I have a sbc 355 with domed pistons and 305 heads on it. I bought it like this and dont know what it has for a cam. There is alot of predetonation when I get on the gas with 93 octane. I have a set of 350 heads that look like they have been shaved. They were 76 cc chambers before this. The current 305 heads should be 58 cc. If I put the 350 heads on, will this fix my issue? The timing is right where it should be.
buy a stock engine you haaaaave a race engine thats gonna need race fuel or your pistons will land in a pile i know how that happens eons ago you could almost get by with pump gas and make something like that run, not for the last 25 years is it posible
Not knowing how much dome.....Ill guess.....If it has a 10cc dome you have around 13.5 to 1 now with 58 cc head...mind you though I dont have exact numbers If you change heads to the 76 cc you'll have around 10.5 to 1....It still has to be cammed properly to run that much with iron heads on todays pump gas..or youll still break it And of course no way in hell can you run pump gas at 13.5 You need to know exact numbers when playing with parts like this....My numbers are below Tony
The guy told me it was 11:1 or 12:1 compression. I plan on putting the 350 heads on, just wondering if it will be worthwhile. I just got the build far enough to drive it this past week and found this issue
58cc heads on a 327 with flattops was a FUN motor...till it popped a piston. 355 with dome pistons is way too high for pump gas.
Its common to put 305 heads on 350s in claimer roundy round for cheap compression bump, but thats with dished pistons...even with 76cc heads you'll want a cam with enough duration/overlap to bleed some of the actual cranking compression to work with pump gas. You can run 10.5cr on 93 with a cam that has 235 duration @.050....but it will detonate like crazy with a stock cam....your calculated CR stays the same no matter what cam, but your CR in addition to your cam determines how much psi is created in the combustion chamber when cranked....and then throw timing in the mix for psi created when running.
A stock piston 350 with 305 heads and a short duration cam makes a good torquey low down street cruiser . The motor is very responsive at just "off the throttle" cruising but will run out of breath at about 4500 rpm. I've built one like this and it was very enjoyable,crisp and responsive! I personally would sell the pop-ups and fit cast dished pistons [ you'll probably get more $$ for the pistons than selling the heads] Leave the race engines for race cars.
I ran 305 vortec heads on the 350 Chevy in my 23, it bumped the compression up about one point. Top end suffers a little due to not as good breathing, but bottom end is better. I ran flat tops and ran on mid grade gas. Don
Yes the 350 heads will help a lot. It might not be enough but you have them and its a fairly simply/cheap swap.. way easier than changing pistons. Only way to know is to do it!
If you are not sure about the pistons just take a spark plug out and use a small flash light and take a look inside to see what pistons you do have . If the guy told you the compression was about 11 or 12 to 1 then you need 100 octane at least or more to run the engine . Now if the pistons are dome you can replace them but the best thing to do would be just sell the engine as a race engine and then just buy another engine . They are very common and cheap for good running engine . A good race engine will sell easily at this time of the year ! Retro Jim
UPDATE. I pulled one of the heads today. The 350 is actually bored .060 over. So its a 360. It has pop-up pistons. The heads were 305s that had 58 cc chambers. The new heads are 350s with 76cc chambers. Hope to swap the heads tomorrow and see how she runs.....
.125 dome with 76cc heads and felpro .040 head gasket will be 9.25 cr. perfect......also polish off all sharp edges on piston tops
I’m surprised the .060 overbore and high compression didn’t end up in a/some split cylinders, especially with the detonation.
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