Block Rigidity and HP

[frnkeore]

There is a thread going on called "Min Bore Wall Thickness" and this is related to that but, my question is:

How much HP is lost if a engine is bored more than the common .030 over? For that matter how much HP are you loosing bore .030?

The consensus is that you should have .120 walls min, no matter the bore diameter and I try to abide to that and think it's reasonable. People talk about lose of rigidity, causing cracking and cylinder pressure leakage causing HP lose but, I never hear of tests to prove that there is actually a lose or a pressure test, to see at what thickness, for what area, a wall will fail.

I'm not advocating that anyone bore a engine below a .120 wall thickness, based on my studying wet sleeve thickness, it's reasonable but, there are one or two wet sleeves that go under that figure.

I'm 77 yrs old and I've seen some things done, that people would consider failure prone today. First the FH Ford, some blocks (truck as I remember) were bore 1/4", regular car blocks were commonly bored 3/16 (I had one). 283 SBC where commonly bored 1/8 but, they only bored 327's .060/.080 usually. Ford's, pre thin wall casting, were bored 1/8, same with Olds & Cad but, I don't remember if the Pont was. My best friend bored his 303 Olds, 1/8 to 324 and ran a 471 blower on it for at least 8 years.

I was a mechanic for 7 years, until 1970 and the only cylinder wall failure, I saw, was the same guy, had a '67 Camero, with a 350 in it and in early '69 it split a cyl wall with a stock bore. This in SoCal w/o freezing temps.

So my question is has there been any back to back, dyno testing, to show HP lose at any given cyl wall thickness or is it "I just know it's true". If there is any testing to that effect, how much lose was there at any particular wall thickness? If that testing exists, please reference it, I would love to read it. Or are there test that show a cyl wall failure at a particular wall thickness by increasing a bore in steps, until it fails?

I think it's a very interesting subject and it's debated for many years. Curious minds, want to know.

[digger]

i would look to what min wall thickness wet sleeves are offered by aftermarket manufacturers and the material of construction to understand this as they would have done these kinds of tests you'd think. Its not a simple non costly exercise to do this in a way that minimises other variables.

[modok]

Cylinder walls should be thicker than your rings, if you want the rings to seal.

Simple but true.

D-wall ring radial thickness is bore size divided by 22

Yep 327 has a lot of wall thickness, and deck thickness, compared to later blocks.

[BOOT]

In my opinion depends how hard/high you push it. I mean we talking a brand new block like back then or same block produced back then block bored now? These days other parts are so good, there are other/better areas/approaches to reach the same or higher goals.

[rebelrouser]

My idea is that the cylinder has to hold and harness the explosion of the air fuel mixture, the better it does this the more energy is used to drive the piston down and rotate the crankshaft. I think a lot of higher end race teams now use a data logger to track cylinder blow by and ring seal. I think it just common sense that the rigidity of the cylinder is directly related to ring seal. An engine under WOT sees a lot of tortional stress and heat. Maybe it is flawed but I don't have the money for a data logger for blow by, plus in NSS racing such electronics are frowned upon. So what I do to try and track it is I leak the engine clod on the engine stand. Then after fire up and a few runs, I let it get cold again, re-leak and compare to the engine stand numbers, they should be a little better. Then I simply periodically leak the engine, when the leak starts to go away I know it is time to start saving pennies for a freshen. Then during a freshen measurements tell me is it just worn rings or did the cylinders distort. This also will show up any valve seat issues you just listen to where the air is leaking from. It is just basic, you have to capture and seal the explosion to make power. All the camshaft, cylinder flow, carb design, etc. gets the air fuel in the cylinder then the explosion has to be contained with as little leakage as possible.

[PackardV8]

I'm your age and have some vague recollection of what you recollect:

I'm 77 yrs old . . . . 283 SBC where commonly bored 1/8

We were racing 301"s, lost a piston and had to install a new stock Corvette 283" hp short block. With eighteen fewer cubic inches and a point less compression, same cam, heads and injectors, it ran faster than the 301". Maybe thicker walls made for better ring seal, maybe the pop-up pistons were bad for flame travel, maybe our hand hone job without a torque plate.

My best friend bored his 303 Olds, 1/8 to 324 and ran a 471 blower on it for at least 8 years.

Then he wasn't making much horsepower. We would split those early Olds blocks down the middle when pushed hard. They didn't like nitro.

So my question is has there been any back to back, dyno testing, to show HP lose at any given cyl wall thickness or is it "I just know it's true". If there is any testing to that effect, how much lose was there at any particular wall thickness? If that testing exists, please reference it, I would love to read it. Or are there test that show a cyl wall failure at a particular wall thickness by increasing a bore in steps, until it fails?

