need some fluidynamics help (related to exhaust)
Registered User
Joined: Feb 2004
Posts: 291
From: Abbeville , LA
The dyno backed up the track results, though. It gained about 3 rwhp when removing the whole 3" cat-back.
I have a theory about the engine being over-exhausted. If you remove backpressure from an engine and it doesn't gain, then the exhaust ports, exhaust cam lobe, and headers may be working too well and overscavenging the cylinder.
Mike
===========================================
anyone actually check "lengths" for comparisons
the Total Exhaust System length with mullfers etc. can make large HP/Torque differences
to compare Apples -to Apples ..the "Internal Lengths" will have to be exactly the same
might even have to consider metal thickness and coatings as exhaust gas temperature greatly influence wave tuning
and exhaust gas velocity
we've dyno tested large CID Big Block Chevys in 900 to 1200 HP ranges with just open headers -vs- complete exhaust systems with Hi_perf mufflers and sometimes the engine will make a few more HP with full length exhaust system , and other times will just be a few HP less .
eveyonce in awhile, even a great flowing muffler and correct diameter exhaust system will show a big Loss if wrong internal length
I have a theory about the engine being over-exhausted. If you remove backpressure from an engine and it doesn't gain, then the exhaust ports, exhaust cam lobe, and headers may be working too well and overscavenging the cylinder.
Mike
===========================================
anyone actually check "lengths" for comparisons
the Total Exhaust System length with mullfers etc. can make large HP/Torque differences
to compare Apples -to Apples ..the "Internal Lengths" will have to be exactly the same
might even have to consider metal thickness and coatings as exhaust gas temperature greatly influence wave tuning
and exhaust gas velocity
we've dyno tested large CID Big Block Chevys in 900 to 1200 HP ranges with just open headers -vs- complete exhaust systems with Hi_perf mufflers and sometimes the engine will make a few more HP with full length exhaust system , and other times will just be a few HP less .
eveyonce in awhile, even a great flowing muffler and correct diameter exhaust system will show a big Loss if wrong internal length
Registered User
Joined: Mar 2002
Posts: 2,743
From: Baton Rouge, LA, USA
Larry, I just removed the cat-back from the y-pipe, so the internal length is most certainly much, much less.
Jon, if you're exhaust port/lobe/header is overscavenging the cylinder, you are pulling intake air and fuel out the exhaust during overlap. This is especially easy to do in a supercharged engine because of the positive intake pressure pushing air/fuel through during overlap. The added backpressure may be helping to keep the air/fuel in the cylinder by 'pushing' in the other direction.
Just a theory. . .
Mike
Jon, if you're exhaust port/lobe/header is overscavenging the cylinder, you are pulling intake air and fuel out the exhaust during overlap. This is especially easy to do in a supercharged engine because of the positive intake pressure pushing air/fuel through during overlap. The added backpressure may be helping to keep the air/fuel in the cylinder by 'pushing' in the other direction.
Just a theory. . .
Mike
Registered User
Joined: Sep 2002
Posts: 655
I'll agree something doesn't add up.
I believe with the proper diameter and length of primary runners,
removing the restrictions will only improve torque in *99.9%* of
engine setups.
It's possible EnginneerMike has the wrong dimensions for header
primaries which is throwing off any sort of "tuning factor" when
playing with cat-back lengths, mufflers, etc.
"Overscavenging the cylinder"?
That void must be replaced with something. If there is a low pressure
area within the cylinder, and enough intake charge ready, VE%
will improve.
Having said that, the intake pressure must be higher to achieve
the "overscavenging" (low exhaust port pressure).
I don't believe exhuast gas velocity can create a "hole" in the process.
IE: xx amount of pressure drop in the exhaust port without xx
amount of pressure increase in the intake runner.
Is there really such thing as overscavenging? Or should we refer
to this as "blowdown" instead?
I believe with the proper diameter and length of primary runners,
removing the restrictions will only improve torque in *99.9%* of
engine setups.
It's possible EnginneerMike has the wrong dimensions for header
primaries which is throwing off any sort of "tuning factor" when
playing with cat-back lengths, mufflers, etc.
