[1968poney]

Is 10:1 compresion on a 1968 ford 302 good or should i try to go higher. i belive i now need to use 93 gas so i dont cause knocking would getting it to 11:1 Give it more power or will i hit a point where i will lose power due to not enough opening in the valves.

Thanks Tom

 

[cindys_sn95]

with a cast iron head you can probably get away with 10.1, but it will require the 93 octane and not alot of timing. you might be better a 9.5:1 and have a little wiggle room on timing. now with an aluminum head you should have no problem at 10:1 and premium. :bigthumbsup

 

[1968poney]

Yes but i heard that aluminum heads tend to warp. i could be wrong but thats what i have been hearing. Do you think i should spend the time on the 289 heads have them polished and ported or would it be cheeper to buy aluminum Z heads or gt40 heads. i know they run about 700 each do you think spending to have the cast irons is worth it?

 

[mccance]

I think you ought to just take a look at any performance magazine at heads. almost all of them are aluminum with the exception of dart and maybe a few others. aluminum heads dissipate heat better so they can tolerate a higher compression ratio. most of them have been designed to flow a lot better and have larger valves than the stock ones as well. even ford performance stepped up to aluminum heads with there top of the line head offering. so I think that while aluminum heads may warp easier than cast iron, under normal operating conditions that will not be an issue. you will gain much more than you will lose. that being said there is a lot that can be done to cast iron heads to make them flow better. as far as your question regarding compression ratio unless you are building a car for primarily racing and do not intend to drive it on the street much I would not go much higher than the 10:1 ratio. but you should be able to run 91 octane pump gas in it without detonation. I did for years in my old engine with no trouble.

http://www.popularhotrodding.com/tech/0901phr_difference_between_pump_race_gas/index.html

this is the best article I could find on the subject. I would however not go any higher on the cr without upping the octane.

 

[The Greek]

Cast iron heads are more resistant to warpage than aluminum if the engine overheats. Since cast iron retains heat more than aluminum, equal performance can be had at a slightly lower compression ratio. Coolant passages are more resistant to corrosion/erosion with cast iron heads over aluminum due to electrolosis from a lack of cooling system maintainance. Performance cast iron heads cost less than aluminum. The cast iron valve guides are more resistant to wear than the bronze alloy guides found in aluminum heads. Spark plug threads don't strip out as easily with the cast iron heads as well as the threaded accessory mounting points. Aluminum heads expand at a greater rate than cast iron which means that head gaskets are subjected to a greater amount of surface shifting when using a cast iron block. The head gasket's cyl. sealing rings can etch grooves into the aluminum head's deck surface due to the shifting. Aluminum heads need to be retorqued more often due to the greater expansion/ contraction rate. Aluminum heads can shave 50 lbs or more from total engine weight. Aluminum heads are easier to machine and port than cast iron. Aluminum heads can be welded or repaired more easily than cast iron.

 

[1968poney]

Thanks, I see what you mean since my car is o=intended mostly for street there wouldn't even be a need to go extreme on a small block. i will mainly stick to stock stuff just interchange year parts. when i save more money for a big block 428 i will go extreme. It looks like aluminum heads tend to be more light but aluminum is also a soft metal cast iron is hard and brittle. I will stick with the cast irons maybe spend some time to have them polished ported and reset the valves. Thanks again for the help.

Tom