Let's talk increasing compression: how much is too much for a stock block?

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Model T Ford Forum: Forum 2017: Let's talk increasing compression: how much is too much for a stock block?
Top of pagePrevious messageNext messageBottom of page Link to this message  By Matthew Atchinson on Friday, June 02, 2017 - 12:15 am:

I located a Sherman Superfire head recently for sale. 8:1 compression ratio. I know the Z and P heads run around 5:1 or 6:1 and people generally say those are safe to run with a stock crank, cam, valves etc. Do you guys think 8:1 for the Sherman head would be pushing it a little too much? This would be on a 26 Coupe so it would have the thicker crank.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Spencer Vibert - Granby ,CT on Friday, June 02, 2017 - 12:22 am:

the Sherman is not 8 to 1 its 6 to 1 with a tail wind


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan McEachern on Friday, June 02, 2017 - 01:20 am:

Sherman heads were available with two different combustion chambers. The lower compression ration one was about 6:1 with stock pistons.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan McEachern on Friday, June 02, 2017 - 01:21 am:

sorry, meant to type 'ratio"


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Friday, June 02, 2017 - 01:57 am:

The recently produced batch of Shermans gives a true 8:1 compression ratio. They have the Superfire name. I think the old alternative version with 6:1 was called Spitfire.

Haven't tried a Sherman yet, but I think the life of a standard splash oiled bottom end would be reduced with a Superfire.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Pat Kelly Montana on Friday, June 02, 2017 - 09:52 am:

Here is a side by side of a Prus head and a Sherman Spitfire head. I emailed Kevin Prus about the Spitfire on a stock T lower and he said don't do it.




Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Friday, June 02, 2017 - 10:14 am:

The combustion chamber volume is about 210cc or 12.8 cu in in the Prus head, that's about 5:1 while Ben Serars modern Sherman Superfire has about 135 cc or 8.2 cu in for 8:1. I wonder what's the volume in the old Spit Fire?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Matthew Atchinson on Friday, June 02, 2017 - 02:09 pm:

So that's at least one "nay" on super fire head. By the pictures it is definitely an 8:1 finned super fire, and says super fire right on it. Maybe I should go with a Prus head instead to avoid any problems. I've heard lots of great things about the super fire though.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Friday, June 02, 2017 - 02:22 pm:

It would also depend on how you drive. The engine will match the applied load, so if you drive like you would a normal Model T it would be fine. If it was pedal to the metal then you would likely break something.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Friday, June 02, 2017 - 02:47 pm:

Agree with Ted, though it's hard to hold your horses when they're available.. :-)

Still curious if someone has experience running the new Superfire with a standard T bottom end? Maybe a high volume oiler would help keeping it running for some really fun seasons?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Friday, June 02, 2017 - 03:51 pm:

So are the Sherman heads available now? Do both the Prus and Sherman head have the same coolant capacity? I have a Z head now and love it but have been thinking of getting a little more compression since I don't have any trouble cranking it Yet.
If Roger is correct the Prus head isn't the 8:1 like advertised either but still better than the Z.

I haven't seen any ads for the New Sherman.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Matthew Atchinson on Friday, June 02, 2017 - 03:58 pm:

The Sherman head was NOS, either original or one of Ben's. It sounds like the lower ratio Prus head is proabably the safer route to go to avoid damage. Honestly the boost I'm lookign for is mainly for hill climbing so the P head will serve me just fine. I just thought it would be cool to own one of the old Sherman heads but if it requires switching out the oil pan to increase dipping and risks crank snapping it's not worth it. At that point I'd be looking at a total engine tear down to add a Scat crank and Stipes cam, etc.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Friday, June 02, 2017 - 04:29 pm:

Gene, I think Ben Serar made a couple(?) of batches in recent years, but has stopped now. The molds are reportedly owned by the original producers son now.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Phillip Maurici on Friday, June 02, 2017 - 05:08 pm:

