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I had dual 2.5'' exhaust with cats, an H pipe, and 'turbo' style mufflers. It just wasn't loud enough for me...
So a couple hours later I have everything including the H pipe off the car and fitted dual 29'' cherry-bomb glasspacks with downspouts. It's much louder. :partyon: maybe too loud... but it seems as though I lost power during acceleration. This was opposite the effect I was hoping for because I figured less stuff blocking exhaust flow=more power. Is the lack of backpressure destroying my low-end torque? Do I need to do any sort of 'tuning' with the carb or mess with the timing?

Thanks :D
 

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Damn. So what can I do to liven up the exhaust note and retain/enhance performance?

Thanks

Use lower diameter pipe prior to the cherry bombs.:thumbsup:
 

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More noise doesn't necessarily equal less backpressure. Glasspacks are known to have excessive backpressure. I would go to the Dynomax web site. They list cfm numbers for all their mufflers. That way you know what you're getting.
 

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It may not be a backpressure problem so much as pipe length tuning problem. Yeah glasspacks are not as efficient as one would think, but if you have dramatically reduced the exhaust pipe length, the change in the reversion waves traveling back up the pipes could have adversely affected your cylinder scavenging. Short pipes are typically best for wide open screaming like drag strips, but longer pipes produce a more usable torque band, especially in variable RPM situations: ie, the street. Your H-pipe also helped a lot in this regard. And truthfully, if you are dumping your exhaust directly under the car, that's not very smart either.....Try putting the old system back on without the mufflers only, see how it acts. The cats do have some muffler effect but it will still be loud.
Also, major changes in exhaust system need to be accompanied by changes in carburation and perhaps timing as well, you may be too lean now.

John
 

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It may not be a backpressure problem so much as pipe length tuning problem. Yeah glasspacks are not as efficient as one would think, but if you have dramatically reduced the exhaust pipe length, the change in the reversion waves traveling back up the pipes could have adversely affected your cylinder scavenging. Short pipes are typically best for wide open screaming like drag strips, but longer pipes produce a more usable torque band, especially in variable RPM situations: ie, the street. Your H-pipe also helped a lot in this regard. And truthfully, if you are dumping your exhaust directly under the car, that's not very smart either.....Try putting the old system back on without the mufflers only, see how it acts. The cats do have some muffler effect but it will still be loud.
Also, major changes in exhaust system need to be accompanied by changes in carburation and perhaps timing as well, you may be too lean now.

John
:agree: That is great advice.

I am sorry to be a pain in your ass John, but at DC we like to avoid telling people that things they have done are not smart. Unless of course someone drives their vette into a tree. I am sure that you could think of a more diplomatic way of getting your point across.

Thank you for your consideration
 

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:agree: That is great advice.

I am sorry to be a pain in your ass John, but at DC we like to avoid telling people that things they have done are not smart. Unless of course someone drives their vette into a tree. I am sure that you could think of a more diplomatic way of getting your point across.

Thank you for your consideration
Woops! Sorry! Not off to a good start on my very first post, eh? :laughing:
That's cool, point taken, I'll be good! :thumbsup:

John
 

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Can I get in on this thread?

Since we are on the same subject...I just removed my Flowmaster Delta 40s and went with super 40's ( have a deeper tone and don't sound as "tinny" when I run out each gear. But I have noticed too that I'm not sure the car is running as well. With this minor change should I recheck the timing and adj. carbuerator? I have no crossover( mufffler guy said I would have to put it right under transmission and then if I had work done on it, they'd have to cut the pipes out.) and I'm running headmanheaders with 2.5 " pipes all around.

Thanks
Marc
 

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Marc,

That is a sweet looking car.

I think you could get some of your performance back with a decent tune up. Also, get a second opinoin on that crossover pipe. For a little extra $$$ you might be able to get some flanges on the end of the X pipe for easy removal. Also, couldn't they just remove the exhaust to work on the trans?
 

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Splanation needed. If the result of an exhaust system back pressure change is a change in air flow through the engine, why would the air/fuel ratio change if the carb is working right? I haven't put a lot of thought into this, but I've heard it for years.
 

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I will try to 'splain as best I can, as I understand it.
(Let me first say I am not an engine builder or even a long time hot-rod guy. Most of my experience is with old harleys, but I read and listen a lot. I never even built a car engine till last year, but I think I did OK: I'm very pleased with the results and my peers have told me they are impressed as well.)

The entire induction/exhaust system must be looked at as a whole. They must compliment each other to work efficiently. Efficiency = power.
Don't think of 'air flow' so much as 'pulses' and 'pressure waves.' At the mouth of the carb you have 14# of static air pressure. You open a valve to a vacuum (bear with me here for the sake of illustration) and the air column is not sucked in: it is pushed in. It is not a smooth flow. It is start-stop-start-stop because of the valves opening and closing. Pulsing.
Same thing on the exhaust side. The exhaust pulses in the pipes are a high pressure 'slug' followed by a low pressure 'blank'. This causes a high speed reverberation of pressure waves moving back and forth in the exhaust pipe system.
The idea is to get the pressure wave pulses of both intake and exhaust timed so as to enhance each other. You want a negative wave from the exhaust to be at the cylinder at the moment when the intake and exhaust valves are open (the overlap) to compliment the high pressure pulse in the intake manifold to help 'pull' more charge into the cylinder. This is 'scavenging.' This reverberation in a long exhaust tract does push some exhaust gasses back into the cylinder, but this is acceptable for the benefits it provides.
Your carb, manifold, cam, heads, & exhaust must all work in harmony to compliment each other.
One key element many people overlook is that the velocity of the gasses for both induction and exhaust is very important. Smaller intake runners and pipes typically work better on smaller less radical engines than big ones because a smaller mass moving faster can carry a stronger 'punch' than a larger mass moving slower. It also takes less energy and time to change it's movement: more efficient 'pulsing'.
However, when you are tuned for a full length exhaust system and you uncork it, the exhaust gas is straight out and you get less contamination of your intake charge due to the lack of back pressures during overlap. The contamination richens your mixture, when you remove it, you now are running lean. Long pipes give a wide torque band, but with that comes some contamination of the intake charge. (EGR: introduce exhaust gasses back into the cylinder for a richer mixture and you then lean out the carb for better emissions.)
There is an awful lot more than this to it, it is the subject of many volumes.

