When do big valve heads out perform small valve heads?
 

I know the large valve heads rule. If you have two identical castings one with the big valves and one with small valves, at what point do the big valve heads make more horsepower is it at 1000 rpm or 2000, or 4500? I am just curious. if it is 3500 or more how much would a everyday cruiser benefit?

Thanks

Just my opinion but everywhere everytime if setup up right

There’s a lot of variables but, theoretically if you put the same heads on the same engine with the only difference being one has big valves vs small valves, the big valve head should outperform throughout the entire rpm range since more air can be moved than the small valve heads.

Ok that was probably a stupid question.

Thanks

Not really.. a small valve can be torquey in the lower rpm. The smaller valve can produce higher velocities in that range in general terms so youre question itself isn't stupid

Moved to Street forum, the engine in question is for a street cruiser.

I honestly doubt you’d see any difference between the big and small valve heads on your Tri-Power street 400 with the 068 or similar cam you’re planning on using.

Keep in mind that all Tri-Power engines had small valve heads and none of them were slouches.

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

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


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Folks underestimate the potential of the small valve heads and still seek out big valve versions.

In many cases a small valve head will be a better bargain, and you can always put big valves in them anyhow.

Years ago we had a 455 short block ready to finish but the custom CNC ported KRE heads and roller cam coming with them were not going to make it in time to get the engine finished and in the car for the Norwalk race.

I reached under the work bench and grabbed up a set of 1969 #46 small valve heads. Did a quick seat and valve grind on them, better springs and screw in studs. Went to the parts room and grabbed up an older Crower flat hydraulic cam and finished the engine and got it installed in time to make the race.

The car was an 1981 Firebird, TH350, Continental converter and 4.10 gears with pretty tall DOT's. Right off the trailer it ran high 11's in the heat of August and it was pretty hot that year. I wasn't expecting much more than low to mid-12's out of it to be perfectly honest. You just never know with these things......FWIW......Cliff

There's no carved in stone answer to that question as it by enlarge depends on how many cid the motor is and how it's Camed.

Some factors are right off the bat the small valve 1968 and up heads are down to 148.5 CCs of Intake port volume from the standard 154 to 156 CC size of the big valve heads, this kicks up the port velocity.

This fact right there brings down the torque and hp peak rpm of the small valve heads by 5% as compared to the big valve heads.
This makes them a great pick for the 326 motors and grocery getter 350 motors with 2.73 rear gears.

On bigger cid motors you can go 5% bigger in Cam duration if you want without having a low speed Torque drop off.

Any way you look at it though if your not with your back up against the wall when running these small valve heads of having too much compression then if your rebuilding them with new valve and need to do a valve job anyway then there's no doubt that installing the bigger valve sizes ( both Intake and Exh in most cases) is very worth more power.

Do a search in this section for what I posted about reworking 2 years ago, search under " The plus side of cheap small valve heads"

Following that logic one could simply up the cam duration by 5 percent to pick up the lost airflow using smaller valve heads.

I know on the engine mentioned above we used a pretty "hefty" cam in it, 308/314, 248/256 @ .050" on a 108LSA. Idled like chit, stinky, but "BRUTAL" mid-range power. It was more or less just a parts engine filled with what we had readily available at the time to make time constraints.

It eventually did get it's custom CNC ported KRE heads and a custom ground solid roller cam which put it solidly into the high 10's. Also interesting is that the new combination doesn't "feel" as strong as the old one if you were evaluating the performance by "the seat of your pants". It's smooth/broad/flat power curve just pulls really hard like a big locomotive and lacks the explosive mid-range power it had with the iron heads and flat tappet cam combination even though it obviously puts considerably more power to the pavement at full throttle thru the 1/4 mile........Cliff

Correct numbers or not the 70 455 with small valve #15s was only 10 HP off the the 70 455 with #64 big valve heads, not sure if there was also a cam difference or not.

Totally true Cliff!

The 70 455 with 64 heads would have had the 068 Cam, the 1968 400 2bbl casting 15 head I can't say without looking it up, but for sure it would not have had the 068 Cam, maybe at best the 066 Cam if ordered in the GTO with that option, or if not that then the standard 2 bbl Cam of the day.

FWIW,

I had a totally stock, but well tuned 1970 Bonneville with the #15 head 455. 3.23 Safe-T-Track and turbo 400 (naturally), ~29" tall tires (P255/70R15 Radial T/As).

It was very fast for what it was. 13.90s at 97. My original 70 T/A RAIII, 3.31 Safe-T-Track, T400 ran 14.31 at 99.8. Runs at Capitol and Budds Creek.

Of course the T/A spun through all of first, the Bonneville was glued.

P cam in Bonneville (i.e. 067).

Bottom line: in mild form small valve heads can run!

Stuart

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

Dennis I always wondered why the non AC/PS 70 motor had such a big WP pulley compared to others.

The 455 block and crank in my 78 TA started out as a '70 Bonneville 455 motor. First RAIV heads now Eddy heads. Kept the long tail shaft TH 400 as a back up core too!

Skip,

This YH 455 was the replacement motor for the original 350 2-bbl engine that we removed in the LeMans 4-speed that has manual steering, so the WP pulley in the picture is from the LeMans' 350 motor. Yeah, they are huge and maybe too big to work properly with PS? I know AC cars had smaller pulleys to spin the WP faster to circulate the coolant quicker.

I'm sure you told us before, but how did your TA run with the RAIV heads on it?

Dennis

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.

I have been running these exact heads on my '67 400 in my GTO ragtop for the past 10-12 years. With an 068 cam, stock q-jet, TH400 and 2.56 gears, it's a great economical high speed low rpm road car. The only thing I did was add screw in studs. Not as strong as it used to be with the 3.36 gear and 670 heads, but it runs on pump gas, which the stock combo wouldn't.

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