I have run both the Z heads and the Prus heads. On my T's.
All I have to compare is top end speed with the GPS. I have found 3 MPH increase in speed over stock with the Z head. A 6 MPH increase in speed over stock with Prus head. I do not have a dyno so I can not be scientific.but there is more torque with Prus head. I have them on 3 of my T's as well as 3 friends are using Prus heads.
If your engine is not worn out and knocking higher compression will not hurt.
Best money you can spend to make your T more drivable in todays traffic.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Gene Carrothers Huntington Beach on Friday, June 02, 2017 - 06:19 pm:

Thanks, Phil, Somebody should be real happy to get my Z head so the upgrade cost won't be that much.

So what really is the C/R of the Prus head?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Pat Kelly Montana on Friday, June 02, 2017 - 07:49 pm:

I talked to a guy over on the HAMB forum and he sad he has seen a Spitfire head on a stock lower in a speedster. Said it was real fast.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Tom Magee on Friday, June 02, 2017 - 08:35 pm:

We had a Sherman SuperFire on our speedster for years. It gave no trouble and ran great. We have a stock bottom end but no magnets on the flywheel. The current Waukesha Ricardo is not even close.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Phillip Maurici on Friday, June 02, 2017 - 09:28 pm:

I am told by Kevin Peus that it is 8:1. He has a dyno sheet of a Z head vs His head. I remember the Z was around 24 HP and Prus about 28 HP. These are not exact numbers just from memory. You can call him for more info.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan Killecut on Saturday, June 03, 2017 - 06:25 am:

I have Z and Prus heads. The quality between the two is night and day. The Z is crude. I had to send the first one back and had to have machine work done on the replacement to make it work. The Prus is almost a work of art in comparison and was just bolt on right out of the box with superior performance.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Nevada Bob Middleton on Saturday, June 03, 2017 - 10:58 am:

6 to1 is about it unless your beefing up the bottom end


Top of pagePrevious messageNext messageBottom of page Link to this message  By Phillip Maurici on Saturday, June 03, 2017 - 03:39 pm:

Bob how do you know 6:1 is the limit. what is this based on? Flat head Ford V-8's run 12:1 on poured babbitt bearings.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Terry Woods, Richmond, Texas on Saturday, June 03, 2017 - 03:52 pm:

Phillip a modified 1932-36 babbit bearing V8 might run 12 to 1, but not a factory stock one, and I've never personally seen a babbit V8 with that much compression. In fact, I've never even seen an 8BA or an 8CM flathead running that much compression. It might for racing, but doubt it would hold up under years and miles of street driving. I have had a stock 32, 34, and 36 flatheads so I know what stock compression is .


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Sherman on Monday, June 05, 2017 - 10:28 pm:

I have an original new spitfire in my garage and will take some measurements this weekend.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les Schubert on Monday, June 05, 2017 - 10:42 pm:

I agree with Nevada Bob's number


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Sherman on Monday, June 12, 2017 - 11:53 am:

I measured the volume of the combustion chambers on a never used Spit Fire head. It was 130 cc's on both of the combustion chambers I measured.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Monday, June 12, 2017 - 12:14 pm:

Hey, that's 8.4:1 in the old Spit Fire head!
Was there a lower compression in the old SuperFire heads, then? Or were one or both available with different size combustion chambers?


Top of pagePrevious messageNext messageBottom of page Link to this message  By Erik Barrett in Auburn Ca. on Monday, June 12, 2017 - 09:26 pm:

It's not the block that is the limiting factor but the crank. 6:1 is about tops for a T crank.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les VonNordheim on Monday, June 12, 2017 - 11:27 pm:

I have a new Z head and Prus head. A friend measured the liquid volume of both heads and found the Z head to be slightly smaller by volume. Used a glass graduated beaker to measure the volume required to fill the chamber of each head.
The Z head measured 230cc
The prus head measured 240cc
Both heads were precisely leveled before each measurement was taken. Our measurements may not be perfect....but close enough to be in the ball park.
I know chamber design plays an important part for individual head performance besides just compression.

