307 to 403 engine swap

[bored030over]

Question...I have an 83 H/O and I am planning on changing the 307 to a mildly built 403. Can I use hook up the 403 to the existing rebuilt 200r4 that is in the car now? The car will see mainly street driving an only once in a blue moon a trip to the track. It already has a 650 Barry Grant carb, an Edelbrock intake and a mild cam; however, it does have a 1" spacer under the carb...so that might present a clearance problem. I know I will have to have the distributor replaced and I plan on changing it over to an all MSD set-up with 6AL box. Lastly, my car already has true dual exhaust, ceramic headers are a must... what size headers will I need to help this 403 breathe better? I was thinking about Hedman or Kooks.

Suggestions appreciated,

Tony
83 H/O

 

[stomis]

The 403 is the same block size as the 307. Meaning everything will bolt up no problem.

As far as needing bigger headers I'd say that depends on your power level not your engine size. I'm sure the headers and exhaust were way over kill for your 307 so they might be fine now. What size primary tubes, collectors, and piping do you run?

 

[bored030over]

On my 83 H/O, the stock cast iron headers dump into a down pipe that goes directly to the stock styled Flowmaster mufflers and stock styled exhaust pipes.

Tony
83 H/O

 

[stomis]

On my 83 H/O, the stock cast iron headers dump into a down pipe that goes directly to the stock styled Flowmaster mufflers and stock styled exhaust pipes.

Tony
83 H/O

O I see now. Your stock "exhaust manifolds" not headers. Get some 1-5/8th primary long tubes with a 2.5in collector.

 

[FE3X CLONE]

1-5/8 would be tough to find for an Olds engine. Most start at 1.75" and go up from there. In my opinion, on a 403 I wouldn't go with anything less than a 1.75" primary tube size.

You may gain a little torque with a smaller primary but the 403 is already going to give you plenty of that so going with a bit larger primary will help up top.

If you want a good quality header for a SBO, look at the American Racing headers. Kooks no longer makes headers for Olds. Dick Miller Racing bought all of their jigs and designs and produces them at his shop. However they never had any for the SBO in a G-body. Just a BBO swap.

Beyond that most all other headers fit like absolute crap. Do not go with Dynomax's. We put them on my brother's 403 and they fit terrible.
And word to the wise, DO NOT buy them ceramic coated. But them unfinished and test fit them first, then send them out to be Jet-hot coated, etc. after you've "massaged" them. 😉

 

[83hurst-olds]

Yep, as they mentioned above, the 403 will bolt up perfectly. I think you were thinking of the taller block deck of the 455 and other "big block" olds motors. The 403 is a great street engine. If you have the budget for them, aftermarket heads will make these scream!
I am thinking of building one for my '83 H/O and keeping the .307 stock.

 

[79loserbluebu]

For the heads if you don't have the scratch for a set of Edelbrocks, you can get a set of 7a's and bump the compression up

 

[dan2286]

Ditch the spacer for the carb. If it doesn't help your power taking it off, it won't be a noticeable loss in power if any. The 200-4R may not hold up for long if it is a stock rebuild and you beat on the car. If you can drive sensibly, it should last.

 

[FE3X CLONE]

I kind of disagree about the spacer, however it will depend on his combo. On my brother's 403 we used an Edelbrock Performer intake at first with a 1" spacer. You pretty much have to run a spacer otherwise the carb linkage will hit the EGR crossover.

It ran great like that. However we then swapped on a Performer RPM and got rid of the spacer. Essentially the RPM acts like a spacer since the plenum is taller. He picked up even more power and mph in the 1/4 with the RPM.

And yes, if the TH-2004R was just a stock rebuild, it may not hold up to the 403, even if you do go easy on it. Mine had a stock rebuild and lasted 3 days behind my 455 before it started slipping.

 

[79loserbluebu]

Yeah if you are using a regular Performer keep the spacer. If you are using a Performer RPM with the taller plenum then get rid of it. Depending on the carb you get you will need a adapter for the intake.