What I learned about exhaust today
03-23-2006, 12:34 AM
#1
Race Director
Thread Starter
What I learned about exhaust today
I reread an 10-page article on race exhaust design today and I think I'm getting an understanding of the basics. (I believe that Vizard might have been the author?)
It's interesting to learn that the velocity of exhaust gases can create negative pressure (a vacuum) which will help suck the intake mixture into the chambers. Moreover, it can actually begin to pressurize air into the cylinders kindof like a mini-turbo/supercharger forces extra boost into each cylinder. The result is an apparent increase in cubic inches.
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
To perform this virtual swelling, primary and secondary tube size must be optimized. In layman's term, I think they mean the tube and collector size of your headers. I was surprised to learn that equal length header tubes aren't THAT important. That's because tube length can vary greatly without harming this suction effect. The size of the tubes, the size of the collector, and the length of the collector sounded most important.
What they didn't say was if a bend in the collector hurts. My guess is that it does. If so, that's probably why long tube headers are considered better. They fire backward and the exhaust escapes STRAIGHT into an extension of the header. On a downward firing shorty, a bend has to take place to re-route the exhaust to the rear of the car.
I can't recall for sure but I think stock shorties (on an L98) are 1 1/2" primaries with 2.5" collectors. Unfortunately, a 2 1/4" pipe is connected to the 2.5" collector in order to get the burnt gases out. This sounds dumb, stupid, ignorant. Here's why....
In the ideal design, an expanded section is added to the rear of the header collector which simulates a "backpressureless", open exhaust. Our L98's shrink at this outlet.
Finally 2.2cfm pipe is adequate to provide routing to get the exhaust to the rear of the car. With straight pipe, I think that 2.5" pipes can accomplish this. Especially on our 350s (vs. 400+HP motors).
So, what really matters is having good headers (which we kinda have stock) and NOT to constrict the exhaust past the collector. It doesn't hurt to create a larger chamber at the header exit. (Maybe that's one reason dual bullet cats are considered ideal for this system).
On an L98, the biggest restriction -- to me -- would be the immediate reduction to 2.25" pipe -- at the collector. That is actually a 25% drop in cross-section. OUUUCH! On top of that, 1980's cats aren't considered particularly efficient. Cough, cough, cough!
While I'm in the process of upgrading my "cat-back" system and my main cat, I now realize that the front "Y" may actually more important!
Though the two pipes merge into a 3" pipe at the mid-section, the gases have had some time to cool and contract. That means the 25% reduction seen when the two 2.5" pipes merge into one 3" pipe may not be all bad. Theoretically, this might keep the pressure constant and the velocity up. And, velocity mean suction! That's a good thing! Also, since no two cylinders fire at the same time, combining exhaust pulses from two banks of cylinders means sharing exhaust route sequentially vs. simulaneously!
If a true dual system were used, it sounds like the pipe diameter should be reduced downstream to maintain this same effect (keeping velocity up). Moreover, the use of dual bullet cats can help make a true dual system the optimum solution to this poorly designed equation.
Beyond basic design, the goal should be to install mufflers and cats that minimize any backpressure that may be induced into the system. Also, ideally, the balance tube sounds most efficient -- if located between the collector's. WOW! This was a surprise since most use X/H pipes to accomplish this function further downstream. However, it's certainly understandable considering with the obstacles which exist between the header collectors (like that big ol' tranny).
So, unless you plan on replacing everything from the headers back (including the headers), it sounds like starting with the front "Y" pipe and cats is the best place to begin attacking the crummy design of the L98 exhaust. In fact, it sounds like this might have a bigger effect than the cat-back upgrade!
Vader86 says this on his website in far fewer words. And, I didn't understand what he was saying -- until now.
Thanks to Vader, Central Coaster, Bogus, and everyone who has pointed me in the right direction and helped me toward this new understanding. (Can't wait to see what a catless 2.5" front "Y", high flow mid-cat (with air tube), and 2.75" cat-back system will do for my performance!
