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383 mild street build

Brett350

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In the middle of a tear down on my 383 after breaking a skirt on cylinder 4. Found out piston selection is pretty scarce for the 383. Ended up ordering the kb400s which is a 6cc dome to try and bump compression. This is what I plan on running:
1967 383 bored 30 over
Edelbrock Performer rpm intake
915 heads 78cc chambers 2.08/1.60 valves
Lunati flat tappet 226/234 @ .050 .494/.513 lift
Kb400 pistons
Had crank turned and rods resized.
Block has not been decked

Was shooting for basically 9.5-1 compression or slightly more if I can get away with it while still running 91 octane.
Seems the compression height of these pistons are lower then factory at 1.908 so taking that into account I'm hoping when I get them mocked up in the block that I will be sitting somewhere under 10 to 1. I ran numbers on a compression ratio calculator and I figure I should be somewhere close but not sure if I might be running the risk of detonation. Not opposed to running different heads or pistons if I really had to but this is what I have and I'm hoping to make this work. Any thoughts? Thanks in advance.

20191111_140911.jpg 20191214_164359.jpg
 
Sounds good, I'd drill out the exhaust valves for a 1.74 to help get the bad air out. Good Luck
 
You might be better to use a flat piston and retain the original compression height.
The quench area is an important factor with the old style combustion chambers.
A larger quench area can promote detonation.
I would say if the compression ratio was the same but quench area increased you might lose power especially with todays modern fuels. If you have to retard the timing to stop detonation the power will definitely be down.
 
Sounds good, I'd drill out the exhaust valves for a 1.74 to help get the bad air out. Good Luck
Thought of that but I might go to aluminum heads later anyway. Already spending a bit on machine work. Heads are good otherwise. Hoping the longer duration on the exhaust side will help a bit.
 
You might be better to use a flat piston and retain the original compression height.
The quench area is an important factor with the old style combustion chambers.
A larger quench area can promote detonation.
I would say if the compression ratio was the same but quench area increased you might lose power especially with todays modern fuels. If you have to retard the timing to stop detonation the power will definitely be down.
I guess I'll have to see how far down in the bore they are sitting and determine the actual compression ratio and factor in quench area to decide if it will work with pump gas.
 
I guess I'll have to see how far down in the bore they are sitting and determine the actual compression ratio and factor in quench area to decide if it will work with pump gas.


That piston will be about 0.025 - 0.030 down in an uncut block.

Buying a piston for a closed chamber iron today will not be the best choice for an aftermarket aluminum head later. The head work is going to cost a bunch anyways to do the 915's right.
 
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I ordered custom pistons for my 340 so I could get things were I wanted them.
It actually made things easier and I did not have much machining.
I am not a fan of carving 20 or 30 thousands off the block it affects the fit of so many other things - I like that deck surface to be as thick as possible.
 
I run the flat top forged Speed Pros that you can get from Summit/Jegs block is decked to get the piston out .004 and using a .039 gasket for a .035 quench. Runs on 93 octane without problems. My combo is pretty close to what you listed above outside of my 1.74 exhaust valve in my 915s and alittle more lift and duration on a Hyd Roller. Works well, Headers, 750, 3.55s in a 69 Super Bee ran 7.98 @86.14mph 1/8mile the only time I had it out. Im going racing April 18 for Mopars at the Rock in North Carolina and hope to make some improvements there. Good luck on the build!
 
IMO, the expense on doing 915's right may or may not be worth it, depending on when you will do the upgrade to aluminum. The piston/CR considerations are pretty different, especially to use pump gas. 915's are a good head, they really want the larger exhaust valve(1.74, maybe even 1.81 with notched cylinders). BTW you need to MEASURE everything to get a true CR. Deck, actual chamber volume(they very a ton). Basic pocket porting gives big power improvements with no downside. Your cam choice looks pretty good.
 
