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Back Yard Junky :)
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Discussion Starter · #1 ·
I have a 350 that i got for free the valve were seized. What I was wondering was I have 400 heads would they do good on the 350 or no good. I also would like to know about 305 heads on the 350 to jump the compresion would it be a good or bad idea to put them on.

Any help or suggestions will be greatly appreciated

Thanks

Ed
 

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Somewhere in the middle would be good. I think most 400 heads have something like 76cc chambers and 305 heads are supposed to have 58cc (?) heads. The 400 heads would make lousy power, but the 305 heads may have a bit too much compression. I think the exact size of the 305 head chambers varries a lot though. If it were me I would try to find some heads with about 64cc chambers.

What are the casting #s of the heads on the 350 and what do you mean the valves are siezed?? Dont think Ive ever seen that before.. Depending on what you have you may want to have the 350 heads worked on.
 

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Back Yard Junky :)
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368 Posts
Discussion Starter · #3 ·
its actually not my motor it was given to my brother and he said that the valves were seized I havent seen it but he lives will my dad and i had a few sets of 400 and 305 heads and he asked my advice. I said dont use either because the compression is probually to great with the 305 heads and the 350 wont have enough ummmph to make the 400 heads work properly. I also told him I was coming down this weekend to visit the family and told him to leave it alone but he insists he wants the 305 heads. Ill tell him to buy some new valves and ill port his heads a little bit for him.

Thanks for the help I just wasnt 100% sure on what to tell him

Thanks

Ed
 

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400smb_s10 said:
I have a 350 that i got for free the valve were seized. What I was wondering was I have 400 heads would they do good on the 350 or no good. I also would like to know about 305 heads on the 350 to jump the compresion would it be a good or bad idea to put them on.

Any help or suggestions will be greatly appreciated

Thanks

Ed
You can't talk compression with the SBC without including the piston crown shape and to some extent knowing the crown to deck clearance which at spec is .025 inch but is seldom that in practice, usually less, sometimes more.

Factory pistons are usually dished to reduce compression. If you have a 350 with OEM pistons and a 76cc large chamber head, you've got low compression 8:1 or less. The same OEM piston under a 64cc head like the Vortec will net around 9:1 or a bit more. The same piston under a 58cc L98 head will net around 9.5:1.

In the aftermarket, pistons are available with different compression heights (center of the pin to the crown edge) stock being 1.56 inches but aftermarket pistons with 1.54 inch compression height are not uncommon as they are used to maintain OEM compression with rebuilder zero decked blocks. So you have to be knowledgeable about this, mixing a 1.54 piston with a 9.025 standard height block will kill the compression no matter what head you use.

400 heads use the standard 76ish cc smog chamber with the compression trimmed by the volume of the 400 piston's dish. These heads when mated to a stock dished piston 350, will produce a compression ratio of about 7.8:1.

305 heads have a much smaller chamber but also smaller valves and ports, so while they boost compression with no other changes to the pistons, they need to be refitted with 350 sized valves and a good porting job in order for them to feed a 350. Back in the late 1970s thru the 80s they were the poor man's way to get a smogger 350's compression up and were quite popular, especially the 14014416 head.

In reality, these days you need to do some careful cost estimating as rebuilding a set of used heads can end up costing as much, or more, as the purchase of a decent set of new heads whether Vortecs or from the aftermarket. Plus, the new heads come with far better performance than can ever be achieved with ported older heads. Much of this is due to newer fast burn chamber designs while some to better port shapes, locations and volumes that just can't be duplicated with a grinding wheel and an old port.

Bogie
 
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