What were your measurements by liquid volume for each head?
This was a sample of one head each....do not know how much chamber volume difference is allowable during the manufacturing process.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Nevada Bob Middleton on Monday, June 12, 2017 - 11:32 pm:

Good safe limit number i known many who run more the 6to 1 and all had bottom end issues with a stock setup
Not an exspert but been messing with Ts 30+ years
Learn from what i have seen and known


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Sherman on Wednesday, June 14, 2017 - 11:51 am:

I measured 2 combustion chambers with liquid and a graduated cylinder. Spit Fire and Super Fire were available in low or high compression. The Dual Fire was high compression only.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roger Karlsson, southern Sweden on Wednesday, June 14, 2017 - 01:29 pm:

Les VanN: That's the figures I get too, that's why I consider both the Z and the Prus heads to give about 5:1 in compression. The Tulsa club got a slightly smaller volume for the Z when they did their comparison ten years ago, making it 5.3:1, but production may have varied over the years. Some have had problems with interference with the pistons - the Prus is milled in the combustion chambers and have better quality overall - well worth the extra $40. And the more efficient shape gave it 2 hp more at top end in a comparison test with a Z head.


Highly interesting, Jerry :-)
Do you have access to any of the lower compression versions for measuring?

Can't remember seeing any picture of a DualFire - how were the spark plugs placed - and I guess the name was cast in on them too?

Would you consider starting production again?
If Ben thought they sold slow, then maybe it was the high price that scared off some potential customers? And the 8:1 only configuration..
$750 certainly scared me off - (found one much cheaper later that needs some work, but would still be interested in a 6:1 version)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les VonNordheim on Wednesday, June 14, 2017 - 02:41 pm:

Roger,
Thanks for providing your findings... it's interesting what compressing ratios are being advertised by after market head manufactures. Using a milled low head and high compression pistons can provide a compression similar to using a Z head at a lower cost....if you already have a low head.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Dan Treace, North FL on Wednesday, June 14, 2017 - 03:53 pm:

Did a measure of the P vs Z heads.

Just used a measuring cup, so my data isn't to the exact cc.

Z head on left, P head on right.



Results of test on Prus head



Maybe 220cc-225cc?

Results on the 'Z' head




Maybe right at 250cc, a bunch more than the Prus

That test was fun, but the better result was I had removed that 'Z' head on my '27 touring, no other changes but to clean off the piston heads of some carbon, and around the top of the block, but that's all.

Bolted on the "P" head, and started up the T, the starter motor knew a bit more compression! Ran test, and WOW, the engine was smoother, used less spark advance, and gained cruise 'sweet' spot of 37mph, up from the "Z' sweet cruise spot of 34mph.

Don't know why so smooth a running, but really think its the CNC machined cylinder chambers, as the Z head is just rough as cast chamber, likely each one is different than the other in capacity.

The Prus head the winner :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Jerry Sherman on Thursday, June 15, 2017 - 04:48 pm:

Hi Roger, I have a couple other Super Fire heads I could measure. I'll get you a picture of the Dual Fire, one plug over each valve. I can measure and get photos this weekend when I visit my mom's property about and hour and a half from my house.

I'd consider making some heads but not right away I'm still sorting through 50+ years of T parts, patterns, wheels, rust etc. Once it's fairly organized I'll clean up some patterns and get with a foundry.

I measured the depth of a Spit Fire, an old Super Fire and a newer Super Fire if you are interested.

application/pdfCombustion chamber dimensions
Sherman combustion chambers.pdf (200.1 k)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Les VonNordheim on Friday, June 16, 2017 - 01:38 am:

If you check the volume the way Dan has shown, your accuracy may vary due to how level the head is and how high the liquid rises above the head surface due to surface tension. If you have not done it, It may be surprising, how much liquid can be added before it spills over the combustion chamber edge. A more accurate way to measure the combustion chamber volume is to use a clear plastic acrylic plate sealed/clamped flat against the head with a small fill hole.


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