Now I need to figure out if this warrants a new chip. Are there any mods that would help the input short of replacing the intake? Yeah, I'm getting a high-rise hood, but the wheels, side-pipes (cat-back), exhaust, paint job, and interior upgrades kinda put a crimp on the budget! Depending on long-term gas prices, the mini-ram and headers may go on someday.
gp
It's interesting to learn that the velocity of exhaust gases can create negative pressure (a vacuum) which will help suck the intake mixture into the chambers. Moreover, it can actually begin to pressurize air into the cylinders kindof like a mini-turbo/supercharger forces extra boost into each cylinder. The result is an apparent increase in cubic inches.
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
To perform this virtual swelling, primary and secondary tube size must be optimized. In layman's term, I think they mean the tube and collector size of your headers. I was surprised to learn that equal length header tubes aren't THAT important. That's because tube length can vary greatly without harming this suction effect. The size of the tubes, the size of the collector, and the length of the collector sounded most important.
What they didn't say was if a bend in the collector hurts. My guess is that it does. If so, that's probably why long tube headers are considered better. They fire backward and the exhaust escapes STRAIGHT into an extension of the header. On a downward firing shorty, a bend has to take place to re-route the exhaust to the rear of the car.
I can't recall for sure but I think stock shorties (on an L98) are 1 1/2" primaries with 2.5" collectors. Unfortunately, a 2 1/4" pipe is connected to the 2.5" collector in order to get the burnt gases out. This sounds dumb, stupid, ignorant. Here's why....
In the ideal design, an expanded section is added to the rear of the header collector which simulates a "backpressureless", open exhaust. Our L98's shrink at this outlet.
Finally 2.2cfm pipe is adequate to provide routing to get the exhaust to the rear of the car. With straight pipe, I think that 2.5" pipes can accomplish this. Especially on our 350s (vs. 400+HP motors).So, what really matters is having good headers (which we kinda have stock) and NOT to constrict the exhaust past the collector. It doesn't hurt to create a larger chamber at the header exit. (Maybe that's one reason dual bullet cats are considered ideal for this system).
On an L98, the biggest restriction -- to me -- would be the immediate reduction to 2.25" pipe -- at the collector. That is actually a 25% drop in cross-section. OUUUCH! On top of that, 1980's cats aren't considered particularly efficient. Cough, cough, cough!
While I'm in the process of upgrading my "cat-back" system and my main cat, I now realize that the front "Y" may actually more important!
Though the two pipes merge into a 3" pipe at the mid-section, the gases have had some time to cool and contract. That means the 25% reduction seen when the two 2.5" pipes merge into one 3" pipe may not be all bad. Theoretically, this might keep the pressure constant and the velocity up. And, velocity mean suction! That's a good thing! Also, since no two cylinders fire at the same time, combining exhaust pulses from two banks of cylinders means sharing exhaust route sequentially vs. simulaneously!
If a true dual system were used, it sounds like the pipe diameter should be reduced downstream to maintain this same effect (keeping velocity up). Moreover, the use of dual bullet cats can help make a true dual system the optimum solution to this poorly designed equation.
Beyond basic design, the goal should be to install mufflers and cats that minimize any backpressure that may be induced into the system. Also, ideally, the balance tube sounds most efficient -- if located between the collector's. WOW! This was a surprise since most use X/H pipes to accomplish this function further downstream. However, it's certainly understandable considering with the obstacles which exist between the header collectors (like that big ol' tranny).
So, unless you plan on replacing everything from the headers back (including the headers), it sounds like starting with the front "Y" pipe and cats is the best place to begin attacking the crummy design of the L98 exhaust. In fact, it sounds like this might have a bigger effect than the cat-back upgrade!
Vader86 says this on his website in far fewer words. And, I didn't understand what he was saying -- until now.
