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jmiska

Do roots-supercharged engines care about exhaust gas velocity?

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Okay, this is a question for the real motorhead geeks out there.  I know that normally aspirated engines are very sensitive to exhaust gas velocity to ensure full cylinder scavenging, and this is directly related to exhaust tube diameter selection.  Too big a tube, and gasses slow down for a given engine, which can hurt lower-end torque.

I'm not sure that's the case with a mechanically supercharged engine, though.

I have the Buick/Pontiac 3800 supercharged engine (gen 3 if anyone cares) and the flex pipe is apparently getting some metal fatigue.  It's getting rattly and buzzy right around the flange, and sounds like a slight leak may be developing.  The replacement downpipe assembly contains the flex joint, converter and the piping. Factory pipe and converter is 2.5" in diameter.  There is a large performance aftermarket for these engines, and so there are a huge variety of exhaust system components available as replacements. One standard upgrade is go to a 3" downpipe/converter assembly. Along with enabling one to run higher manifold pressure (which I don't intend to do), it also supposedly reduces the likelihood of knock on stock engines. This engine runs at rather high cylinder pressure and knock is a constant concern.

While I'm not hot-rodding the car (it's my daily driver), I'm wondering if there is any disadvantage to simply going with a 3" converter/pipe instead of the factory 2.5".  If it were a normally aspirated engine, I'd stick with stock for the torque, but I've read where supercharged engines don't use the exhaust system for cylinder scavenging, and the less restriction in the exhaust, the better.  Still, most people doing exhaust mods are going on to other mods as well, and are less concerned about daily torque and more about high RPM horsepower, so their performance improvement claims may not apply to all cases.

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On your stock car you shouldn't notice any difference at all and will be ok with the aftermarket pipe. Only thing you will likely run into is that it may not fit well or last as long as an oem replacement.

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Is it turbocharged or supercharged with a Belt driven blower?  If turbocharged is go bigger, which will probably help with boost lag and a stronger power band.  I miss my turbos.

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The exhaust is only going to flow as good as the smallest orifice in the system, such as manifolds or cat restrictions while under boost. The diameter of the tube won't make to much of a difference on a supercharged motor with a full exhaust system. You probably only make a few pounds of boost anyways. Now when we get into turbocharging a large exhaust down pipe off the turbo outlet is beneficial as we want to get the hot exhaust out of there asap. The up pipes however you don't want to increase the diameter over stock as it will lead to increased turbo lag and drivability issues. The newer diesel trucks such as the Titan with the Cummins motor use a compound turbo setup that utilizes a large low speed turbo that builds most of the boost and uses a small high speed turbo that fills in at low rpm to reduce boost build up time. By high speed it can spin up to 104,000 rpm.

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Thanks guys. I guess I'm going to go ahead and order the aftermarket pipe setup.  It's actually quite a lot less expensive than the stock assembly, and the one I'm looking at seems to be high quality stainless with good on-line reviews.

On a normally aspirated engine I would not be inclined to change the exhaust system unless I were doing other things to improve breathing (and wanted higher RPM performance), but I do expect that with the supercharger, that doesn't necessarily apply. Still, this is the only forced-induction engine I've ever owned so I don't really know what I'm doing.  The Buick 3.8 typically ran about 8 psi or so of boost. While it's not a ton, I was thinking that it would be enough to overcome the loss of exhaust velocity, and the less restrictive converter could slightly help cooling the heads and manifolds. According to the guys who build these engines, the flex pipe and converter are the main restrictions in the exhaust, hence the change to 3" being so typical.

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If the 3" will fit under the car neatly and stay up out of the way then for sure the larger exhaust and converter. A boosted engine does not rely on exhaust induced velocity to assist cylinder scavenging. Easy flow is the way to go on any boosted engine. As pointed out earlier on a turbo stay stock forward of the turbo to retain gas velocity.

Joe

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1) I'd argue that, yes, exhaust still plays a role in cylinder scavenging on a supercharged engined.  Being intake boosted does not play a role on the exhaust stroke, intake valves are closed during that interval, so whatever can be done to promote flow to clear the combustion chamber should be pursued.  However, pulse harnessing on a *turbocharged* engine is not really possible as the turbine itself chops up the pulses and their energy, making scavenging difficult if not impossible.  I think that's where the scavenging not being important on a boosted engine statement comes from.  As you know, there's no mechanical interference on the exhaust side of a supercharged engine, so pulses remain intact and scavenging can happen.

