Minimum cylinder wall thickness on boosted motor

[Hemi Joel]

On a cast iron block, half filled, at 22 lbs boost +/- what would be the minimum safe wall thickness? THis is an O/T engine, a Lincoln 337 flathead.

[Joe-JDC]

It seems folks are afraid to make a statement on this, but if you have had a thorough sonic check, and are comfortable with your readings, then I personally would not consider any thrust wall with less than .125-.130" as useable at 22psi.  I really would prefer more, even with the half fill, but that will help stabilize the walls.  I am not comfortable with the boost amount on "old iron".  Joe-JDC

[machoneman]

Unless a custom forged steel crank is on order (and a damned good one at that!) I'd be equally worried about spitting out a crank and/or tearing out the mains!

[WConley]

Agree with Bob!  That three-bearing main setup is a weak link.  Even the Flatfire Bonneville flathead "only" uses 16 lbs of boost.  They use a Moldex billet crank.

[Hemi Joel]

This will have a billet crank with five mains, and a full  main girdle with saddles and caps built in for the extra two mains.

[Hemi Joel]

The stock bore and stroke is 3.5 x 4.375. The class allows 375 cubic inches. I'd like to get close to that on the cubic inches. So I'm trying to figure out if I can overbore .100 or .125.  Or am I better off pushing the stroke out to 4.625 and keeping the bore at 3.560 to maximize cylinder wall stability.

[cjshaker]

This will have a billet crank with five mains, and a full  main girdle with saddles and caps built in for the extra two mains.

Sure would be interesting to see how you did the 5 main bearing set-up. That would help tremendously on bottom end strength. Bore doesn't gain you near as much CI as stroke does, so trading bore size for cylinder wall strength seems like a good idea, on a boosted engine. Is there any way to bore the engine for a higher strength dry-install sleeve? Cylinders should be pretty thick, but it's still 70+ year old iron.

[frnkeore]

First, I don't know anything about the 337. It is heavy and not many used it for racing.

All the FH's, that were raced (221, 239 & V8 60), could be bored at least .100 NA. Some 24 stud blocks, of the early '40's, could be bored .250 over. So, your going to have to use a sonic tester and find out what you have. Maybe that heavy block can be bored at least 1/8".

That said, for the FE, Ford castings, were suppose to have a min of a .170 wall, that to allow a .060 over bore or .140 min. I might use that spec, for a min wall on a blown engine? Arbitrary, at best!

The other thing about FH's, is the port. Can you get your 22 lb boost, threw that port and into the fairly big cylinder or, will it just be 22 lb on the blower side and restricted into the cyl? The port would have to be much larger than the 24 stud engine, to fill it. But, of course any positive pressure will help to do that.

[Hemi Joel]

Here is the sonic test data

[Stangman]

They look pretty thick, obviously that’s before boring. Why can’t you go 20 or 30 over and keep them thick.

[Hemi Joel]

I'm trying to figure out if it's better to have small bore, thicker walls, longer stroke or bigger bore, thinner walls, shorter stroke. If I bore it a minimal amount, I'm going to have to go up to a 4-5/8 stroke.

[WConley]

I think you're going to want thick walls.  That's a lot of combustion pressure with so much boost!  Unless you install good sleeves, you'll be fighting blow-by.

There are always trade-offs!  Sounds like you are building a stout bottom end, so you should be able to handle a decent stroke.  Extra torque will always be welcome since you're going to be limited up top by that FH port flow.

[Hemi Joel]

So sleeves are stronger than the original iron, even though the original iron has to bored thinner to fit the sleeves? How thick of sleeve is best, and what material? Lack of port flow is why all the boost. I have about 175 cfm intake.

[frnkeore]

Is your CFM rating drawn from the cylinder, or just in the valve head area?

FH flow has to change direction and have only ~180* of the curtain to flow. So, realistic boost, into the cylinder itself, might only be 10-15lb. You already know that since your saying that's why you want 22 lb.

To reach your 375 ci goal, your going to need something in the area of 3.6 x 4.6 to 3.56 x 4.7.

Is this for a salt flat's car and are you going to use turbo's?

[My427stang]

If you need to hit 375 cid, I would almost always choose a thick wall over a thin one, so I think I would stroke, assuming no odd piston or rod combos.

However, I have zero modified flat heads of any kind under my belt LOL only maintenance and stock experience, and it's a long time ago to boot.  Keep parts light and balance well