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Thu May 24, 2007 7:33 am

I have my 2007 together now and have been reading up on proper sized exhaust for it.
the consensus seems to be that I need a 1 1/2" exhaust. (Based on displacement and exhaust valve size of 35.5 mm).
One thing that confuses me is that the smaller ID exhaust will cause a large engine to run hot. The other part that confuses me is that too large a diameter exhaust will hurt low end torque.
I realize that the smaller ID will cause higher exhaust velocities and hence higher frictional losses, but I can't understand how that makes a material difference in the running temp of the engine. On the other side, I can't understand the principle at work that causes a drop in low end torque if exhaust velocities are too low.
I would appreciate if someone could explain the principles at work here.
Thanks

Thu May 24, 2007 10:07 am

If the diameter is to small its going to cause excess back presure hence stoping the hot gases escaping, olso making the engine work harder as a pump to get the gases out.

Also if I am correct tis not just the diameter on the pipe but its length, so the internal area before the collector. But there is to much differance in length on vw headers, just real merged headers are a bit longer.

:D

Thu May 24, 2007 11:35 am

I did some quick calcs on a pressure drop program I use for pipelines.
Based on a flow rate of 30 e3m3/d through each pipe at a temp of 200 C and 1m of pipe and an average pressure of 50 kPa:
Although the 1 1/2" exhaust only has 16% larger flowing area than a 1 3/8 exhaust, it has 35% less pressure differential
The 1 5/8 has a 15% larger flowing area than the 1 1/2", but has a further 26% less pressure differential than the 1 1/2" exhaust.
At 30 e3m3/d, the program showed a gas velocity of 390 m/s, which seems pretty high. Sonic velocity is only 518m/s at those conditions.

The 30e3m3/d was a SWAG, but if the flow is significantly higher, the exhaust gasses could reach sonic velocity which would cause huge pressure differentials.
Anybody have any reasonable estimates of the volume flow from each cylinder? How about the pressure inside the exhaust pipes?

Thu May 24, 2007 2:09 pm

LOL, you asked the question like you didant know much and then come back with that. :shock:

I have no idea what 30 e3m3/d is, but I know from a desktop dyno that 48 ida webbers flow about a top end of 1000cfm, so you playing with less air than that in your calcs.

There are also head flow charts on Darren heads at his site http://www.drdracingheads.com/l3fs.html

:D

Thu May 24, 2007 2:34 pm

also remember that to little backpressure can burn an exhaust valve... ask me how i found that out :oops:

Matt

Thu May 24, 2007 2:40 pm

Didn't mean to be too pretentious. I can model pipelines - an exhaust is kinda like a small pipeline :) , but I have a lot to learn about engines.
I am having a tough time understanding how a rather small pressure differential (10 kPa, 1.5 psi) can make a large difference in the performance and heat retention of an engine. I'm not doubting that it does, I just don't understand the physical principles driving it.
Same thing with the exhaust being oversized and hurting low end torque - I don't doubt the advice of you guys who know WAY more about these engines than I do, I just don't understand HOW it works.
It makes me crazy when I don't understand the concept.

Thu May 24, 2007 2:45 pm

ok, im a slight moron when it comes to exhausts but i see it this way(measurments for example only!):

with a 1.5 inch exhaust your port velocities are up at even the lowest RPM. this allows more intake charge to be sucked into the cylender. but it can limit the high-RPM flow.

with a 1.75 inch exhaust has less low-RPM port velocity but can flow more at high-RPM, hence the loss of low-RPM power.

with a 2 inch exhaust you loose even more low-end but you can gain loads of top end if your motor can flow to the potential. back-pressure is the least and you could possibly burn an exhaust valve.

Matt

Thu May 24, 2007 4:16 pm

People under estimate the amount of importance that exhaust flow plays into pulling the intake charge into the cylinder, especially on a high performance engine with a cam that has a lot of duration and overlap.

During valve overlap, when both the intake and exhaust are open, it is the exhaust flow that helps pull the fresh intake charge into the cylinder and gets the intake charge moving.

When the exhaust is too small, the flow is under back pressure, this cause trouble with exhaust escaping the cylinder. 75% of the heat produced in the cylinder leaves via the exhaust Poor flow equals heat in the head, less power, more fuel burned for less work.

If the exhaust is too large, then exhaust velocity is lost. Think water from a fast moving river when hits a pond or lake. It slows down. when a fluid or gas is a small tube, it flows faster( until the point to where the tube can't flow enough volume).

With a four into one header, the work is done in the collector. Each exhaust pulse pulls the pulse behind it. The faster the velocity, the greater the pull. When we get everything right, great things happen. we getting these pulses moving very fast at about 1400 ft per second. They move so fast that they pull pull a vacuum behind themselves.( good headers have equal length primaries and equal number of bends in each primary, so as to insure each exhaust pulse gets to the collector in time.)

So with a proper designed header, there is a vacuum at the exhaust valve before the ex valve even opens. When the ex valve opens, the spent gas is quickly pulled/ sucked from the cylinder by this vacuum. and then this ex gas charge enters the primaries and pulls a vacuum behind it self on the cylinder. Now the cylinder has vacuum on it, so as soon as the intake valve opens the intake charge can start, even before the piston starts down the bore creating it's own vacuum.

On the very best designed header systems at some rpms, there are actually pumping gains. This is when not only has the header pulled a vacuum on the cylinder, but has done so while the piston is moving up on the exhaust stroke. This Sucks the piston upwards, making hp on the exhaust stroke!

