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Discussion Starter #1
Trying to calculate the compression on our recently acquired '69 GTO. Freshening up the engine. It has OEM 48 heads..and 8 valve relief pistons. I don't see any specs anywhere that tell me what the volume of the valve relief cuts are. Has anyone cc'd their pistons of know what that number is? Thanks...Stan M.

...also. I'm considering a Voodoo cam (ends in 702). It's a 4 speed with 3:55 gear. Thoughts?
 

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Discussion Starter #4
Thanks OMT...
I wondered about the lack of replies. I thought these were OEM pistons. What does everyone use with #48 72 CC heads.
Head and piston options other than stock are expensive, especially for a stockish street build. I'm fixing this car for my wife...she had one when she was in her early 20's. So it needs to be street friendly. She is the equivalent of the little 'ole lady from Pasadena...and can still hit a gear with the best of them at 66 years old...but we have a 454 SS Chevelle that isn't too streetable, and just want this one to be a more or less stock, and I'm installing Vintage Air as we speak. She'll probably park her Jeep and drive this thing often..lol.
It's a very nice cusom build (18 years ago, nothing but weekends and parades since) Platinum grey with ghost flames and Weld Racing wheels. The engine was over cammed but other wise runs pretty good...so I thought I would freshen it up while were recamming it. (notice cam question above...).
I would be very interested in knowing what these engines compression is in stock form. ..but cant seem to find that either. Lars and others suggest factory specs are higher than reality.
Thank for any information or suggestions for this project
Stan...
 

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Discussion Starter #5
....oh, according to a question asked by a customer on Summit's web site..Summit says theses pistons have 15 cc's valve relief volume.
 

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My guess would have been pretty close - 14 cc's, but it was just a guess. The typical pistons used have two valve reliefs. The typical relief volume is 7 cc's. Advertised stock compression on your car was 10.75:1, but measured would be closer to 10.25:1. However, if you change two stock type pistons you will be back to compression too high for today's gas. I think it's worth the money to spec your pistons for the CR that you want.
 

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Discussion Starter #7
Thanks for the reply Taylor, most appreciated.
The build sheet when this engine was overhauled years ago..say OEM Sealed power .020. ( part # 411P .020). So I assumed they came that way stock (8VR) only standard bore?
Did these engine come stock with flat top/2 relief pistons? This engine has .445 deck cl., which I don't like much...with a .040 head gasket. Looks like the block would need to be milled to have any quench /or flame travel to amount to anything.
Wish I had some idea of what pistons I need with these heads...I'd just have the block work done and order my parts ..and assemble it myself. I was a garage owner from the early 70's to late 80's, and have plenty old equipment/tools. Machine work these days cost more than it should imho, or more than I wanted to spend on a relative low performance stock type motor for crusing.
Thanks...Stan
 

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"Did these engine come stock with flat top/2 relief pistons?" Yes. Why not just freshen up your existing combination? My guess is that the pistons used were to drop the compression in an inexpensive manner? It's also possible that they put in a larger cam to reduce the dynamic CR.

Your deck clearance of 0.045 is fine as long as the engine wasn't pinging (knocking). I'm assuming that is the total deck clearance counting the gasket thickness. If you go to true replacement pistons your compression will be too high. I paid "the big bucks" to have special pistons made to get my compression where I wanted, and to keep a tight quench. My quench is 0.038 (IIRC), and my piston relief volume is 20 cc's.

DSCN4172-Ross small.jpg
 

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Discussion Starter #10
Ok ...you said it did come with 2 vr flat tops...thanks for clarifying that ! I couldn't find that out anywhere!

The piston to deck clearance on this engine is .045 by itself...it's really down in cylinder .045 ! ...then the head gasket is another .040 to boot. The build sheet has the part number for the head gasket which I looked up.. the specs are (.040) crushed.

What heads did you use to get the .038 quench with your custom pistons?
About what did the pistons cost, where'd you get them...and did you buy rods other than stock?
Thanks again for the repies...
Stan..
 

