[geeteeohguy]

Originally Posted by Skip Fix

I always thought #15s would make a good pump gas 400. Why I kept those off my 455, but probably will never use them.

Mine cc'd at 87cc, so the CR is about 9-9.3:1 with the .030 flat top forged pistons in my engine. No regrets.

[Will]

I would argue that "theoretically" the big valve heads are superior in just about every respect.

When set up properly they have better flow numbers at all lifts due to the larger area and 30 degree seats promoting better flow at lower lifts. So, just looking at flow numbers, they *should* make better power in any engine. However, whether the engine can make use of the additional flow is another question.

Port volume does come into play as others have pointed out. Heads with big valves and small valves have different shape ports with different volumes. There are no apples-apples comparisons here. You can't say "an engine with identical heads other than valve size" because Pontiac never made any heads like that. There is a relationship between port area (particularly the throat just above the valve) and the valve size that needs to be maintained in order to have decent flow at lower lifts and to promote good flow velocity which is especially important on the closing side of the cam lobe.

When choosing an iron head, I would be concerned mainly with chamber size so I could get the compression ratio I wanted. If it's a small valve head, just have the larger valves installed. It's not a big deal for a machine shop to open up the throats to the appropriate size for the larger valves and then they'll flow very comparably to stock large valve heads. The only other downside is that prior to '74 all the small valve heads came with pressed in studs that will need to be converted to screw-in to avoid problems if running larger than stock cams.

If you're on a tight budget and just want to build a good performing street engine, stick with the small valves. As others have pointed out, they won't hold the larger engines back all that much and might be a better choice for smaller engines (326 & 350).

[Dragncar]

Growing up we had a big boat of a 66 Catalina station wagon. What do those things weigh ? A LOT.
It was a 389 2 barrel engine so it had to have small valve heads. My dad went to a junk yard and got a AFB intake and carb off a GTO. That thing got 19mpg. Had to be at least fairly efficient to get that mpg out of that tank of a car. It did have a 3 on the tree though.

[SD455DJ]

Originally Posted by Skip Fix

Very conservative dyno through a full exhaust(real mufflers not dyno big mufflers) made 420 HP with an UD 288/296 HFT cam HO intake and 73 SD SR Q jet. In the car with stiff autocross suspension and sticky DOTs and 3.42s ran 12 teens @ 117 through a 3" H pipe and 2.5" tailpipe exhaust system Weight was about 3950. Was a blast with road race slicks autocrossing also.

I did have to run race gas mixed in it even dropping the CR my shaving the pistons and a 0.080 down the hole deck.

Nice! 420 hp on at the rear wheels? Did it have RA manifolds or headers?

Dennis

[SD455DJ]

Originally Posted by geeteeohguy

Mine cc'd at 87cc, so the CR is about 9-9.3:1 with the .030 flat top forged pistons in my engine. No regrets.

My #15 heads were 89 cc's with a truing cut and new Ferrea valves.

Dennis

[firebirdcrazy1959]

Originally Posted by Skip Fix

I always thought #15s would make a good pump gas 400. Why I kept those off my 455, but probably will never use them.

They do,i run them on my 73 400,068 cam,stock intake and exhaust manifolds,2.5" pipes.

[phil400]

So for conversation, on a .030 or .060 400 which would make more power, say a small valve head(retaining small valves like '69 #46 heads)with more compression like 9.5/10-1 Or a large valve with less 8.5/9-1 compression?(6x-4)

[PDC]

Originally Posted by SD455DJ

I totally agree with Stuart (Slick), Skip, and Cliff. These engines are sleepers and don't need large valves when the engine is fairly stock. Here is the dyno sheet of my '70 YH 455 360 hp stock rebuild motor with the original small valve #15 heads, a Summit 2802 cam and stock log manifolds. The compression was just a smidge under 10.0 to 1.

I believe it could have cracked 400 hp with more cam, but I was happy with the 390 hp/512 tq it made. I also believe it would have made 420 hp/525 tq with stock #64 large valve heads throughout the pull range.

Dennis

With all the big-displacement roller-cammed fuel-injected dream-builds these days, it’s important to remember how much fun you can have with a flat tappet carbureted engine that churns out power like this.

[Skip Fix]

420 flywheel but had a fan and exhaust.

