390 piston compression boost?
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390 piston compression boost?
Im contemplating a turbo build on my efi 300l6. It's gonna run about 10-15 psi with a intercooler I hope. Wiil do a rebuild before I do it thought about the 390 pistons, I cant really find a approximate compression ratio when Used on 300 rods. Dont want to pay for custom forged 300. Any advice will help to help it withstand a turbo. Will end up running a aftermarket ecu to tune it properly
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Hello 300 fan. I'm not completely sure I understand what it is you're asking.
From my experience, it's best to look at what compression ratio you want and then work backward from there.
What CCs are the chambers in your head after its rebuilt?
How much are you boring it over? (The more bore, the higher the compression).
How much are you planning on shaving down the block?
Which head gasket? (thickness/diameter?)
Then, you can calculate what CCs you need the pistons to be to reach your desired compression ratio. Generally, turbo builds require lower compression, so you may be perfectly fine with stock 300 pistons.
From my experience, it's best to look at what compression ratio you want and then work backward from there.
What CCs are the chambers in your head after its rebuilt?
How much are you boring it over? (The more bore, the higher the compression).
How much are you planning on shaving down the block?
Which head gasket? (thickness/diameter?)
Then, you can calculate what CCs you need the pistons to be to reach your desired compression ratio. Generally, turbo builds require lower compression, so you may be perfectly fine with stock 300 pistons.
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Frenchtownflyer, honored to get a reply from you. I have been peaking around here and Ford six for years. Need to join Ford six. I plan to do a race fuel, or at least 91+ octane boost. I'm rebuilding the engine and plan to run a smaller turbo. I want to put big. Valves in the head and just do a simple clean up port/polish. Its a farm/toy truck now. Will do some sled pulling but it's mainly just for fun.
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Abandoned bronco, no bore if I can help it. I'll have it honed, low milage engine just had some blow by caused by a bad pcv and dirty k&n air filters. I'm running paper or drop in from know on. Also sparked knocked it because I assumed counter clockwise rotation for a Ford cause I'm a gm mechanic! Had the Timing well up there!
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If you do want to raise compression, you can always shave a little off the block too.
Otherwise, what I did with mine was to search out some 1965 - 1968 connecting rods. They use a 0.912" wrist pin instead of the 1969+ 0.975" wrist pin.
The 0.912" is the same size as the 351w V8 and allows for a wide selection of pistons that will fit right in.
Only issue you'll run into is that you'll more than likely be stuck with 2 spare pistons. From what I found, though, eight 351 pistons are usually less expensive than six 300 pistons.
However, again, with a turbo setup, you usually don't want too high of compression, so you should decide on what you want your final CR to be. I think the stock EFI was already 8.5 - 8.8 or so, which is plenty for a turbo setup.
Otherwise, what I did with mine was to search out some 1965 - 1968 connecting rods. They use a 0.912" wrist pin instead of the 1969+ 0.975" wrist pin.
The 0.912" is the same size as the 351w V8 and allows for a wide selection of pistons that will fit right in.
Only issue you'll run into is that you'll more than likely be stuck with 2 spare pistons. From what I found, though, eight 351 pistons are usually less expensive than six 300 pistons.
However, again, with a turbo setup, you usually don't want too high of compression, so you should decide on what you want your final CR to be. I think the stock EFI was already 8.5 - 8.8 or so, which is plenty for a turbo setup.
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Fleet Mechanic
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If you end up with a set of V8 pistons make sure that they have ZERO PIN OFFSET or you will have a very noisy motor. At least two holes will have the pin orientated in the wrong direction.
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From: Boise, Idaho
Can you elaborate on this? I used 351 pistons in mine and don't ever remember any mention of this, nor do I know whether mine do or not.
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Some engines have the pin offset by about .050" to make the engine a little quieter by putting a little preload on the thrust side of the piston to prevent piston slap on compression as the piston rocks across TDC. The preload on a six should be toward the left hand (drivers) side of the engine. Since V8s generally use the same cylinder head on both sides of the engine the valve orientation, valve notches, dishes and domes will be mirror imaged from side-to-side. In the case of a V8 piston with no valve notches, dishes or domes - i.e. a true flat topped design - it would matter little and all pistons could be oriented correctly in the six. But with a dish or a dome if you wanted to keep the dish / dome alignment with the six's combustion chambers it would be necessary to turn two of the V8 pistons around, thereby not only negating the effects of an offset pin but making the piston slap worse by offsetting the piston in the wrong direction.
Another consideration is effective rod length. Pin offset affects effective rod length and piston speed through its travel, especially influencing the accel rates as it approaches TDC. So it has a slight effect on scavenging in the overlap phase.
Another consideration is effective rod length. Pin offset affects effective rod length and piston speed through its travel, especially influencing the accel rates as it approaches TDC. So it has a slight effect on scavenging in the overlap phase.
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