"Overscavenging the cylinder"?
That void must be replaced with something. If there is a low pressure
area within the cylinder, and enough intake charge ready, VE%
will improve.
Having said that, the intake pressure must be higher to achieve
the "overscavenging" (low exhaust port pressure).
I don't believe exhuast gas velocity can create a "hole" in the process.
IE: xx amount of pressure drop in the exhaust port without xx
amount of pressure increase in the intake runner.
Is there really such thing as overscavenging? Or should we refer
to this as "blowdown" instead?
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by Zero_to_69
I'll agree something doesn't add up.
I believe with the proper diameter and length of primary runners,
removing the restrictions will only improve torque in *99.9%* of
engine setups.
It's possible EnginneerMike has the wrong dimensions for header
primaries which is throwing off any sort of "tuning factor" when
playing with cat-back lengths, mufflers, etc.
"Overscavenging the cylinder"?
That void must be replaced with something. If there is a low pressure
area within the cylinder, and enough intake charge ready, VE%
will improve.
Having said that, the intake pressure must be higher to achieve
the "overscavenging" (low exhaust port pressure).
I don't believe exhuast gas velocity can create a "hole" in the process.
IE: xx amount of pressure drop in the exhaust port without xx
amount of pressure increase in the intake runner.
Is there really such thing as overscavenging? Or should we refer
to this as "blowdown" instead?
I'll agree something doesn't add up.
I believe with the proper diameter and length of primary runners,
removing the restrictions will only improve torque in *99.9%* of
engine setups.
It's possible EnginneerMike has the wrong dimensions for header
primaries which is throwing off any sort of "tuning factor" when
playing with cat-back lengths, mufflers, etc.
"Overscavenging the cylinder"?
That void must be replaced with something. If there is a low pressure
area within the cylinder, and enough intake charge ready, VE%
will improve.
Having said that, the intake pressure must be higher to achieve
the "overscavenging" (low exhaust port pressure).
I don't believe exhuast gas velocity can create a "hole" in the process.
IE: xx amount of pressure drop in the exhaust port without xx
amount of pressure increase in the intake runner.
Is there really such thing as overscavenging? Or should we refer
to this as "blowdown" instead?
Exhaust scavenging due to primary and secondary pipe length tuning is doing it's job from the time exhaust valve opens when the intake is closed. This is the time of max exhaust velocity and pressure which should cause the max tuning.
Intake tuning for positive pressure at the intake valve usually peaks about intake closing time when the exhaust is closed. One doesn't affect the other until the only time they are both open during overlap when the tuned exhaust can achieve a lower pressure at the valve for some rpm range and help the intake as well as maybe "overscavenge" somewhat. More overlap gives more opportunity for this.
Larry's reasoning (apples to apples) makes a lot of sense.
FWIW, poor flowing mufflers cause most of the losses in an exhaust system. The difference between the best flowing (almost no back pressure caused by them) to the worst flowing (significant back pressure) is huge (or at least a lot!).
Vizard sometimes describes muffler flow by relating it to the flow of an straight tube of a certain diameter. That's a good analogy, IMO.
A free flowing 3 in. muffler flows just about like a 3 inch smooth tube, but a poor flowing one flows like a 2 inch or 1-3/4 inch smooth tube, or maybe less! Imagine your system with 2.5 or 3 inch y into a 3.5 or 4 inch pipe with a 1-3/4 inch tube stuck on the end of it! You'd lose your lunch if you saw someone install that on your car, but restrictive mufflers even of 3 inch or more inlet size do just about this. When you change just a muffler and pickup significant hp, you demonstrate this.
My $.02
Registered User
Joined: Sep 2002
Posts: 655
I am referring to the overlap period when I mentioned the "hole theory".
As the piston exhausts the charge and the exhaust valve is opening,
there is velocity created.
As the piston reaches TDC and the intake valve opens, the energy
of the exhaust gas velocity has created a low pressure area at
the exhaust port...
With both valves open, the intake charge *must* replace the "void"
in the cylinder/chamber left by the exhaust gas, otherwise a "vacuum"
would be created.
