Which head makes the most power?

[e30 gangsta]

Hello friends I have a 1978 xs1100 that I am currently working on. I'm dropping off one of my cylinder heads to the machine shop on Saturday, and I'm curious which one I should drop off to be redone.

I have a small valve head and a later model big valve head.

I'm aware that the later model heads come with bigger intake and exhaust valves, at the cost of slightly lower compression due to a bigger dome area.

I plan to run the 1978 cams for sure but I'm torn on which head to run.

Will the larger valve head out perform the smaller valve head even with the lower cr ratio?

Or should I just use the smaller valve head?

Everything that I've read states that smaller valve heads keep the air velocity up, vs the same head with bigger valves.

Any help or insight is greatly appreciated. Thanks guys!

[red bandit]

There has been a lot of discussion & testing on this subject. IIRC if you have 78 pistons go with 78 heads.

[e30 gangsta]

Alright last question. I dropped off my jugs and 78 head along with the cruzin image 1196cc big bore kit off to the machinist. Now that I am going to be running this big bore kit, should I still use the early 78 head? I have a 81 head as well that I can use. I plan to run the hotter 78 cams regardless which head I run.

I just want to maximize the setup as much as possible to make some decent power, especially since i'm at higher elevation. I found this post on here that leads me to believe that the later bigger valve head, might be the better options than the early model head. Thoughts?

Dan Hodges:
Originally Posted by Dan Hodges
"The domes on the late pistons for the standard bore measure 6 cc's and the domes on the Wiseco pistons for the 74 mm bore measure 6.6 cc's. The domes on the Wiseco's are shorter because the bore is bigger and the dome volumes are essentially the same. A cheap upgrade for a 78-79 combo would be to use the late piston with the early head which would increase the compression ratio by about three quarters of a point or about 3% more power.
The late head in stock form will flow about 10% more than a 78-79 head in stock form which equates to about 5% more power.
The early cams will make about 3 HP & 2 ft. lbs. of torque more than the late cams from 4,000 rpm up, about the same at 3,000 rpm and less from idle to 3,000 rpm. I've checked this on the dyno and a fellow member named Gene from Florida found similiar results. I've posted the timing specs and event angles on these cams previously and so I won't list them again. Installing the cams on the stock marks will work/run but you should degree the cams no matter which you are using. I used a "Bridgeport" to slot my cam sprokets but you can buy them new from Mega-Cycle already slotted.
If you have a bagger with the 750-850 FD you don't want the early cams but should use the late ones with the intake advanced 5 degrees because it's low end torque that is most important to you because of the high gear ratio. You can fiddle with the intake valve closing and static compression but it's 150-160 pounds of pumping compression that you are after in a torquer motor. All things equal, the quicker the intake valve closes the more pumping compression you will have."

[DiverRay]

The head will in part will be determined by the piston tops. If they are flat, use the '78, if domed with valve relief, use the big valve head. The other way to insure max power is to match the ports with the gaskets, both intake and exhaust. You can pay for the machine shop to do this, or do it yourself. All you are trying to do is have a clean transition from the carb boot to the head, no lip as in stock form. it should NOT look like a mirror, just be smooth from boot to head. The exhaust SHOULD look like a mirror, and be smooth from the gasket to the head with no lip. It's a lot of work for about .2 seconds in a 1/4, but all drag racers I've read about do this, including instructors from the '70's when I went to Arizona Automotive Institute.

[e30 gangsta]

The new pistons are domed with the valve reliefs. I guess that means I should go with the later head then. I was just thinking in my dense head that the earlier model head with the smaller chamber would give me the higher cr. The problem is I don't want to put money into the head go to install everything then have piston to valve contact lol.

I think running the later model head with the bigger valves and the 78 cams is the ticket.

[DiverRay]

If your not sure, do a test on the head BEFORE bolting it down. Put a little clay on top of two pistons, 1&4 will do. Place the head down, install cams without chains, and slowly rotate cam so the intake and exhaust valves are fully open. Make sure to set valve lash first! The clay should allow you to measure how much room you have between piston and valve. IIRC, you need about 2-3mm, but do look this number up.