Here's some more food for thought: Imagine a scenario with a stock/mild engine that seldom sees past 5000 RPM and has less than 325 hp at its peak, with all things being relative and not constantly talking about 500+ hp drag cars. In this sub 5000/325 scenario, who here honestly believes that this Would perform no better than or equal to this when considering the science behind this , saying nothing about opposite bank pulse gathering (scavenging).
Actually a turbo engine is a closed loop system and it's ability to make horsepower is affected by the intake and exhaust system including backpressure more than any other configuration. A header pipe feeding the turbine still creates a positive pulse during the blowdown part of the exhaust cycle followed by the negative pressure pulse designed for scavenging but the turbine can produce very high backpressure which raises the average exhaust pressure. I had the graph from our data logger which showed that both the positive and negative pulse amplitude stays the same but the centerline they ride on moves up or down represented by the backpressure. The ability to begin charge flow when the intake valve opens is about the difference between the intake port pressure and the cylinder pressure and not solely on boost pressure. The turbine housing is responsible for the backpressure and any exhaust system pressure added on after the turbine just adds to the problem. A turbine housing with a small A/R (less than .7) will make torque quickly at low rpm but also chokes sooner and limits HP. This is a case where the exhaust pressure can exceed the intake pressure by over 2:1 A large A/R turbine housing has low restriction but requires a lot of exhaust volume to spin the turbo so it will not spool till higher rpm and is better suited to make a lot of HP at high rpm. Our turbos had turbine housing A/Rs well over 1.00 and at 27 lbs of boost the exhaust pressure was less that 16 lbs. On a turbo engine turbo PSI is a very bad indicator of horsepower.
Thanks Paul, and this is why my engine with the small turbos makes great power in the low and mid RPM range the turbos have no lag as they are sized well for a street car. I have a few Vizard books I can look through and pull out into about exhaust systems but for now I found these articles on line:
Yes, being that I currently own a twin turbo, and a supercharged vehicle, and in the past stuck a couple KB Whipple chargers on some things, I totally understand that. What blows me away, is you can take a relatively stock car, run 9's at 140 and weighing close to 4,000 lbs.
Something I never really thought about, was why there was such a big difference in boost pressures on turbo and blower applications. Maybe if my turbo engine had been a max effort I would have. Now I think I see it. On a blower motor boost is boost. You get what you get. If boost is 16psi you are basically cramming double the amount of air into the cylinders, though clearly scavenging can modify that. Still, the exhaust is going to perform about the same all the time. On a turbo engine though, wow how different. Any pressure between the ports and the scroll is going to subtract from the intake boost to get actual boost, and there has to be pressure there for the turbo to work. Any pressure in the remainder of the system is going to cut the pressure differential across the turbine as well and reduce shaft speed. Wonder why I never thought about that? Maybe just because I'm hooked on instant response. As for practical use of all this theory, I think it's far from useless. I've built headers. I studied before I did the first set, asked people who knew, and in the end they worked beyond my expectations. I still use that set and they are still great headers. OTOH I've seen some seriously screwed up attempts where obviously the builder never asked anybody anything, and it shows. Building headers is not hard to do, anybody who can use a mig welder can do it. There are only a few things that need to be reasonably close to right in order to have them work well. I've built equal length fenderwell headers, a set of tri-Y turbo headers, a heavy wall 4 cylinder header, and will undoubtedly make at least one more set. Just because most headers are purchased does not mean that's the only option by any means. Often a commercial set can be sliced and diced for specific uses. In any of those cases knowing a little theory helps to avoid major blunders. And if you do find yourself building a custom set it's nice to know that you can get better results than almost any commercially available header if you pay attention to what you are doing. David Vizard is a smart guy. I've read a lot of his