Building a low compression 390?
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From: Pittsburgh
Building a low compression 390?
I'm looking to build a 390 from the ground up for my Thunderbird. Deal is, I'm looking to go forced induction on it, so the motor is going to need to be of a decently low compression ratio. So far I'm thinking 8 or 8.5:1.
My question is how did the factory go about doing this? I know the last of the 390's were only 8.6:1. Did they run a larger chamber, alot of deck, or a super deep dish?
I'm hoping to be able to get that low with off the shelf pistons, preferably designed for the 390. I see how I could use 351C or W pistons if I rebushed my rods.
So basically how did the factory get the 390 down to 8.6 to one and what is the most effective way to get a 390 down to that CR?
Justin
My question is how did the factory go about doing this? I know the last of the 390's were only 8.6:1. Did they run a larger chamber, alot of deck, or a super deep dish?
I'm hoping to be able to get that low with off the shelf pistons, preferably designed for the 390. I see how I could use 351C or W pistons if I rebushed my rods.
So basically how did the factory get the 390 down to 8.6 to one and what is the most effective way to get a 390 down to that CR?
Justin
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From: Iowa
The factory used 410 pistons that were .112" below deck. They had a nice quench type head, but with .112" deck clearance, it might as well have been an open chamber.
You probably want forged pistons for a forced induction motor, right? If so, they'll probably need to be custom. I would suggest a step dish design similar to the KB150. If you dont need forged, the KB150 and a larger combustion chamber may be enough.
You probably want forged pistons for a forced induction motor, right? If so, they'll probably need to be custom. I would suggest a step dish design similar to the KB150. If you dont need forged, the KB150 and a larger combustion chamber may be enough.
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From: Pittsburgh
Summit has Ross forged pistons for a 410 for 569 a set. I did some figuring and with the flat top i'd be right between 8.5 and 9:1 depending on what the deck clearance actually wound up being.
Sounds like my best bet for price and availabilty.
Justin
Sounds like my best bet for price and availabilty.
Justin
Last edited by hoxiii; Apr 25, 2007 at 06:18 PM.
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From: Iowa
But then you lose all the advantages of having a quench head. You'd have to run more timing, *even less* compression, etc.
What you might do, is some head work and open up the chambers a bit. Unshroud the valves, etc. This will increase combustion chamber volume. Then maybe cut a dish into a set of TRW L2291's?
What you might do, is some head work and open up the chambers a bit. Unshroud the valves, etc. This will increase combustion chamber volume. Then maybe cut a dish into a set of TRW L2291's?
Last edited by rusty70f100; Apr 25, 2007 at 07:23 PM.
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i realize it loses the quench zone, but under boost is it really that necessary to maintain? The other turbo motor i've built maintained a quench zone, but that's because i did it on an aircooled VW so it's really easy to get all kinds of different deck heights using stock cylinder sets.
Otherwise i figured that if i ran stock 68cc heads I would need a 390 piston with a 38cc dish. Even if i reworked the head a bit and hit 72, it would still take a 34cc dish.
Can i easily get a 34cc dish out of a L2291? Depending on how thick the head of the piston is it might cause an issue.
Justin
Otherwise i figured that if i ran stock 68cc heads I would need a 390 piston with a 38cc dish. Even if i reworked the head a bit and hit 72, it would still take a 34cc dish.
Can i easily get a 34cc dish out of a L2291? Depending on how thick the head of the piston is it might cause an issue.
Justin
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Shouldn't some of the later model engines 72, 73 and later have low compression pistons? Maybe their reduction in compression was due to the heads. I don't know. Its worth the research. Plus, you might be lending some thought as to the cam that will best suit the forced induction with your ideas on compression too. JMC Motorsports (John's Mustangs and Classics) out of San Diego does a lot of FE block building. It might be a worthwhile phone call.
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From: Pittsburgh
Rusty already said the factory used 410 pistons which comes out right at 8.6 compression when i run the numbers, which is what the 76 390's ran.
68cc heads and 410 pistons work out just right.
Anyway, i'm still looking and trying to find something that'll work. It's not like i need pistons tomorrow.
But somethign else i thought about are 391 pistons. I know the 391 ran 7 or 7.5:1 with dished pistons, so anyone happen to know if 391 pistons are cast or forged? It's easier to bring compression up a point than down a point while watching quench.
Justin
68cc heads and 410 pistons work out just right.
Anyway, i'm still looking and trying to find something that'll work. It's not like i need pistons tomorrow.
But somethign else i thought about are 391 pistons. I know the 391 ran 7 or 7.5:1 with dished pistons, so anyone happen to know if 391 pistons are cast or forged? It's easier to bring compression up a point than down a point while watching quench.
Justin
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From: Iowa
FT pistons are cast, and also quite heavy.
This is why it's not easy. Certain other members were able to cut quite a bit off the top of a 2291; I'm confident it could take a dish.
But then, figure up what it would cost to have that done, and compare that to a set of custom pistons.
And yes, quench will help. Forced induction is not an excuse to throw all engine building theory out the window. It's still subject to pinging, probably even more so than a normal engine. Quench should be priority #1.
Headers will still help a lot, more power will still be made with better flowing heads, etc.
This is why it's not easy. Certain other members were able to cut quite a bit off the top of a 2291; I'm confident it could take a dish.
But then, figure up what it would cost to have that done, and compare that to a set of custom pistons.
And yes, quench will help. Forced induction is not an excuse to throw all engine building theory out the window. It's still subject to pinging, probably even more so than a normal engine. Quench should be priority #1.
Headers will still help a lot, more power will still be made with better flowing heads, etc.
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Post Fiend
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From: "Islander"
C4AE-G heads will be 75 plus cc's, I've seen them at 80cc's plus you'll have the larger ports.
What about KB 150 pistons and lower the piston top plus make the dish larger in diameter leaving the quench step area alone?
What about KB 150 pistons and lower the piston top plus make the dish larger in diameter leaving the quench step area alone?
Post Fiend
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From: Iowa
391 pistons were actually interesting units, with an extremely large dish, but normal compression heights. See here:
http://webpages.charter.net/beckracing/slvpg51.htm
Note the listed compression ratio. That's with FT heads though. Also note that some of 'em have an iron top groove.
If you dont need forged pistons, and are going to keep it down to 5000 (maybe 5500) rpm, FT pistons might work.
http://webpages.charter.net/beckracing/slvpg51.htm
Note the listed compression ratio. That's with FT heads though. Also note that some of 'em have an iron top groove.
If you dont need forged pistons, and are going to keep it down to 5000 (maybe 5500) rpm, FT pistons might work.
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Allright, I've never gone through and ordered a set of full custom pistons, so I have a quick question. I know I can order them with however deep or shallow of a dish I want, but is there a manufacturer out there that will cut the dish to the exact shape I want, like a mirror dish?
If not I'd be better off buying a set of off the shelf TRW flat tops and then send them off to a shop with autocad files of how i wanted the dishes cut.
Justin
If not I'd be better off buying a set of off the shelf TRW flat tops and then send them off to a shop with autocad files of how i wanted the dishes cut.
Justin