At the end of the day the hypereutectic pistons are still cast pistons. For a little more why not do forged. I see a lot of people going with hypeutectic...I have run them when it was my ONLY choice. I would try returning the kb400s and look at getting these...
https://uempistons.com/i-21959-icon...cc-dome-fits-indy-edelbrock-and-oe-heads.html

Couldn't open the link. But if those are the 687's, that's what I was referring to above. Probably the right piston for an 80 cc aluminum head. Might not work with the 915 heads and pump gas
 
Ended up ordering the kb400s which is a 6cc dome to try and bump compression.

I’m curious if that box of pistons in the pic says they’re a 6cc dome, and whether or not those are current stock.

Both my old 2016 catalog and the on line catalog show the KB400 to have a net 0cc effective dome volume.

D89CEB94-5CB8-4AC8-9303-2FB261343919.png
 
I’m curious if that box of pistons in the pic says they’re a 6cc dome, and whether or not those are current stock.

Both my old 2016 catalog and the on line catalog show the KB400 to have a net 0cc effective dome volume.

View attachment 894210


Noticed that. Maybe the dome is 6 cc, and the valves reliefs are -6 cc, netting zero.
 
From what I understood the effective volume of the valve reliefs and dome equals 0cc. The dome itself is 6cc
 
If your dome and valve reliefs basically cancel each other out, you should be close to what my tossed together 383 is. As I recall, my flat tops were .020 on the hole, and I ran a .039 gasket. My heads were closed chamber 516's, but had nail head valves which reduces CCs a bit, also had the 1.74 exhaust and a fair bit of professional porting. The bottom end was pure junk, including the Summit .465/.488' cam. It made "0" power past 5,000 rpms, but still ran a 12.3 on its 3rd pass in a 67 Barracuda, which calculated to about 400 HP at the crank.

My compression I believe was 9.96/1 static, yours should be close to that. My combo was a disaster for some reason, you have a much better cam, same intake. I'd spend some time on the heads or go E street, they are about $1,100, and flow better than most ported iron. I'd expect about 450 HP from your combo with good headers. Just my 2 cents.
 
You might be better to use a flat piston and retain the original compression height.
The quench area is an important factor with the old style combustion chambers.
A larger quench area can promote detonation.
I would say if the compression ratio was the same but quench area increased you might lose power especially with todays modern fuels. If you have to retard the timing to stop detonation the power will definitely be down.
Very good point.
 
Very good point.

There are no original compression height pistons that are available for the 383.

The pistons that are worth considering are all pretty close to one another in compression height. Pick what works best for your situation , and cut the block to get it where it needs to be.
 
If your dome and valve reliefs basically cancel each other out, you should be close to what my tossed together 383 is. As I recall, my flat tops were .020 on the hole, and I ran a .039 gasket. My heads were closed chamber 516's, but had nail head valves which reduces CCs a bit, also had the 1.74 exhaust and a fair bit of professional porting. The bottom end was pure junk, including the Summit .465/.488' cam. It made "0" power past 5,000 rpms, but still ran a 12.3 on its 3rd pass in a 67 Barracuda, which calculated to about 400 HP at the crank.

My compression I believe was 9.96/1 static, yours should be close to that. My combo was a disaster for some reason, you have a much better cam, same intake. I'd spend some time on the heads or go E street, they are about $1,100, and flow better than most ported iron. I'd expect about 450 HP from your combo with good headers. Just my 2 cents.
Any idea which flat tops you were using? Did they have valve reliefs?
 
Any idea which flat tops you were using? Did they have valve reliefs?

Old TRW no valve relief forged, .030" over. I talked to a fellow that had the same piston to deck clearance as me, same gasket, same valve size. He ran the old 292 509 Purple cam, and had sufficient clearance, but not by a ton. I had intentions of using the MP 528 solid, which should actually have better clearance, but never got around to swapping it in. If your cam has .500" lift or more, I'd really suggest checking clearance with a no valve relief piston.
 
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