Putting the Super High Output in SHOs

By Timothy Wright a.k.a. "Buford T. Justice"

(Parts One and Two of a three part series, as published in the SHO Club Magazine, but with color pictures in high resolution.)

(picture above: start with one V6 SHO head, add in band saw and you get sliced aluminum with all the passages open to see.  Click on all the pictures to see the full size version, note: we kept them high resolution.  Those on dial-ups may spend a little time waiting for downloads, but without the detail, the pictures are not as valuable or fun.  We cut two heads, one V6 and on V8.  They each got Three cuts.  two cuts so we could see a thin slice through both sides of the ports, and then with one end of the head we cut down the long axis to reveal some different passages.  Study the V6 and V8 images at the start of this article to see how this was accomplished. -- Ed)

I invested much of my misspent youth reading car magazines. Back then when engines were less efficient per cubic inch and much of the HP magic involved trick heads. Cleveland heads made more HP then Windsor heads. Chebbies had the magic 202 LT1, angle plug heads, closed chamber and open chamber heads. Pontiacs had oval port, D port and Ram Air heads. The choices basically were either conservative heads that made fair torque for daily driving or special, rare and expensive heads that flowed great near redline, but also required a radical cam shaft, a new carb/manifold and stiff gearing to keep the engine near redline all the time. The art was to choose from about 50 casting numbers to find something you could live with, find the optimal balance radical enough to win on the track without being so radical you would lose so much low end that you lost low end punch in day to day driving.

If selecting the perfect head casting is as obscure to you as helping your spouse shop for shoes I have some great news. If you have a SHO you have few options, but then again you don't need any. SHO heads, either V6 or V8 are as exotic as it gets and perform just great from idle to redline and make so much HP per cubic inch that I no longer lust after the rare and exotic heads of my youth. While the lust for obsolete heads may have waned, intelligent reverse engineering and discussion of SHO heads is hard to come by. This is not, and need not be a "how to improve your heads" article since ours work very well thank you. But if you are interested in how the put the Super High Output in your SHO this is both your primmer and anatomy book. If anyone wants to develop the skills to do competition head work, valve seats, porting and polishing this article may serve as a guide.

(A look at the V8 head after the attack of the band saw!)

 As mentioned historically the trade off has been small valves and ports restrict flow at high RPM, or valves and ports too large which cause loss of midrange punch as air velocity drops through the ports. Until recently most cars had only 2 valves per cylinder. Normally one large valve to let the fresh air and fuel in, and a smaller valve is sufficient to let the hot exhaust escape since it is under great pressure was sufficient. The V6 SHO was not the first 4 valve DOHC sedan but it was exceptional in its day. It is very difficult and expensive to produce very high power per displacement from a two valve design. At 6,000 RPM a valve has to open and close 50 times a second. Double or even triple valve springs maybe required to control large heavy valves. If the revolution to over head valve engines at least doubled the output per displacement 70 years ago, the revolution to over head cam engines during the last 20 years has doubled the specific output yet again. Problem is we have a lot less information about our heads than I am used to enjoying, few good photos or drawings of the heads for the technically curious. That is just is not acceptable, it is time for SHO heads to be exposed.

Family Resemblance

The V8 and the V6 have almost the same stroke but the V6 has 2 less cylinders and ¼" more bore. As a result a bare V8 head weighs 31.16 pounds vs. the lighter V6 head which weighs exactly 25.00 pounds. That is actually very light, an iron small block head can weigh 50-60 pounds and an iron big block head can weigh 100 pounds. Because the V8 cams are hollow I suspect a set of either heads fully outfitted must weigh about the same.

(The V6 head showing the exhaust outlets and the front or passenger side of the head facing left.  This would be the head closest to the radiator)

The general layout and design of the two heads are also similar but not identical. Both are Double Over Head Cam (DOHC) designs so named because each head has two cams directly acting on the valves. Some Ford DOHC Indy V8 engines had center exhaust and side intake layout. It might be wild to have a tangled mess of exhaust headers sprouting out of our hoods but visibility with a front engine car requires the center intake, side exhaust layout. The combustion chambers are similar but not identical pent roof designs, well known for power, low emissions, gas economy and very high efficiency. One important big difference not visible is in the cooling systems. The V6 SHO cools the block first then the warm water cools the head and returns to the radiator. The V8 in contrast sends the coolest water first to heads then the block. This is not a trivial difference. In a high output engine the exhaust valves are the most difficult to cool hotspot that limit prospects of further HP multipliers like NOS or blowers. So is the V8 cooling superior or did a superior system become necessary during development? I don't know.

