I am working on building an olds 403 out of my 79 trans am. Not going for a ton of power, something pretty street friendly hopefully around 300 hp maybe a little more.
So far I have a set of olds 350 #7 heads which have been machined and the valve guides have been hardened and I am getting a 850 cfrm carburetor. I just have 3 questions left that I thought I would ask people with more experience,
First I have been considering whether to do flat top or dish pistons, 17cc dish pistons will likely be 9.1:1 with the 70cc heads while flat tops will likely be 9.6:1 and notably more expensive. How much power would I get out of flat tops? and are they worth it?
As for camshaft I was considering something like the edelbrock performer pro which is 204/214 with .449/.473 lift. With the high compression would I be better going with something bigger or different?
Finally is there anything else I should change? I considered a different intake but don't want something like the perform RPM where I would have to modify the hood clearance. Do I need any header changes or anything else?
Thanks!

 

850 is too big for almost ANY 403.
True flat tops and #7 heads will get you way more than 9.6:1.
That 204/214 grind came over on the Mayflower. There are better grinds out there.
RPM is the better intake for that, there are drop bases available to run that intake on a TA.

 

Thanks for the info!
With a little more research it looks like maybe 10.6:1 on the flat top pistons, which would still run on 93 octane but is a lot higher than 9.1, would you recommend just sticking with the dished?
For the cam would you recommend something more like a 268/268 or a 280/290?

 

What gas are you going to run?
How much power are you looking to make?
Are you sticking with the 7111 intake or doing something different?
You need to decide the direction you ultimately want to take, then go from there.

 

X2 on the carb, but see if you can grab a Rochester carb that was correct for at 79 Hurst Olds and the Pontiac Trans Am 403, I think the number was 17059253 (?). They are out there, check out E Bay auction 266896724637 for a quick look. If they are rebuilt right IMHO they are a suitable carb for street use.

 

If the bottom end is healthy and with your power goals keep the stock pistons. With a 64cc chamber with factory pistons .025" in the hole and Felpro head gaskets, you will be right at 9 to 1 compression. I ran #6 heads milled to 64cc with 2.07 intake valves and the bowls opened with a cutter, Performer RPM intake, full length 1 3/4" Chinese stainless full length headers, 2.5" dual exhaust, a custom tuned with a AEM wide band 800 cfm Qjet and the 204/214 cam in my 70 Cutlass with a 2200 stall and 2.78 gears. It felt like 300 HP and 350+ ft-lbs of torque. It felt stronger than my current 9.6 to 1 Olds 358 with the same cam and SEFI. You can fit 2.07/1.625" valves in those heads but the bowls should be opened to take advantage of larger valves. The 204/214 is very mild in a 403 but would meet your goals with supporting parts like headers and dual exhaust.

 

Fellow member 79T/A has a dyno proven formula to extract 300 flywheel hp from a previously stock 403. However he was actually disappointed in the 20% gain in power. https://classicoldsmobile.com/forums...esults-103967/

His biggest gripe is a seemingly unintentional drop in torque. But in all my years of studying aftermarket cams on mild engines this is to be expected. The factory focused on maximum torque not hp with their cam profiles. While aftermarket cams focus on peak hp production first and foremost... One of the reasons the aftermarket crowd lauds modern cam profiles over muscle era ones.

The reality is he had a great gain. And if he track tested the T/A from bone stock to true duals to mild build. He would see that he took the car from 1987 442 performance to 87 GN performance to GNX performance. Quite impressive when you view it through the proper perspective.