In 1969, Pontiac unveiled it's Trans Am model Firebird, and since racing rules required a sub-305 cid engine, Pontiac unveiled the 303 for racing models only, never available to the public. Bore And Stroke were 4.12X2.82. It was rated at 475 gross hp.


I'd be happy with 475 hp out of my 305, wouldn't you!?!

 

notice the bore size and the stroke, 305's have tiny bores which cause shrouding of the valves.

 

yeah it would be nice to have a 305 make 475 horse but it would take alot of work to do it. The bigger the pistons the easyier it is to make power. Like you can get the same power out of a 305 as a 350 but it would take alot more money and work you might as well just get a 350. And probably the easiest way to get 475 horses out of a 305 is get a good blower for it, or put nitrous on it.

 

Large bore allows larger valves which allows more HP. The tiny bore of the 305 can't compare.

A Ford 5.0 makes more power than a Chev 5.0 because of the bore/stroke configuration. A Chev 302 can make a lot more power than a 305 also because of the larger bore.

It's not just about CID but how it's obtained.

All things being equal, bore makes HP, stroke makes torque.

The short stroke 303 probably also made that 475 hp at over 7000 rpm for road racing

 

Anyone else notice this? GROSS HP! That's comparing apples to oranges there. Also, notice the size of the solid flat cam used on that? And the compression ratio they needed (and the leaded gas)?

A big bore/short stroke helps with the valve size (for a given displacement), but it's terrible for avoiding detonation. Little known fact (for everyone here that LOVES oversquare engines...) larger bore = more prone to detonation. For example, if I were to turbocharge an engine, and was limited to 310 cubes, I'd rather have a 305 based engine, vs a 302 (SBC) based engine

 

the bore on that is HUGE, stock 350 bore is 4.000" and like a 3.500" stroke?? if i'm not mistaken. 4.120" is a decent size to say least lol.

 

well us guys with the 305's can still dream if we had loads of money what we'd do lol. Prolly still better off swapping though.

I just would have liked to see that car with the 303 in action! :-)

 

The Pontiac Trans Am motor from the 60s was never competitive, even back then. It was too heavy overall and they had to use a very very long rod with the tall deck height and super short stroke crank.

Chevy built their Trans Am 302 by de-stroking a 4" bore 327/350. Not a huge change, but it was workable and they had considerable success. And it was still a zero-torque engine in street form.

Pontiac had even further to go- destroking a 400 to 303ci. That was an even bigger leap. It didn't work out so well for them on the track and there was no way they were going to even attemp to market that pig in a street-going form. That's why the street "Trans Am" never had a detuned version of that race engine- they got 400s instead.

 

my 305 fairs well enough...and it didn't cost an arm and a leg either.

 

i solved my bore/stroke handicap...i just gave it some laughing gas

 

While we're bench racing with non-street engines. I wonder if anyone has built either of these big-bore short stroke rpm engine combos:
347: 400 block and 327 crank or a
320: 400 block and 302 crank.
The larger bore should make up for some of the lack of torque of the short stroke and stock rods should be able to be pushed to higher rpm with the smaller rod angles.
It would be interesting from a theoretical point of view...

 

Honestly, for a street car, I'd actually prefer the 305 over the 302/303. Its so much more driveable. Yes, the 302/303 made like 450hp, but thats at way over 6500 rpm. The CrossRam (dual Holley 650) 302s made 480hp, but that was at nearly 8500 rpm. Trains accelerate faster from a stop.

Most people give the 305 a bad wrap because it was always lame in stock form, unlike some other motors (350 for example). The 305 can be made pretty healthy for not really any more money than any other small block. Clean up the heads, put in a healthy cam, good intake and headers, and its not really difficult to get near 300hp. The other problem is just that it does cost the same to build a 305 as a bigger small block, so most would build the bigger one. The 305 isn't a bad motor, there are just better ones for the same money is all.

A friend of mine has a BONE STOCK '85 5.0 TPI engine - completely stock, even exhaust - with only a ProCharger setup making a mere 8psi, and is making nearly 300rwhp IIRC.
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True. Lots off BBC guys that build street motors have problems with this. Especially the big bore/short stroke motors (427 for example). BBCs have it twice as bad though, as they have the big bore causing detonation problems, but also the enormous domed pistons they use to bring the compression up compound those problems.