So my thoughts on overscavenging are:
It's a good thing. It will totally clear the chamber of residual exhaust
gas and continue to pull fresh charge in.
The point at which the exhaust valve shuts to trap the fresh charge
doesn't seem critical...so long as there is a continuous stream of
intake charge to replace it.
From what I gather...there is a trade off between wasted charge
blowing down the exhaust runner and the ability to increase VE%
Another angle would be to ask:
Can you overscavenge so much that fuel is no longer supplied by
the injector pulse causing a lean condition?
I don't see this as much of an issue with a carbureted setup because
the fuel is pulled into the stream according to pressure.
As the piston exhausts the charge and the exhaust valve is opening,
there is velocity created.
As the piston reaches TDC and the intake valve opens, the energy
of the exhaust gas velocity has created a low pressure area at
the exhaust port...
With both valves open, the intake charge *must* replace the "void"
in the cylinder/chamber left by the exhaust gas, otherwise a "vacuum"
would be created.
So my thoughts on overscavenging are:
It's a good thing. It will totally clear the chamber of residual exhaust
gas and continue to pull fresh charge in.
The point at which the exhaust valve shuts to trap the fresh charge
doesn't seem critical...so long as there is a continuous stream of
intake charge to replace it.
From what I gather...there is a trade off between wasted charge
blowing down the exhaust runner and the ability to increase VE%
Another angle would be to ask:
Can you overscavenge so much that fuel is no longer supplied by
the injector pulse causing a lean condition?
I don't see this as much of an issue with a carbureted setup because
the fuel is pulled into the stream according to pressure.
Thread Starter
Registered User
Joined: Oct 2002
Posts: 1,639
From: Toledo, OH
Zero...I believe you may be speaking correctly in a N/A setup...but in a blown application, like Mikes, he has no problem of pushing the remaining exhaust gases out during the overlap period, and if there is too little backpressure, he could be pushing TOO much and forcing strictly a fuel/air mix out the headers.
Sounds like a fix for this would be a cam with slightly less overlap. Just my thinking though.
Sounds like a fix for this would be a cam with slightly less overlap. Just my thinking though.
Registered User
Joined: Oct 2002
Posts: 2,931
From: Upstate NY
Originally posted by Zero_to_69
I am referring to the overlap period when I mentioned the "hole theory".
As the piston exhausts the charge and the exhaust valve is opening,
there is velocity created.
As the piston reaches TDC and the intake valve opens, the energy
of the exhaust gas velocity has created a low pressure area at
the exhaust port...
With both valves open, the intake charge *must* replace the "void"
in the cylinder/chamber left by the exhaust gas, otherwise a "vacuum"
would be created.
As the intake starts to open 20-30 degrees(*) before TDC there is still + pressure in the exhaust flow because the piston is still rising. Of course what the piston does as it passes TDC and starts down is create a pressure lower than atmospheric so the outside air pushes intake charge in. (the so-called vacuum). Initially the exhaust valve is still open (overlap) for maybe 20-30* so flow can go both ways depending on the pressure at any time.
At idle or low rpm all the exhaust doesn't get out and it mixes with the intake charge and there is reverse flow back up the intake tract. Depending on the valve event timing and the resulting ovelap, this continues up thru the rpm range until you reach the range where your cam timing is most effective.
When you reach the "tuning" point (or range) of your engine, the exhaust tuning pulses are timed to have a lower pressure at the exhaust valve to help accelerate the intake charge.
So my thoughts on overscavenging are:
It's a good thing. It will totally clear the chamber of residual exhaust
gas and continue to pull fresh charge in.
The point at which the exhaust valve shuts to trap the fresh charge
doesn't seem critical...so long as there is a continuous stream of
intake charge to replace it.
From what I gather...there is a trade off between wasted charge
blowing down the exhaust runner and the ability to increase VE%
Another angle would be to ask:
Can you overscavenge so much that fuel is no longer supplied by
the injector pulse causing a lean condition?
I don't see this as much of an issue with a carbureted setup because
the fuel is pulled into the stream according to pressure.
Whew.
