Question about 383 pistons

[markj]

I wanted to ask some advice about the engine in my 1966 Coronet 500. I rebuilt the 383 a 3 years ago and put new .030 oversized cast pistons in that the machine shop provided with the rebuild kit. Not having put an engine together for many years didn’t realize that they are down below the deck about .060 of an inch when I had the heads off last summer. I put Mopar shim head gaskets in to raise compression some rather than putting Fel pros back on it. It’s got rebuilt 906 heads, 1 7/8 in Hooker Comp headers, Weiand dual plane Action plus intake and 750 Edelbrock carburetor. Runs good but would it be worth all time and money to pull it back out and put pistons with a higher compression height? How much more power would I gain if the pistons were close to even with the deck. The machine shop surfaced the deck and heads. I’m not sure exactly how much. I have open 8 3/4 rear end w 3.23s.

 

[Lefty71]

Ummm-K!

 

[1 Wild R/T]

interesting question....

 

[Charger Dana]

I’d say: probably.

 

[1 Wild R/T]

I’d say: probably.

Good Answer.... LOL..

 

[Bobby Sixkiller]

What question?

 

[Lefty71]

Ummm-K!

Sorry to be a smartass.... but there was no question when I first saw your post. My fellow smartasses will have to re-read as well.
So, the important thing is that your pistons develop the proper quench. This is key to power and efficiency in a big block Mopar. I personally dont get too hung up over the actual compression. but we may need to know a little more about your car and it's intended use. Also, if you have build details, and maybe some paperwork and measurements from the motor builder, we should be able to make some educated guesses as to what you have now, and whether it's really worth pulling back apart. How does it run now, By the way?? And are these flat top pistons, no dish? Ever go to the track for any kind of baseline?

It looks to me like (depending on your budget) you would definately gain from a closed chamber head, and get the quench much closer to ideal without any risk of clearance issues to the valves. Compression would likely still be good for pump gas, even if you used one of the super thin Cometic head gaskets and iron heads (or aluminum for better flow) would be the cheap way out for a dual purpose car. And you wouldnt have to tear the whole thing out/down.

 

[1 Wild R/T]

With 906 heads & pistons .060 in the hole you have no quench & about 7.8-1 C/R

With the pistons .060 in the hole you could bolt 516 or 915 heads on with steel shim head gaskets & still not have any quench... But your C/R would be to 8.8... A little milling & 9.3 would be safe & easy...

To gain quench you either need taller pistons, longer rods or a shorter block....

Personally, I'd grab a set of 516 heads, mill them .030 use a steel shim gasket & live with it...

 

[markj]

Sorry to be a smartass.... but there was no question when I first saw your post. My fellow smartasses will have to re-read as well.
So, the important thing is that your pistons develop the proper quench. This is key to power and efficiency in a big block Mopar. I personally dont get too hung up over the actual compression. but we may need to know a little more about your car and it's intended use. Also, if you have build details, and maybe some paperwork and measurements from the motor builder, we should be able to make some educated guesses as to what you have now, and whether it's really worth pulling back apart. How does it run now, By the way?? And are these flat top pistons, no dish? Ever go to the track for any kind of baseline?

No problem on the smartass I wanted to show a few pics but accidentally sent it before i could write my questions. Anyway it runs good. Just a street cruiser. It has a mild Comp cams XE 262H hydraulic. Electronic ignition now. They are flat tops not dish. I would have to go back try and find paperwork from machine shop for details. It has 727 that i also had apart put in new friction rings and steels as I burned it up coming home from work a year ago. Considered putting maybe 3.73s to get better take off i don’t drive it too far. Just play around with it close to the house.

 

[Jerry Hall]

I think you would gain significant performance with closed chamber heads with 2.14 1.81 valves and a little port work, with thin head gaskets. If you want a little more my preference would be a Mopar Performance 509 cam & lifter set. I see no need to do anything to the bottom end.

