[Nicks67GTO]

So I remember reading some years back about a Pontiac dealership "over the counter" high compression piston kit you could get for SD 455 engines. Does anyone have any literature or info on that?

[johnta1]

Don't know about the Pontiac dealership thing, but TRW made the SD piston, and came out with a domed piston just for the SD rods.
I used them and they worked great when modified.
(HO Racing I think had an article on modifying the dome)

[starlightblack]

Originally Posted by johnta1

Don't know about the Pontiac dealership thing, but TRW made the SD piston, and came out with a domed piston just for the SD rods.
I used them and they worked great when modified.
(HO Racing I think had an article on modifying the dome)

Did they happen to look like this dome?

[johnta1]

Yes.


That small protrusion at top of dome needed work or taken completely off.

[starlightblack]

Originally Posted by johnta1

Yes.


That small protrusion at top of dome needed work or taken completely off.

Why? Quench?

[johnta1]

Some possibly.
I think it hindered 'swirl' in chamber?

[Sirrotica]

The flame propagation was impaired by the large dome, so there was a modification that knocked most of the top of the dome off, and if memory serves me there was a groove from the area off the spark plug across the dome.

The compression was raised to 12 to one with the new TRW pistons, but because of the reduced flame front, power was reduced. The modification was to reduce the compression back down in a trade off for better flame travel. The engines made more power with less compression and a better flame front.

Large dome pop up pistons have always had a negative effect on Pontiac engines, flat tops have in most circumstances on Pontiacs made more power.

The SD had such a large combustion chamber that it took a real large lump of aluminum to fill it up, and get 3 plus points of compression. The engineering wasn't really given enough time as TRW rushed these pistons to market, probably less that 6 months after the SD engine debuted. I recall the local speed shop owner telling me that they were available shortly after the cars were available. I'm fairly certain it was in the spring of 1973 that they became available.

3.5 compression points would give about 30-35 HP on paper over the factory flat tops, it turned out less because of the reduced flame front. I seem to remember that they were fairly heavy too as TRW made the OEM pistons and just added a rather large dome to the flat top design, instead of re-engineering the whole package, making them even heavier that the OEM parts.

That's the way I remember it in 1973, I know I'm getting up there in age, but I believe my recollection is pretty close to how the TRW 12 to 1 pistons ended up not working out as they were billed by TRW.

[Brewster]

Brad, cool information.

[starlightblack]

Yes, thank you Brad

[starlightblack]

Was it possible for these TRW high-compression pistons to be fitted to any Pontiac rod or are they a SD-455 fit only? Meaning if an engine has these pistons are you 100% assured they are swinging on Super Duty rods?

[Sirrotica]

Since the SD rod can be used in any stock journal sized crank you could have any rod attached to the piston. Back in the day the SD rod was the best bang for the buck (at least that's what we thought) but the bad heat treat on some of the rods coming from the vendor kind of soured that. There was a run on those rods as soon as the Pontiac racers found out they could be had from GM sources.

A run on the SD rods was answered by GM, and the vendor that you could only purchase 4 rods at a time and you had to supply the VIN number to even get them ordered, been there done that. Years later I was able to purchase 8 SD rods through a connection I had that got them from Jack Shaw Pontiac. I had the Rockwell tested on mine and it came back within the specs. I still have those rods NOS sitting on the shelf in my garage.........

Back to the question, if the pistons are in an original SD engine chances are they're connected to SD rods. I have never seen those pistons used without the SD heads because that's what they were designed to be used with. Now it's completely possible that someone would upgrade a SD engine to better rods such as Crower or Oliver if they had a serious racing program.

[johnta1]

The ones made for the SD rod are good only for the SD rod.
They made another one just like it for all the rods. (different part #)
The SD rod is thinner than a stock rod. SD rod can be used with standard pistons but standard rod won't fit SD pistons.

The SD piston was rated at 10.5 CR in the 455 SD.
(111 cc heads)

There are 2 protrusions on the pistons. The one on the dome is what causes problems.

