[MDGOAT69]

I have a 1971 455 that came in my 79 Trans Am that I am really wanting to run next season. *Please understand that I know this is less than ideal, my plan is to pick up another engine to build properly, but I want to drive this car next season and sending this engine to a machine shop for a proper inspection and new rotating assembly just about guarantees that won’t happen.*

The engine is a previous rebuild that has never been run. I pulled the 66 heads off and I confirmed that to be true. Now for the sad part. It’s bored .060 and the pistons in it are the Sealed Power NP417s, with the valve reliefs and the beveled edge (YUCK). Rods and crank appear stock.

Before I found out about the pistons I picked up a set of 6X-4 heads to swap on and was hoping for 9.3-9.5 compression and a nice fun engine with a RAIV cam swapped in. Now that I know I have these pistons, I know that CR ain’t happening.

So the big question is this, now that I know about the pistons; I know the CR is getting killed with the beveled edge. My question is how do I figure out how low this is going to drop my compression?

[25stevem]

It’s my understanding that those pistons do not come with a chamfer from what I just saw on the summit site, so that chamfer must have been added.

I would bet that the chamfer was done to add a volume that would allow a head of less then 90 CCs to be run on a 455.

Unfortunately to nail down a compression ratio with any head you might choose to use your going to need to CC one cylinder with the piston close to TDC.

A makeshift way to do it would be to get. A 100 CC short type ( wide ) plastic beaker , get some clay and then fill in and level off with a razor blade both the chamfer and the valve notches.
Removing and balling up the clay and dropping it the beaker filled up to the 50 Cc level would then by means of displacement tell you the total CCs you are dealing with to use in a compression formula.

By the way, you do not need a rotating assy, just new pistons and a balance job, and yes that’s about a 700 buck cost assuming you can reuse the rings.

Another possibility is that running the -4 heads now might get you a 8.5 comp and since your goal for next year is to just use the car then just put that RA4 came on the shelf for latter use in your next motor and just get a smaller cam for now.

Something close to a 068 factory cam duration wise but with more lift should work with a 8.5 comp and still give you some tire burning fun!

[MDGOAT69]

I’m hoping this picture works, these are the pistons in question.

[MDGOAT69]

Originally Posted by 25stevem

It’s my understanding that those pistons do not come with a chamfer from what I just saw on the summit site, so that chamfer must have been added.

I would bet that the chamfer was done to add a volume that would allow a head of less then 90 CCs to be run on a 455.

Unfortunately to nail down a compression ratio with any head you might choose to use your going to need to CC one cylinder with the piston close to TDC.

A makeshift way to do it would be to get. A 100 CC plastic beaker , get some clay and then fill in and level off with a razor blade both the chamfer and the valve notches.
Removing and balling up the clay and dropping it the beaker filled up to the 50 Cc level would then by means of displacement tell you the total CCs you are dealing with to use in a compression formula.

By the way, you do not need a rotating assy, just new pistons and a balance job, and yes that’s about a 700 buck cost assuming you can reuse the rings.

I wonder if I’m potentially wrong about the brand then? Here is a photo of them.

I assumed you couldn’t just swap pistons without basically starting over with the whole rotating assembly, but maybe that’s a discussion I need to have with my local machine shop instead of assuming lol.

[Jay S]

The 417NP cast pistons are modeled from the 73 and later pistons. The 72 455’s that I have seen did not have the bevel on the top. I just worked on a original 72 a few days ago. The 417NP pistons have a fairly low compression height (1.481”) , and the piston top is likely .020”-.030” below the deck. That range is depending on if your engine has ever been decked, but it likely falls somewhere in that range unless it was zero decked.

From what I recall the total cc’s is suppose to be between 10 and 11 cc’s on those pistons. The compression will be about 9:1, maybe upper 8s.

[77 TRASHCAN]

I'm not reading. ALL of your ideas/intentions, possibly???

Do you plan to re-use the 66 heads (some of the largest chambered Pontiac heads made?)

The double chamfered pistons are not desirable, BUT in you case, wanting a ONE year temporary engine, they would be ok (my opinion). Use those pistons (even with a low compression, large chambered head).


