350/255HP Help please

[Chipper Chapman]

I'll get it up to temperature and do another leakdown. Don't have access to a Dwyer guage. Cam is in at 103. What you guys are saying about 35-40 and 600rpm down is exactly what I have been figuring but can't find why. Engine is a factory block but fresh at .065 with plate. Pistons are a couple years old, Manley rods, (light piston heavy Rod)

[Alan Roehrich]

Originally Posted by Chipper Chapman

I'll get it up to temperature and do another leakdown. Don't have access to a Dwyer guage. Cam is in at 103. What you guys are saying about 35-40 and 600rpm down is exactly what I have been figuring but can't find why. Engine is a factory block but fresh at .065 with plate. Pistons are a couple years old, Manley rods, (light piston heavy Rod)

Is the block poured?

To the water pump holes?

Piston to wall clearance?

Hone finish?

Oil clearances?

Oil weight?

Oil pump type?


A Dwyer gauge is pretty cheap, find one online, probably for less than $100. They're easy to hook up. And they tell the tale about real ring seal with the engine running.

[Chipper Chapman]

-Block is not poured,
-.005" piston to wall as per CP
-Plateau Honed with torque plate, cannot remember exact grit but we spoke to total seal and did what they said
-Bearing clearances .0025 rods, .0037 mains
-Broke in and dynoed with amsoil break in 30 weight
-now running amsoil dominator 5w20
-M55A pump with stock spring,
-Champ CP40 pan

Cam specs
Bullet CHS 307/315-06H duration @.050 262/270

[Alan Roehrich]

Originally Posted by Chipper Chapman

-Block is not poured,
-.005" piston to wall as per CP
-Plateau Honed with torque plate, cannot remember exact grit but we spoke to total seal and did what they said
-Bearing clearances .0025 rods, .0037 mains
-Broke in and dynoed with amsoil break in 30 weight
-now running amsoil dominator 5w20
-M55A pump with stock spring,
-Champ CP40 pan

Cam specs
Bullet CHS 307/315-06H duration @.050 262/270

Okay, in my opinion, for whatever it is worth, what I have highlighted above is problem one. There is no production block that I know of that will stay round under stress at 0.065" over. I've never seen an 010 or other block thick enough to do that.

If you want to run that big a bore size, you need to pour the block with Hard Blok or Embeco 885. At a minimum. You really need a BowTie or Dart to make it work well.

Ring seal is the most critical and fundamental part of restricted engine performance. You simply do not have one single solitary cfm to waste, the engine is suffering pumping losses just to breath, you cannot waste what it breathes. The bores have to stay round and straight, period.

I do not know if you can salvage what you have by pouring that block. You could try it, and maybe open the clearance up to 0.006-0.0065. If you pour a block one side at a time with either cylinder heads or torque plates, it will sometimes get a little smaller. Not every time. And if you cannot get it back round and straight by 0.0065" clearance, you'll have to change blocks.

I run tighter oil clearances, and use a lighter oil ring with a Napier cut second ring. No more than 0.001" per 1.0" of journal size. Of course, the bearing bores have to be dead round and stay that way. But it allows around 10-12# oil ring tension with no oil consumption.

Next, with a cam ground on 106 and in at 103, you're going to have an early HP peak, especially considering low lift and relatively low compression. You might consider a cam ground on 108 and in at 104-105. You can try moving the cam you have around.

That's about all I can offer you on the forum. Take it for whatever you think it is worth. Good luck.

[Chipper Chapman]

Cam is in at 103 as per bullet's instruction. Really can't see the block being the issue, none of the other engines around here out powering me are aftermarket or filled blocks. I can understand where you are coming from there and if there was an extra 3k in the budget it would have got a dart or bowtie.

[Alan Roehrich]

Originally Posted by Chipper Chapman

Cam is in at 103 as per bullet's instruction. Really can't see the block being the issue, none of the other engines around here out powering me are aftermarket or filled blocks. I can understand where you are coming from there and if there was an extra 3k in the budget it would have got a dart or bowtie.

Okay. Good luck.

[Alan Nyhus]

Chipper, where are you at with the intake manifold?

Cylinder walls on a non-filled production block can really rattle around at high rpms.

[Ed Wright]

Originally Posted by Alan Nyhus

Chipper, where are you at with the intake manifold?

Cylinder walls on a non-filled production block can really rattle around at high rpms.

I would listen to Alan. Everything he stated is dead nuts.

[Alan Nyhus]

This is a non filled '72 vintage block +.070 from my Nova Stocker that split the #4 cylinder. Will section the cylinder wall this week just to see how thin it really is. At high rpms, the walls in a block like this gotta' be shakin' like a female intern in the Clinton White House.......

[Rich Biebel]

OEM blocks vary a huge amount in hardness as well as wall thickness.

OEM blocks are often way offset from center on the cam and crank as they relate to the cylinders and other issues as well.

OEM blocks often have a lot of core shift......cylinders are thin on one side.


With the power levels being what they are even in stock engines and also the cost of the parts to build a good engine.....any OEM block considered for use should be hardness tested and sonic'ed for wall thickness....

We did this in the shop I worked at 30+ years ago.....and a soft block or a thin block was just not used....

I would not hesitate to fill the block no matter what.....but as was stated filling a block that is already machined and at size is not the right way to go....it should be done before the block was finished....

My last 350-255 was a stock 010 block at .040 over...filled block.....ran 11.80's in G/SA with TRW pistons....stock rings.....untouched heads other than old school legal VJ and CC prep and untouched intake.....old Lunati cam with std type lifters....that was 1996 and it was a good 4-5 tenths slower than the fastest cars....

I shifted that 350 at 6600-6800 and it was a powerglide trans. 3 speeds were not legal back then..

MPH was never better than 112....usually less