Luke, when you radically shortened your exhaust, you changed the scavenging effects the engine had been tuned for and thus the mixture being introduced into your cylinders.

That's my theory....Comments from those more knowledgable than I? I would like to hear any...

John

(edited)
 

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I will try to 'splain as best I can, as I understand it.
(Let me first say I am not an engine builder or even a long time hot-rod guy. Most of my experience is with old harleys, but I read and listen a lot. I never even built a car engine till last year, but I think I did OK: I'm very pleased with the results and my peers have told me they are impressed as well.)

The entire induction/exhaust system must be looked at as a whole. They must compliment each other to work efficiently. Efficiency = power.
Don't think of 'air flow' so much as 'pulses' and 'pressure waves.' At the mouth of the carb you have 14# of static air pressure. You open a valve to a vacuum (bear with me here for the sake of illustration) and the air column is not sucked in: it is pushed in. It is not a smooth flow. It is start-stop-start-stop because of the valves opening and closing. Pulsing.
Same thing on the exhaust side. The exhaust pulses in the pipes are a high pressure 'slug' followed by a low pressure 'blank'. This causes a high speed reverberation of pressure waves moving back and forth in the exhaust pipe system.
The idea is to get the pressure wave pulses of both intake and exhaust timed so as to enhance each other. You want a negative wave from the exhaust to be at the cylinder at the moment when the intake and exhaust valves are open (the overlap) to compliment the high pressure pulse in the intake manifold to help 'pull' more charge into the cylinder. This is 'scavenging.' This reverberation in a long exhaust tract does push some exhaust gasses back into the cylinder, but this is acceptable for the benefits it provides.
Your carb, manifold, cam, heads, & exhaust must all work in harmony to compliment each other.
One key element many people overlook is that the velocity of the gasses for both induction and exhaust is very important. Smaller intake runners and pipes typically work better on smaller less radical engines than big ones because a smaller mass moving faster can carry a stronger 'punch' than a larger mass moving slower. It also takes less energy and time to change it's movement: more efficient 'pulsing'.
However, when you are tuned for a full length exhaust system and you uncork it, the exhaust gas is straight out and you get less contamination of your intake charge due to the lack of back pressures during overlap. The contamination richens your mixture, when you remove it, you now are running lean. Long pipes give a wide torque band, but with that comes some contamination of the intake charge. (EGR: introduce exhaust gasses back into the cylinder for a richer mixture and you then lean out the carb for better emissions.)
There is an awful lot more than this to it, it is the subject of many volumes.

Luke, when you radically shortened your exhaust, you changed the scavenging effects the engine had been tuned for and thus the mixture being introduced into your cylinders.

That's my theory....Comments from those more knowledgable than I? I would like to hear any...

John

(edited)
Thanks for a very good explanation. If I've got this right, you still want to minimize the backpressure to minimize flow resistence as long as you tune the exhaust system to maximize scavenging in the operating range that you are looking for, consistent with the rest of the system components. With the carb adjusted, Luke's engine may, in fact, be putting out some additional power up high in the rpm range, but the low end has been compromised and that's what he feels now. Disregarding any benefits of a crossover pipe, with proper tuned length headers and shorty mufflers, you could still get more power/torque where you want it as long as the lengths are right(??). I would think this A/F ratio effect might be even more pronounced in a single plane manifold, than a dual plane, because of the more direct connection of adjacent cylinder intakes. Just speculating. Thanks.
 

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I think you've got the jist of it, Fred.

Where the 'H' & 'X' pipes come in is to reduce the pressure & pressure waves, both positive and negative, in the pipe by effectively increasing the volume of the pipe. The high pressure wave on one side will typically correspond to a low pressure on the other. By connecting the two, you create an alternative route for some of the pressure waves thus reducing the effect they have upstream towards the valve, plus reducing pressure differentials downstream in the pipe. This is why H & X pipes sound quieter. If you hold your hand on the end of a single pipe while the engine is running, you notice it flaps back and forth: pushed out by the pulse and then sucked back in. This is the 14# atmospheric pressure pushing in to the negative pressure pulse. The crossovers reduce this also.
The main difference between an 'H' and an 'X' is that an H lets most of the actual exhaust gas pulse travel down it's original pipe, while relieving the pressure waves. The X actually takes one pipe and splits it into two. You can hear the difference between all three: individual pipes have a distinct alternating 'thump' from each pipe. An H system will still have an alternating 'thump' but not as distinct--some of the noise & gas will come out the other pipe, but the most noticable difference would be less 'back-sucking' if you hold your hand over one pipe. The X system splits all pulses into both pipes: instead of individual alternating 'full thumps', you get continuous 'half thumps' from both pipes. They sound almost like a 12 cylinder, a soft purring instead of a nasty rumping. That's one reason some guys don't like them. But they do the best job of smoothing out a pulsing exhaust flow.

:thumbsup:

John
 

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Discussion Starter #20
Thanks a lot John that was interesting and educational. I've never thought about exactly what goes on in these situations and you have summed it up very well. :thumbsup:
 
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