Every valve event is critical, and EC isn't the most critical, but it's far from unimportant. IC probably is #1.
Scavenging is good, but overscavenging in the power band means (to me) too much of a good thing; cam timing isn't optimum.
If too much of the air/fuel intake charge is "sucked" out thru the exhaust, the A/F ratio probably doesn't change, but the VE could certainly go down. It doesn't just remove the fuel to make the charge lean.
I don't see how carbed and FI mixtures are much different as they enters the cylinder. If injector shot and carb jetting are both good, the engine certainly doesn't know how the fuel got mixed with the air.
Doesn't a carb add fuel by creating a lower pressure in the carb venturi (due to increase in velocity) so the fuel in the float bowl is at atmospheric pressure, and the outlet in the airstream at less pressure than that so the fuel flows toward the lower pressure at the venturi?
If, at peak torque rpm to peak power rpm and beyone you are still drawing a significant amount of intake charge out the exhaust, you need to rethink your valve timing.
Don't get lost trying to explain "why" to yourself. Keep you mind open. My unsolicited, entirely free advice...and worth what you pay for it!
I am referring to the overlap period when I mentioned the "hole theory".
As the piston exhausts the charge and the exhaust valve is opening,
there is velocity created.
As the piston reaches TDC and the intake valve opens, the energy
of the exhaust gas velocity has created a low pressure area at
the exhaust port...
With both valves open, the intake charge *must* replace the "void"
in the cylinder/chamber left by the exhaust gas, otherwise a "vacuum"
would be created.
As the intake starts to open 20-30 degrees(*) before TDC there is still + pressure in the exhaust flow because the piston is still rising. Of course what the piston does as it passes TDC and starts down is create a pressure lower than atmospheric so the outside air pushes intake charge in. (the so-called vacuum). Initially the exhaust valve is still open (overlap) for maybe 20-30* so flow can go both ways depending on the pressure at any time.
At idle or low rpm all the exhaust doesn't get out and it mixes with the intake charge and there is reverse flow back up the intake tract. Depending on the valve event timing and the resulting ovelap, this continues up thru the rpm range until you reach the range where your cam timing is most effective.
When you reach the "tuning" point (or range) of your engine, the exhaust tuning pulses are timed to have a lower pressure at the exhaust valve to help accelerate the intake charge.
So my thoughts on overscavenging are:
It's a good thing. It will totally clear the chamber of residual exhaust
gas and continue to pull fresh charge in.
The point at which the exhaust valve shuts to trap the fresh charge
doesn't seem critical...so long as there is a continuous stream of
intake charge to replace it.
From what I gather...there is a trade off between wasted charge
blowing down the exhaust runner and the ability to increase VE%
Another angle would be to ask:
Can you overscavenge so much that fuel is no longer supplied by
the injector pulse causing a lean condition?
I don't see this as much of an issue with a carbureted setup because
the fuel is pulled into the stream according to pressure.
Whew.
Every valve event is critical, and EC isn't the most critical, but it's far from unimportant. IC probably is #1.
Scavenging is good, but overscavenging in the power band means (to me) too much of a good thing; cam timing isn't optimum.
If too much of the air/fuel intake charge is "sucked" out thru the exhaust, the A/F ratio probably doesn't change, but the VE could certainly go down. It doesn't just remove the fuel to make the charge lean.
I don't see how carbed and FI mixtures are much different as they enters the cylinder. If injector shot and carb jetting are both good, the engine certainly doesn't know how the fuel got mixed with the air.
Doesn't a carb add fuel by creating a lower pressure in the carb venturi (due to increase in velocity) so the fuel in the float bowl is at atmospheric pressure, and the outlet in the airstream at less pressure than that so the fuel flows toward the lower pressure at the venturi?
If, at peak torque rpm to peak power rpm and beyone you are still drawing a significant amount of intake charge out the exhaust, you need to rethink your valve timing.
Don't get lost trying to explain "why" to yourself. Keep you mind open. My unsolicited, entirely free advice...and worth what you pay for it!
