I have a '42 Chevy Aerosedan with a 1974 Olds 350 with #8 heads, it runs well but. I have a set of heads from a 1969 Olds 350 that are #5's and I would like to swap them to my '74 engine. I know the combustion chamber is smaller on the #5 heads and am wondering what kind of performance increase I might get by swapping. I would add a better cam and new timing chain while in there. I am wondering if that would be a worthwhile thing to do or not? I could afford to do this way before I can afford to rebuild my 425 and would like the extra power. If it would not m,ake a very noticeable difference I will wait. Oh I have an Edelbrock Performer intake and true dual exhaust already. The engine runs great but I want MORE POWER! I am posting pics of the car I am working on.

 

The head swap will bump up the Cr almost a point and that will let you run more cam. All in all, probably 30 HP and 40 ft/lbs, a noticeable gain, but certainly won't match the output of the 425. How much does that car weigh?

 

It weighs 3200 lbs without me in it. It moves decent now but would like more. I am still going with the 425 eventually, but this would be something I could afford this summer. 30 horsepower would be enough to justify it to me, the heads are good and I will just do a better cam and timing chain at the same time.

 

Since you have yet to bolt the heads on, or even tear off the #8's, take the #5's to the machine shop and have them freshened up. While he is at it, have him back cut the intake valves, and put some fresh springs on it (i would try to talk you into some 2 inch intake valves with back cuts).
May as well cut them .030", too.
You may find more than 30.
Do not bolt on unknown, possibly bad, or needy heads, with worn out guides and valves.
JMHO
Jim

 

I will freshen them up, how much more is it to go to the bigger valves usually? If you cut the heads down that much do you have to do anything to anything else, cut the intake that sort of thing? Does backcutting the valves help alot?

 

#1 Around $100, after the cost of the valves. Shop prices may vary.
Focus on the intake side!
#2 No. Not if you keep it .030" max, you should be fine.
#3 Back cuts are an inexpensive way to promote extra low lift flow. Just do the intakes, the exhausts will not respond too much with this trick. At $2 each ($16 total!!!), I think it is well worth the 5-10 hp you will get.
#4 Think about filling the heat crossover with aluminum, and run an electric choke. 20-25 hp.
Jim

 

Thank you, you guys are great. I can't wait to make it quicker.

 

Have you found that filling the crossover is worth that much on a low rpm 350? I usually do it using the E-brock plugs, none of my hot-rods have chokes on them anyway.

I agree with Warhead on the larger intake valves, that along with the resulting bowl blend is a good, cost effective upgrade that you will notice. You want to build for throttle response and low end torque.

 

Thanks, throttle response and low end torque is exactly why I was asking. Thanks