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OMT please correct me if I'm wrong but the way it was explained to me is that the piston with 8 valve reliefs is a generic design. It can be used for late or early heads and can be installed in any direction. 2 valve relief pistons must be used with the intended head and usually have an arrow to point the direction of installation.
Another reason the 8 vr pistons aren't used much is that they have sharp ridges across the piston face, These ridges tend to cause early detonation.

I'm using Icon forged pistons with a 14cc relief on my 400 build. Most recommend using forged rods but if the car is intended for just cruising the cast rods will do fine if new rod bolts are used and the rods are thoroughly checked out.
 

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OMT please correct me if I'm wrong but the way it was explained to me is that the piston with 8 valve reliefs is a generic design. It can be used for late or early heads and can be installed in any direction. 2 valve relief pistons must be used with the intended head and usually have an arrow to point the direction of installation.
Another reason the 8 vr pistons aren't used much is that they have sharp ridges across the piston face, These ridges tend to cause early detonation.

I'm using Icon forged pistons with a 14cc relief on my 400 build. Most recommend using forged rods but if the car is intended for just cruising the cast rods will do fine if new rod bolts are used and the rods are thoroughly checked out.
That is correct. If you put in new rod bolts, make sure to have the rods resized.
 

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Ok ...you said it did come with 2 vr flat tops...thanks for clarifying that ! I couldn't find that out anywhere!

The piston to deck clearance on this engine is .045 by itself...it's really down in cylinder .045 ! ...then the head gasket is another .040 to boot. The build sheet has the part number for the head gasket which I looked up.. the specs are (.040) crushed.

What heads did you use to get the .038 quench with your custom pistons?
About what did the pistons cost, where'd you get them...and did you buy rods other than stock?
Thanks again for the repies...
Stan..
To put it bluntly, your quench is terrible. You need to get to about 0 deck height. If you order new pistons then you can specify the dish and the pin height to get to zero deck height. Otherwise you've got to deck the block. Custom pistons like that would probably be about $800, but in the long run you will forget about the cost because you got it exactly the way you wanted. Otherwise just do a cleanup on what you have - however, I'm surprised you don't have a big time pinging problem.
 

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Discussion Starter #14
It may have a big time pinging problem...lol! ...and yeah, like I said I've "never" seen an engine with the piston so deep in the cylinder..never? (and I'm 72) and had a lot of cars.

I agree on needing pistons..but it doesn't stop there if "only forged pistons" are available, forged rods follow that and , and ...so on. I'd like to just get some cast aluminum dished pistons, and build a nice little more of less stock engine...but there's not any cast pistons "that I can find" for one of these engines that will provide the deck cl., and compression needed for today's fuel.

I was thinking I'm just not looking in the right places...but maybe there is no way to build a stock cast motor "right". My other fun cars have forged internals, lopey cams and excellent heads...but was to be driven more often .

If I have to buy forged pistons, I probably couldn't stop there ..I'd have to do the Edlebrock heads or something and get forged rods and probably a roller cam too. I'm kinda anal that way...it's a sickness lol! The idea was to just build a good stock engine and give the car to my wife.

So..It's all cast ..or all forged. Bummer! (or that's what w e used to say 50 years ago..lol)

Regarding pinging..It's a beautiful car ...but the cam was so ratty I only drove it a mile or two!
It had a cam where the power band cam alive in third and then car got real squirrely, with lotsa wheel hop.(easy fix), so if it was pinging I couldn't have heard it.

It had one of the Edelbrock AFB clone carbs with an chinese single plane intake. I plan on using a Performer RPM intake with a Holley 770 Street Avenger with vac advance...or a Quadrajet. Already have one of each. The Holley is new, and Q-jet is one I built last year for a 427 chevy.
Camshaft..well maybe a Lunati 10510702, with a lsa of 112. (I appreciate any thoughts on this cam too).
It's a 4 speed car with 3:55's and I'm installing Vintage a/c as we speak.
Whoever built the car, knew what he was doing...then someone else tried to make a hot rod out of a low compression stock type engine. I've seen it happen several times...mismatched everything engine wise with a 10K paint job...and stellar attention to detail.

I say all the above ...for opinions on what is the "best" way to build this engine for "the intended use". After all the chips are in ..there is only "one" all around best! lol...

