A pair of 5" mufflers from a semi. On this 366 HP engine we lost 13 HP, and 20 torque.
I would have never thought there was that much back pressure.
I would be interested in seeing a before and after graph. I'll assume that you're losing 13 peak horsepower. I don't even think I'd feel the difference.
But my understanding of exhaust tuning was that there was an optimal pipe size for a given engine size/cam timing; if the cam was tuned for peak torque at 4500 RPM, there was a very specific pipe size for that tuning. And the size/flow of the muffler should match the exhaust pipe. And the distance from the exhaust valve to the muffler should also be adjusted to match the peak RPM.
The muffler will create reflections that travel back to the exhaust valve. If you can get the reflection to hit the exhaust valve at just the right time, you can get it to extract extra exhaust gas from the cylinder. But, even though this should be a good thing, it will change the airflow, and that will cause a change of air/fuel mixture. In a closed-loop EFI system, that's not a problem; the computer will read a lean spot, and increase the fuel. A carb can't do that.
Cross-over's and their placement also affect torque production; the crossover causes similar reflections as well as improving velocity. But it, too, will require a change of carburetion.
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Paul Menten
Last edited by pcmenten; 12-21-2006 at 08:27 PM.
Reason: typo
Somehow I missed the intake manifol type/manufacturer. Looks like a single plane. Does the engine make enough torque down low to work on a street truck auto with say a 2300rpm stall?
The intake manifold is a CHI high rise single plane manifold made for the CHI-3V heads. While this isn't optimim for low RPM operation, it still makes over 400 ft-lbs of torque@2000 RPM. With 59 degrees of valve operlap, the vacuum at idle is around 10 psi. This may require a vacuum can for power brakes.
Instead of a vacuum can run a vacuum pump. They can be found on 80's Chevy Cavaliers and other vehicles. It is a nice self contained unit that can be wired up to run when the engine is running. It senses vacuum levels and only runs when needed. It will work for cruise controls, HVAC, brakes, etc.
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Have you tried a dyno sim using the CHI flow #s from TMI's site?
With the head flow numbers from Tim's site, I get peak torque = 520 ft-lbs@4500 RPM, and peak HP = 511 @5500 RPM. The torque@2000 RPM is 428 ft-lbs. This is a flatter curve with more low end torque than either the previous dyno sim or the real dyno.
I really need to see the numerical data from the dyno in a spreadsheet, but it appears that the real dyno data is steeper with less low end torque, than the sim data. I would expect higher RPM and higher peak power from the real data. It looks like something is happening at about 5000 RPM to reduce the peak power. It is in this area where we see the pressure waves in the manifold vacuum. Tim and Mark were able to hear this disturbance during the dyno runs. It is also in this area, that the sim predicts the VE to exceed 100%. I think we need an intake manifold expert to look at this.
I think that you have hit the answer, but it is not necessarily the exhuast that causes reversion. Here is something that I picked off the Internet.
Nearly everyone even remotely associated with high-performance engines is aware of the so-called "ram" effect as applied to engine induction and exhaust systems. This condition is generated by sonic pulses that continually rattle about in an engines induction system, exhaust system and at times, in the combustion chamber area, and a favourable ram effect will occur when these pulses become aligned in direction and magnitude to cause a larger-than-normal charge of air/fuel mixture to be pumped into the cylinder from the induction system. A similar alignment in direction and magnitude of pulses causes a larger-than0normal volume of exhaust gases to be pumped from the cylinder through the exhaust system. In simplest terms, pressure reversion could be defined as diametrically opposite a favourable ram effect, a condition in which the air/fuel mixture is forced away from the cylinder, back up the induction system toward the atmosphere. A similar condition can exist on the exhaust side whereby exhaust gases can be forced back toward the cylinder from the exhaust system. It is quite reasonably felt by some experts in the field that the pressure reversion in the induction system is caused by a pressure reversion in the exhaust system, with the combustion chamber area as the connecting link between the two during the overlap period. However, later indications, are that pressure reversions in either system can occur independently of the other, although their magnitude seems considerably less than a combined induction-exhaust reversion.
Pressure reversion usually manifests itself visibly as liquid fuel or fuel stains on some surface at or near the upstream sides of the carburetors. Sometimes it doesn't get as far as the atmosphere, in such cases being visible as a sort of "ball of fog" extending from the intake manifold and perhaps into the carburetor while the engine is operating through its normal speed range, preferably at full throttle. Sometimes it may not be visible at all due to the intake manifold configuration, which would cause the reversion pulses to be damped and contained within the manifold. Happily, there are times when a pressure reversion condition does not exist at all within the normal engine speed range.