The dome pistons made a noticeable HP increase compared to the flat top pistons. Seems like at least .3 ET reduction in my car.
(been a long time ago though and for the 1/8th mile)

Also 400 heads will not work on those pistons.

[sd7369]

I have a 72 TA where the previous owner had those pistons and a ra IV cam installed....worst running HO engine I ever had. Built somewhere around 78-79 time period. Still have the car but all stock parts back in and runs as strong as they always do when stock parts are used Don't know what the engineers were thinking, those pistons really SUCK.. Anybody want them cheap? If so I have to locate them. Heavy as hell, amazing the stock cast rods held together.

[Formulajones]

I built a 72 455HO recently that has a set of custom made dome pistons installed on a set of H-beam floating rods to bump compression to 9.98:1 with the large chamber HO heads.
The idea was to keep from chopping a set of expensive factory round port heads beyond the point of no return.

We went back and forth about the dome and how it might affect power output. Talked to Paul extensively about it and his experience dyno testing these things. In the end it turns out to be a lot of internet hype, and as John noted they helped his car.

The engine made 507hp 571 tq with a smallish 239/243 .050 hydraulic cam with a very nice flat torque curve on 91 pump gas, through the stock exhaust manifolds, intake and carb. Seems the dome worked pretty good, albeit it is a more modern design compared to the old SD offering.

[starlightblack]

Thanks for that info John, very helpful

Yes Formulajones that does look like a much more thought-out engineered piston, part number? manufacturer? Thanks for pic.

['ol Pinion head]

Custom AutoTec's with slight dome?

['ol Pinion head]

Years ago tore down a 72 455 HO engine i picked up with L2394's in it that each dome had the flame slot. Believe that practice was known decades ago in Pontiac engine builder circles; know the 2394's in that engine were installed in the mid 70's, lot of taper to the cyl walls in that block, really glad the pistons in that block were std's, as an .030 to .040 remachine may have been a tough deal.

Realize, some have had decent results with old 2394's & 2440(?)'s, but was not impressed with those HEAVY boogers & them needing wide piston to bore clearance. A few years later, late 90's, was offered a set of .030 2394's that had been significantly lightened. For my intended use, was still wary of the wide cyl wall clearance & passed on purchasing them.

[Formulajones]

Originally Posted by starlightblack

Thanks for that info John, very helpful

Yes Formulajones that does look like a much more thought-out engineered piston, part number? manufacturer? Thanks for pic.

Originally Posted by 'ol Pinion head

Custom AutoTec's with slight dome?

They were a custom made piston for this engine, from Mahle I think. I don't have the paperwork in front of me. They were a one off deal and have no part number on them, just a job number molded into the bottom side. When I contacted them for some information we were looking for, they couldn't even look them up using the job number, they had no record on it.

Nice light piston with small ring package. Also an odd bore size for a Pontiac not offered off the shelf. 4.200" so we had more ring package choices available in that size.

[njsteve]

The original 1972 455HO I rebuilt for the "black car" that I later sold to BentwheelBob had the regular rod version of those high domed pistons. TRW part number 2394N. The engine had been rebuilt in the late 1970s or early 1980s with those crazy pistons. The engine made some great power but rattled so badly with preignition that the rod bearings were ruined from the shock. It didn't matter how high the octane race gas was I used even 110 - it just wouldn't burn smoothly and you could feel the engine trying to blow itself up.

When I rebuilt it I put the forged versions of the stock pistons in it and it would run on pump gas no problem. I ended up selling the domed pistons for a couple hundred bucks.
Here's some photos of the pistons.

[Douglas Willinger]

What if they instead used the standard SD 455 piston, flat, no dome, with the single-straight trough design value relief cut, instead of the 'eyebrows'?

Of course they would need a smaller chamber SD 455, which would exist in prototype form, or closely as a ported Ram Air II or Ram Air IV set of heads.

Originally Posted by johnta1

Some possibly.
I think it hindered 'swirl' in chamber?