work and he usually knows what he's talking about. But not always. He's just as human as the rest of us and he does screw up from time to time. He is almost always factual with his info which is very commendable but sometimes opinion sneaks in. So I read what others have written too. On exhaust systems one of the gurus I pay particular attention to is Ed Henneman. Ed is definitely old school, but he also was upsetting the applecart back in the day and promoted a lot of controversy in exhaust design. But his stuff worked. Reconciling his theories with David's has been a very interesting mental exercise and on many aspects the jury is still out. That's delving into some of the finer details though and isn't at all necessary to build a good exhaust. It's easy to thread a path between them that satisfies your needs. But knowing some of this theory helps in making decisions that can have a great effect on how difficult the job is to complete. Put it this way, if you learned that using 1-5/8" tubing would serve your needs as well as 2" tubing would, which one takes less effort? And again in terms of the exhaust and tailpipe, if we know that the flow characteristics are suitable for the intended use of the car, then there is no reason to vary from that plan unless there is a different goal. In the muscle car era a 300 hp sedan used a single 2-1/4" exhaust and a 2" tailpipe. This was typical. For performance this was often doubled to twin 2-1/4 " and 2" pipes and these were more than adequate for those engines. The flow characteristics promoted power, economy, and a reasonable noise level and sound. Could somebody please explain to me why it is now considered so necessary to use 3" and larger exhaust pipes? (A pair of 3" tubes will carry four times the volume of a single 2" tube.) The only reason I can think of is to increase the noise level. Oh, and about that 2" tailpipe. As the exhaust makes it's way towards the bumper it cools. A lot. Exhaust gas temperature at the head can easily be 1500 degrees. At the rear bumper it might be a little above boiling. Even if it's 300 degrees that's only 1/5 of the temperature at the heads. Since volume decreases in direct proportion to decreases in temperature, obviously the pipe does not have to be as large at the outlet as at the inlet and any pressure drops accordingly. At the same time, the pulsations begin to even out, reducing the areas of higher pressure. An ideal exhaust would have no pressure pulsations at the outlet. We are far from that ideal. Now if the exhaust pulse was half of one crankshaft rotation and was even over that span, with 4 pulses per bank the pulses could join to form one solid exhaust stream, meaning that to maintain flow and velocity the exhaust pipe could be the same size as the primary tube. Since that is not the way we do it, it seems clear that the typical car exhaust is a crude extractor system, designed to suck. So you get into formulas for expansion chambers as the tubes merge into a collector and then decrease in size towards the exit. If you are in fact building an extractor this makes sense, crude as the system may be. If you are not, if you are just dumping into the largest tube you can fit under the car and giving up any possible benefits from secondary pulsations, then all you are really accomplishing is a crude and ineffective megaphone. Incidentally, those secondary pulsations are what makes a 2 stroke extractor so very effective. Jim
I watched those two videos on the mufflers and any back pressure that was put into the system caused the power to drop off and that was proven with the 3 inch system on the 350 Chevy. The engine made the same power and torque with the 2 or 3 inch system then when they put the Thrush muffler on it lost some power. That muffler has more chambers as compared to the Magnaflo muffler. The big block with the 600 hp needs the 3 inch as was proven there. No exhaust made more power. So you can go with the Magnaflo's or the Walker Dynomax which got good hp ratings on the dyno with a 2 1/2 exhaust system and you can't go wrong. If you want the engine quieter go with one of the other types that control that more and you may lose a little HP but that may not matter if you are just a cruiser. You can put the 3 inch on if you like but that system does weigh more, but will not hurt your power. If I used the Magnaflo's or any straight thru type pipe I would put in a X-Pipe and that will quiet down the drone/loudness at low rpm's. This really quieted mine down a lot as I have the large Flomasters. The second video really proved that 2 1/2 or 3 inch exhaust system did not make or break any HP or TQ in the 350 Chevy engine.
