Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

[Larry E.]

Question for some of our Engine Pros on this Board:

Been a fan of NHRA Pro Stock Class for years. I noticed in the past 10 years or so that the majority of
wins and pole wins have gone to the teams using the GM DRCE Block which is based on the Chevrolet
B/B. The Chrysler Hemi in most cases is always slower. Why is this so? Does it have to do with the fact
the Hemi is heavier then the DRCE(that way the teams can put the weight where they want to)or is the
Semi-Hemi head design better then the Hemi design in carburated form? Thanks in advance--Larry

[Dick W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Can't give you a definite answer, but factory money seems to help.

If you are interested in hemi heads, I just received an email about a set of 32 valve SB hemi heads. Have no idea who made them, where they are at, or how much they cost. But if I had the money to play with right now, they would be on the way to my shop. I wanna build one mo' play toy, then again they might look good on the White Ghost of Dave Gray and KC Strawmeyer at Bonneville next year.

[Clem Z.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Can't give you a definite answer, but factory money seems to help.

If you are interested in hemi heads, I just received an email about a set of 32 valve SB hemi heads. Have no idea who made them, where they are at, or how much they cost. But if I had the money to play with right now, they would be on the way to my shop. I wanna build one mo' play toy, then again they might look good on the White Ghost of Dave Gray and KC Strawmeyer at Bonneville next year.

chevy made some for the 302 trans am program but they did not add any HP over the std heads and they weighted more so they dropped them. there is company out there selling hemi heads for both BB and SB chevies. http://www.guinns-engineering.com/Ri...ds and Cam.htm BBchevy hemi heads. http://www.racecarparts.com/images/AriasBlown.JPG

[Duke W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

I recall way back in the seventies "Moser" heads for the SB had four valves per cylinder operated by pushrods. I have no idea if they are still around.

Pure hemi heads (near hemispherical with no quench area) may offer an advantage in valve size for a two-valve design, but the large chambers have less detonation resistance than a smaller chamber with quench area, so the pure hemi probably can't run as high a CR. Most so-called "hemis" are really "semi-hemis" with the valves offset to one side of the chamber and a quench area on the other side in an attempt to make the chamber more compact so CR doesn't have to be compromised.

One of the reasons why narrow angle four-valve heads (as originally developed by Cosworth in the sixties from older wide-splayed four-valve designs) are so widely used on both road and racing engines is that four valves provide more "curtain area" (valve circumference times lift, which is usually the limiting factor on flow, especially in a racing engine) than any two-valve design you can stuff into the same bore size. A narrow valve angle with quench areas along the sides provides provides a very compact chamber with a centrally located spark plug, which is ideal from a thermal efficiency standpoint. That's why modern four-valve heads require less spark advance than vintage two-valve quench chamber heads. The less spark advance required for a given bore size, the quicker the charge burns and approaches the ideal of combustion at constant volume, and that's why four-valve heads will usually make more torque/power with lower brake specific fuel consumption.

But there are disadvantages, too. For a given bore and stroke a four-valve DOHC engine is bulkier, heavier, and has greater internal friction, which is an issue in road engines that spend most of their time at idle and part load.

Ultimately, though, the biggest factor for a racing engine where you are looking for maximum power for a given bore size is port flow - just like on a road engine. Back then designing ports was somewhat of a black art - try this and flow test.

Nowadays computational fluid dynamics (CFD) programs can accurately predict port flow, and it's a lot easier to change the port geometry to obtain a better flow coefficient in the computer rather than building a port and testing. This is why the modern LS engines are so much better than vintage SBs. The design was 99 percent optimized - small package volume, light weight, high specific output, and low specific fuel consumption - before they even built the first prototype.

Duke

[David G.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Can't give you a definite answer, but factory money seems to help.

If you are interested in hemi heads, I just received an email about a set of 32 valve SB hemi heads. Have no idea who made them, where they are at, or how much they cost. But if I had the money to play with right now, they would be on the way to my shop. I wanna build one mo' play toy, then again they might look good on the White Ghost of Dave Gray and KC Strawmeyer at Bonneville next year.

