389 Horsepower Possibilities
 

Have I done it, no. Do I think it can be done, yes. Remember while carb size limits HP it is not a hard limit. A carb rated at XXX cfm will flow more that XXX cfm at a high than rated depression. This means you are giving some HP up to get the extra cfm, but HP can still increase. All of that said I would be looking at the stock exhaust manifolds.

Stan

My understanding is that you can get a little better than 1hp per cu in with upgraded airflow-- I'm not expecting much more than 500 out of my 461, for instance and that'll have new everything.

What if you did three NO2 cheater-plates and sprayed it? Maybe with a dedicated fuel cell that has race gas in it? Or three supercharger hats and do a tripower procharger setup? Amazing.

If you want to make much more than 400hp, you have to figure out what you want to change, right? Is your goal to keep it stock-looking or acutal stock? How fast do you want to spend?

Just numbers in a check book!Tom

399 horsepower 1965 389 with factory AFB intake and carb running 3-tube headers:

https://forums.maxperformanceinc.com/forums/showthread.php?
t=836191

As Stan has already pointed out the log manifolds might be an issue you’re going to have to work around. The use of three tube headers in the link above might give you some hope that it’s possible with reworked log manifolds to achieve 400 horsepower or get pretty close.

The idea would be to look and stay close as possible to stock. Keeping the AFB and intake and exhaust manifolds. Mostly relying on a carefully selected camshaft and precision balanced rotating assembly. I tend to believe 350-375 might be attainable and low 400's with tripower.

Thanks b-man I just saw your post as I just wrote mine.

Oooh- I thought of another idea: Put an LS in the trunk! ;)

I would say use a 4" stroke crank, .030 over bore, ported heads, tripower and ram air exhaust manifolds, 450hp would be easy.
Basically building a 421 out of your 389.

389
 

That's more A/FX than C/S :--)
I have this handy book to help keep the theme.

https://imagizer.imageshack.com/v2/1...922/OW1IUk.jpg

Is your car a stick or automatic and what are you going to use as a rear gear ratio? Using the 65 heads you need to convert to screw in rocker arm studs. Good flowing heads,then think about the cam, hydraulic, hydraulic roller, solid. The intake and exhaust are restrictive. How about 67 ram air exhaust manifolds and oversize exhaust. How about sneaking on Edelbrock Performer intake and removing the lettering and painting it Pontiac blue. Larger AFB. If your in the NYC area look up Richie Hoffman or Larry Hansen over in Jersey. 400-450 rwhp should be obtainable.

Some older but pertinent info I guess.

You mention stock Exh, are you also taking about the 2" head pipes here?

straycat, I appreciate your ideas, looking to keep that stock look and function so not even Ram Air exhaust manifolds.

Yes using the factory exhaust manifolds but 2" head-pipes are to restrictive IMHO.

2 1/2 inch pipes would be important if shooting for 400hp,. to bad Gardner doesn't 'make it. :---) Thanks

If you try to recreate the car using the 1965 rules for stock class cars, you will be limited as to what can be done to the heads as they must remain essentially stock. The chambers need to be CC'd to 65 cc's so they are all the same. You can port match them. 3-angle valve job. Good head prep/rebuild.

Here is a suggested list of improvements.

The Carter carb should have stock .0935" primary jet and .078" secondary jets. Open the secondary jets up to .086" for an increase in HP.

The ignition system is where HP can be found.
Use a good low resistance spark plug wire.
Use AC44 spark plugs gapped to .032" (or Champion J-10 plugs)
Dial in the distributor advance curve. These numbers are distributor degrees as used by a distributor machine. X2 would be degrees at the crank/balancer.
Stock - 6 degrees initial
.5 - 2.5 degrees @ 450 RPM
7 - 9 degrees @ 1,000 RPM
8 - 10 degrees @ 1450 RPM
10 - 12 degrees @ 2300 RPM
Modified - 14 degrees initial
1 degree @ 250 RPM
5 degrees @ 550 RPM
9 degrees @ 900 RPM
Vacuum advance 8" - 10" starts the advance
14" - 16.5" @ 10 degrees for full advance
10 degrees @ 1200 RPM

You can run forged pistons but must be same as stock type - 10.75 compression. So go with a light forged piston with light piston pin all to save weight and rev higher/faster.

Use aftermarket forged rods - the lighter the better.

Balance the assembly.

Next is the "Bobcat" treatment.
Thin .020" head gaskets.
Blocked exhaust crossover intake gaskets.
Poly locks to make the non-adjustable valve train adjustable and "zero-lash" the hydraulic lifters for higher RPM's and usable power. Be aware that your heads will have press-in 3/8" bottle neck studs and adding a larger lift cam could pull the studs out or break them. The best route will be the addition of 7/16" screw-in studs added to your stock heads for more radical cams.

NHRA did allow for headers in the stock class. But you can use stock log manifolds if you want to go that route. Have them polished inside to remove all rough castings, part lines, etc.. Port match. I would modify a set of head pipes to use a 2 1/2 exhaust system. You can use the smaller flange to use the log manifolds and then use the exhaust reducers in reverse to size up off of your flange to the 2 1/2" pipe. I have done this on a couple cars making my own head pipes, but not on a 1965 GTO. A muffler shop should be able to help create this head pipe.

Use the factory cold air open hood scoop and make a lower pan to work with your carb as the tri-power did. You can also go with a modified cold air set-up as used by the 1966 Olds similar to this set-up from Spectre https://www.summitracing.com/parts/s...SABEgIemvD_BwE

I would also suggest adding the upper/lower control arm braces to stiffen the rear chassis. These are available in a stock type used in 1967 and up, or aftermarket. I like and purchased the UMI set-up as it is adjustable and clears the floor pans. https://www.jegs.com/i/UMI+Performan...SABEgJf7_D_BwE

PontiacJim, that recipe is spot on.

Straight from 1965 - magazine article in building a B/Stock GTO. Used 4.33 gears & close ratio trans, and slicks. It'll be high on compression so you will want racing gas. The build used a factory cam & anti-pump up lifters and the rocker arm holes were welded up and re-drilled with smaller holes to keep too much oil from flowing. RPM for the engine build was 6,700 RPM's with stock prepped rods, tri-power, and the air tub. Good for getting the car in the 12's.

I question the primary jet size as it seemed a bit large for factory, but I could not find anything in my reference collection to verify or provide the correct jet sizing.

So you could go Stock in class and build accordingly by the book and have fun, or tweak a little here and there and add a little more tire smoke and speed.