Yep! Just to add As discussed the effective exhaust system has to be consider right to the end of the tail pipe or collector extension if open system. However on a turbo application with respect to pulses, backpressure and thermal energy, the exhaust system terminates at the turbo turbine. Any thing that is added beyond the turbo is backpressure and not part of the tuning scheme. This makes the exhaust manifold or header solely responsible for the response to the emerging exhaust gas. There is very little or no additional piping between it and the turbo. If you have a simple log manifold there are pressure pulses traveling back and forth in the log that will most certainly present high cylinder pressures during valve overlap in some cylinders. You can add pumping loss as there will be higher cylinder pressure as the piston returns to TDC during the end of the exhaust cycle. A long tube header adds traveling distance to the pressure pulses which lends to cylinder isolation and you get a negative or scavenging pulse that subtracts from the average backpressure created from the turbo and beyond. Better intake charge motion during valve overlap and far less pumping loss. Engine compartment room and budget is always a problem so you compromise at best. A supercharger is a positive displacement pump so whatever the displacement rating/rev, rpm and pump VE at a certain engine rpm, that is what will be forced into the engine. However engine volumetric efficiency still plays a roll. If you map the intake manifold pressure curve over the torque curve you will notice they are inverse. In other words the higher the engine's VE the less pressure it takes to stuff the cylinder. If the supercharger works less then the torque required to turn the charger is less leaving more net torque at the crankshaft . We run vintage class 7 liter hydroplane. The heads are DART 320s assembled and ported by Al Dicksen that flow over 400 cfm. Straight pipes for exhaust. We follow the GP rules that restrict us to a 671 large housing blower @ 20% overdrive. In days of old the original engine made almost 1200 hp and the blower pressure was around 20 psi. Same blower size and gearing, almost identical cam but lesser heads. Now we never see over 17 lbs of pressure and make more power.
I have used Flowmaster Y merge pipes with good results they have to be better than the factory style merge.
In reference to post #61 Almost sounds like a trick question. LOL Connections that "T" whether it is an exhaust pipe connection, port entry into a log manifold or cylinder head Siamese ports are very low on my list of acceptable configurations
I concur. The abrupt turns and gas introduction that can encourage reversion and/or turbulence aren't exactly conducive to promoting optimal performance. Adding to this list of potentials would be a muffler with a similar disruptive flow configuration, which is the main culprit behind performance loss even when used in conjunction with an exhaust system that would otherwise be overkill many times over (as previously communicated). Connecting the dots can be fun! (and can help with presenting an image that might otherwise be obscured due to bias or misunderstanding) Lots of good info in this thread. Thanks to all for their contributions.
I finally got around to watching the second video and I am under-impressed with it. I think it would have been more meaningful to the target viewers if they compared cheap and expensive versions of the same muffler configuration. That is, two or more versions of turbo-style mufflers or two or more versions of straight-thru or "lazy-S" mufflers. Of course, maybe they already tried that and found that there was little of no difference as far as power production goes. Also, wouldn't a 2.25" or 2.50" diameter exhaust system have been more in line with what most 1 HP/cu in Chevy small block street/strip Camaro's, etc owners install?
My perspective is different of course. To me, with a car that weighs under 2800 lbs, 400hp is a lot of power. 7 lbs/hp in fact. Many cars in my group weigh under 2400 lbs, for them it would be 6 lbs/hp. For a 3400 lb car that's like almost 600 hp, and I don't know all that many people who build those kind of small block engines and then drive them on the street. But we don't have room for full length headers and moderate length header pipes. Our exhaust systems just kill engine performance, and very few decent solutions have been created. An engine that you guys would get an easy 400 hp out of might give us 300, so it's worth something to figure out how to modify the system to get good performance. In all these hundreds of cars there may be half a dozen with full length headers. few of the remaining ones have even one primary tube much over a foot in length. So they are in effect, tube built log manifolds. I think the OEMs have learned how to get the most out of this type of manifold. They still use them in spite of the proven superiority of headers, and build some very fast cars doing it. Seems to me they must know what they are doing. But I believe what we have here are two very different systems that operate under two different sets of rules. Where the header system creates suction at the port by using momentum and a long tube, and then needs to keep pressure at the outlet as low as possible, I think the manifold system uses reversion pulses between the muffler and the ports to create a low pressure secondary pulse at the ports in time with the exhaust opening, which works well with a log manifold. My theory is that shortening the length of the header pipe as we do on our conversions upsets that system and destroys the scavenging that OEMs have designed into their cars. It might be worth moving the muffler to a position behind the axle to see what it does, but then the header pipe might be too long. Something of a dilemma. But out of curiosity, could someone with iron exhausts measure the length of their header pipe, between the manifold and the muffler? It could help a lot just to know what that measurement is. Jim
I believe that by using a system that was way overkill (actually, both systems were way overkill, as a 2 1/4" press bent (not mandrel) would have been more than enough for the power output of this engine), but it seems they had those systems left over from the 600+ hp big block they tested them with showing that the smaller of the two was a restriction vs the larger when used with this much power. The real educational aspect to extract from this second video is the muffler type, understanding that a turbo muffler that doesn't have curved deflector plates has TWO harsh 180* turns that the exhaust must make to negotiate its way out of the muffler and back into the tail pipe, where it has been considerably slowed down (both from temperature loss and turbulence inside that style of muffler). This point is accentuated by the fact that they used the 3" system to show it, and so exemplified this phenomenon by eliminating the rest of the exhaust system as a restrictive factor.
