Is the 403 out of a 79 Trans Am the same size and mounting as the 307 in an Oldsmobile. I have found a 403 and am thinking of putting it in my 83 H/O instead of the 350 that I was planning to install. Thanks Larry

 

very easy swap,dress it with all the 307 accessories

 

Externally they're the same, but the 403 cylinders are siamesed. This just means make sure your cooling system is operating well, which is good practice anyway.

 

May I ask why? I am also of the school that every motor loves being bigger but 68-70 350s can be had at a reasonable price and if you find a good running motor you have the benefits of higher compression, good flowing heads and (please correct me if I'm wrong) better main webbing if you want a more substantial build. I'm in the final assembly of a 350 swap into an 83 H/O as well and wish you luck. Parting words, I found my 350 (which had a nice intake/cam/hei set up) for 500 bucks with the help of a CO member so include the classified sections in your search even if you want to go 403.

 

At any HP/cubic inch level, the 403 starts off with an extra 35-50 HP over a 350. As long as you are not going wild, extra power will be with the 403 for no extra cost.

My opinion.

 

A 403 into a 78-88 A/G body is a great swap. You can have a bone stock 403 that will give good gas mileage and decent performance all the way to a mid 11 second street strip set up if you build it right. The 403 is a fine choice if you stay within its' inherent limits.

 

I have to respectfully disagree, I would be very surprised if a 79 403 83cc head engine made more power than a 70 350. Factory listing aren't gospel but an error variance of 100hp seems unlikely.

 

Who said the 403 was going to keep it's crappy factory heads? Swap on your 1970 #6 heads along with a performer rpm intake an a well chosen cam and watch the 403 spank that 350.

 

Well the debate started with 'at any hp/inch level" How about stock? I can't debate you Dave as I've owned 77 less Oldsmobiles' but I gotta stand my ground that the 500 dollar motor I pulled out of the 70 cutlass would outperform a 79 403. Without knowing the ambitions of Larry's build we really can't find neutral field to argue in so my offering is this: I paid $500.00 for a 350 that would outperform a 403 from a 79 trans am at a similar price point. My heads are stock which means I would not have to incur the added cost of a core motor+rebuild+ head upgrade as is the case with your model. I think the best option is to ask Larry exactly what he is looking to do and at what budget.

 

I said hp/cubic inch, you stated 70 model year 350. I'd put any stock 403 against any 71 later 350 and not worry. If the compression/everything else the same, the 403 has the edge. I see your point with the stock heads, and weak compression, but I like the 403's for a reason.

 

I didn't aim to start a war of words. The price of each engine was basically the same. The 403 wasn't running and stuck from setting and no one had ever heard it running. Had been setting about 10 years inside. The 350 has been setting about a year but seller has heard it run(Oldspackrat}. Got to thinking the 350 with the better compression would be a better fit. Wouldn't have to worry about the 403 keeping cool. Originally thought that the 403 in the 83 H/O would be a better sell but now not so sure. Anytime you buy a used engine without physically hearing it run you are taking a gamble. Oldspackrat has a good reputation on CO and that goes a long way. Anyway buying a engine that is stuck can end up being a costly endeaver and I feel the 69 350 will do me a good job for what I want. I'll just take the 307 and set it back in the corner in case I ever sell the car and buyer wants to try and convert it back. Thanks to all who helped me make my decision. Also thinking about getting a Lunati Voodoo series cam to stick in before I drop it in just to give it a little more. Larry

 

Olds engines were very durable compared to other makes but are 50+ years old in some cases. Problem is most rebuild them with cheap cast replacement pistons. The 24cc dish pistons leave the compression miserably low on a 350 even with early heads. The 403 with it's huge bore and 18-21cc replacement pistons with early 350 heads will 9 to 1 will mild milling. My 8 to 1 403 feels stronger than my 9.5 to 1 350 with ported heads and the same cam. The 1/8 mile will tell the real story. If the 350 is from a trusted source and actually a 69 350, it will perform well. I would swap out the lame factory cam as it will probably need a new timing set and all gaskets including a rear main seal. I would go rubber for rear main.

 

Pretty much my thoughts and plans. Just looking to make a decent and dependable driver that will get out of its own way.

 

You are not comparing stock-to-stock here as you stated your 350 has performance upgrades.
And FYI, I have first hand experience with 403 Trans Ams vs. a 1970 350 engine: my then-stock 1970 Cutlass Supreme 350-4bbl was left behind by many 403 TAs back in high school (late 80s).

 

Point taken.