We sometimes like to think of the air/fuel mixture and exhaust gases as smoothly-flowing, but such is not the case. The sonic pulses, or pressure waves, as you prefer, which incidentally, occur in all engines, cause violent disturbances to the air/fuel mixture and exhaust gases within the cylinder and induction and exhaust systems. These pulses represent energy, quite a bit of energy, in fact, and when they can be made to work favourably for and with an engine, as they can in the correct application of the ram principle, engine performance comes to life. However, when they work against an engine, as in the case of reversion, engine performance tales a gigantic nose-dive. If these pulses were one-directional downstream pulses, that is, from the atmosphere, through the induction system and into the cylinder, then from the cylinder through the exhaust system to the atmosphere, things would be lovely. However, for every downstream pulse, there is a reflected upstream pulse of lesser magnitude and these are the ones that do the damage, particularly when they become so unsynchronised or out-of-phase as to cause a pressure reversion, a highly-undesirable, performance killing condition.
The consensus is that these sonic pulses are initially generated by the opening and closing of the valves, although when either or both valves are open, the piston crown cannot be ignored as a possible secondary source of pulse generation. It could also be that the piston is a primary source of pulse generation within the combustion space when both valves are closed. The latest data corroborates earlier findings in that the pulses are basically sonic in velocity. But sonic velocity varies with the density, temperature and pressure of the working fluid; therefore the actual pulse velocity in the induction system will vary greatly from that in the exhaust system, with the combustion space serving as a transition between the two extremes. In addition, there is a thought that downstream pulse velocity should be added to downstream gas velocity, while upstream pulse velocity should have downstream gas velocity subtracted from it. With all this downstream/upstream gas/pulse thrashing about going on simultaneously, there is little wonder that some disagreement exists between experts in the field, but the biggest wonder is that the engine runs at all.
Pressure or pulse reversion exists most prevalently to a performance-damaging degree in engines equipped with individual runner (IR) induction systems where each cylinder has its own isolated carburetor throat and intake manifold runner and there are no interconnections between carburetor throats or manifold runners. This applies to the L-series Datsuns because this type of system is used most frequently for Datsun race engines, and to some extent, for modified street and dual-purpose engines with Datsun-available 44mm or 50mm Mikuni/Solex side-draft carburetors and manifolds, and sometimes with Weber carburetors.
The reversion problem shows up at its worst when the induction and exhaust systems appear to be "clean"; that is, when the carburetor throats, manifold runners, cylinder head ports, exhaust header pipes are all nicely matched and blended to their mating pieces. It may be clean in fact as well as appearance, but unfortunately, it is clean in both directions, so reversion pulses have and easy time of it.
Four separate and distinct areas require possible reworking to minimise the effects of pressure reversion, if not eliminate them completely. First, the exhaust system flange and primary pipe should be about 1/8-inch larger on all sides than the port opening in the cylinder head. Second, the intake port face in the cylinder head should be about 1/8-inch (1/4-inch on the diameter) larger than the intake manifold runner, then the port should be funneled down to more normal dimensions as it approaches the intake valve. Third, the intake manifold runner should be about ŧ-inch larger in diameter than the carburetor throttle bores, and the runners funneled down to a smaller dimension at the manifold mounting face.
The idea is to make deliberate mismatches at these three points. The reasoning behind this is that there is pretty conclusive evidence that the downstream pulses (the good guys) take the shortest distance to get where they're going, while the reversion pulses (the bad guys) stay close to the walls of the carburetor, intake manifold runner, intake port, exhaust port and exhaust pipe. The deliberate mismatches make abrupt changes in cross-sectional area, which are highly beneficial in damping the unwanted reversion pulses. In addition, the air/fuel mixture traveling downstream is pumped into areas of lower-than-normal pressure, which in itself, helps induce a larger volume of mixture into the cylinder, and the same is true on the exhaust side. Edelbrock Equipment Company has made a couple of prototype manifolds incorporating the mismatch concept for the L-16, L-18 engines with encouraging results for a first attempt in damping reversion pulses.
The fourth are that may require a change is valve timing. By itself, valve timing can have rather dramatic effects upon the presence or absence of pressure reversion.
If a reversion problem exists, the changes should be made one at a time and in the order shown until the problem disappears completely or is at least helped considerably. At the points of mismatches, leave the edges square and sharp. DO NOT ROUND OFF THE SHARP EDGES! Perhaps strangely, there are highly modified L-series engines with no reversion problems at all within the normal operating speed range.
