I was amazed at how you all raved about the 781 heads. I didn't know they were that good and could be good for so much horsepower. I have both a set of open 781's and a set of closed chambered 702's. I passed on the 781's and put on the 702's on my 460 to bump up my compression since they both have the same valve sizes. It seems to run quite nice but I'm wondering if the 781's have more or better runner volumes. I'm going to be bumping up to a roller hyd cam down the road and want to make sure I use the best head I have on the shelf. I also want to know, since I have to have the spring seats cut for the bigger springs, if it would be a good idea to put in bigger valves at that time. Willl either of these heads work well or better with bigger valves, or are the 2.06/1.72 the biggest I should go regardless of which casting I decide to use???? Thanks!!!!

Imho, the deal here is that a smaller closed chambered head burn's slightly better so that a schosh more compression can be ran holding timing the same! But it doesn't breath quite as good as the open chambered head. This and from a smog standpoint, they have more chamber wall which quench's the flame and make's them emissions dirtier than the open chambered heads.

Only problem with the open chambered head is that you have to run some sort of piston dome to get any sort of decent compression out of them and this hurt's combustion eff. some vs a small chamber and flat-tops. More timing....

And as far as adding bigger valves, stock small ones that have been bowl-blended and port casting flash cleaned up will flow about the same as just untouched heads that have big valves installed in them. So you won't see any pick up unless you unshroud, bowl-blend and casting-flash clean them up after you install the bigger valves.

Then you will need to check their cc b/c unshrouding them will more than likely make the chambers bigger which will necessitate planing them down smaller if you want them to cc what they did before you worked them over?

I really wish W/P's would step up to the plate and make a 90 cc open chambered, 290 cc port true large oval cast-iron head that would use stock valve-train stuff and that would sell for what their cast-iron Merlins sell for.

pdq67

Agreed.....but even better than that would be that I would finally win the powerball and be done with this thing my wife calls my "garage budget". I'm thinking the Merlin 632 for my daily driver...:thumbsup:

pdq67.I thought I read an article from Dave Vizard comparing valve sizes in the bbc vs sbc.He mentioned the bbc was undersized when comparing displacement.I think your a fan of the hp How to hot rod bbc books.Do they not also recommend enlarging the valves if you can?I think those oval ports would benefit with 2.190/1.88,s.Would they not increase the umbrella area at mid to full lift?I agree with you..they are a great oem head cleaned up.:)

I agree George!

I like that old HPBook book and right, the BB's are valve dia/cylinder volume ratio'd smaller than the SB's if I remember right.

As long as you also do everything else to them that you would do to the small valve heads to use them. Otherwise, they probably will only run as good as the smaller valved, worked over heads.

It's funny tho b/c I think it say's that GM thought that their stock, over the counter, 2.300" valves were too big for even the 495's or whatever their big CanAm engines were back then that ran the old Crower Calliope mechanical FI unit that had different length stacks.

But some guy's swear by them..

pdq67