(V8 head with all the cut parts assembled.  Again the exhaust ports are showing and this would be the head closest to the radiator with the portion to the left on the passenger side)

Both heads use simple 4 bolt per cylinder bolt pattern. With twice as many ports as an OHV engine, a lot more complex internal oiling system and much higher cooling requirements it has always been difficult to find places to locate the head bolts to give even and sufficient clamping force without getting in the way of oil galleys, water passages and intake or exhaust ports. With SHO cylinder liners there is not enough room for a head bolt between cylinders, that space is taken by water passages. From above one can see the bolts are directly in line with the spring perches and on either head the cams must first be removed before the heads can be removed. I always associated head studs with quality construction but both sets of heads rely on bolts not studs and are proven trouble free. The V8 the bolts are use once and throw away. Maybe they know we would be reluctant to remove and discard studs?

Photos at 11:00

Let's take a look at the V6 combustion chamber first. The small 2 valves seats on the top are the home for exhaust valves. The big valves seats on the bottom are where the intake valves live. The big round hole to the upper right is an oil drain back hole. We will see how the oil lubricates the cams and valve shims, when it does it pools in head had has to get back to the oil sump quickly. That is where the oil drains back to the sump from the head. The center hole is threaded and is where the spark plug lives. No projected nose plugs here. The plug is virtually flush. All the other holes are coolant passages in this case the coolant comes up from the block into the head. The bottom (intake side) has a slot for coolant which outlines the cylinder and disappears in the block. Is this to aid cold engine drivability? In contrast to the meager coolant trail on the intake side the exhaust side has abundant coolant passages. We will see more of them in greater detail again. Note the roof of the combustion chamber is mostly valves. Beyond the valves the head surface is flush creating "squish" areas. Imagine as the piston rises the gas in the center compresses, but on the sides the piston almost touch the head and at the last moment (TDC) the air fuel mixture is squished into the center like a grape run over by a truck. That last moment puff promotes turbulence, aids knock resistance and improves power.

The V8 SHO combustion chamber is similar. Valve seats are in the same relative positions, we have the same four bolt hole locations.  No coolant trough on the intake side, it has been replaced with a simple rectangular hole. On the exhaust side the holes are more ample if anything (this is more evident in cross section).

The combustion chamber is also a pent roof with some interesting details. Note two little flame troughs from the spark plug hole. The intake (bottom) side has an "eyebrow" extending the squish area which shrouds one valve but exposes the other? Which is the primary and which is the secondary valve? If the secondary valve is inoperable till the IRMC opens the butterfly valve at 3000 rpm then we have a 2 mode combustion chamber much like the V6 but this asymmetry is a sophistication we don't yet have figured out.

Part II

I found this brief and intelligent discussion of pent roof heads at Replika Maschinen a well known Californian machine shop that has a good reputation for advanced head work.

The modern, now almost universal low pent roof, narrow valve angle style four valves per cylinder combustion chamber, was designed, or re-invented etc., by Keith Duckworth of Cosworth engineering in 1966 for the incredibly successful Cosworth DFV Formula One engine. This engine dominated world Formula One racing from 1967 to 1983 and produced one hundred and fifty-five world championship victories. The advantages to this type of combustion chamber are focusing the compressed charge to the spark plug, raising an engine's knock threshold and the ability to run higher compression ratios. Additionally, higher port runner angles and of course more valve curtain and port area are also major advantages.

So what am I talking about when I say that a four-valve combustion chamber has more valve curtain area than a similar sized two-valve cylinder? A quick and dirty theoretical example should help explain this. Say we put a single 2.08-inch diameter intake valve into a four-inch diameter cylinder. This would be a fairly typical high performance small block Chevrolet cylinder set up. Now we take our theoretical four-inch diameter cylinder and remove the large single valve and in its place we install two smaller intake valves of 1.200-inch diameter each. We would have a valve diameter equivalent of 2.400 of an inch, more potential airflow and more potential power. When you add up the respective potential valve curtain area and the supporting port area you can quickly see that two is greater than one. - Don Redmon (http://www.speedoptions.com/articles/2327/)

It is a good reading article, but let me make my own stab at explaining valve curtain. At low valve lift the area available to let air in or out is related not to valve area but the circumference of the valve. As the valve lift increase the "curtain" is shaped like the sides of a little cylinder, air can not move through the bottom of the valve. Now the bigger that curtain the more air you can flow and two small valves have more curtain than on big one. Smaller valves are also much lighter, easier to control and by using a sophisticated tuned intake system we can have the advantages on one small valve (which keeps air velocity high and improves filling) at low RPM and two valves for maximum flow at high speeds. One paradox, the V8 has 6-13% more displacement but 12% less intake curtain and 3% less exhaust curtain. The V6 engines have much larger valves as may be expected because they are far more over square. What may be unexpected is the very large exhaust valve lift of the V8 which may be why it is an interference engine. What must be emphasized is if the heads look and function similarly, the actual dynamics are not the same. Someone who designed this part of the V8 concluded they wanted a smaller valve with more lift not the other way around. This choice has serious consequences for V8 owners.
                                     

How about if we cut open our two SHO heads to see what they look like in the inside?