What's amazing here is that once again, this turns some magazine dogma on its head. For years the magazines would preach that you need to blend/match the intake runners to the intake manifold. Once again, they were well intentioned, but wrong. And it's also interesting, but I've heard before that a mismatched intake works well on some engines.
The advice to leave the sharp edges in the mismatched ports reminds me of another techinque that I've heard of. This one involves how the valves are ground. The intake valves should leave sharp edges on the valve face to help control reversion, and the exhaust valves should round the edge on the face, but leave the angle near the valve stem unblended.
This also brings to mind the stepped header designs where the primary pipes start out at 1 5/8" diameter, and are fitted into 1 3/4" diameter. Also, the tri-y header design would have twice the anti-reversion effects of a 4 into 1 design.
I'm curious to hear how valve timing and LSA alters reversion.
__________________
Best regards,
Paul Menten
Last edited by pcmenten; 12-23-2006 at 12:10 AM.
Reason: typo
no, it's not the exhaust that causes reversion, it's just a phenomenon that occurs. i think the valve event timing in relation to piston position, direction and speed plays a key role though.
does anyone remember the AR (anti-reversion) type headers from back when? i think Cyclone was the brand name? they had an oversized primary tube with a smaller cone inside that matched the exhaust port size. the reversion would be diminished by the "dead" area between the cone and the outer primary tube, almost like a heart valve that doesn't move.
quick and simple test on the dyno would be to try a set of big tube Cleveland 4V headers to see if the condition improves. "one test is worth a thousand expert opinions"~Smokey Yunick
Not just match/blending the ports, how about using the intake gasket as a porting templet? As if the opening somehow provides an ideal cross section. I've referred to it many times as "Porting for Dummies".
If people would bother to read the directions included with the Edelbrock Performer intakes they would know it specifically states, do not port match as this will decrease performance.
One of the reasons my 4V heads make significant mid range torque is because the headers I'm using are tri Ys.
I don't think Dan's problem is about exhaust reversion because the EFI didn't have the stumbling problem.
Dan, if you contact CHI about the intake you may also want to ask about the ignition timing. They should be able to tell you how much other guys are running.
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I have been fighting that port matching, gasket matching, magazine dogma for years. I have seen the results of those "steps" in testing, computer simulations, and even some clear parts with high velocity smoke running thru them. Notice I mentioned trimming that gasket in this thread: http://www.ford-trucks.com/forums/55...chi-heads.html
After a while I get tired and just let the people port match to their hearts content because some abrasive manufacturer wants to sell product. Then laugh when their engines fall on their face under certain conditions...
Always go from small to large, even the tiniest protrusion of a gasket or slightest mismatch where the gas stream "sees" a ledge or shelf causes standing waves and reversion. Very nasty when a person can actually see it! Keep those edges sharp, Do not radius or deburr the trailing edge with one of those nice deburring tools. A burr or dent causes nasty things also.
And yes I do remember those anti-reversion headers. Several performance books speak of them but I have never held a set in my hands. The inner pipe still had to be larger than the exhaust port tho. They did seem to make it hard to get adequate space for bolt heads around the header pipes.
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"Beam me up Scotty. There's no intelligent life down here..."
I don't think that this is exhaust reversion. We had two different sets of headers, and it made little difference. We had the EFI setup in place without this problem.
The problem comes from the velocity of the intake charge through the high flowing heads and manifold. This high velocity causes the desireable Ram effect, but the flow is also very good in the reverse direction, so the reflections propagate through the manifold as well. When we had the more restrictive EFI intake, the velocity was lower and the reflections were less or none. The velocity is a function of the volume of air pulled through the manifold and tubes, a more restrictive intake will flow less volume and have a lower velocity through the same size tube.
The Cobra TB on the EFI setup is rated at 1089 CFM, and should be adequate for this motor, but the air intake has a 90 degree bend and other restrictions. I will have to investigate that when I get it in the truck.
Jon the owner of CHI is suppose to be by my shop either tuesday or wednesday. I am going to ask him about the intake sounds we are hearing. It's to bad we had another engine to get dyno'd, otherwise we could have left Dan's engine on the dyno and let Jon hear it.
Jon the owner of CHI is suppose to be by my shop either tuesday or wednesday. I am going to ask him about the intake sounds we are hearing. It's to bad we had another engine to get dyno'd, otherwise we could have left Dan's engine on the dyno and let Jon hear it.
Hey Tim,
Maybe you can get him to cast up some dual plane intakes just for the 400 crowd. It would be nice to have something kick-ass off the shelf instead of needing spacers. There was one 400 at the EMC this year and with your work, maybe that's all CHI need to get them to develop a good high rise dual plane.
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