caves in it. Anyway - here's the first test fit:

https://www.youtube.com/watch?v=IpAibLBmGUA

A high lift /long duration cam will affect compression pressure at cranking speed. Cam timing would also affect it. The condition of the pistons and rings and how well lubricated they are during a compression test will affect how much pressure is developed. A newly built engine could be expected to develop lower compression pressure than the same engine after complete break in. Elevation and barometric pressure will affect compression readings, as will cranking speed. The type of spark plug used can also affect readings. So can the type of valves and how high the valves sit in the seat.

WillsT wrote: ↑

Sat Mar 15, 2025 10:48 pm

based on comments on this forum i selected the Prus head.
Head arrived yesterday and decided to do a fitment test today before bolting it up, good thing because could hear the interference
Piston looked like standard one and piston head shape and size appears match to indentation in head.
Looking to figure out amount of interference I doubled up the gaskets, 1 new 1 old, head laying loose on top. Interference went away and head allowed some lateral movement with piston all the way up.

So question to you experts is where to go from here, ask for replacement head, or take head to local machine shop and mill a larger piston cavity and then the question is by how much. Other options ?
Thanks for your input in advance.

Expert certificate lacking, but have fitted alum high compression "Z' and the Prus heads.

The Prus head made no interference on 2 motors, both had block surfacing and new stock pistons, .030" over.

The "Z" fit didn't work as there were piston knocks after install (had not learned to fit with loose head bolts first). So took off the "Z" and painted the chambers with Prussian Blue paste. Fitted back with a few bolts and cranked over, that showed on the top of a couple of pistons where the piston hit the chamber dome.






So maybe do the same, also measure how much height you pistons rise, and if the pistons are .060" or more like .080 as there maybe contact on the edges of the chamber. There can be variations of how high pistons rise at TDC, like the one below:



This engine shows piston height at TDC much lower, and fit of a Prus head wasn't compromised, and worked great.



By using blue paste, you would see the issue. Maybe only one or two chambers are affected. Use judgement to see if alum has to be removed from the chamber(s).

I'd use caution removing any metal from pistons. Thinning sections of a piston above the top ring can lead to hot spots. Balance issues might also occur.
The cylinder head is probably thicker, and it is water jacketed, and balance is no issue. The head is also more likely to be non-uniform than the pistons are.

I have not heard of anyone having a problem with the Prus Head.

I have not heard of anyone having a crack in a Prus Head.

I do know the difference between the stock head and Prus Head is several horsepower.

I do know that Model Ts climb hills much better with the Prus Head!

The actual number is not even important, and should not be listed in the ads, or worth discussing by all these Model T Ford engineers, as most of them do not even know the proper method to verify that number!

Compression ratio and compression pressure are not the same thing, as was pointed out

Compression pressure is measured with a pressure gauge, and is subject to many variables besides head chamber volume.
Compression ratio is fixed for any given engine, as assembled, and it is calculated mathematically.

You need to calculate the swept volume of an engine cylinder, then calculate the volume of any dome or recess in the piston head, then measure the volume of the combustion chamber in the head, and allow for the head gasket installed thickness. If you have a flat head piston that TDCs level with the block deck, the ratio between the actual combustion chamber volume at TDC and the swept volume, plus the installed head gasket thickness, less any intrusion of the valve heads or spark plug into the chamber is the compression ratio. You cannot measure that with a compression gauge.

If you calculate the volume of any part of the T piston that is above the deck at TDC, and subtract that from the combustion chamber volume, and calculate the ratio of that volume to the swept volume of the cylinder, that ought to get you fairly close to the compression ratio.

Flathead engines with a chamber design similar to the Prus head and domed pistons, like many of the flathead Ford V-8s, had a stock compression ratio around 7:1.