Registered User
Joined: Jul 2001
Posts: 752
From: Double Oak TX
I'm short on the physics and also short on some of the hp you guys are making, but pretty long on real world dyno testing.
We dynoed my car with open headers and then dynoed it with dual 3" pipes, X-pipe, resonators and Dynomax mufflers......there was no difference.
With two Car Sound cats added it lost between 8-10 hp/torque with all of it above 5000 rpm. Below 5000 there was almost no noticeible effect. Maybe a couple of hp or so.
Sometimes you can have your cake and eat it too.
We dynoed my car with open headers and then dynoed it with dual 3" pipes, X-pipe, resonators and Dynomax mufflers......there was no difference.
With two Car Sound cats added it lost between 8-10 hp/torque with all of it above 5000 rpm. Below 5000 there was almost no noticeible effect. Maybe a couple of hp or so.
Sometimes you can have your cake and eat it too.
Registered User
Joined: Mar 2002
Posts: 2,743
From: Baton Rouge, LA, USA
I think some people don't understand what blow-down is and how scavenging works.
Blow-down is when the exhaust valve opens towards the end of the power stroke. The high pressure gas blows down to atmospheric, hopefully before the piston begins the actual exhaust stroke.
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC. At this point, the exhaust is exiting the exhaust port at high velocity. As the piston decelerates nearing TDC, the mass of the exhaust gas in the long header tube 'wants' to continue traveling at the same, high velocity due to its momentum. The piston stops, but the exhaust continues exiting, thus pulling a vacuum on the chamber. This, in turn, helps pull intake air/fuel in when the intake valve opens.
Overscavenging can occur if your header/port/exhaust lobe work very well to pull a vacuum on the chamber, but you have alot of cam overlap, and good low-lift flow in the head. The air/fuel goes into the chamber then out the exhaust port.
Remember, the object is to get as much air/fuel in the cylinder as you can, not out the exhaust.
Don't believe in overscavenging? Ask Larry Meaux (MaxRaceSoftware), who dyno'd a Hemi that had 140% VE. However, the engine didn't make much power because 20% of the air/fuel were scavenged right out of the exhaust. They put in a cam with less overlap, the VE dropped to 120%, and the power increased.
Mike
Blow-down is when the exhaust valve opens towards the end of the power stroke. The high pressure gas blows down to atmospheric, hopefully before the piston begins the actual exhaust stroke.
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC. At this point, the exhaust is exiting the exhaust port at high velocity. As the piston decelerates nearing TDC, the mass of the exhaust gas in the long header tube 'wants' to continue traveling at the same, high velocity due to its momentum. The piston stops, but the exhaust continues exiting, thus pulling a vacuum on the chamber. This, in turn, helps pull intake air/fuel in when the intake valve opens.
Overscavenging can occur if your header/port/exhaust lobe work very well to pull a vacuum on the chamber, but you have alot of cam overlap, and good low-lift flow in the head. The air/fuel goes into the chamber then out the exhaust port.
Remember, the object is to get as much air/fuel in the cylinder as you can, not out the exhaust.
Don't believe in overscavenging? Ask Larry Meaux (MaxRaceSoftware), who dyno'd a Hemi that had 140% VE. However, the engine didn't make much power because 20% of the air/fuel were scavenged right out of the exhaust. They put in a cam with less overlap, the VE dropped to 120%, and the power increased.
Mike
Registered User
Joined: Sep 2002
Posts: 655
First of all, let me apologize for the confusion. Sometimes I forget
that people aren't mind readers ...so trying to figure out my backyard
physics must be difficult for many of the regulars here!
When I describe my thoughts, I am speaking of one aspect - the
scavenging....or overscavenging in this case.
I realize there is reversion at low RPM; I realize things change
when RPM increases and there is a definite outbound flow occuring.
Scavenging occurs at the point which the exhaust pressure is lower
than the intake pressure. To my knowledge, reversion and scavenging
cannot occur simultaneously at any RPM...all else being equal.
I can understand that too much charge "spilling" down the exhaust
runner is a waste.