The 068 or the Summit cams 802 would be sufficient for this "temp" engine. Melling sells their SPV- 7 cam, it's the 068 copy. A great thing about using it IS, it utilizes stock springs.

Your engine would run well enough, in my opinion. Allowing you to step up to something more powerful in the future.

[25stevem]

If I was the OP I would be making a call to sealed power to get the CC info on that piston.

All the on line photos I have seen of that piston show no chamfer and far smaller in width valve notches.

If what he has there is only a total of 11 CCs he is lucky!

[MDGOAT69]

Originally Posted by Jay S

The 417NP cast pistons are modeled from the 73 and later pistons. The 72 455’s that I have seen did not have the bevel on the top. I just worked on a original 72 a few days ago. The 417NP pistons have a fairly low compression height (1.481”) , and the piston top is likely .020”-.030” below the deck. That range is depending on if your engine has ever been decked, but it likely falls somewhere in that range unless it was zero decked.

From what I recall the total cc’s is suppose to be between 10 and 11 cc’s on those pistons. The compression will be about 9:1, maybe upper 8s.

9:1 I could definitely live with for now, I don’t know if it was zero decked, but I doubt it. I’ll have to confirm that, as well as a few other things it seems.

Originally Posted by 77 TRASHCAN

I'm not reading. ALL of your ideas/intentions, possibly???

Do you plan to re-use the 66 heads (some of the largest chambered Pontiac heads made?)

The double chamfered pistons are not desirable, BUT in you case, wanting a ONE year temporary engine, they would be ok (my opinion). Use those pistons (even with a low compression, large chambered head).


The 068 or the Summit cams 802 would be sufficient for this "temp" engine. Melling sells their SPV- 7 cam, it's the 068 copy. A great thing about using it IS, it utilizes stock springs.

Your engine would run well enough, in my opinion. Allowing you to step up to something more powerful in the future.

Oh no, those 66 heads are not going back on!

My plan is to use the 6X-4 heads with the 93ishcc chambers. My issue then became I was planning on a 041 style cam but now I don’t think I’ll have the compression ratio for it to really make sense.

I think I need to do a little more digging on these pistons and my setup so I can get the CR more accurately figured, then I can make a solid cam choice and enjoy it for awhile until I build the new engine. Leaning towards a 400 based stroker I think.

Originally Posted by 25stevem

If I was the OP I would be making a call to sealed power to get the CC info on that piston.

All the on line photos I have seen of that piston show no chamfer and far smaller in width valve notches.

If what he has there is only a total of 11 CCs he is lucky!

They definitely look nothing like the photos I see online of the current NP417 pistons, that’s what makes me think the CCs on these may be even more drastic than I thought on top of the fact they are likely .020-.030 in the hole from what I’ve read as well.

If these are even worse than I thought, I’m wondering if I would actually be better served going to one of the older 72cc heads if I can determine the CR properly…

[Jay S]

Looking closer Steve is likely right, they probably are a couple more cc’s than 11. It will run fine as long as it isn’t over cam’d.

I think that “802” cam 77trashcan mentioned was intended to be the summit 2802. It would be a better choice than a SPC-8 (RA4 cam).

What do you have for gears and converter?

[Stan Weiss]

Originally Posted by SD455DJ

The most recent 455HO that Dan built and Dyno'd is the one for my GT-37. It too is .040 over, 8.6 to 1 compression, Summit 2802 (224/234/114 @ .466"/.488"), untouched stock heads and factory RA manifolds made 430 hp @ 5000 rpm and 535 lbft. torque @ 3500 rpm. So, fairly similar numbers, but this 455HO isn't a max build for the Pure Stocks either. With this cam, it still pulled 20" vacuum at 1200 rpm and 18" at idle...very minimal lope if any...sounded very stock.

Dennis

Stan

[PAUL K]

I would proceed as planned but either change the cam or install Rhoades lifters. Either will work and you will have a strong engine to use until you build the new one the way you want.

[MDGOAT69]

Originally Posted by Jay S

Looking closer Steve is likely right, they probably are a couple more cc’s than 11. It will run fine as long as it isn’t over cam’d.