The above does not consider several design features which have as much effect as cylinder wall thickness:

1. Block deck height - Common US OHV8s vary from 8.2" to 10.625". There's a lot of stiffness difference in a cylinder of the same wall thickness when it's two-and-a-half inches (30%) shorter/taller.

2. Top deck, bottom mains and middle lifter valley of the block reinforcements. The earlier OHV8s; the aforementioned Olds, Pontiac, Packard, et al, were strong in the walls, weak up the middle. Later designs and most all aftermarket blocks have twice the iron through the center lifter valley area.

3. Number of head bolts. The design of the head bolt to block interface makes a difference. The SBC is infamous for the head bolts distorting the internal cylinder wall. Rings can't seal a wavy wall. That's why some weaker blocks mandate honing with a torque plate The Studebaker, Packard and several others attach the head bolts to the outer walls of the block and will show minimal internal distortion when a torque plate is pulled down.

jack vines

[mag2555]

I need to take issue with how you have used the term " fuel explosion ", the the air and fuel mixture burns, and yes it's at a fast rate but not at a explosive rate by any means!

Detonating = explosion

[frnkeore]

Jack, I don't want to take this to far off topic but, no, he didn't run nitro, it was a street car! The rest of the story on the bored Olds is that it also had '56 heads, cast blower pistons (probably just a shorter CH) and a 280° Herbert roller cam. The blower, with it's two AFB's put out 14 lb and the engine would rev to 7k, in 1st, 2nd and 3rd (never took it to the top of 4th). I would guess the HP was in the +/- 500 range, in those days. The 14 lb of boost, would approx double the HP and cylinder pressure.

Here's the pictures. One is my own Instamatic and the other from HR mag.

[frnkeore]

I will add, that it was driven 8 years with the blower but, a additional 3 years with 4 x 2 manifold, still with the Herbert roller.

[Ks Fats]

Frank,
I have a 59Ab block that is 3&3/8 +.030, it shows evidence of one sleeve installation that appears to have been a rod failure. I can't imagine that it didn't have some flex at that bore size. It has not been sonic checked to see if it is worth completing or not; it came to me out of a dirt car so I'm sure it didn't live a stress-free life. Jahn's cast pistons B.T.W., got a partial set of 3&7/16 (Jahn's also) with it, perhaps for a freshen -up? Only failures on the block were the customary cracks around some head bolt holes, none in the valve bowls or seats. Had the factory relief cut in. Probably taken out due to a flat Isky 400 junior that was in it.

[PackardV8]

Frank, I have a 59Ab block that is 3&3/8 +.030, . I can't imagine that it didn't have some flex at that bore size.

IIRC, the 59AB was 3.1875" bore, so a 3.405" bore is .2175" overbore. I'd imagine it would have run really hot when raced with walls that thin.

FWIW, the magic 59-series block is supposedly the rare 59Y, claimed to be "harder" and safe to 3.4375".

[frnkeore]

The Ford FH, might be the perfect example of where or not, a engine looses power with what we would call today, extreme overbores.

If the larger bores didn't increase power, like Jack infers, regarding the 1/8 over 283 then, with as many racing FH's as there where in the 50's, that would have surfaced. There would have been results that showed, for intense, that a .060 over would go faster and be more durable, than a 3/16 over engine.

Regardless, what I was hoping to find, was documented testing, to show that something failed or lost power on the dyno, at a particular wall thickness and maybe the length of the unsupported span involved.

[allencr267]

Regardless, what I was hoping to find, was documented testing...
If you can't find that, would the SB&BBC cast to aluminum block comparison stuff be similar enough suffice?

[Ks Fats]

I don't think I've encountered any documented A/B tests regarding that Frank; I think we all had our self-imposed limits back in the day. Most of my limits were set by guys older than I and with more experience; at my age I suppose
I've become one of those guys. Unfortunately, the choices when it comes to stock blocks are becoming slim; I've seen a lot of money spent bringing blocks back to life that would have been relegated to the iron pile in years past. Whenever aftermarket blocks are accepted, I encourage folks to go that route but sometimes there is no alternative but to keep searching until you find a good core.

[Ken_Parkman]

The FH would probably not be comparable to a higher output engine when considering block stiffness. A big factor has to be how much cylinder pressure, and a lower specific power application would be far less of a problem. A big overbore on a FH may gain power due to the increased cu in, where a higher specific output engine may lose power due to the loss of strength.

I know my experience is firmly never overbore an engine any more the the absolute minimum. It does not seem to matter much on a street car, but anyplace on a higher power application that is consistently raced where there is a larger overbore you get eventually get cracked cylinder walls in my AMC stuff.