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Dyno tuning with different SUs and velocity stacks

We were recently doing some engine dyno testing of a 3/4 race "fast road" TR-4 engine destined for a Morgan. We had what I thought were some very interesting results.

The engine is an 87 mm bore Hepolite piston engine with standard size valves, 10:1 CR, 280 degree seat to seat cam (similar to the Competition Dept" D" cam), a "pocket ported" head with a 3 angle valve job. The testing was done with both HS-6 carburetors on a long manifold (these carbs and manifold were dead stock and in perfect shape with the float bowls mounted on rubber using MG-B mounting insulators) and with a fully race prepared set of H-6 carbs on a race prepared short inlet manifold. The engine had "Tri-Y" style headers and was run both with an open exhaust and with a straight through English style muffler.

The first and most important thing that we found was that the first day of testing we could not get the engine to repeat power runs accurately enough to learn anything at all until we modified a 45 DCOE Weber "soft mount" and put it in between the HS-6 carbs and the inlet manifold to insulate the carbs from engine vibration that apparently was upsetting the fuel delivery to the engine. As soon as this was done, we were able to start to get some testing results.

The next surprise was when we added velocity stacks to the HS-6 carb set up. We tried the old aluminum parabolic stacks that I had seen on the Competition Department TR-4s, and also a set of stacks that I had made many years ago that look like Weber Velocity stacks. Either stacks just killed the power above 5500 rpm. I mean it just took a nose dive, whereas without any stacks the power would peak at about 5400 rpm and then stay almost level until 6,000 rpm. Unfortunately we did not have a set of "stub stacks to try.

Apparently the 13.2" length of the intake and carbs is very much a "tuned length, at least on a 3/4 race engine. At least that is what I concluded. What a shame, because the Velocity stacks look so cool! We then found that with no stacks, the stock HS-6 carb setup made a higher torque peak by almost 8 ft. lbs. than the race prepared H-6 setup .Above 5,000 rpm the H-6s on the short manifold was slightly superior. The H-6 carbs did not need the "soft mounts" and the "weber like" velocity stacks worked well on them.

It made very little difference whether the muffler was on or the engine had a straight open exhaust! But remember the muffler we were using is a very free flowing unit, we have used it on engines making up to 150 hp and this engine was in 120s. Our best runs made 123 hp with a very broad peak that gave over 120 hp from 5,000 to 5700 rpm and 138 ft lbs of torque with over 130 lbs. from 3300 to 4700 rpm and with the peak around 3900 rpm. All of the testing was done on 91 octane pump gas. The best power was with 34-36 degrees total advance measured on the outside of the flywheel.

Greg Solow

I had a similar experience a few years ago with HS6 carbs. I could not get any power over 6000 rpm on the track. I just could not get them to flow enough fuel. I put air cleaners on to add some restriction and gained 300 rpm on the straights. I did not, however, go so far as to add the weber style spacers.


My work with velocity stacks was always with FULL RACE engines. The purpose of the stack use was to help contain the fuel standoff which appears at the bottom of the rev scale (around 4000 revs) where the long overlap of the camshaft tends to reject the air/fuel charge. But we were fortunate to make very good gains in the entire rev range.

My back to back tests with the GT-6 engine showed a improvement of 6-8 horsepower at 8000 rpm. In these tests, I held the velocity stack up to the carburetors with my hands as the engine was a full throttle and top revs, then removed the velocity stack an watched the torque arm fall, pushed the stacks back into position and watched the torque go back up and stabilize. We were all pretty brave standing right next to the engine at full power, but trusted the engine builder (me). Again this is with full race engines and big camshafts.

I think Gregs tests were great and give added information to all that there are variables in this world of engine development and also points out the wonderful information available from dyno testing. So Joe, adding the air filter reduces the amount of air and therefore indicates to me that there is either insufficient fuel supply (too small a needle and seat) or a lean mixture at that higher rev point (needle shape).

kas kastner

Subject: velocity stacks and stand-off

While we were dynoing this 3/4 race engine (280 degree duration cam) installing the velocity stacks in every case made the fuel "stand-off" disappear. Without the velocity stacks there was a cloud of fuel in front of the carb inlets that you could feel the wetness on your hand up to over 1 ft away. The fuel seemed to be in constant motion into and out of the carb throat. the fuel did not seem to "blow away" into the dyno room. When the velocity stacks were installed, the cloud was no longer there, and the engine made slightly more power from the point where it came on the cam, ie. about 3200 rpm up to around 4500 rpm. Above that rpm the power was always less with the stacks than without them.

As I mentioned before, we did not have any "stub stacks" as recommended by David Vizard as being a benifit on Minis and MG engines. So they are a questionmark as far as this engine is concerned. It is interesting and puzzliing when emperical results do not agree with what should the theoretical results. Everthing I have ever read says that the stacks should make things better everywhere, not worse. The square corner of the carb inlet should act to in effect reduce the size of the inlet and reduce the air flow. Maybe the power level of this engine does not require any more air and so that is why it did not help. On Weber carburated engine, every time we have gone up in venturi size, the power has gone up. But again this was on a full race engine making well over 170 hp on this same dyno. By the way even on this engine, with either set of carbs, the air valves were up all the way they could go by 4500 rpm. So from this point on the actual taper of the mixture needle is no longer having any effect on the fuel metering. as only the exact point of the needle where it enters the jet is controlling the mixture and that is no longer changing as the revs change. Of course the size of the needle at this point is important and is controling the amount of fuel flow into the engine.