 

[markj]

Thanks much for the replies. Sounds like good advice to go with closed chamber heads. I’ll keep a look out for some 516s
I think for just a cruiser i’ll leave the pistons in it. It sounds great have glass paks on it. I’m not wanting to pull the engine back out. Maybe build another big block for the car at some point.

 

[Jerry Hall]

Thanks much for the replies. Sounds like good advice to go with closed chamber heads. I’ll keep a look out for some 516s
I think for just a cruiser i’ll leave the pistons in it. It sounds great have glass paks on it. I’m not wanting to pull the engine back out. Maybe build another big block for the car at some point.

516s shouldn't be very pricey, 915s IMO are a better choice, but still won't break the bank. Also IMO the 383 is a very under rated engine and good for a lot more RPMs before coming unglued.

 

[1 Wild R/T]

516s shouldn't be very pricey, 915s IMO are a better choice, but still won't break the bank. Also IMO the 383 is a very under rated engine and good for a lot more RPMs before coming unglued.

I have 915's on my 383 & I have a set for a 440 I'm building... They are getting pricy, if you don't scrounge around they can be very pricy but even with scrounging these days everyone knows what they are & demand a premium...

516's are cheap & unless you want to spin past 6K the 516's are gonna have better velocity so they will feel stronger down low...

 

[markj]

I have 915's on my 383 & I have a set for a 440 I'm building... They are getting pricy, if you don't scrounge around they can be very pricy but even with scrounging these days everyone knows what they are & demand a premium...

516's are cheap & unless you want to spin past 6K the 516's are gonna have better velocity so they will feel stronger down low...

Sounds good. Would you consider using new aluminum like some edelbrocks or stick with iron Mopar

 

[Jerry Hall]

I have 915's on my 383 & I have a set for a 440 I'm building... They are getting pricy, if you don't scrounge around they can be very pricy but even with scrounging these days everyone knows what they are & demand a premium...

516's are cheap & unless you want to spin past 6K the 516's are gonna have better velocity so they will feel stronger down low...

I agree, but the last 383 I built, I was shifting at 7K on the track. It was a bit more radical than what I recommended to the OP. It was making peak HP at 7K and only dropped 25 HP between there and 8K. That's been 20 years ago, maybe more and a lot has changed in that time. You guys are talking things today that were never brought up when I was seriously involved at the track.

 

[Jerry Hall]

Sounds good. Would you consider using new aluminum like some edelbrocks or stick with iron Mopar

Depends on your budget. Iron Mopar is all I have ever used, but I see a great following for the aluminum heads here. I'm old school and aluminum heads were just starting when I changed from racing to collecting. I'm sure there are plenty of answers for you on here.

 

[1 Wild R/T]

Sounds good. Would you consider using new aluminum like some edelbrocks or stick with iron Mopar

Thing about going to Aluminum is you can't/shouldn't use a steel shim gasket, guys have done it but it leaves brindling damage to the head where the gasket is embossed...

And that composition head gasket costs .3 in C/R...

Other than that, yeah Aluminum is great....

 

[Paul Secoy]

Edelbrock E Street heads are available in a 75 CC version with a more modern chamber design, and are about $1,100. You will have most or all of that in a set of 516's with bigger valves and even bowl port work done by a pro.

The last 383 I did was .030", flat tops with no valve reliefs, .020" in the hole, 516 heads milled .010" with the 1.74" exhaust valve added, (nail head style which reduced chamber volume), and with a Fel-Pro gasket, it was still under 10-1 compression.

I think you would notice a fair difference if you could raise your compression. I would not want to tear into a fresh bottom end either, and a milled set of small chamber heads with a thin gasket is about your only choice.

Good luck, nice car by the way.

 

[markj]

I’m on a fairly tight budget.I will be putting some money aside this winter for the heads hopefully in the spring i can get it done. You definitely have given lots of good info. Think i like the idea of 516s milled .030 with shim. It’s a toy never run on a track Just thrilled to get it out on the street Thanks again for the replies.

 

[1 Wild R/T]

Just know that when you cut the head .030 you need to cut the intake .037 and you should measure as there is a good chance you will need shorter pushrods