[firebirdmi]

Tore down the top half of my engine today and found the short block stuffed with these compression killing pistons. Any idea of the manufacturer and how many cc's to account for the valve reliefs when calculating compression?

Thanks

[71HOT/A]

I would guess Badgers, I wouldn't worry as much about the cc's as all the sharp edges.

[screamingchief]

Around 14cc or so,give or take depending on who's answering the question,some wil say more,others say less,but that's a good ballpark number to use for the valve relief CC's,especially seeing as those are the ones with the large chamfer around the circ. of the piston.

What you also have to account for is the fact that they also typically sit down in the hole like .030" or more (which is a BUNCH!) as that too kills the CR.

IMO those are the worst pistons ever available for a pontiac,whoever thought up that bright idea needs to be kicked in the nuts hard.

HTH

Bret P.

[72LuxuryLeMansLa.]

Originally Posted by screamingchief

Around 14cc or so,give or take depending on who's answering the question,some wil say more,others say less,but that's a good ballpark number to use for the valve relief CC's,especially seeing as those are the ones with the large chamfer around the circ. of the piston.

What you also have to account for is the fact that they also typically sit down in the hole like .030" or more (which is a BUNCH!) as that too kills the CR.

IMO those are the worst pistons ever available for a pontiac,whoever thought up that bright idea needs to be kicked in the nuts hard.

HTH

Bret P.

Who ever designed those had no nuts!

Karl

[gtofreek]

Those are what I call PINGERS! They drop your compression a couple points but yet make it more likely to detonate.

ScreamingChief said it correctly when he said they are the worst piston ever made for anything! Total crap and I talk anyone who wants to run them, out of them.

Karl, the person who designed them doesn't have any nuts because I done already kicked them up into his throat! You'll recognize him on the street because he's the guy with three Adams Apples!

[lust4speed]

Had a friend that I couldn't talk out of running them in his 70 GTO, and he had a pretty nice cam -- never did get out of the 16's.

[J GLASGO]

We have redone many motors that had those pistons,and they all had one thing in common. They were all slugs,and performed poorly.

[firebirdmi]

Thanks for all the replies. Right now the car runs very high 13's very low 14's.

For the short term I'm planning to put on a set of #62 72cc heads just to squeeze out a little more power (I just removed 1971 #96 heads). Long term I'll be doing a complete build up of a large c.i. motor.

[firebirdmi]

Regarding these pistons being known to cause pinging, is there anything I could do to the piston, like knocking down some of the sharp edges slightly, to reduce the possibility of detonation or is it not worth the time and just deal with it by adjusting the timing?

[Half-Inch Stud]

Always thought the Post #1 slugs looked generic, thus ugly. But, gots no technical basis to say they are non- optimal, ping, or are bad.

See, They might promote swirl, might help something that a good looking flat-top simply doesn't do.

However, me thinks the installed height is paramount to ANY slugset behavior. Of course the thorough detail on slug design, ring land detail, and material matters in fact.

Material Preference: Forged slugs having high silicon content for CTE control and strength.
Ring stack preference: Race; Chevy stack, Street/Strip Chevy stack or PMD stack with lathe cut in slug landing between #1 & #2 rings. Street; PMD stack

Compression; proud slug top vs deck, matched to NEVER contact the head for intended maximum adult-driven RPM. THEN pick Dish/ pocket cc's to compliment the CHOSEN EXPENSIVE HEAD (that flows).

[gtofreek]

Originally Posted by firebirdmi

Regarding these pistons being known to cause pinging, is there anything I could do to the piston, like knocking down some of the sharp edges slightly, to reduce the possibility of detonation or is it not worth the time and just deal with it by adjusting the timing?

The problem isn't necessarily all the valve reliefs and ridges[they don't help], but the fact that they sit so far down in the hole. Any piston needs to be near the top of the deck at TDC to help with detonation.