Above is the exhaust side of the V6 head cut top to bottom. It shows a cross section of a pent roof combustion chamber. Notice the abundance of cooling passages above the combustion chamber and surrounding the sparkplug well. Those water passages go deep on this side and cover the exhaust side well leaving a pillar of aluminum supporting bolt hole. The "second story" of the head is where the oil pools, drains and then returns to the sump.

The V8 cross section of the exhaust side shows a little more curvaceous roof in the combustion chamber than the V6, the two tiny flame troughs jutting from the sparkplug hole are unique to the V8. Just like the V6 the coolant passages go deep on the exhaust side, leaving pillars to support the head bolts. One unique development is the system of unique ribs and struts hidden inside the coolant passages indicating a computer aided finite element design. Those are tiny tweaks beyond what a good designer can do with experience, simple formulas and rules of thumb. The round hole is an oil gallery which feeds the final cam bearing above it. As before the second floor collects oil, and a hole in the back of the low side on the right hand side of the photo is the location of one of the oil return holes. An increase in the complexity and detail of the casting, particularly inside the coolant passages is evident. Note both castings have those big plugs in about the same relative locations; some direct lineage may be evident. Why is the wall thickness of the sparkplug asymmetrical?  Might we have a touch of core shifting?

Above, the V6 photo is on the left, the V8 photo is on the right, both of the intake side, the other half of the above photos and frankly less is going on. The water passages only go back as deep as the bolt holes. On the V8 photo you can see the oil passage where it rises to feed the chain tensioner with oil. Again the interior of the V8 has more complex detail.

I hope I have saved the best part for last. These are the cross sections I wanted the most. First is the V6 head with the intake on the left side and exhaust on the right side. (the picture below is a cross section of the V6 head.)

We cut the head like a bologna through the valve guides once, then move over and cut it again. Most bizarre, the intake starts off with two ports, which share an open window (and hence a common plenum). Below the window is where the fuel injector lives. Even with the intake butterfly valve up stream closed both intake valves will open and both the injector wets both ports. Who would anticipate a 2-1-2 port design? The injection is close to the intake valves. If you remember the head sits on a 60 degree V block the intake port is almost vertical and is very straight. Note the minimal size of the coolant passage on the intake side. The round hole above the intake port is an oil gallery which provides oil for the intake side camshaft bearing. You can see valve guides, above them the well where the springs live. The finished trough of the spring is called the spring perch. The spring perch on the intake side is more robust (bigger valves) then if you tilt the page because the head fits on a V engine the oil drains from the cool intake side down to the hot exhaust side. Better that than soaking the intake side with hot oil? The semicircle is the bottom of the cam bearing. You can see how the oil first lubricates the cam bearing, leaks down and covers the valve shim, cools the spring, pools and finds a return hole. Down the center is the sparkplug well and the window in the center is the coolant passage. Now the valve guides are mild iron and they wear so your valve stems don't have to. As engines age sometimes the engine rings are fine but your valve guides may leak oil, and here is a good look at them rascals. More difficult to spot are the valve seats seen here in cross section. They are round like a wedding ring and get pressed into the combustion chamber. When the head of the valve closes that is what it seals against. Think of a hard steel valve closing 50 times a second with a fair amount of force. If the head were either mild iron or aluminum the valve would hammer the hell out of it. So the plan is to make this hard insert that is replaceable if the head gets rebuilt. The exhaust port on the right side is short, the two ports merge, before the exhaust manifold. The ports themselves have a rough sand finish where two cores seam together. As you might expect the big water passage in the lower right is the exhaust side. As the exhaust ports pull together to center this cross section gives a smaller than maximum cross section. The intake valve is inclined 21° from vertical, the exhaust valve is inclined 24° from vertical.

The photo above is of the V8 head sliced open, and has the same orientation as the V6 photo.  The intake side is to the left, exhaust side to the right. Above the intake port is mounted (not shown) the lower intake manifold which has the butterflies valves with the fuel injectors well above that. Therefore the fuel injectors are much further away from the intake valve with the V8 than the V6. I am guessing the big coolant passage on the intake side may help with fuel atomization and cold engine drivability. The intake runners are truly independent. Much is very similar. The oil galleys feed the cam bearings, which eventually ends up in the oil trough on the far right and goes down the drain. Water passage on the exhaust side under the port is bigger than it photographs. This is a better photo of the valve seats showing a two angle cut. The intake valve deviates 19° from vertical, the exhaust is 22° from plumb which has 4° less included angle than the V6. So the roof of the combustion chamber is a little flatter. The same sand seam is apparent in both ports but less evident than the V6. The port blending above the valve seat on the intake side is much nicer than on the V6. 

On either head, if you stand the head on one ear, and look through the intake port, you can see straight to the valve seat. And with both the V6 or V8, when the head is mounted on the block, in the car, the valve is basically pointing straight up, the valve stem vertical to the ground..

Stay tuned for part three coming soon!

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