Thanks to Vader, Central Coaster, Bogus, and everyone who has pointed me in the right direction and helped me toward this new understanding. (Can't wait to see what a catless 2.5" front "Y", high flow mid-cat (with air tube), and 2.75" cat-back system will do for my performance!
Now I need to figure out if this warrants a new chip. Are there any mods that would help the input short of replacing the intake? Yeah, I'm getting a high-rise hood, but the wheels, side-pipes (cat-back), exhaust, paint job, and interior upgrades kinda put a crimp on the budget! Depending on long-term gas prices, the mini-ram and headers may go on someday.
gp
03-23-2006, 01:10 AM
#2
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St. Jude Donor '05
IMO there is no need or huge benefit for having true duals, the 2.5" Y to 3" main-2.5"rear Y is more than enough to handle most applications.
:flame suit on:
:flame suit on:
03-23-2006, 01:45 AM
#3
Race Director
I like what you wrote here ! Recently, Popular Hot Rodding had an article titled. "Exhaust systems demistified" Your post reminded me of it, you said some of the same stuff I read there.
The tri Y design is supposedly the best... 4 into 2 into 1... have you read about that exhaust style / system ?
The tri Y design is supposedly the best... 4 into 2 into 1... have you read about that exhaust style / system ?
03-23-2006, 02:00 AM
#4
Team Owner
Originally Posted by cuisinartvette
IMO there is no need or huge benefit for having true duals, the 2.5" Y to 3" main-2.5"rear Y is more than enough to handle most applications.
:flame suit on:
:flame suit on:
03-23-2006, 02:01 AM
#5
Team Owner
Originally Posted by GREGGPENN
I reread an 10-page article on race exhaust design today and I think I'm getting an understanding of the basics. (I believe that Vizard might have been the author?)
It's interesting to learn that the velocity of exhaust gases can create negative pressure (a vacuum) which will help suck the intake mixture into the chambers. Moreover, it can actually begin to pressurize air into the cylinders kindof like a mini-turbo/supercharger forces extra boost into each cylinder. The result is an apparent increase in cubic inches.
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
To perform this virtual swelling, primary and secondary tube size must be optimized. In layman's term, I think they mean the tube and collector size of your headers. I was surprised to learn that equal length header tubes aren't THAT important. That's because tube length can vary greatly without harming this suction effect. The size of the tubes, the size of the collector, and the length of the collector sounded most important.
What they didn't say was if a bend in the collector hurts. My guess is that it does. If so, that's probably why long tube headers are considered better. They fire backward and the exhaust escapes STRAIGHT into an extension of the header. On a downward firing shorty, a bend has to take place to re-route the exhaust to the rear of the car.
I can't recall for sure but I think stock shorties (on an L98) are 1 1/2" primaries with 2.5" collectors. Unfortunately, a 2 1/4" pipe is connected to the 2.5" collector in order to get the burnt gases out. This sounds dumb, stupid, ignorant. Here's why....
In the ideal design, an expanded section is added to the rear of the header collector which simulates a "backpressureless", open exhaust. Our L98's shrink at this outlet. Finally 2.2cfm pipe is adequate to provide routing to get the exhaust to the rear of the car. With straight pipe, I think that 2.5" pipes can accomplish this. Especially on our 350s (vs. 400+HP motors).
So, what really matters is having good headers (which we kinda have stock) and NOT to constrict the exhaust past the collector. It doesn't hurt to create a larger chamber at the header exit. (Maybe that's one reason dual bullet cats are considered ideal for this system).
On an L98, the biggest restriction -- to me -- would be the immediate reduction to 2.25" pipe -- at the collector. That is actually a 25% drop in cross-section. OUUUCH! On top of that, 1980's cats aren't considered particularly efficient. Cough, cough, cough!
While I'm in the process of upgrading my "cat-back" system and my main cat, I now realize that the front "Y" may actually more important!