Regards, Greg Solow <>

That's pretty interesting. I might make note that the higher revs the higher the air velocity the higher the velocity the lower the pressure, therefore the flow of fuel will be HIGHER even though the needle has not risen any higher in its taper. Funny stuff huh!!!.

kas kastner

IOW, when the piston hits the top of it's travel, you suddenly have a fixed venturi with a fixed jet ... just like a real carb <g>


But with no method of preventing over enrichment like the air corrector jet on a Weber!

Greg Solow

Just wondering; what was the shape of the outside lip on the stacks you used. They should be rounded back toward the outside of the stack (fully radiused). If these happened to be of the variety with a sharp outside edge, this could be one source of trouble. If you check the airflow into various stacks, I believe you will find quite different characteristics between the sharp edged ones and the ones with a fully radiused outer end. I dont have the explanation, but they result in very different air-flow patterns.

Tom Strange
#4 White

The "parabolic" velocity stacks have an extremely large radius bell that flares out a full 90 degrees but does not roll back on iteself. The outside diameter where the edge of the bell ends is 3 5/8" whereas the throat of the carb is only 1 5/8" in diameter, so I don't think that the fact that they don't roll back on themselves is hurting the power or airflow. Since Kas set up TR-4 engines that made over 160 hp with 1 3/4 SU carbs on them, I suspect that at the 125 hp level, the air flow requirements of the rest of the engine are such that they are not close the limit of the carb size. If that is the case, it would make sense that increasing the air flow potential of the carbs would have no effect on the power output of the engine. Other factors like "tuned length " of the inlet tract might have a greater effect on power than air flow potential of the carbs. We also did some back to back testing of high lift rockers vs. standard lift rockers and found that even with an almost stock head, the engine really liked more lift. Going from total lift at the valve of .375" to .415" was worth about 5 hp from 4500rpm on up to 6,000 rpm, which was the maximum at which we ran this engine. Consequently, we are now working on a new cam grind which will still have seat to seat duration of 280 degrees, (which idles well at 800 rpm with SU carbs) but will have faster lift rates and more total lift while still being relatively gentle on the valve train so it will not require hemongus spring pressures like a real "race" cam does.

Greg Solow

The early velocity stacks that I did had a sharp edge and no roll back of the outer edge. They did not make the big difference that we achieved later with a rolled over edge. In tests we found the inlet air was coming from the back of the carburetors. I'll explain our tests another time if interest is demonstrated. It's no big deal.

kas kastner

The tests I did for the air movement was first on the flow bench. I mounted the stack on the air inlet then used smoke to get the indication where the air was coming from. For a simple test I put my hand over the stack and it didn't make a bit if difference until it was about 2" away. The smoke indicated that the air was coming from the back of the stack thus the manifold side. In the dyno with the engine running under full power at about 5000 I used a flat board and slowly moved it up to the stacks, nothing happened until I was ABOUT AN INCH away from the stack again indicating that the air was coming from the manifold side. After this I then made the stack with big roll on the edge, that is when I picked up the big change and power improvement. Like I said, simple minded tests. I would think that stacks on Webers that had rolled edges would improve the flow. You could probably do that by mounting in a lathe, heating the edge with a torch, turning the lathe on the back gears so that it was low revs then use a wooden paddle to roll the edge over. OR just buy some that do the job. The latter sounds best.

What all this did was relieve me from worrying about the air inlet being too close to the inner fender well. But it also showed the worth of having a heat shield between the headers and the carbs and manifold.

kas kastner

Ah yes, free horsepower from ram air. This is a favorite subject of mine. Lots of experiments and study of the effect.

The prime thing I found was that cooler air, near the roadway, was a better giver of free bhp than anything else. It takes a lot of speed to actually get an ram effect. Now if you use a corrugated flexible tube you just tossed all chance of ram out the window for as the speed increases the wall drag of the tube and reduces the effective diameter to half of or less, how disappointing. Smooth tube works though.

If the ram tube is led directly to the carburetor inlets you can have the problem that the car will not run very well at anything but light throttle. Why? The slight ram and I mean slight will give the carburetor more pressure than the float bowl fuel sees, thus the float bowl will not flow the fuel.

Fix this by making a box for Webers that includes the float chamber inlet and on S.U.'s just connect the float lid overflow pipe to the air tube with a piece of hose. Still problems with miss fire etc, poor high speed running? Probably it is the turbulence in the box and the carburetors are not getting equal air, and in fact may have choked off some of the inlets opening. Fix? Cut a hole in the back of the box the size of ONE carb butterfly (if that is better but still a little problem, increase the hole size 50%) and make certain the box extends well past the inlet of the rear carburetor inlet. When you are all finished, you'll have spent some money, some time, some labor and testing and for this probably will not gain a single horsepower because these boxes (car shape) do not operate at the speeds needed for true ram effect.

Cool air? Yes, now you're talkin'.

Never Be beaten by Equipment
Kas Kastner


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