I don't believe OEM's are making leaps and bounds with log manifolds, maybe overall improved performance over 50-60's cam and head designs with branched manifolds and improved airflow. These are production engines with an emphasis on smoothness, computer control, emissions, reliability, etc., that have restrictive exhausts yet. Wide LSA cam timing coincides nicely with both computer control as well as blowing a cylinder down into a pressurized system. Improving power via better scavenging and more idealized cam timing is really low hanging fruit. I've found that a long branched tri-y fits nearly anywhere as a large single pipe from a log dump without giving up secondary branch length or collector length. It's just expensive for the fabricator, so there's that...
I like the way you think, Jim. If one were to opt out of the use of headers (for which there are various reasons for doing this), the exhaust plays a more crucial role in how the gasses are eliminated from the head(s) to the tailpipe(s). We know that headers do most (if not all) of the work in a scavenging environment in any exhaust system using them, and that the exhaust system itself does most (if not all) of the work of any scavenging effect when using manifolds, whether they be 'log' style or otherwise. Anything that can be done to keep velocity up without creating too much pressure and work off using alternating pulses from firing cylinders will aid in this. While it won't be as effective as using headers themselves, it's better than relying solely on gas temperatures and momentum for evacuation. This is why it is said to be best to put any merge/crossover pipe as close to the engine as possible, so that temperatures and pulse strength/speed are at their highest. The type of merge/crossover is also very important in its design as to how effective it will be. It is also said that muffler placement as far away from the engine as possible makes the muffler more effective at sound reduction and is the least restrictive location (gas temperatures and speeds at their lowest). I agree with these assessments. Many exhausts will be limited to physical dimensions rather than idealist preference and so this type of discussion is good for offering up alternatives for those who experience this. You wouldn't happen to have photos of your exhaust system, would you?
My car is far from typical for these conversions. With 1-5/8" x 32" equal length primaries, glass packs and short side dumps it is very much a conventional hot rodder's system and works extremely well. You can see some of the primaries in my avatar photo. The 455 buick MGB-GT that we built is a better example, with tube log manifolds, a short dump to glass packs, over the axle tailpipes and glass pack resonators back by the fuel tank. The system doesn't look particularly restrictive but where we were expecting to see something in the 400-500 hp range on the dyno it was actually under 300. I suspect two things about the exhaust could be hurting us, both by reducing or negating the effects of reversion pulses in the header pipe. First, the glass packs have no boundary to generate reversion pulses in the first place. Secondly any reversion that was generated would be out of phase at the valves due to the short header pipes. Jim
I suspect there are two main reasons for the exhaust hurting performance, and it doesn't sound like it's so much 'restriction' as it is lack of scavenging from the valves to the tailpipes. This would be exacerbated (bigtime) with a camshaft design that has considerable overlap that relies heavily off scavenging to perform optimally. 1) log style tube 'manifolds' (IMO) would perform worse than the factory Buick manifolds, unless they were designed with similar engineering and improved dimensions for pulse placement and volume movement; 2) After this, mufflers too close and/or no merge section to help try and overcome what the lack of headers is introducing into the overall system's behavior/performance is making the engine work against itself. If there's little room for a merge section after a manifold or short primary pipe designed manifold type tubing, the only real solution would be longer primary tube headers (might as well go with a 'tri-y' setup since it's all custom work anyway, to try and get the most out of it). Those short primary tubes coming off the heads and "T"ing into a huge pipe is killing it. My 2c. I'm sure there are others here (such as Tony and Paul) who could add to this. (any more pics of the entire system?) P.S.--my intention isn't to knock or bash anyone's hard work, but to offer my insight into this particular setup