2) I'd argue that the statement of "bigger is better" for a turbocharged engine, in terms of exhaust, comes with caveats.  On a street driven car, exhaust length from exhaust port to tailpipe is considerable.  Exhaust velocity is related to thermal management of the gasses, as related to Boyle's law and Bernoulli's principle, where thermal drop of the exhaust gasses as they travel down the long exhaust pipe results in a drop in pressure, and a drop in pressure reduces the effectiveness of the low pressure seen at the tailpipe and causes the gasses to slow down (high pressure seeks out low pressure.). So, bigger on the exhaust pipe is only better when length is taken into consideration to offset the natural cooling effects and what that does to the internal pipe pressures, suggesting that a slight *decrease* in pipe diameter along its length on a street car may be best for maximizing gas velocity, and reducing backpressure against the engine.  On a turbocharged car that approach reduces kinetic energy losses, and on a non-turbo engine it also encourages maximum scavenging effect.

3) On a street car, a swing of 10hp either way won't be felt or easily measured, making the above analyses moot.

 

So, moral of the story: go for the easiest repair route here.

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7 hours ago, Toddavid said:

 

So, moral of the story: go for the easiest repair route here.

And likely stock will fit better.

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I had a Grand Prix GTP 3800 super charged. Go with the bigger down pipe.  I was into modding that car. Look for forums on 3800's. My car went from 260 hp, to 325 hp. Still ran strong with 150,000 miles on it. So with the 3" down pipe in your car you should feel the change, it's like 20 hp. Been there done that. 

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On 11/23/2016 at 11:43 AM, Slowhand said:

I had a Grand Prix GTP 3800 super charged. Go with the bigger down pipe.  I was into modding that car. Look for forums on 3800's. My car went from 260 hp, to 325 hp. Still ran strong with 150,000 miles on it. So with the 3" down pipe in your car you should feel the change, it's like 20 hp. Been there done that. 

That's my exact car (GTP version). It's fully stock, except for some slight modifications to improve airflow through the engine compartment and cool the supercharger, which was cooking hot on a typical Florida summer day. It is a great little car in most all respects, and fun to commute in. Thanks for the real-world advice.

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On 11/19/2016 at 10:56 AM, Toddavid said:

...Exhaust velocity is related to thermal management of the gasses, as related to Boyle's law and Bernoulli's principle, where thermal drop of the exhaust gasses as they travel down the long exhaust pipe results in a drop in pressure, and a drop in pressure reduces the effectiveness of the low pressure seen at the tailpipe and causes the gasses to slow down (high pressure seeks out low pressure.). So, bigger on the exhaust pipe is only better when length is taken into consideration to offset the natural cooling effects and what that does to the internal pipe pressures, suggesting that a slight *decrease* in pipe diameter along its length on a street car may be best for maximizing gas velocity, and reducing backpressure against the engine.  On a turbocharged car that approach reduces kinetic energy losses, and on a non-turbo engine it also encourages maximum scavenging effect.

3) On a street car, a swing of 10hp either way won't be felt or easily measured, making the above analyses moot.

 

So, moral of the story: go for the easiest repair route here.

Todd, you think exactly like I do! In the end, I came to the same conclusion: easiest repair route.  Plus, the aftermarket pipe is less than the stock one, and, according to Slowhand, might show some performance improvement on my exact car, so it becomes a no brainer.

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On 11/23/2016 at 11:43 AM, Slowhand said:

I had a Grand Prix GTP 3800 super charged. Go with the bigger down pipe.  I was into modding that car. Look for forums on 3800's. My car went from 260 hp, to 325 hp. Still ran strong with 150,000 miles on it. So with the 3" down pipe in your car you should feel the change, it's like 20 hp. Been there done that. 

By the way, I'm going to hijack my own thread on two fronts:  First, I noticed in your signature that your boat was lost in a warehouse fire. Was that in the Bethlehem Steel plant fire? My brother lives in Hamburg about 2 miles from there. He kept his RV there, but it just happened to be out at the time by a crazy stroke of luck. His neighbor across the street lost a gorgeous vintage racing hydroplane in there. I'm actually shocked that the magnitude of the loss there hasn't made bigger news. There were probably many millions of dollars in classic cars, RV's and boats in there, and the only reason I know about it here in Florida is through my brother, who has used the place for years.

On a different note, you mention modding your 3800.  This engine has been around for about 50 years. About 25 years ago I built up three different versions of the Buick 231 for a potent Chevy Monza with a T5 transmission. What a fun little car. The last version of the motor I built was actually a 1965 225 block (from before Buick sold it to Kaiser Jeep) with a steel crank and rods, 10.5:1 compression, 1979 stage 1 heads (I ported myself based on Smokey Yunick port templates), a custom intake and cam, custom oiling system and a 650 carb I built for the engine. It was a surprisingly strong little motor, and was a real blast with a five speed in that tiny car. Fun stuff.

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No, my boat was lost on Tonawanda Island in a warehouse fire 2 years ago. My car mods were fun and I beat a lot of 5.0 Mustangs and old car too. lol 

Picture 00009.JPG

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Could always put a Whipple on the 6.2L in the 255 SSi...imagine the looks on the faces of some Baja owners would be worth it.

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There is a difference when discussing the exhaust back pressure at different rpms & a exhaust systems peak / resonant tuned rpm.

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