Though the two pipes merge into a 3" pipe at the mid-section, the gases have had some time to cool and contract. That means the 25% reduction seen when the two 2.5" pipes merge into one 3" pipe may not be all bad. Theoretically, this might keep the pressure constant and the velocity up. And, velocity mean suction! That's a good thing! Also, since no two cylinders fire at the same time, combining exhaust pulses from two banks of cylinders means sharing exhaust route sequentially vs. simulaneously!
If a true dual system were used, it sounds like the pipe diameter should be reduced downstream to maintain this same effect (keeping velocity up). Moreover, the use of dual bullet cats can help make a true dual system the optimum solution to this poorly designed equation.
Beyond basic design, the goal should be to install mufflers and cats that minimize any backpressure that may be induced into the system. Also, ideally, the balance tube sounds most efficient -- if located between the collector's. WOW! This was a surprise since most use X/H pipes to accomplish this function further downstream. However, it's certainly understandable considering with the obstacles which exist between the header collectors (like that big ol' tranny).
So, unless you plan on replacing everything from the headers back (including the headers), it sounds like starting with the front "Y" pipe and cats is the best place to begin attacking the crummy design of the L98 exhaust. In fact, it sounds like this might have a bigger effect than the cat-back upgrade!
Vader86 says this on his website in far fewer words. And, I didn't understand what he was saying -- until now.
Thanks to Vader, Central Coaster, Bogus, and everyone who has pointed me in the right direction and helped me toward this new understanding. (Can't wait to see what a catless 2.5" front "Y", high flow mid-cat (with air tube), and 2.75" cat-back system will do for my performance!
Now I need to figure out if this warrants a new chip. Are there any mods that would help the input short of replacing the intake? Yeah, I'm getting a high-rise hood, but the wheels, side-pipes (cat-back), exhaust, paint job, and interior upgrades kinda put a crimp on the budget! Depending on long-term gas prices, the mini-ram and headers may go on someday.
gp
It's interesting to learn that the velocity of exhaust gases can create negative pressure (a vacuum) which will help suck the intake mixture into the chambers. Moreover, it can actually begin to pressurize air into the cylinders kindof like a mini-turbo/supercharger forces extra boost into each cylinder. The result is an apparent increase in cubic inches.
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
To perform this virtual swelling, primary and secondary tube size must be optimized. In layman's term, I think they mean the tube and collector size of your headers. I was surprised to learn that equal length header tubes aren't THAT important. That's because tube length can vary greatly without harming this suction effect. The size of the tubes, the size of the collector, and the length of the collector sounded most important.
What they didn't say was if a bend in the collector hurts. My guess is that it does. If so, that's probably why long tube headers are considered better. They fire backward and the exhaust escapes STRAIGHT into an extension of the header. On a downward firing shorty, a bend has to take place to re-route the exhaust to the rear of the car.
I can't recall for sure but I think stock shorties (on an L98) are 1 1/2" primaries with 2.5" collectors. Unfortunately, a 2 1/4" pipe is connected to the 2.5" collector in order to get the burnt gases out. This sounds dumb, stupid, ignorant. Here's why....
In the ideal design, an expanded section is added to the rear of the header collector which simulates a "backpressureless", open exhaust. Our L98's shrink at this outlet.
Finally 2.2cfm pipe is adequate to provide routing to get the exhaust to the rear of the car. With straight pipe, I think that 2.5" pipes can accomplish this. Especially on our 350s (vs. 400+HP motors).So, what really matters is having good headers (which we kinda have stock) and NOT to constrict the exhaust past the collector. It doesn't hurt to create a larger chamber at the header exit. (Maybe that's one reason dual bullet cats are considered ideal for this system).
On an L98, the biggest restriction -- to me -- would be the immediate reduction to 2.25" pipe -- at the collector. That is actually a 25% drop in cross-section. OUUUCH! On top of that, 1980's cats aren't considered particularly efficient. Cough, cough, cough!