I think that “802” cam 77trashcan mentioned was intended to be the summit 2802. It would be a better choice than a SPC-8 (RA4 cam).

What do you have for gears and converter?

Car is a 4 speed, so I think that gives me a little more wiggle room?

Earlier in the week I picked up a cam from a member here for very cheap that I thought would work pretty well with this lower compression build, a Crane H272-2, I’ll attach the specs, but I think it may be a good match? If not, I’ll definitely go with the 2802, I know that’s been a popular option.

Originally Posted by Stan Weiss

Stan

Good info there, impressive numbers

[PAUL K]

Originally Posted by Stan Weiss

Stan

That 224/234 cam is a good choice for an 8:1 455 using a tight converter and numerically low gears. Crane offered that cam and a more modern version 222/234 467/494 lift that worked very well, also especially with a 1.65 rocker on the intake side.

[MDGOAT69]

Originally Posted by PAUL K

I would proceed as planned but either change the cam or install Rhoades lifters. Either will work and you will have a strong engine to use until you build the new one the way you want.

I appreciate the input, makes me think I’m not being too unreasonable at least.

Thoughts on using the Crane H-272-2 that I posted above?

My plan was to use Rhodes lifters and 1.65 roller rockers with the RAIV cam, but I figure I’ll probably stay with the stock style valvetrain now depending on which cam I end up using

[PAUL K]

Originally Posted by MDGOAT69

Car is a 4 speed, so I think that gives me a little more wiggle room?

Earlier in the week I picked up a cam from a member here for very cheap that I thought would work pretty well with this lower compression build, a Crane H272-2, I’ll attach the specs, but I think it may be a good match? If not, I’ll definitely go with the 2802, I know that’s been a popular option.




Good info there, impressive numbers

That cam will work fine for your project, especially since you already have it. If you want to optimize the combination, retard it a couple degrees and add 1.65 rocker arms.... You will have a strong running engine.

[MDGOAT69]

Originally Posted by PAUL K

That cam will work fine for your project, especially since you already have it. If you want to optimize the combination, retard it a couple degrees and add 1.65 rocker arms.... You will have a strong running engine.

That sounds like a good plan to me! Would you still recommend Rhodes lifters? Im guessing that vacuum will he no issue at idle, but I hear they are high quality lifters.

[PAUL K]

Originally Posted by MDGOAT69

That sounds like a good plan to me! Would you still recommend Rhodes lifters? Im guessing that vacuum will he no issue at idle, but I hear they are high quality lifters.

No, I would run that cam with standard lifters. JMO

[25stevem]

Here’s some more info.
Stock 6X head wise ( valve spring installed height) will not work with that .467” lift, you will be into coil binding those stock springs .

The stock spring / installed height set up many times is really only good for .425” lift before the stock spring is .060” away from coil bind.

Even if a freshen up valve job was done you would likely be too close to coil bind for used stock springs to hold up, you would also only have at the very best 105 psi of seat pressure, and with used high mileage springs more like a pity full 90 psi.

Block / piston wise 455s are spec out to have from the factory a deck clearance of .013”.

The compression height of the factory piston is 1.497” to 1.510” from what my notes I have taken over the years have shown me.

Math wise if you wanted a 9 to 1 compression out of your .060” over motor you would need a total CC volume with the piston at at TDC of 121 CCs.

Assuming a stock deck height of .013” would make for a CC volume of 3.77 CCs.
The usage of a Fel pro head gasket makes for 9.7 CCs, so now we are at 13.47 CCs.

If we add that to your cut 6X heads of 93 CCs we are at 106.4 CCs.
This leaves 14.6 CCs left out of that 121 CCs that we are shooting for to have a 9 to 1 compression for your 455.

Now the CCs of your piston notches and chamfer need to be determined, and hopefully that will not exceed 14.6 CCs.

[Stan Weiss]

Bore = 4.21"
Stroke = 4.21"

Total Volume against CR

Stan

[25stevem]

Don’t forget that as seen in his photos his block still has some intake chamfers left even after a .060” overbore and I did not factor in if he has any dish in his valves being used and I did not factor in 1.5 CCs in ring land volume.

All of this I would estimate to be a easy 3 CCs.