I only built one engine with those. It was a 67 400 in a GTO. I tried until I was blue in the face to talk him out of them and into some TRW's. He wouldn't budge. He wound up with a GTO that would barely run a 16 second 1/4 mile and it pinged like crazy with any timing in it. Finally, he saw the light and we put in TRW's. He raised his compression 1 1/2-2 points and it then ran a 13.99 1/4 and never pinged with optimal timing.

The thing with detonation is all the fuel needs to be burned by 14° ATDC. This is where max cylinder pressure occurs. If it is not all burned by then, whatever is left, detonates from the pressure. When the piston sits too far down in the hole, the flame front has a further distance to travel in order to complete the burn by 14°ATDC and just can't do it. So it detonates/pings. By putting the piston near the top of the cylinder, the flame front has a shorter distance to travel in said amount of time to complete the burn.

[gtofreek]

Originally Posted by Half-Inch Stud

See, They might promote swirl, might help something that a good looking flat-top simply doesn't do.

They really don't help anything but the engines ability to ping!

[PONTIAC-ONE]

Use as ashtrays...but then again most people don't smoke nowadays so that renders them useless.

[jamaca85]

the 62s should help alot.

[firebirdmi]

Thanks for the info everyone. I would love to start replacing the short block but I need to keep reminding myself to be patient. It would be easy to spend a bunch of cash and have someone build me a beast of a motor but first I should get some experience with swapping out parts and putting the info from this site (and Jim Hand's book) to work. Then after I learned a few things buy some big money parts and go after the real horsepower.

Mike

[gtofreek]

Originally Posted by PONTIAC-ONE

Use as ashtrays...but then again most people don't smoke nowadays so that renders them useless.

They could be used to hold E-cigarettes! Not quite useless yet.

[screamingchief]

Zero decking the shortblock would help alot.

But that's a pretty serious haircut just to fix the main problem with those pistons.
(IE: their $#!tty deck height)

Next best option,use some .027" thick Cometic headgaskets.

That moves the pistons slightly closer to the heads,much the same as decking does.

Just remember the Cometics require a specific RA finish for the deck & head surfaces.

And yeah,smaller chamber heads will help to overcome the added valve relief CC's.

Just remember that they do nothing to help the deck height ~ quench issues.

There's more to it than just the final CR you end up with,improving the overall quench dynamic is the important thing that you're striving for.

FWIW

Bret P.

[gtofreek]

Originally Posted by screamingchief

Zero decking the shortblock would help alot.

But that's a pretty serious haircut just to fix the main problem with those pistons.
(IE: their $#!tty deck height)

Next best option,use some .027" thick Cometic headgaskets.

That moves the pistons slightly closer to the heads,much the same as decking does.

Just remember the Cometics require a specific RA finish for the deck & head surfaces.

And yeah,smaller chamber heads will help to overcome the added valve relief CC's.

Just remember that they do nothing to help the deck height ~ quench issues.

There's more to it than just the final CR you end up with,improving the overall quench dynamic is the important thing that you're striving for.

FWIW

Bret P.

This is exactly why engines like 390 Fords and certain Dodge engines have pinging issues at 8:1 compression. The pistons sit an 1/8" down in the hole. Don't think they realized how important quench is back then.

[68WarDog]

Originally Posted by jamaca85

the 62s should help alot.

Won't help at all. 15/16 years ago, I mistakelnly tried that same remedy(stock 62's..03 400/ .031 compression gasket), MSD variable timing(attempted) with 750 holley roller coaster jetting. Still ran like a FAT PIG( couldn't keep up with wife's TR6). That engine now resides in a friend's barn and functions as a pigeon/mouse habitat.

[77 TRASHCAN]

Originally Posted by firebirdmi

Thanks for the info everyone. I would love to start replacing the short block but I need to keep reminding myself to be patient. It would be easy to spend a bunch of cash and have someone build me a beast of a motor but first I should get some experience with swapping out parts and putting the info from this site (and Jim Hand's book) to work. Then after I learned a few things buy some big money parts and go after the real horsepower.

Mike

What ever you build or buy, use forged pistons PLEASE, non of those hypereutetic jobbies, that some highly tout.