1970 390 Bored 90 Over

[68AMX@AZ]

Hello I purchased 1968 AMX Go- pak about 3 years ago and have been working on it ever since. I purchased a 1970 dogleg 390 that is completely rebuilt but is bored at 90 over? Would I have problems with perhaps overheating thin cylinders? Also would sleaving be an option???

[bigbad68]

What are you going to do with the car? If you are just going to drive it on weekends to shows and/or cruising then sleeving would be fine. If you plan to race it, then I would not sleeve it. But I do think you will have a problem with overheating the engine being bored over 90, so sleeving would be a good idea.

[68AMX@AZ]

What is the MAX bore size for the 70 390. And also would sleaving all cylinders make them weak where a once in a while heavy foot whould be a fatle mistake? My plan is to do a mild hop up using a 0 to 5,500 RPM Edleblock intake and cam {COMP} and using stock heads with Keith Black Hyporutectic 10 to 1 Pistons, 6.1 roller tip rockers a set of headers and a 650 CFM Holley Double Pumper. I'm not going to drag race maybe get on it every once in a while mainley I would like to take it to Reno once ayear for the big show,and to cruse...

[donsjave]

What is the MAX bore size for the 70 390. And also would sleaving all cylinders make them weak where a once in a while heavy foot whould be a fatle mistake? My plan is to do a mild hop up using a 0 to 5,500 RPM Edleblock intake and cam {COMP} and using stock heads with Keith Black Hyporutectic 10 to 1 Pistons, 6.1 roller tip rockers a set of headers and a 650 CFM Holley Double Pumper. I'm not going to drag race maybe get on it every once in a while mainley I would like to take it to Reno once ayear for the big show,and to cruse...
I have been told and I believe that the Tech Manual states that you should not bore out the cylinders more than 60 over. Anymore than this will cause serious problems with the tempature of the engine when running. AMC blocks are mostly the same no matter whats the cubes. 390's are definitely already pretty maxed out as they are. The only difference that I have found comparing 390 and 401 engines are their stroke is slightly different. The bore sizes are the same. Second thing Seal Power makes a good 9.5 to 1 forged piston that seems to tolerate pump gas fairly well. Going to 10 to 1 would necessiate a fuel additive being added to the gas pump stuff. I currently run Seal Power forged pistons in my 401 without nary a problem. I am carrying about 170-175 lbs of compression and the car scoots down the road decent. :)

[Big Bad AMX]

Max overbore is .060, but over .040, the cylinders get thin and the coolant can boil around the cylinders which will cause oil burning in the cylinder and excessive wear in a short period of time. You really must have a custom 4-row radiator or a crossflow radiator to cruise in Reno in the summertime. A high volume water pump is a good idea too. 10-1 pistons WILL require octane booster, especially in hot weather. Sleeving the cylinders is just fine if the shop does it right. They will not be weak. FYI: Double pumper carbs are not that good to use on the street as they dump lots of raw gas down the intake in city driving. A far better choice for what you are intending to do with the car is an AVS Edelbrock carb - no more than 650cfm at max.



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