[25stevem]

Who knows at this point!
Maybe the 455s made without the chamfer where for slightly increased compression for High altitude applications , lol!

[tc]

If the champer is 1/8 inch and 45 degrees, the cc around the piston is very close to 1.5

[25stevem]

So your talking .125” wide and .125” deep?

The actual angle of that chamfer could change that CC volume greatly as that angle is what sets the depth and width of that chamfer.

By my math a chamfer of .125” by .125” on a 4.210” bore would add 1.638 CCs.
That would not change the compression ratio by much at all which would make me question why the factory would pay for a added machining step for what amounts to no effect at all.

With a 4.150” bore it takes a 4 Cc change to produce a 1/4 point change in compression.

[Stan Weiss]

If the bevel was done with a .125" radius cutter it would remove less than 1 cc of material. Along with the bevel was the piston to bore clearance above the 1st ring increased?

Stan

[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.

[Formulabruce]

Amazing how DYNO charts start at >> 3500 RPM?? Who DRIVES at 3500 RPM or Higher, ALL the time?
Building an engine to an estimated HP number at some point at high RPM doesn't make sense unless you regularly race it.
455 can have 500 ft lbs of Torque at LOWER rpm than this chart.
There is a TON of Torque left on the table at cruising ranges......

[MDGOAT69]

Had some time to get out to the garage, and my big question was how far in the hole the pistons may be. To my surprise they seem to be flush to the deck surface to me, so Is this zero decked?

[android 211]

I recall a Cars magazine test of the 76 TA with 455 HO engine. They said the HO was in name only and it was the same engine that went in the station wagon 455. They said the compression ratio was 7.5 or 7.6:1. The bevel would have to be to lower compression to some level it could meet emission standards. The Cars staffers advanced the timing, richened the carb and lowered the headpipes away from the exhaust manifolds and got in the 14s anyway. The bevel would ruin any beneficial 'quench' or 'squish' effect in the combustion chamber,

[Stan Weiss]

Originally Posted by MDGOAT69

Had some time to get out to the garage, and my big question was how far in the hole the pistons may be. To my surprise they seem to be flush to the deck surface to me, so Is this zero decked?

If in fact the top of the piston is flush with the deck than they are zero decked.

Stan

[Lee]

Originally Posted by Formulabruce

Amazing how DYNO charts start at >> 3500 RPM?? Who DRIVES at 3500 RPM or Higher, ALL the time?
Building an engine to an estimated HP number at some point at high RPM doesn't make sense unless you regularly race it.
455 can have 500 ft lbs of Torque at LOWER rpm than this chart.
There is a TON of Torque left on the table at cruising ranges......

On most engine dynos [which I have limited experience with] there are characteristics about how it functions that make it so it needs to be running at a certain rpm (usually 3500+rpm) for accurate readings. If anyone knows particulars, please share.

Things are different on a chassis dyno. With an automatic transmission, you have to worry about the transmission downshifting. When a car downshifts on the dyno at WOT, it can be a little unnerving!

The car I performed the testing on has a THM350. I've been able to start testing at/below 3000rpm at times, but not consistently without a downshift. Even when I can, the chassis moves around, the carb takes a moment to transition and settle down, etc. making the first few hundred RPM "funny" at times - so it is usually about 3500 before I have "clean" data. With a modern computer-controlled transmission, if you know how to program it, you CAN make a dyno program which will allow testing to start a much lower RPM.

If I have a car with a manual tranny, then I can test at any RPM. I've attached another dyno graph. This is a 79 T/A with an 8.3:1 400, R/A exhaust manifolds, Performer intake and a Qjet. You can see where the secondaries start opening around 2600rpm.

But I do agree with you. I see MOST people throwing away a lot of mid/low-range power in exchange for a little bit of a HP gain. My own car (from the "start at 3500" pulls) has 3.08 gears and a stock torque converter, yet it still has traction problems off the line with 255/60 MT drag radials.

[KEN CROCIE]

Third times the charm ? My third attempt at replying to this post.
We at HO tore down virgin, never been apart, engines.We occasionally came across a champhered piston.
If I recall, the chafer was to promote the burning of the unburnt fuel between the top of the ring and the cyl, wall. This was thought to reduce emisions. Since it was used only 2-3 years, it might not have been a great idea. The pistons might have been a part of a Cal. emissions pkg. I believe TRW sold theese with a 3000 series P/N. Theese pistons have been discussed before on this forum, but I can't find them.

[25stevem]

Thanks for this insight Ken!
Since this topic came back up I have been wondering if the factory tried this emissions fix for CA cars before they went with the 5s heads , as in the CA heads with air injection, or maybe they found that for CA cars they needed both the air pump and the chamfer!