While I'm in the process of upgrading my "cat-back" system and my main cat, I now realize that the front "Y" may actually more important!
Though the two pipes merge into a 3" pipe at the mid-section, the gases have had some time to cool and contract. That means the 25% reduction seen when the two 2.5" pipes merge into one 3" pipe may not be all bad. Theoretically, this might keep the pressure constant and the velocity up. And, velocity mean suction! That's a good thing! Also, since no two cylinders fire at the same time, combining exhaust pulses from two banks of cylinders means sharing exhaust route sequentially vs. simulaneously!
If a true dual system were used, it sounds like the pipe diameter should be reduced downstream to maintain this same effect (keeping velocity up). Moreover, the use of dual bullet cats can help make a true dual system the optimum solution to this poorly designed equation.
Beyond basic design, the goal should be to install mufflers and cats that minimize any backpressure that may be induced into the system. Also, ideally, the balance tube sounds most efficient -- if located between the collector's. WOW! This was a surprise since most use X/H pipes to accomplish this function further downstream. However, it's certainly understandable considering with the obstacles which exist between the header collectors (like that big ol' tranny).
So, unless you plan on replacing everything from the headers back (including the headers), it sounds like starting with the front "Y" pipe and cats is the best place to begin attacking the crummy design of the L98 exhaust. In fact, it sounds like this might have a bigger effect than the cat-back upgrade!
Vader86 says this on his website in far fewer words. And, I didn't understand what he was saying -- until now.
Thanks to Vader, Central Coaster, Bogus, and everyone who has pointed me in the right direction and helped me toward this new understanding. (Can't wait to see what a catless 2.5" front "Y", high flow mid-cat (with air tube), and 2.75" cat-back system will do for my performance!
Now I need to figure out if this warrants a new chip. Are there any mods that would help the input short of replacing the intake? Yeah, I'm getting a high-rise hood, but the wheels, side-pipes (cat-back), exhaust, paint job, and interior upgrades kinda put a crimp on the budget! Depending on long-term gas prices, the mini-ram and headers may go on someday.
gp
03-23-2006, 08:43 AM
#8
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The balance tube has been used by (old) racers for a long time, it works.( one side will help scavenge the exhast from the other as it fires) I remember seeing my first crossover tube from GM on a 58 big block biscane i had, this was stock GM stuff then. Tri y's work up to a hp point, the reason our l-98's exhast sucks is simple....no room and cost.
Last edited by bluealtered; 03-23-2006 at 09:03 AM.
03-23-2006, 11:38 AM
#10
Race Director
Thread Starter
Certainly, the 3" center section becomes the biggest constraint IF you increase the front pipes to 2.5" or larger.
After gaining this knowledge (in post#1), I would conclude that replacing the front "Y" pipe and main cat are the single most important first steps in modding an L98 exhaust. And, I bet that the catback portion is actually the least important! That's because the APPARENT cross-section of the rear "Y" is actually LARGER than the 3" center section in front of it. Unless you release pressure upstream, lowering it downstream would have little effect.
The center cat should be included as a first priority if it's as restrictive as reported in this forum. I have not found imperical data to support/deny the claim but it sounds reasonable to assume much has been learned since the 80s. The decision to use bullets in the front section (in place of the factory precats) would be up to the modifier -- and would be based on their need to maintain emissions conformity. (Same is true for main cat considerations).
Once the front "Y" and cats are upgraded, then it would seem that a conversion to a larger center pipe or a conversion to true duals would be in order. That's when you should be looking at the rear section. I have to agree with Bogus that 350HP would probably be the threshold for that consideration.
Swapping out the rear "Y" before changing anything upstream should be done solely with the expectation of altering exhaust tone and/or visual "bling". (HP gains aren't likely to occur).
Edit: Rear, one-outlet mufflers are also worth replacing in early stages of modification. However, the entire cat-back wouldn't need to be.