Let me rephrase based on your new information:
Intake Valve Opens
Piston moves downward
Pressure drops in the cylinder
Velocity is created in the intake runner
Intake valve shuts
Intake charge is compressed
Power Stroke
Exhaust Valve Opens
Piston rises
High pressure within cylinder
Exhaust gas is forced out
Exhuast runner velocity is created
Low pressure in the exhaust runner
Piston continues to reach TDC
Intake Valve Opens
Piston is at TDC
** at an optimum tuned RPM ...or range of RPM **
High pressure in the intake runner is sucked into the cylinder
from the fast moving exhaust pressure pulse (low pressure)
VE% may increase depending on valve timing, tuned pipe lengths, RPM, etc.
Exhaust valve shuts
Pressure pulses at the exhaust port and intake port are timed
with the valves and piston movement to improve VE.
Pipe diameters and length determine the RPM or harmonic intervals
at which these pulses (wave lengths) help improve (or degrade) cylinder pressure (VE).
Clear as mud?
that people aren't mind readers ...so trying to figure out my backyard
physics must be difficult for many of the regulars here!
When I describe my thoughts, I am speaking of one aspect - the
scavenging....or overscavenging in this case.
I realize there is reversion at low RPM; I realize things change
when RPM increases and there is a definite outbound flow occuring.
Scavenging occurs at the point which the exhaust pressure is lower
than the intake pressure. To my knowledge, reversion and scavenging
cannot occur simultaneously at any RPM...all else being equal.
I can understand that too much charge "spilling" down the exhaust
runner is a waste.
Let me rephrase based on your new information:
Intake Valve Opens
Piston moves downward
Pressure drops in the cylinder
Velocity is created in the intake runner
Intake valve shuts
Intake charge is compressed
Power Stroke
Exhaust Valve Opens
Piston rises
High pressure within cylinder
Exhaust gas is forced out
Exhuast runner velocity is created
Low pressure in the exhaust runner
Piston continues to reach TDC
Intake Valve Opens
Piston is at TDC
** at an optimum tuned RPM ...or range of RPM **
High pressure in the intake runner is sucked into the cylinder
from the fast moving exhaust pressure pulse (low pressure)
VE% may increase depending on valve timing, tuned pipe lengths, RPM, etc.
Exhaust valve shuts
Pressure pulses at the exhaust port and intake port are timed
with the valves and piston movement to improve VE.
Pipe diameters and length determine the RPM or harmonic intervals
at which these pulses (wave lengths) help improve (or degrade) cylinder pressure (VE).
Clear as mud?
Last edited by Zero_to_69; Mar 12, 2004 at 08:27 PM.
Registered User
Joined: Sep 2002
Posts: 655
Mike, I just realized you snuck in a response before my post.
Thanks for clearing up the "blow-down" term. I didn't pick up
that angle from reading the text. I figured it meant "gas flowing
down the tube" (overscavenging if you will).
The reduction of overlap causing a lower VE% BUT higher output power confuses me.
My understanding of VE% is the amount of charge stored into the
cylinder after the intake valve closes. Something closely realted
to DCR?
If VE% reaches 140%, but 20% is "dropped" through overlap...
resulting in 120% total...
How can 120% VE (total) make MORE power?
Would it not be similar, or possibly slightly less if some charge was
to "overscavenge"?
Thanks for clearing up the "blow-down" term. I didn't pick up
that angle from reading the text. I figured it meant "gas flowing
down the tube" (overscavenging if you will).
The reduction of overlap causing a lower VE% BUT higher output power confuses me.
My understanding of VE% is the amount of charge stored into the
cylinder after the intake valve closes. Something closely realted
to DCR?
If VE% reaches 140%, but 20% is "dropped" through overlap...
resulting in 120% total...
How can 120% VE (total) make MORE power?
Would it not be similar, or possibly slightly less if some charge was
to "overscavenge"?
Last edited by Zero_to_69; Mar 13, 2004 at 03:23 PM.
Registered User
Joined: Mar 2002
Posts: 2,743
From: Baton Rouge, LA, USA
VE is typically:
(measured airflow through the engine)/(cid/2/rpm)
So, you can measure 140% VE, but only actually trap 110% in the cylinder.
