[Prototyper]

Hi All,
Pulled the 197 Heads from my 71 455HO recently. Found it has domed pistons fitted as per the attached pic. Can anyone identify them?
I've used a mirror and can see stampings SPDMS, A & 5 only under crown
Forged slipper pistons with skirt support ribs 4.185 bore x 4.250 stroke, approx 0.015" down hole +10cc dome volume 0.235" high. Dome perimeter profile matches cylinder head chamber.
Out of curiosity I'd like to know what they might be??
High cranking pressure and pinged under load with previous single pattern 224/286/111/109 HFT on pump gas, speccing up a new cam and wondering if a 9.7:1 static comp with bigger duration split pattern wider LSA HFT on a 73/74ish degree IVC will be ok from a pump gas DCR perspective if anyone has run a similar setup?
Regards,
Jarrod

[mgarblik]

Any way you can get a pic or 2 of the skirt support area and pin box? Venolia is one of the few brands I am familiar with that used heavy rib supports on the skirts.

[Prototyper]

Engine is still in vehicle so I can only get a pic of the lower skirt at BDC, have to contortion myself to see any more via a mirror on a stick let alone get a pic!!
I've seen pics of Venolias with similar ribbing but they had full skirts, these are partial skirt slippers
Thanks,
Jarrod

[mgarblik]

Thanks for trying. We ran some slipper skirt Venolia's with the supports for several years in our drag car. But they were flat tops. Possibly Aries Pistons as well. They made a lot of pop up pistons, some with skirt support. Anyway you go, that piston design is a super detonator, with an iron Pontiac head. You would need very high octane fuel to run them without rattling the poor thing to pieces, IMO. If you really wanted to use these pistons in a street car and the pistons are a solid dome and thick enough, you could mow the dome off in a piston vise and a mill. That would improve the situation some at a minimal cost.

[PAUL K]

They look like Venolia and your markings sound like Venolia. I'll check later and see if I can confirm that. But the SPDMS is the clue.

[Prototyper]

Thanks,
Detonation is my concern, I'm pretty sure they're solid domes so will keep that up my sleeve

[Prototyper]

Thanks,
Detonation is my concern, was hoping I may be able to compensate with later IVC.
I'm in Australia and run 98 octane which is the same as your 93 but it did ping previously with a less than optimised combo.
I think they're solid domes so I could consider milling them flat.

[PAUL K]

That's a poor cam choice for your combination and is contributing to your pinging issues much more than the dome. I'd smooth the edges of the dome and re-use them with a cam that matches the rest of your combo. Your current cam may not be a bad choice if you remove the dome.

[PAUL K]

Those are Venolia pistons. It looks like someone dropped the ball by making those valve reliefs so large.... Absolutely zero reason to make them that big with a 197 head. Unless your wanting an excessively large dome.

Piston in the picture made about 10.5 with a milled 103 cc SD-455 head. The dome is roughly .250 tall.Those pistons are strong but old. We ran nines in a 3900 lbs. street car with a shot of spray about thirty years ago. Our small valve reliefs allowed plenty of room with a 270 ° solid roller cam.

[Prototyper]

Thanks for that info Paul.
I always knew that cam was a poor choice since I've owned this car, 71 TA 4 Speed....from memory was around 210 cranking psi, lots of torque with very little throttle input, could only run 28 degrees total timing on a slow advance curve and would run quite hot at sustained highway speeds.
Intake crossover and timing cover was leaking coolant externally so thought I'd pull the cam while I removed those items and confirmed my thoughts. Intake was an Eddy performer with a 750 DP and it only had non RA/HO/SD gaskets! It also has the original exhaust manifolds in place still and I have the original HO intake, just picked up a SR SD455 Qjet which I'll rebuild.
I debated pulling the heads as part of speccing a new cam, glad I did in the end.
I don't really want to remove and strip the short block, looks like it's done very little work since whenever it was built. I would tape everything up and smooth the sharp edges on these pistons and on the heads too which have barely been cut so close to 110cc chambers.
My thought now is will a 286-288 intake lobe on a 110ICL paired with a 292-296 exhaust lobe on a 118ECL work or should I go bigger on the intake lobe to try and mitigate the detonation risk? It is a 4 speed and car is very limited use so I can tolerate some reduced low rpm manners or having to run octane booster on highest octane pump gas isn't too much of a problem.
Regards, Jarrod

[PAUL K]

YW..... I like your plan. Advertised numbers can be a bit deceiving I'd look at something around 235/240 at .050 on a 112. If you prefer a smoother idle I'd. add a few degrees of exhaust duration and go with your suggested 114. Generally those heads perfer a bit of a softer lobe. I'd see what Crower has to offer then consider Bullet and Lunari, they still offer the Ulradyne lobes. Once you price those cams the Summit RAIV copy might look awfully appealing and would work well, especially if 1.65 rocker arms are an option. JMHO

Sounds like you have a great car!

I'm curious to you know what manufacturer your current cam came from?