390 ?????
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Freshman User
Joined: Jan 2007
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From: Newport PA
390 ?????
Hello, I am building my 1st engine the numbers on the block have been ground off so I dont have a clue what the block is other than it is newer than 64, it has 4 mount holes. the crank is for a 390 ( 2U ) but the rods are 428 part #'s.Info from HP book the pistons from the machine shop are Fed.Mog. 381P .030. Can anyone tell me what the comp.ratio will be ? thanx Charter
Cargo Master
Joined: Aug 2000
Posts: 2,931
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From: SW Michigan
very low compression
Charter, the 390 and the 428 do in fact use the same connecting rod, so there is no problem there. And the 2U is a 390 crank. But those 381Ps are a very poor choice. The 381P is a copy of the 410 mercury piston Ford used in the 70s to lower compression in the pickup truck 390. If you build a 390 with those and use the FelPro 1020 head gaskets most of us do, ( .043 thick ) you'll end up with 7.6 to 1 static compression. That just isn't enough. Don't even think of using them. Not even if the shop gives them to you for free, not even if the shop pays you to take them ! Yes, they really are that bad. Depending on how much compression you had in mind, you could use a set of H304Ps or a set of H395Ps. How much compression are you looking for ? DinosaurFan, on the old work 'puter
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Freshman User
Joined: Jan 2007
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From: Newport PA
I'm not sure what comp.ratio will be best, the cam (comp cam 33-222-3 ) and a Edelbrock RPM Performer intake are going on.The machinist said it would be in the 8.5-9 to 1 range. I think he is getting felpro gasket set. I can say this what I hope to have is 10 MPG strong low end with a little top end. The pistons you listed which one would you use to reach my idea? I do know that tires and gears play a big role in MPG. I will be using this in a 68 F250 as often as possible but not a daily driver also I have a slide in camper and will tow my boat.
Needsmorecoffee.
Joined: Oct 2004
Posts: 3,624
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From: CT
Same rod number yes but some have larger dia bolts. C7AE-B's in came both sizes 3/8" and 13/32".
As for the block some did not have casting numbers. So it possible it was not ground unless you see grind marks where it was. LOL. Check the date code on the block.
As for the block some did not have casting numbers. So it possible it was not ground unless you see grind marks where it was. LOL. Check the date code on the block.
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Freshman User
Joined: Jan 2007
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From: Newport PA
I checked Summit,they have a piston that will give me around 9.5 comp.,what will they do for this engine? Will I be able to use 89 octane with this 390?
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Post Fiend
Joined: Jan 2003
Posts: 8,600
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From: Iowa
Summit doesn't have a clue. Neither does your machinist. You need to measure everything up, and calculate it yourself. Here's a calculator:
http://www.kb-silvolite.com/calc.php?action=comp
Here's some dimensions to get you started:
Deck height = 10.17"
Fel-pro gasket thickness and bore: .041" and 4.4"
Stroke = 3.784"
Rod length = 6.488"
381p piston compression height = 1.660"
Probable head CC = somewhere around 70
Plug it into the calculator and see what you really get.
http://www.kb-silvolite.com/calc.php?action=comp
Here's some dimensions to get you started:
Deck height = 10.17"
Fel-pro gasket thickness and bore: .041" and 4.4"
Stroke = 3.784"
Rod length = 6.488"
381p piston compression height = 1.660"
Probable head CC = somewhere around 70
Plug it into the calculator and see what you really get.
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Cargo Master
Joined: Aug 2000
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From: SW Michigan
pistons
Charter, if I were going to tow a boat and sometimes carry a piggyback camper, 9 1/2 would probably be too much for 89 fuel, but the H395P piston would get you there. For trailer pullin and camper carrying, the H304P will be less than 9 to one, and probably more low octane fuel freindly. The KB150 makes a nice work piston for trucks as well. DinosaurFan, @ work on lunch
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Freshman User
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From: Newport PA
Kurt, I put the #'s in along with my bore (4.080) and it did'nt work. It showed 1.359 as the ratio. My machinist is saying because I want MPG's and towing is why he recomended this piston and cam,however he does say the edel. RPM is too much that the Performer dual plane w/ a holley 650 vac.2nd dairies would work better what is your opion? Dino I will look into the other pistons.
Cargo Master
Joined: Feb 2004
Posts: 2,076
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From: Central Texas
I went with the KB 150 piston.
In order for Summit to estimate comp ratio, or anyone else for that matter, they have to know your combustion chamber volume. Truck heads usually have bigger chambers than car heads. Which one does Summit and others base the advertised compression from? You don't know. So you have to calculate it yourself, and it isn't hard to do. I'd stay at 9:1 or less for what you want.
In order for Summit to estimate comp ratio, or anyone else for that matter, they have to know your combustion chamber volume. Truck heads usually have bigger chambers than car heads. Which one does Summit and others base the advertised compression from? You don't know. So you have to calculate it yourself, and it isn't hard to do. I'd stay at 9:1 or less for what you want.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
Originally Posted by FFR428
Same rod number yes but some have larger dia bolts. C7AE-B's in came both sizes 3/8" and 13/32".
As for the block some did not have casting numbers. So it possible it was not ground unless you see grind marks where it was. LOL. Check the date code on the block.
As for the block some did not have casting numbers. So it possible it was not ground unless you see grind marks where it was. LOL. Check the date code on the block.
Hotshot
Joined: May 2003
Posts: 11,141
Likes: 25
From: south louisiana
If building a healthy 390 along with a good sized (long duration/overlap) cam, you really need to stick to the 66-70 4 bbl 390 flat tops for the passenger cars (not pickup truck). These will give you a 10 to 1 ratio with C8/D2 heads, slightly less with earlier or LR/CJ heads.