[Will]

Originally Posted by phil400

So for conversation, on a .030 or .060 400 which would make more power, say a small valve head(retaining small valves like '69 #46 heads)with more compression like 9.5/10-1 Or a large valve with less 8.5/9-1 compression?(6x-4)

I think you could get more peak power with the lower compression, better flowing heads. You can do some things with cam timing to help make up for the lower compression, in part.

You would probably trade low speed torque though and the lower compression engine would likely feel softer at lower speeds and need more converter and gear to make up for it.

[footjoy]

I built a 400 with #15 heads I had the heads redone I put screw in studs larger springs and all the other correct stuff. It was the first pontiac motor I played with it had good response but I missed something in the build. Maybe bad tune wrong cam, carb. My choices were not good ones. I didn't have the heads ccd before hand and found out after I took motor back apart the heads were 90 cc combustion chamber. Which gave me 8.6 compression and I was hoping for 9:1
I didn't do my home work.
I think I will put these 15s on the shelf I tried to sell them online but all I got was stupid comments on small valve heads.
But I think these heads would work great on a 455 or a 400 with the right parts.

Thanks Greg

[phil400]

Originally Posted by Will

I think you could get more peak power with the lower compression, better flowing heads. You can do some things with cam timing to help make up for the lower compression, in part.

You would probably trade low speed torque though and the lower compression engine would likely feel softer at lower speeds and need more converter and gear to make up for it.

Alright cool, thanks.

[Cliff R]

"So for conversation, on a .030 or .060 400 which would make more power, say a small valve head(retaining small valves like '69 #46 heads)with more compression like 9.5/10-1 Or a large valve with less 8.5/9-1 compression?(6x-4)"

Compression is a big player in power production (torque). It also allows for a larger cam. For the example above the 10 to 1 compression ratio engine will make more power in the upper mid-range and top end even with smaller valve heads. The full point compression ratio increase allows for 10 degrees more camshaft with zero negatives anyplace, about the same cranking compression, idle quality, low speed power, etc as the lower compression engine with a smaller cam. From there on up to the shift point the higher compression/larger cam engine will make more power at every RPM. The slight loss in air flow using smaller valves wouldn't even be noticed........Cliff

[steve25]

Phil the main fact to keep in mind in regards to your question and if both motors are within 1/2 point in compression ratio is that a in regards to output power a motor is nothing more then a air pump.

Above 4000 rpm the more air it moves in and out of itself per revolution then the more power it will make.

[slowbird]

There is a reason why GM stuffed in huge valves for the bore size on LS engines and there is a reason why Ford went to a 4 valve design. More valve area equals more power.

[geeteeohguy]

Originally Posted by PDC

With all the big-displacement roller-cammed fuel-injected dream-builds these days, it’s important to remember how much fun you can have with a flat tappet carbureted engine that churns out power like this.

This has been my M.O. for the past 40 years and it works for me. The engine in my '65 is flat tappet and carbureted and has been running just fine since I assembled it in...1981. No roller failures, no spun bearings, no lifter failures, no flat cam lobes. Just miles and decades of low-tech dependability and good power. The original 400 still in my '67 with the #15 heads gets me 20-21 MPG at 75-80 mph consistently (2.56 rear gear) and has 253,000 miles on it.....90,000 of which have happened since I overhauled it for the first time.....in 1988. Stock Q-jet and original points ignition. No high-strung, short lived, super high performance engines for me.....these are classic muscle cars for STREET driving. I LOVE not working on broken engines. It saves a lot of money, too. Amazing how well the GM engineers figured things out back then.

[455abody]

Its interesting to see the power numbers from that YH 1970 motor made. That is pretty close the the setup I have.Its nice to see it made almost 400 hp. Nice

[65sport]

Originally Posted by Steve C.

http://www.wallaceracing.com/hp-valve-size.php

http://www.wallaceracing.com/intake_valve.php


.

Do you know what "FPS" means as an input on valve size ?

[slowbird]

Originally Posted by 65sport

Do you know what "FPS" means as an input on valve size ?

Feet per second

[SRR]

I once had a 70 Sport with the stock 350P with 17? (small valve) heads and one day a rocker stud broke. I had a fresh pair of 62's (big valve,screw in stud) so I bolted them on along with a stock Q-jet intake and added dual exhaust. 3.23 GTO diff. It felt like I added 100HP to the car!