This vacuum created by the header is why we can run a large duration cam that opens the intake way before TDC. without a header a large cam wouldn't work at all. The motor is a package. That's why people say it's all about the "combo", heads, carbs, cam and exhaust. Changing one can kill the others.

With a header that is too large, the pulse can't get up to speed, they spread out and get lazy. They don't work, so the whole combo goes to hell.

I made 210 hp on jakes dyno with a 1-5/8 exhaust header. But the good part is it made 200 ft-lbs of torque! course it also fell on it's face at 6500 rpm. At that point, the small primaries just couldn't flow enough.

A larger exhaust would have shifted the whole torque curve up in the power band.

This is also why an exhaust leak can cost you tons of HP, especially if it's at the head.

There is a lot to write on exhaust, and diameter, primary and collector length all play into it. I posted a formula to figure out primary diameter here about a year ago.

On small cams, exhaust sizes too large do not have that big of effect, because of the small over lap.

Another myth, which somebody is sure post while I'm writing this, is you need back pressure to keep the fresh intake charge in the cylinder during valve overlap. Complete nonsense. The intake valve is open long after the exhaust valve closes, any intake charged pulled out will be replaced. The overlap may waste gas, but it will not hurt power.Back pressure is NEVER a good thing in an exhaust system when we are talking HP.[/b]

Thu May 24, 2007 4:17 pm

That's pretty much it

mat3833 wrote: ok, im a slight moron when it comes to exhausts but i see it this way(measurments for example only!):

with a 1.5 inch exhaust your port velocities are up at even the lowest RPM. this allows more intake charge to be sucked into the cylender. but it can limit the high-RPM flow.

with a 1.75 inch exhaust has less low-RPM port velocity but can flow more at high-RPM, hence the loss of low-RPM power.

with a 2 inch exhaust you loose even more low-end but you can gain loads of top end if your motor can flow to the potential. back-pressure is the least and you could possibly burn an exhaust valve.

Matt :D :D

Thu May 24, 2007 5:02 pm

Fabulous!
Thanks - it's going to taske me a little while to wrap my brain around all of this, but that's exactly what I wanted.

Fri May 25, 2007 6:35 am

So, James... What effect would putting an 1 5/8" merged header on a 1600-1641 motor?
Good, bad, or no change?

Fri May 25, 2007 6:56 am

jamestwo wrote: That's pretty much it

mat3833 wrote: ok, im a slight moron when it comes to exhausts but i see it this way(measurments for example only!):

with a 1.5 inch exhaust your port velocities are up at even the lowest RPM. this allows more intake charge to be sucked into the cylender. but it can limit the high-RPM flow.

with a 1.75 inch exhaust has less low-RPM port velocity but can flow more at high-RPM, hence the loss of low-RPM power.

with a 2 inch exhaust you loose even more low-end but you can gain loads of top end if your motor can flow to the potential. back-pressure is the least and you could possibly burn an exhaust valve.

Matt :D :D

OMG! i was actually right?? time to go celebrate! haha, good to kno i gained some knowledge from this place!

Matt

Fri May 25, 2007 7:25 am

atye wrote: So, James... What effect would putting an 1 5/8" merged header on a 1600-1641 motor?
Good, bad, or no change?
I'm going to try to answer this to see if I have absorbed any of the knowledge.
Here goes - the 1 5/8 exhaust will have little effect on a 1641. The fact that it is unlikely that a 1641 would have a long duration cam, the size of the exhaust will have little effect. There may actually be a slight decrease in performance as the exhaust pulse will dissipate in the pipe before it has had a chance to act on the next exhaust expulsion.
A 1600/1641 would have to turn out 10K RPM before it could begin to make use of that size of exhaust.
Close?

Fri May 25, 2007 7:35 am

you learn fast! the exception being what is on the 1641. if it has a cam, dual carbs, or even a turbo then you could gain some power with the exhaust, but if not i would limit tubing size to 1.5 inch max for stock sized engines. trust me a 1&5/8 on a stock engine with no muffler = bad things...

Matt

Fri May 25, 2007 7:49 am

define "bad things" :wink:

the reason I asked is... I have a brand new 1 5/8" ceramic coated merged header and muffler just sitting until I get around to installing my stroker. Sure would sound a lot better than what I am currently running. I just don't wanna ruin my motor or anything just for a header.

Fri May 25, 2007 7:55 am

if you have a muffler and dont mind a loss of low-RPM power add it on!! i ran my empi U-bend without the supertrapp on it while at the dunes for a week and lets just say that week ended with a set of new heads and a piston or 2... it pains me to talk about it, let alone remember it. i kept if off the forums for fear of being shunned... hopefully the forum gods dont ban me for my stupidity.

Matt

Fri May 25, 2007 8:00 am

I may just try it and see, if it won't cause any damage...

I'd sure love to go from this:

to THIS:

... especially for just a little labor, no $$.
:wink:

Fri May 25, 2007 8:02 am

lol, remember, backpressure is needed in a small ammount. as long as you have a muffler youre ok.

Matt

Fri May 25, 2007 9:35 am

another reason why i like diesels. no back pressure required.

Fri May 25, 2007 10:43 am

Its been a long wait but finally smaller engine can have a good exhaust.

http://www.cbperformance.com/catalog.asp?ProductID=1634

:D