Use a cam with less overlap and measured VE goes to 120% and it traps 120% in the cylinder -- power increases.
The overscavengine occurs when the intake valve opens (during overlap), rather than the intake valve closing, which affects DCR.
Mike
(measured airflow through the engine)/(cid/2/rpm)
So, you can measure 140% VE, but only actually trap 110% in the cylinder.
Use a cam with less overlap and measured VE goes to 120% and it traps 120% in the cylinder -- power increases.
The overscavengine occurs when the intake valve opens (during overlap), rather than the intake valve closing, which affects DCR.
Mike
Registered User
Joined: Apr 2003
Posts: 1,591
From: PA
Originally posted by engineermike
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC.
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC.
Mike,
I tried looking at the extremes of rod/stroke ratio, but can't come up with a case where the max velocity would occur that close to TDC. Is that a typo, or am i missing something here?
-brent
Registered User
Joined: Mar 2002
Posts: 2,743
From: Baton Rouge, LA, USA
Originally posted by 94formulabz
Mike,
I tried looking at the extremes of rod/stroke ratio, but can't come up with a case where the max velocity would occur that close to TDC. Is that a typo, or am i missing something here?
-brent
Mike,
I tried looking at the extremes of rod/stroke ratio, but can't come up with a case where the max velocity would occur that close to TDC. Is that a typo, or am i missing something here?
-brent
The piston reaches max velocity at close to 70 BTDC with a 5.7 rod and 3.75 stroke.
Mike
Registered User
Joined: Jun 2002
Posts: 888
From: Langley, BC,Canada
Originally posted by engineermike
I think some people don't understand what blow-down is and how scavenging works.
Blow-down is when the exhaust valve opens towards the end of the power stroke. The high pressure gas blows down to atmospheric, hopefully before the piston begins the actual exhaust stroke.
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC. At this point, the exhaust is exiting the exhaust port at high velocity. As the piston decelerates nearing TDC, the mass of the exhaust gas in the long header tube 'wants' to continue traveling at the same, high velocity due to its momentum. The piston stops, but the exhaust continues exiting, thus pulling a vacuum on the chamber. This, in turn, helps pull intake air/fuel in when the intake valve opens.
Overscavenging can occur if your header/port/exhaust lobe work very well to pull a vacuum on the chamber, but you have alot of cam overlap, and good low-lift flow in the head. The air/fuel goes into the chamber then out the exhaust port.
Remember, the object is to get as much air/fuel in the cylinder as you can, not out the exhaust.
Don't believe in overscavenging? Ask Larry Meaux (MaxRaceSoftware), who dyno'd a Hemi that had 140% VE. However, the engine didn't make much power because 20% of the air/fuel were scavenged right out of the exhaust. They put in a cam with less overlap, the VE dropped to 120%, and the power increased.
Mike
I think some people don't understand what blow-down is and how scavenging works.
Blow-down is when the exhaust valve opens towards the end of the power stroke. The high pressure gas blows down to atmospheric, hopefully before the piston begins the actual exhaust stroke.
During the actual exhaust stroke, the piston reaches max velocity about 30 deg BTDC. At this point, the exhaust is exiting the exhaust port at high velocity. As the piston decelerates nearing TDC, the mass of the exhaust gas in the long header tube 'wants' to continue traveling at the same, high velocity due to its momentum. The piston stops, but the exhaust continues exiting, thus pulling a vacuum on the chamber. This, in turn, helps pull intake air/fuel in when the intake valve opens.
Overscavenging can occur if your header/port/exhaust lobe work very well to pull a vacuum on the chamber, but you have alot of cam overlap, and good low-lift flow in the head. The air/fuel goes into the chamber then out the exhaust port.
Remember, the object is to get as much air/fuel in the cylinder as you can, not out the exhaust.
Don't believe in overscavenging? Ask Larry Meaux (MaxRaceSoftware), who dyno'd a Hemi that had 140% VE. However, the engine didn't make much power because 20% of the air/fuel were scavenged right out of the exhaust. They put in a cam with less overlap, the VE dropped to 120%, and the power increased.
Mike