projectresto83

I currently have some 78 heads with 69cc chambers on my 306. I don't know what my compression ration is right now but how much do you guys think that it will go up when I put on my GT40 heads with 65cc chambers?

mjr46

alot will depend on your pistons if there is a slight dish if so how much ..... with the gt-40 heads you'll be around 9 to 1 but If I were you...I'd look for a set of dooe or c90e heads

projectresto83

I do know the pistons are not dished or domed (at least I couldn't tell by a quick visual inspection). They have the normal valve reliefs like H.O. motors do but I don't know the specs or brand of the pistons. I already have the GT40 heads in my garage so I am gonna do some work with them and use them till this motor goes and then drop in a 351.

tyler72

Very generally, if you go from a 69 cc head to a 65 cc head, you will probably see a bump in compression of about .5. So, right now you should be at about 8.5:1, and with the GT40s, you should be at about 9.0:1, like mjr46 noted.

projectresto83

Thats what I figured.
Ok, another question. What if I was to get the gt40 milled down .010 or .020? How much would that help bump up the compression? I know from head to head it would vary do to chamber design. I know I can put new pistons in it but I am not putting that money into this motor. When it goes it goes. There isn't anything special about it so ehh.

mjr46

at .020 mill it might put it at 9.3 or so?? just a guess....honestly it may not yield much of a diff..if you plan in the future to do a better motor then just bolt what you got together and save the extra for a better build

projectresto83

Yea I know what you mean. I do need to get them cleaned up from the rust and stuff but I can do all that my self with a dremel and wire wheels. I think the valve guides are in good shape so I don't think I need to get them replaced. I kinda want to get new valves for them but I have been getting all the carbon cleaned off pretty good so they seem to be in good shape. I figured since I am not gonna get the guides replaced I might as well keep the valves because of the wear patterns for each valve go together. I just gotta get the time to get them done with the wife and kid needing attention all the time. I should just take them to work since we just since in our office all day anyway lol. I just need to get some bits to do a little smoothing in the ports to help that flow.

mjr46

just gring down the thermactor humps in the exhaust ports and widen the exhaust port a little to match gasket opening and smooth out the jagged edges in the intake runners and clean up in the bowls and rough areas around guides and your good to go...but I hear ya on the wife and kid thing!! same boat here!!

projectresto83

Yea thats what I am gonna do. I havn't gotten much info on working the bowl itself and the blending. I am gonna port my intake to match the ports on the GT40's while I have it apart. At least you have a shop you can work on things. When I go to work we have nothing to do so we are there make 2 hours a day then we go home. So I work maybe 3-4 hours a day and then I am home with the family. Thats the nice thing about salary pay, I don't even have to go to work and I get paid the same. One of the FEW things I like about the army. But I DON'T get paid overtime. lol[:@]