After gaining this knowledge (in post#1), I would conclude that replacing the front "Y" pipe and main cat are the single most important first steps in modding an L98 exhaust. And, I bet that the catback portion is actually the least important! That's because the APPARENT cross-section of the rear "Y" is actually LARGER than the 3" center section in front of it. Unless you release pressure upstream, lowering it downstream would have little effect.
The center cat should be included as a first priority if it's as restrictive as reported in this forum. I have not found imperical data to support/deny the claim but it sounds reasonable to assume much has been learned since the 80s. The decision to use bullets in the front section (in place of the factory precats) would be up to the modifier -- and would be based on their need to maintain emissions conformity. (Same is true for main cat considerations).
Once the front "Y" and cats are upgraded, then it would seem that a conversion to a larger center pipe or a conversion to true duals would be in order. That's when you should be looking at the rear section. I have to agree with Bogus that 350HP would probably be the threshold for that consideration.
Swapping out the rear "Y" before changing anything upstream should be done solely with the expectation of altering exhaust tone and/or visual "bling". (HP gains aren't likely to occur).
Edit: Rear, one-outlet mufflers are also worth replacing in early stages of modification. However, the entire cat-back wouldn't need to be.
Last edited by GREGGPENN; 03-23-2006 at 12:04 PM.
03-23-2006, 11:43 AM
#12
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Originally Posted by GREGGPENN
It's interesting to learn that the velocity of exhaust gases can create negative pressure (a vacuum) which will help suck the intake mixture into the chambers. Moreover, it can actually begin to pressurize air into the cylinders kindof like a mini-turbo/supercharger forces extra boost into each cylinder. The result is an apparent increase in cubic inches.
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
The cubic inch displacement of an engine is dependant upon only two things: stroke and bore (okay, okay: THREE: number of cylinders....happy now?
) Unless you change at least one of these, the displacement of an engine will remain the same.Exhaust tuning is an arcane mixture of art, science, and intuition. It CAN increase the volumetric efficiency of an engine, giving the same power (torque) output as a larger displacement engine...but only in a fairly narrow rpm range.
I love exhuast systems.
Larry
code5coupe
03-23-2006, 12:09 PM
#13
Race Director
Thread Starter
Originally Posted by rocco16
...Exhaust tuning is an arcane mixture of art, science, and intuition. It CAN increase the volumetric efficiency of an engine, giving the same power (torque) output as a larger displacement engine...but only in a fairly narrow rpm range.
Larry
code5coupe
Larry
code5coupe
Sounds like the L98 to me!
gp
03-23-2006, 12:35 PM
#14
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Originally Posted by GREGGPENN
So, changes to a system that would let an engine APPEAR larger at lower RPMs and perform better at mid-RPMs would be an improvement over one that's just a torquey low-RPM motor, right?
The only problem with this scenario is that no system can be tuned to give optimum results over a wide range of rpm. (e.g. both low and mid rpm)
An exhaust can give maximum torque improvements at one place in the engine's rpm range...the more you are willing to compromise on power increases, the wider that place can be.
For instance: you can achieve a 20ft/lb increase at 2000-2300rpm, or you can achieve a 10ft/lb increase at 1900-2600rpm. Just as with cams, or intakes, you have to know what you want with an exhaust.
Your power increase will always be a peak with tuning (not so with more displacement), we have to decide if we want that peak to be tall and steep or smaller and more gentle.
There are a few rules of thumb; Less backpressure is better than more IF by backpressure we mean resistance to FLOW. Some people use the terms "backpressure" and "reverse pressure waves" synonymously, and this can be confusing. They are not the same thing.
IC engine exhausts can be fascinating subjects, study them and you will find jewels of wisdom and you will find BS. The hard part is separating the two.
Larry
code5coupe
Last edited by rocco16; 03-23-2006 at 12:39 PM.
03-23-2006, 02:31 PM
#15
Race Director
Thread Starter
Originally Posted by rocco16
An exhaust can give maximum torque improvements at one place in the engine's rpm range...the more you are willing to compromise on power increases, the wider that place can be.
