Those aren't real SB hemi-heads, but rather, "mini-BB, splayed valved" type instead.
I don't think Chevy ever made a true MOPAR hemi-head for the SB..
If I was so inclined to try to install HH's on a SB, I would look closely at the '50's MOPAR hemi-head motors and try to find one that had as close a bore spacing as the 4.4" bore spacing the SB has and then figure out a way to redrill the decks and bolt them on!! Then figure out how to create the valvetrain and intake so everything would work...
BUT it would be a heavy bugger just like the old MOPAR hemi motors were back then!! AND it would breathe like there's no tomorrow!!!
Tony the pics in John's book don't really show the dome well. They actually had a dome over the entire top of the piston. The domes were cut quite deeply to accomodate the splayed valve angles. I purchased a new never run set of standard bore pistons from Yunick some years ago. I gave up on putting the engine together because I felt there were better choices out there for the cost. Most of the obstacles can be overcome, you can have a camshaft custom ground, have a custom set of headers built, clearance the lifter bores for the pushrods angles, gaskets can be made, a sheet metal intake can be fabricated and you can use the standard 302 crossram lid.
A splayed valve head is still available from GM, but no hardware for the valve train is offered. Moroso makes a valve cover for the head.
Back to the pistons, I sold them a few years ago, still unused. The only picutre I still have of them has one of them out of the box and turned sideways and you can see the intake relief cut into the dome extends almost all the way down to the top compression ring groove. Someone at Smokey's shop had written some numbers and letters in marker on the pistons, but I couldn't tell where it meant anything really.
The book Camaro Untold Secrets has a little better picture on page 122 of the dome of the piston part number 3865534. The engine isn't really a true hemi, but rather a modified semi hemispherical design as the book states, closer to the big block Chevy. But it is an interesting experiment.
By the way the pistons I purchased a set of the standard 302 pistons from Yunicks that were very heavy modified, with a lot of material removed to lighten them. The "hemi" pistons probably weren't modified but due to the age and oxidation you wouldn't be able to easily tell. Smokey apparently liked to mill a lot of material from the piston including the skirt and balance pad areas below the pin bore and the 302 had a very low pin location to begin with. He also had cut the valve reliefs deeper to allow for additional lift camshafts, he liked to use higher ratio rockers like the 1.7:1s from what I've read. The "hemi pistons" didn't appear to be reworked in this area.
I've never bothered to set up a site to post pics from, but if someone here has the ability to post pics and is interested I can send a copy of the pic of the pistons, but I think the Camaro Untold Secrets book has a better picture and it's highly recomended reading.
Forgot to answer your question regarding comp ratio. Really not sure, all I've read is Camaro Untold secrets says 12.5:1, but also notes 12:1. I've also seen somewhere mentioned in another article as 13:1.
Tony they look like flat tops more than they look like the dome we're used to seeing on SBC pistons. They are actually a mild radius across the face of the piston with the high point in the center that creates the dome, not like the large sharp dome most SBC pistons are known for.
Hey PDQ, your idea is one that my cousin and I worked on in the mid 70's. We took a 331 hemi head, and attempted to fit it onto a 340. We thought, what could be better than a HEMI-340? High winding, great breathing, etc. Some guys had done it with the 440, redrilled and installed 426 heads on it. It seemed to perform pretty good. But what a nightmare the 340 HEMI project turned out for us. We finally abandoned it, the expense was getting outrageous, and no machine shop seemed to want to grasp the concept and run with it. Also about then, MOPAR came out with the W2 head, which took care of the 340's breathing problems. But we were there! Also, I always kinda figured that Smokey Yunick would have adapted a hemi head onto the small block Chevy if it could have been done. So I don't have too much hope there. What do you think?
Chevrolet made around 8-10 sets of hemi heads and testing showed that a set of 186 heads ported by Traco made the same power as the hemi heads. So it wasn't worth the investment to make them since they wouldn't work on a street engine anyway. I'm not sure what the pistons looked like but they were some sweet looking heads.
Dang, I just did a search and there IS a company making true Hemi heads for small block Mopars! Not re-welds! Now I want to see independant testing of them versus the W2 head, or some other aftermarket aluminum heads. If they do outperform wedge heads, then maybe they will make them for the small block chevy, also, someday! We can only hope!
I figure that the reason hh's didn't perform all that much better then reworked wedge's is b/c our motor's bores and overall size is just too small compared to a B&S MOPAR hemi race motor.
May be wrong, but why go to all that trouble to install them unless something is going to improve big-time!!
Now the idea I have had about putting 409 "W" motor heads on a 454 motor and using figure-8 dished pistons to create max. torque like Mr. Fueling did intrigues me to no end!! BUT only from a low cost standpoint deal.. "Shade-tree" deal is all.....
As does Pete Ardema's SOHC conversion on stock, SB heads....
I think there are 3-4 of the 302 hemi motors assembled. Charley had one in a car, Floyd's museum had one and I forget if there was 1 or 2 others. In Wayne's book, the pistons are domed with serious valve reliefs cut into them for clearance. They are listed as 12:1 compression pistons. They are not flat-top pistons.
Super Chevy said 5 302's went to Yunick. I have been told however that of the five engines 2 of them were 350's and 3 were 302s. I spoke with a gentlemen who purhased one of the canted valve 350's from Smokey. He assured me the heads were still available in bare form and at the time when I contacted Chevrolet you could still buy the bare castings, but no valvetrain was available for them. Carl Dwiggens supposedly bought the other canted valve 350 engine.
I also was told that the 302's were owned by museums at Daytona and Talladega and Smokey had one remaining, again this was some years ago.
Doing the math however things don't add up because this doesn't account for the engine that went into the Price car, the yellow 69. But I've been told that this motor had the later GM Performance versions of the heads. There are others on this site that know the history of this car and maybe they can shed some light onto it's history, I don't really know anything about it other than what I've been told, or read. Even if that were true it would've taken the special design pistons and special ground camshaft and crossram base to put it together. So I still feel there were more of these, albeit maybe in parts forms rather than complete engines, that were given out.
Just after Smokey's passing they began to auction off all the stuff at the shop. I purchased the unused set of "hemi' 302 pistons from his daughter. This leads me to believe that while only 5 canted valve engines were given to him to tweak on there was probably a pretty good supply of spare components bundled with them.
There is more confusion to add as I know there was one 302 "hemi" put into a 68 Z/28 and sold through a Hemmings ad approximately 14 years ago. Also I've been told that the GM tech center had a fully assembled 302 "hemi" on display there for some time, could there have been more? In Hooper's book there are at least two full engines on display that an individual owns near here. Start doing the math and there are easily quite a few more of these than supposedly went to Smokey.
I wouldn't swear to any of this, it's just what I've learned through some discussions. I'm sure it can be disputed. The one guy that knows for sure unfortunately is gone now.
hey everyone u may be interested to know that the guy buildin my engine goes way back with engines and cylinder heads of that sort and he actually has an EXTREMElY rare pair of HHs that chevy made for a small block chevy. dont know if they ever went into production, but he had connections with chevy internals and somehow he got a pair. (has them mounted and everything...worth tons too). I think they must have been some kind of prototype SBC HHs, but anyways he has them. I will try and get some pics for the site.
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