Hi
Thread Starter
Registered User
Joined: Mar 2021
Posts: 7
Hi
Hi from the UK
we have a 403 oldsmobile engine in our 1970 pontiac lemans.
will be after some engine advice later on best pistons ect .
It has 455 heads thats what we know so far ,but everything ive read is about 350 heads on the 403.
.
we have a 403 oldsmobile engine in our 1970 pontiac lemans.
will be after some engine advice later on best pistons ect .
It has 455 heads thats what we know so far ,but everything ive read is about 350 heads on the 403.
.
Old(s) Fart
Joined: Mar 2007
Posts: 50,815
From: Northern VA
Welcome. The 455 heads will flow much better than 350 heads, but there are two things to watch out for. The first is compression ratio. The 455 heads have about 80 cc chambers. The 350 heads have about 64 cc chambers. You need to either mill the heads or use flattop pistons to get to an appropriate CR with that combo. The second is port mismatch. Since the 455 heads have larger ports, there will be a mismatch with 350 intake manifolds. Only a very few intakes have sufficient material to be port-matched to the 455 heads. The E-brock Performer RPM is the most common. The ancient E-brock OL4B is another. Most factory iron manifolds should have enough material also.
Thread Starter
Registered User
Joined: Mar 2021
Posts: 7
Welcome. The 455 heads will flow much better than 350 heads, but there are two things to watch out for. The first is compression ratio. The 455 heads have about 80 cc chambers. The 350 heads have about 64 cc chambers. You need to either mill the heads or use flattop pistons to get to an appropriate CR with that combo. The second is port mismatch. Since the 455 heads have larger ports, there will be a mismatch with 350 intake manifolds. Only a very few intakes have sufficient material to be port-matched to the 455 heads. The E-brock Performer RPM is the most common. The ancient E-brock OL4B is another. Most factory iron manifolds should have enough material also.
Thread Starter
Registered User
Joined: Mar 2021
Posts: 7
Block is 554990 (1977) 403
Cylinder heads are E 403686 (68-70) 455
E brock rpm intake
cam is Elgin pro stock E-908-p
will open engine up to see what's been done ,I do know guy said standard pistons,and roller rockers,double valve springs.uprated oil pump,new chain.
Cylinder heads are E 403686 (68-70) 455
E brock rpm intake
cam is Elgin pro stock E-908-p
will open engine up to see what's been done ,I do know guy said standard pistons,and roller rockers,double valve springs.uprated oil pump,new chain.
Out of Line, Everytime😉
Joined: Dec 2006
Posts: 10,129
From: Melville, Saskatchewan
How does it run? You will still be around 8 to 1 compression with stock replacement pistons. Measure how far below deck they are at TDC. Those heads will have a slightly smaller chambers, my 4A heads were 86cc. They also have a better flowing intake port and a much better exhaust port than the stock 4A heads. A set of KB Hyper 6cc or Autotec forged pistons would wake it up a bunch. Looks like a copy of the W30 cam in a possibly 8 to 1 motor. Even with a piston upgrade, I would run a 2500+ stall converter with that cam.
Last edited by olds 307 and 403; Mar 2, 2021 at 05:13 AM.
Thread Starter
Registered User
Joined: Mar 2021
Posts: 7
How does it run? You will still be around 8 to 1 compression with stock replacement pistons. Measure how far below deck they are at TDC. Those heads will have a slightly smaller chambers, my 4A heads were 86cc. They also have a better flowing intake port and a much better exhaust port than the stock 4A heads. A set of KB Hyper 6cc or Autotec forged pistons would wake it up a bunch. What are the specs on that cam?
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