For instance: you can achieve a 20ft/lb increase at 2000-2300rpm, or you can achieve a 10ft/lb increase at 1900-2600rpm.
Larry
code5coupe
For instance: you can achieve a 20ft/lb increase at 2000-2300rpm, or you can achieve a 10ft/lb increase at 1900-2600rpm.
Larry
code5coupe
03-23-2006, 03:21 PM
#17
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Originally Posted by GREGGPENN
Can you give specifics or examples to illustrate what elements factor into creating a narrower/wider "bandwidth".
When an exhaust valve opens, there is ..... never mind; this could take way too long and I'd lose you (if not myself) before it was done.
I'll despense with the specifics and go to examples:
The tri-y or 4-2-1 header is one. Stepped headers are another. They can give you a softer 'hit', but over a wider rpm range, than 4-1 will. X/H pipes soften/spread the hit. Stock manifolds and pipes give you the softest, but widest hit of all.
As bogus said, there are lots of books out there than can explain how it works far better than I can. They make very interesting reading.
(they will also show you why "anti-reversion" headers are counterproductive.) Good luck!
Larry
code5coupe
Last edited by rocco16; 03-23-2006 at 03:24 PM.
03-23-2006, 03:48 PM
#18
Team Owner
Originally Posted by bogus
there are about 20 books, all about 300 pages, that outline the black art called "exhaust design."
I am not kidding.
I am not kidding.
03-23-2006, 04:08 PM
#19
Drifting
Originally Posted by GREGGPENN
I gather this effect is called scavenging. (I always wondered what this was)! Anyway, it can increase our 350ci displacement to something closer to 373's. That's because a higher amount of air is compressed for each explosion. (Don't get touchy 373 owner, yours can be increased too)!
gp
gp
I think you are trying to say the the "Compression Ratio" is increased.
This would be true but, marginally and only at a specific narrow rpm range.
This is where the "RAVE" exhaust came into play. I can't explain it but, I do know it works,and it works well. Too bad it is for 2 stroke motors.
03-23-2006, 06:59 PM
#20
Race Director
Thread Starter
Originally Posted by Midnite 88
Actually, you can't increase the displacement without increasing the bore or stroke.
I think you are trying to say the the "Compression Ratio" is increased.
This would be true but, marginally and only at a specific narrow rpm range.
I think you are trying to say the the "Compression Ratio" is increased.
This would be true but, marginally and only at a specific narrow rpm range.
"Negative pressure waves can scavenge the combustion chamber during the valve overlap period that exists at the end of the exhaust and beginning of the intake stroke.
To understand how this increases an engine's breathing, let us consider the cylinder and combustion chamber volumes of a typical high performance 350 Chevy. In traveling down the bore, each piston displaces 727cc. If the compression ratio is 12.5:1, the total combustion chamber volume above this 727cc is 63cc. If a negative pressure wave sucks out the residual exhaust gases remaining in the combustion chamber at TDC, the cylinder can draw in 727+63=790cc. The result is the the engine now runs like a 385 CID instead of a 350. But there is more to it than just scavenging the chamber. If enough energy is put into the incoming charge by the exhaust, it is possible to cause the cylinder to fill above atmospheric pressure at the time of intake valve closure. Compared with intake, exhaust tuning is far more potent and can operate over as much as ten times the rpm band."
David goes on to explain how exhaust pressure can enable race motors to draw an intake charge into the cylinder up to 100mph during the intake stroke. He dubs this a "second induction event". And, he says it can "endow your engine with a 20-40 HP advantage". To be fair, he doesn't say how much this applies to a street car.
Still, you can (and should) conclude that -- with proper exhaust tuning -- the air can be compressed to a higher degree (i.e., the compression ratio is increased). But he also says the motor performs like a larger one and over as much as ten times the rpm band. So I'm not sure if you're right.