Dick, Right now we have our hands full getting more speed out of our 770 hp aluminum small block, but in a year or two I am sure we will be looking at some
"Mountain Power". Keep posted
Dave Gray

[Dick W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Last week, I saw a complete engine with those heads. Made another stab to buy it. I don't know what I would do with it though. Piggins told me they would make power, but the ports were so large they would not accelerate worth a hoot. Ran fine at 8,000 though. VERY narrow power band

[Bill M.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Question for some of our Engine Pros on this Board:

Been a fan of NHRA Pro Stock Class for years. I noticed in the past 10 years or so that the majority of
wins and pole wins have gone to the teams using the GM DRCE Block which is based on the Chevrolet
B/B. The Chrysler Hemi in most cases is always slower. Why is this so? Does it have to do with the fact
the Hemi is heavier then the DRCE(that way the teams can put the weight where they want to)or is the
Semi-Hemi head design better then the Hemi design in carburated form? Thanks in advance--Larry

Found this on yellowbullet. This applies to pure racing cams:

"The hemi will not allow you to put in the cam you want due to contriants caused by valves touching, plus you have to worry about ex valve flex due to exhaust backpressure. So you end up putting in a cam that fits. Now you have to tune around that cam and try to make it work. With the wedge you can use the cam you want, tune it the way you want and end up with a way more consistant car."

[Dick W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Found this on yellowbullet. This applies to pure racing cams:

"The hemi will not allow you to put in the cam you want due to contriants caused by valves touching, plus you have to worry about ex valve flex due to exhaust backpressure. So you end up putting in a cam that fits. Now you have to tune around that cam and try to make it work. With the wedge you can use the cam you want, tune it the way you want and end up with a way more consistant car."

Way back when, we ran some VERY wicked cams on the short tracks and for qualifying. Believe it was Racer Brown STX 13 grind. Instructions from Racer Brown/Chrysler stated the cam was for 100 mile races max. The profiles were so radical that they recommended that you replace the valves, retainers, and springs after a race

[Michael H.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Last week, I saw a complete engine with those heads. Made another stab to buy it. I don't know what I would do with it though. Piggins told me they would make power, but the ports were so large they would not accelerate worth a hoot. Ran fine at 8,000 though. VERY narrow power band

Disk,

I have the GM dyno sheets on the semi-hemi 302 inch Trans-Am heads that engineering was playing around with in 1968 or 69. The peak HP was slightly higher than with conventional heads but the peak was raised about 1000 RPM and the mid range was reduced.

[Dick W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Disk,

I have the GM dyno sheets on the semi-hemi 302 inch Trans-Am heads that engineering was playing around with in 1968 or 69. The peak HP was slightly higher than with conventional heads but the peak was raised about 1000 RPM and the mid range was reduced.

The engine here is a 350, I am assuming they are the same heads as the T/A engine. The ran it on the dyno at JR Johnsons shop and told me that it would not accelerate??

[John H.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

Here's the 302 Semi-Hemi; as noted above, it was very expensive and complex, and didn't provide any usable benefit over the production small-block head design.


cd_3.jpgd8_3.jpg

[Duke W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

John - do you have any info on the detailed differences between these heads and the Grand Sport twin plug hemi heads?

I wonder why they didn't just resurrect the twin plug heads instead of starting a new design.

Duke

[John F.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

My old high school buddy's son builds some mean racing motors for some class of pro stock. Check out his website at Uratchko Racing Engines.
John F

[John H.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

John - do you have any info on the detailed differences between these heads and the Grand Sport twin plug hemi heads?

I wonder why they didn't just resurrect the twin plug heads instead of starting a new design.

Duke

Duke -

I think the twin-plug Hemi was another experiment that never made it out of the dyno lab; it never showed up in any of the Grand Sports. Zora did a number of pretty exotic experimental engine designs - that one is at the bottom right of the photo below. That photo was shot in 1967, in GM's 12 Mile Road warehouse, across the street from the Tech Center, where most of that stuff was stored back in the day (and subsequently disappeared or was scrapped).


ZoraEngines2.jpg

[Duke W.]

Re: Semi-Hemi Vs. Hemi in Carburated Maximum Horsepower Race Question

I remember that Hot Rod cover. I think it was earlier - while I was still in high school - that they did a similar cover on Pontiac experimental engines, but I can't remember if Pontiac's chief engine engineer - I recall his name was McKeller - was on the cover.

I think Ludvisgen's book has a photo of the twin-plug hemi on the dyno, but it was never developed sufficiently - and there were probably too few parts to get it into the Grand Sports before management clamped down in early '63, which is why they used modified production heads on the revised 377 CID Grand Sports that went to Nassau in December that year.

When I went to work for Pontiac as a production engineer in 1968 I asked one of my mentors about those engines. He took me down to the basement of the engineering building and over in a far corner, there they were in a heap covered with dust. So sad!

That same day I also saw the two Banshee prototypes that were in car sized wooden crates out in a back corner of the engineering parking lot - forlorn and forgotten. (I was surprised when I learned from an Autoweek article years later than one had gotten into private hands.)

Pontiac had tunnel port heads on the dyno, but they got killed, too. The party was ending in 1968, and it was all about emissions and safety. I was born about five years too late.

Duke