is 8* overlap enough for 10.5 comp?
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Lead Driver
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is 8* overlap enough for 10.5 comp?
just as the topic states, i have 10.5:1 pistons that were priced to well to pass up. its a 351w, i only have 8 degrees of overlap on the cam im running, its a voodoo 1000-5500 rpm cam. im worried that i wont have enough overlap for these pistons, and dont have the funds to buy another cam if its not 100% necessary. i was thinking i could cheat the timing off to keep it on pump gas.
any thoughts/experience?
any thoughts/experience?
Last edited by Zookie400; Jul 29, 2007 at 04:25 PM.
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Lead Driver
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ok...how high of a comp ratio would you put in with 8 degrees of overlap?
im not looking to make this perfect right now, it just needs to run until my brother gets more money to match the rest of the engine to the high compression. at the moment we are running on the "if its free its for me" budget.
im not looking to make this perfect right now, it just needs to run until my brother gets more money to match the rest of the engine to the high compression. at the moment we are running on the "if its free its for me" budget.
Hotshot
Joined: May 2003
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From: south louisiana
Is that 8* at .050" or 8* at advertised duration? There's a huge difference there. Eight degrees at advertised duration isn't enough. Eight at .050 might be OK. You also need to factor in all the other variables in calculating the final ratio. Head gasket volume, chamber volume, deck clearance,etc. Just because a piston's rated at 10.5 to 1, doesn't mean it will be in your engine.
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Lead Driver
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8 degrees at .050. stock heads and napa gaskets for about 6 months until he will be able to buy a nice set of afr or gt40 heads with big chambers to calm it down some. personally i think i will be able to cheat it for those 6 months without running race fuel, or adding torco. i guess time will tell.
thanks baddad457
thanks baddad457
Hotshot
Joined: May 2003
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From: south louisiana
What stock heads ? Most aftermarket heads have the same chamber volume as the stockers. Only the late 70's D8OE heads had larger (70 cc) Aftermarkets come anywhere from 58-64 cc. 8* @ .050 is better. But then again I looked it up on Lunati's site and its an EFI grind and more than likely you're going to have to retard the timing and/or run higher octane with that much compression. Polish the chambers before bolting them on and remove any sharp edges to eliminate hot spots.
Last edited by baddad457; Jul 30, 2007 at 06:32 PM.
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Lead Driver
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D8OE??? thats tha casting on his stock heads, but they have those EGR pickups in the exhaust ports....??? should i be keeping these heads and porting them?
i dont have the cam card in front of me, ill get that and post the specs asap.
i dont have the cam card in front of me, ill get that and post the specs asap.
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Postmaster
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they have 69cc chambers i think
what's the combo ?
there are all kinds of calculators on line ..but you need all the info...
with those heads you might be ok.
what's the combo ?
there are all kinds of calculators on line ..but you need all the info...
with those heads you might be ok.
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Hotshot
Joined: May 2003
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From: south louisiana
The bumps in the ports are Thermactor Air Injection bosses. Nothing to do with EGR. Just take a die grinder and remove the bump flush with the port roof. If you really want to get adventurous, open the port sides out to match the header gasket, and ditto for the port roof. Leave the floor alone. Blend this down into the bowl under the valves. Swapping to aftermarket heads will raise the comp ratio more, you already have the largest chamber volume of any Ford or aftermarket head here. They're not particularly good heads, but for reducing the comp ratio, you can do no better. What pistons are you trying to use ? Know the pin height ? piston top volume?
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Lead Driver
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well we found a buyer for the pistons, and also found out that we need to bore the block .06 over, so the new pistons are going to be stock compression cheap as they come. i appreciate all the help.
off hand does anybody know what the compression would be with stock pistons, and those d80e 69cc heads?
off hand does anybody know what the compression would be with stock pistons, and those d80e 69cc heads?
Postmaster
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That well all depend on the compression heigth of the new pistons and the valve relief cc's, and trust me there are all kinds of different CH pistons out there. If you have a link for the pistons ..post it. It cam make a huge difference in compression..I've sent pistons back because of it, now I make sure I have the right piston.
what year 351? the early ones had shorter decks on them so it make a big diff in the compression.
get the specs on the pistons and post them
I'll set it up and get you a number.....
what year 351? the early ones had shorter decks on them so it make a big diff in the compression.
get the specs on the pistons and post them
I'll set it up and get you a number.....