Low compression 400 HP upgrade challenge
 

The challenge is to get as much streetable HP out of this block and heads.
I read about these builds with low compression heads and I hope to tear this down this winter and make some changes.

With a crower 60240 cam this engine dynod at 217 hp to rear wheels

I have a 400 bored .040 fresh(2000 mile)
Deck helght .021
The block is built and not going to change

The heads are # 15 small valves 87cc right now about 8.6:1 SCR

cam is optional I own a 2801 cam never been used but I am open to other cam selictions.

Automatic overdrive with 3:73 gears
Stock exhaust manifolds 2.5 dual exhaust

Carburetion wil be a Pontiac tri power.

I think I have covered everything.

I am open to head work and new cam.

Thanks
Greg

Switch to nice headers an option? Or better manifolds?

or switch to big valve heads an option? i'd prefer a set of common 6x-4 big valve heads & shave them some to get closer to 9-9.5 comp. would be able to take advantage of better cam options.

When looked at in terms of pre 1971 gross HP numbers stock D port non RA type exh Manifolds have been proven to allow some 360 hp on a 9.5 to 1 400 motor, if your looking for HP numbers above that then it's atleast time for RA type Exh Manifolds or headers!

My former “spare” Pontiac 400 was a low compression (7.50 to 1) deal. The cam used was an Elgin version of the Summit 2800. I used 1.6 rockets, a Performer manifold with Qjet, and headers. It was a kick to drive. My rear gear was 3.42 with a 4 speed and 82 inch circumference tires.

Lets take the heads what is the most I can do with these heads to get 9+ compression?

Thanks

We got a stock 400 with 7J2 heads to run mid 14’s with a Summit 2800 cam. That was after I drove it to school, 60 miles round trip every day for 4 years. Then it sat for 10 years.


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Speed pro/TRW pontiac ForgedFlat top 4VR .040
https://www.summitracing.com/parts/s...w/make/pontiac



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I had a throw together 400 once. We put the 87cc 15 heads on it with a 68 intake and a worked up 78 qjet. Hooker headers and 3” torque tech exhaust with flow masters. 4.10 gears and a Lunati single pattern cam with 235 .050 507” lift. On bald futures street tires it ran 13.20 @ 104mph. Tranny was a stock th350 with a shift kit and a cheap summit 2500 stall. Car was a 79 z28 that weighed 3600lbs.

I would throw on some 16’s, 62’s etc with a healthy cam and headers or ra manifolds with a big non restrictive exhaust.

Another interesting thought would be to port your heads and mill them but I’m not a fan of milling. My brother ported a set of 72cc 15’s in the garage for his 69 firebird and he picked up 4 or 5 tenths and he kept the stock small valves too.

if you are at 8.6 now, should be pretty easy to mill them to get 9:1. heres a chart showing the cc's removed for any given amount milled. ive had good success with milling 6x heads .030-.050 & did not have an issue bolting up a performer intake on an old 455 i had back in the 90's. others have had issues milling heads & had to mill the intake to fit. looks like you can get an easy 5cc removed with only ~.025 milling.

you can gasket match the heads & do light porting iof youre comfortable doing that as well, but i still say to get some better heads with bigger valves to help them flow better. could probably sell the 15's for as much or more than what a set of 6x will cost.

http://www.wallaceracing.com/headmill.htm

By the title, I take it you're trying to get this thing to at or above 400hp? I'm not certain how attainable that will be without a head change. Large valve, small combustion chamber heads with your 2801 and an upgrade to either headers or RA manifolds should get you there. I kind of get the feeling from your posts that you're not really keen on the head swap though?

What's your budget? Do you already have the intake for the tri-power and carbs?

I know it's not everyone's cup of tea, but based on the power desire, what would be needing changed to make that a reality and already having a good engine running, I'd maybe think about a small turbocharger. Your static compression ratio is already in a decent range for that and from the sound of it, the cam is probably decent for that application as well.

There's obviously more fab work involved with that deal, but you could see an honest 400 hp to the tire both reliably and maintain the same streetability the car currently enjoys.

I messed the title up How do I get more horsepower out of this low compression 400

I messed the title up How do I get more horsepower out of this low compression 400
 

NEW TITLE


How do I get more horsepower out of this low compression 400 ci motor

ok. mill the 15 heads, gasket match or light port work, change to big valves.

pick a good cam to match the compression. consider 1.65 roller rockers if you use the cam you have for a little extra dur/lift.

use as thin of head gaskets as possible to keep the quench & comp as good as possible.

headers or good manifolds & good exhaust with free flowing mufflers.

those types of basic mods will be the cheapest way to increase power, unless you want to add a turbo....

I have used that summit 2801 in several low compression 400s and it works very nice

In the one in my 81 TA and is nice slight lope works A/C etc well.

Footjoy, I just dyno'd a 400 with stock small valve #16 heads (78 cc's) that is .030 over with the 2801 cam and it made 360 hp/425 lbft torque through the stock exhaust log manifolds, cast iron intake and Q-jet. The pistons are .0015" in the hole and using Felpro 1016 .039" gaskets netted .0405" quench and 9.94 static compression.

I think you can mill your #15 heads 0.036" to reduce your chambers to 81-82 cc's and use a .025" Cometic head gasket to get a reasonable .045" quench and 9.5 static compression. Enlarge the pushrod holes to accept 1.65 rocker arms and keep the cam you have. RA exhaust manifolds are nice and will help add power throughput the range. I think you should get a true 300 hp to the rear wheels with a good tune and 93 octane fuel.

Dennis

Although I didn't find a .025" Cometic gasket, they do carry a .027" MLS, which is pretty much as low as you an go without o-ringing the block.

Cometic C5769-027 has the compressed thickness along with a 4.2" bore to minimize gasket volume down to 4.77cc. The .047" quench is not great, but much improvement over what you probably have right now.