Many thanks guys...Stan
 

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Factory pistons are indeed 4-valve reliefs so they could be installed on either left or right bank. 2-valve relief pistons woukd require 4 pistons for the left bank and 4 pistons for the right bank. They will have some form of indication, ie a dimple/notch or 'F" for front. The 2 valve reliefs simply reduce a few cc's as compared to factory pistons, and 2 cc's would indicate aftermarket forged pistons.

For a smooth daily driver, you want 9.0 compression. Pontiac factory pistons are typically .015"-.020" down in the bore. .045" isn't optimum. Add the .040" head gasket and quench/squish is out the door. A tight quench/squish area helps in the area of detonation and can aid in a cooling effect for the cylinder temps. I like to see .040" - .045" quench.

#48 heads, will be around 72 cc's and generally a couple cc's more.

The only way to get the compression down will be with custom piston having the cc's required to get the compression down. Most likely you'll want a dished piston.

And as you say, it doesn't stop there once you begin to do a rebuild and do it right. The engine may already have forged rods, but you won't know until you tear into it.
 

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Discussion Starter #16
Thanks Jim...
It looks like it has OEM rods with arp bolts if the last build sheet is right. So far it has been spot on.

The assembly was balanced too...but I can't imagine a machine shop building and engine with that much deck clearance unless the customer insisted on it and the low compression 8 vr Sealed Power pistons?

Scat has a couple rotating assemblies that might save some dough...but I'm not sure how to read the specs to figure compression. They give rod length...but I'm unsure how to determine how deep in the block the pistons are (f
or deck clearance).
I like Scat stuff better than eagle, they are generally lighter...have built two chevy strokers in the past that worked out well.

Has anyone used Scat /Summit stocking rotating assembly kits that the compression worked out well without losing 5 weeks for custom piston delivery (or more)?
...or has anyone used rotating assembly Pontiac 400 kits at all...?

Thanks...Stan
 

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Thanks Jim...
It looks like it has OEM rods with arp bolts if the last build sheet is right. So far it has been spot on.

The assembly was balanced too...but I can't imagine a machine shop building and engine with that much deck clearance unless the customer insisted on it and the low compression 8 vr Sealed Power pistons?

Scat has a couple rotating assemblies that might save some dough...but I'm not sure how to read the specs to figure compression. They give rod length...but I'm unsure how to determine how deep in the block the pistons are (f
or deck clearance).
I like Scat stuff better than eagle, they are generally lighter...have built two chevy strokers in the past that worked out well.

Has anyone used Scat /Summit stocking rotating assembly kits that the compression worked out well without losing 5 weeks for custom piston delivery (or more)?
...or has anyone used rotating assembly Pontiac 400 kits at all...?

Thanks...Stan
Butler Pontiac is where you may want to get a complete rotating 461 stroker kit. They will guide you along with part selections so everything matches. Not sure how long any parts take with todays screwed-up nation. How old did you say you were????? LOL
 

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Call Roy Johnson . He has 3 warehouses full of rebuilder inventory.
I got some stock 400 forged pistons . NOS . 1.714 Compression height . TRW IIRC

Price was good.


517-321-6460
 

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If you wish to keep the engine as a stock build I recommend the 068 cam. This was the factory cam for the 68 HO engine and is an excellent overall cam without going crazy with modified heads or special valve springs. Makes good power with the factory heads, intake and QJ. I paid $95 for a Melling SPC7 (068 clone) on line with O'Reillys.
 

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Discussion Starter #20
Butler Pontiac is where you may want to get a complete rotating 461 stroker kit. They will guide you along with part selections so everything matches. Not sure how long any parts take with todays screwed-up nation. How old did you say you were????? LOL
Ha haaa ha !! ...good one Jim !...I'm 72 and still going pretty strong. But every once in a while ...I can tell I better get busy of I want to finish all my projects.

I talked to Butler about a cam last week...and you may be right on parts delivery taking some time. They don't know when they will get Johnson lifters, if I ordered any with a cam. I imagine it'
s sorta the same with engine kits "especially " if it requires "custom" pistons.
 
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