[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.

[Jay S]

Good point Paul. I think all the beveled pistons I have seen in 455s are aftermarket cast oversized. They show up a lot in stock reman’s.

I was thinking Lee Atkinson on the forum had a OEM set in a 455 with the beveled pistons. But I was mistaken, his are aftermarket oversized with that beveled. FWIW, he has very low compression with his 66 heads and a bigger cam with tri-y headers, and has done multiply pulls on his wheel dyno with it. It runs well, if it is just temporary anyway, the 66 head can work too. Lee’s makes over 300 at the rear wheels.

[MDGOAT69]

Originally Posted by android 211

Pontiac installed the beveled pistons in the 75 and 76 455s. They went into a lot of Trans Ams, Grand Villes and station wagons.

Originally Posted by PAUL K

I have disassembled dozens of those engines and never seen a factory beveled 455 piston. They have less than 8:1 compression with a flat top as it is. Do you know any specific applications or block codes they were used in?

Originally Posted by Jay S

Good point Paul. I think all the beveled pistons I have seen in 455s are aftermarket cast oversized. They show up a lot in stock reman’s.

I was thinking Lee Atkinson on the forum had a OEM set in a 455 with the beveled pistons. But I was mistaken, his are aftermarket oversized with that beveled. FWIW, he has very low compression with his 66 heads and a bigger cam with tri-y headers, and has done multiply pulls on his wheel dyno with it. It runs well, if it is just temporary anyway, the 66 head can work too. Lee’s makes over 300 at the rear wheels.

I really just don’t understand why these pistons exist with the bevel, these 455s were already low compression so why lower it anymore on a rebuild?

[Lee]

Originally Posted by Jay S

I was thinking Lee Atkinson on the forum had a OEM set in a 455 with the beveled pistons. But I was mistaken, his are aftermarket oversized with that beveled. FWIW, he has very low compression with his 66 heads and a bigger cam with tri-y headers, and has done multiply pulls on his wheel dyno with it. It runs well, if it is just temporary anyway, the 66 head can work too. Lee’s makes over 300 at the rear wheels.

Jay, here is are dyno results from my testing.

The red lines are for the bevel-edged cast pistons, and I still had a Q-jet and HEI on the car during this run. This is the best pull the motor made in that configuration. I calculated the CR to be right at 7.8:1.

The blue lines are with Keith Black mini-dome hypereutectic pistons and SCAT H-beam rods. These kicked the CR up to a whopping 8.3:1! I had also added a FAST XFI Sportsman throttle body system with ignition control. The cam, heads, headers, exhaust, stock non-adjustable valvetrain remained the same between the two tests.

I did use a Performer on the "red" pull, and a P4B on the "blue".

I also attached a pic of the KB pistons I used. If I can find a photo of the beveled pistons, I'll come back and add that.

Oh, the motor was bored 0.030" oversize when I got it, so the pistons were aftermarket. The bore looked fine and the KB's fit perfectly, so the block was not bored further when I made the change.

[ts3189]

Doug,slap a set of low compression heads on it and send it. You already know you will be building something else!

[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?

[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 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

[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

[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 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