I agree the Prus head appears to be very well made although I have never run mine. The chart I did at the Tulsa Website (https://mtfctulsa.com/Tech/head_design.htm) I explain how to account for piston popup, head gasket thickness, milling, overbore and stroking. I always use a plexiglas plate (as shown in photo) to get an accurate head volume measurement. I measure 215cc for a Prus head and 203cc for a Z head which is in line with what others have found. So both heads are a little more than 5:1 compression. I'm currently running a Sherman head that measured 214cc. I measure 220cc for a Chaffin Rajo. I have complained about truth in advertising before. How can someone make an informed decision without this basic number?

As has been noted, cranking pressure is not a precise number since it depends on thing like valve timing (camshaft). It makes a difference whether the exhaust valve opens at 35 deg or 55 deg BBDC. Also, Art is absolutely correct that you have to take the compression ratio to a power. This is fact not "theory" because the compression is not at constant temperature (the air heats up).

Larry,
Thanks for supporting me on the compression calculation. I thought about explaining how the heat of compression increases the pressure, but just didn't feel like arguing with others.

Under ideal conditions the ratio would be raised to a power of 1.4 for air. Under realistic conditions, some of the heat that is generated from the compression escapes to the surfaces of the combustion chamber and to the piston. Empirical data ahows a factor of 1.1 to be fairly accurate.

I have thought checking the compression with a cold engine versus with a hot engine. I have not yet woke up my car from it's winter nap to try this. I am wondering if anyone has made this comparison.

Art Mirtes

First of all I need to make a correction on my “interference” problem with the Prius head.
This was a newby operator problem as I did not have the head centered over the pistons, after correcting this no more interference.

So all this started because I had water in my oil because of head corrosion on my old head.
So I wanted to check to see how well the new head and gasket was sealing, so first I did a compression test using my starter as power source and all cylinders without plugs in
So all cylinders came out very similar at 70 PSI, no spark plugs firing so engine was barn cold
Then I started engine up with 3 plugs in and 1 measuring PSI all 4 cylinders showed around 80 PSI, engine now warmed up obviously
Don’t know is difference in PSI was due to temperature change or RPM change but here is a data point.

So hopefully my water leak is fixed

Thanks John,
I will assume that is the Waukesha- Ricardo head.
Craig.

30 years ago, I got 72 psi with domed pistons.

You don't hear much about them anymore, but they did make hill climbing easier.

Not to be a spoil sport, but motor-head CR calcs for everything have about 20 odd ways to ‘xplain! Depending on where you learned your knowledge...lol

There are some who just take full volume at bottom of stroke, and then remaining volume at top of stroke on a compression stroke. That’s one way.
There is a totally different way that gets a little scientific in the early Van Dykes...That’s another way.
There are those who like to use the old ‘rule of thumb’ of maximum pressure developed divided by ambient pressure (not including an absolute pressure and only using gauge pressure for the compressed value). Divide the two = CR.
There is yet another way to factor in the thermodynamics of gas changes due to combustion and that one throws all of these others out the window by not even being close!
Each way has survived with equal weight (lol) for most of my life as a motor-head! The easiest way is to forget CR as a ‘moniker’ and go for the volume using the Montana 500 way to measure-couldn’t be easier. Less volume = better ‘explosion’ with rapid expansion. With tongue in cheek, all else the same, ignoring a ‘squish/swirl’ value argument as it just gets complex, if you want to be all you can be...pick a clearance...make a mylar shim set that is that clearance while including gasket height...blue the head up...and polish out the bumps. To me, ain’t worth that kind of worry ‘cause a Model T ‘breathes’ so bad to begin with its all muck but performance is better with a blueprinted engine – right? Simple as that, I'd guess.

A final thought...for those that want to quantify --- ever ask yourself...at what point in these amazing reported CR's should you switch over to hi-octane with anti-ping??? Yeah, I know - - -goes back to that breathing problem and has nothing to do with these old school backyard thoughts... More precision = greater results.

My biggest worry would be that 1.5 inch crankshaft. To much load on that and your day may not go so well.