i have a 350 in one of my cars that has 305 heads on it(that is the way i got it). and i was wondering if that could be causing the severe blowby that it is having. It looks as though the motor has just been rebuilt so i don't think anything is worn out, i think it just has too much compression. what, if anything, can i do to solve some of the blowby, without swapping heads. This setup runs awesome so i don't want to change it too much. This motor runs real smooth with no noticable cam and ran a 9.6 at the track spinning, but i have no idea what internals it has. the head casting # is 14014416 and the block casting # is 3970010.
any help would be great.

 

Well it isn't running well if it has a lot of blowby. Did you do a compression test with a guage? Swapping heads would be in your best interest. Maybe it has a leaking head gasket since the 305 heads have small chambers?

The heads actually shouldn't cause blowby, the piston rings or bore surface is the cause (also poorly fitting pistons). If you have too much compression for the fuel you run you may have detonation, not blowby.

 

Check to see if your PCV valve is working. If not then replace it. Second, Do a compression check and a leak down test.

 

i'd run a cranking compression check if I were you, like he said. I was thinking of running the same combo by using the 416 heads off my 305 on my 350, does anybody know how many cc's that head is and about how much is that time equivalent to in the 1/4 mile. I'd look for between 160 and 180 psi of cranking compression for a stock motor, it you have more than that it may be caused by the smaller heads or the cam being installed to far off centerline, both of which could cause excess cylinder pressure.

 

The most accurate conversion that I have used is multiply 1/8th times 1.56.

9.6 X 1.56 = 14.976

 

well the oil breather was stoped up and when you take the pvc valve out of the valve cover, a whole bunch of smoke is shooting out. and if you get on the car really hard oil comes out of the dipstick tube, the cheapo guage says the car only has about 50psi when you get on it, so either the guage is wrong or the compression is pushing oil out of the dipstick tube since the breather was stoped up and the pvc valve couldnt keep up.

 

how did you test the compression? I'm not real clear on your method there. Also note that your heads have 58cc chambers and with normal flat top 350 pistons that could yield nearly 11:1 compression, theirs no way street gas could hold up to that with stock heads even on a really well tuned motor unless the cam was phased to compensate for that.

 

sounds like blowby to me

 

i had alot of blowby coming out of my pvc valve and also from the dipstick. It ended up being my piston rings.

 

since you said it was a recentely rebuilt engine could the rings be installed up side down? do you know who built it? do you know the quality of their work?

 

I run 416 305 heads on a 355 with flat tops. Pump gas works fine. If he has that much blow by (constant smoke out of the breather, and pushing out the dipstick) I vote rings. Either worn out, improperly installed, or improperly broken in. My $.02

 

My brother had that problem on a recent rebuild. turned out piston was broke at one of the groves where the rings are installed. compression test will tell u which one.

 

well i am going to pull the engine this weekend anyways and freshen it up, the guy i got it from lied about alot of things so he probably lied about the rebuild, also i have a friend that insists that is a 302 since it is a 69 model block and ran mid 14s at the track. so i want to find out for sure.