tbennett017

OK... just so I can understand.. how does removing exhaust restrictions actually decrease torque? How is backpressure related to this? I have been thinking it through since reading a post on it yesterday and it has yet to make sense to me...

Bill Verburg

it's all about keeping gas velocity high. At each point in the rpm regime used there is an optimal size that keeps the gas flowing best and turbulence least. Ideally you would want straight long narrow pipes at low rpm and shorter but wider pipes at high rpm. Ain't gonna happen, so you compromise.

Mufflers, bends, merges and step transitions in the flow path all tend to slow down the gas and induce turbulent flow. More so at high rpm than at low.

there are also acoustic effects that can crop up at the darndest times.

So to your question, at low rpm the pipes are too big to keep gas flow velocity at it's best if the pipe is sized to keep the flow maxed at high rpm.

There is also a bit of illusion involved, anything that enhances the kick at the top will make the rest seem anemic.

Indycam

http://headerdesign.com/index.asp

N51

Bill's given a good summation.
Back pressure loads the engine and produces torque.
It's all about compromise. Load early and top end hp suffers as the engine labors.
Study what Porsche has done with the GT3. High rpms, low torque. No one does it better, regardless of the claims.
The low torque/high hp engine benefits from proper gearing. You can see the $$$$ adding up.

I defer to Porsche's experience.

NineMeister

It is a common mistake to think that the backpressure of a smaller header pipe which results in an improvement in torque, in fact it is the gas velocity which does the job. In general, for conventional header designs, at any chosen rpm design point simple flow mathematics predict that a smaller header pipe = higher velocity = lower pressure = better scavenging = more torque. The improved scavenging is as a result of the lower pipe pressure extracting the remaining exhaust gas from the cylinder and initiating the flow in the intake tract during the cam overlap cycle. If the header pipe diameter is sized for optimum torque at say 4500rpm, it could be restrictive at peak power of 6000rpm which would lead to a limit in the hp numbers.

I think it is well known though that, because of resonant lengths, longer header tubes favour lower rpms/torque and shorter tubes favour high rpms/power, so assuming that you do not have the budget to run tapered header pipes (which effectively can give you the best of both worlds) there is a design compromise to be chosen between the pipe diameters and lengths for either peak power or peak torque.

psiegel

so where does a turbo fit in this discussion?

Mike Murphy

This is a very complicated subject: fluid dynamics. An engine is basically an air pump. Generally, the more air in and out of an engine develops more torque. If you can pump more air at a higher RPM, you'll get more torque at a higher rpm, and therefore more horsepower.

You want to tune your engine to pump most effectively at the RPM range which you have chosen to be your ideal range, whatever that might be. In many cases, you want the exhaust gases to be completely expelled effortlessly from the engine, so you want low back pressure. If the engine cannot breathe easy in this manner, then the pistons are actually pushing the exhaust out, which causes a decrease in efficiency. Too small headers can cause this, but if you put on too large a header, then you can't take advantage of another tuning secret: exhaust gas velocity can cause a vacuum, which helps suck the intake charge into the engine will greater force, resulting in more air/fuel density during combustion.

A turbocharger does cause backpressure (sometimes quite a bit), but that loss is offset by the intake air, which is boosted and allows a greater air/fuel density on the intake side.

NineMeister

Without a doubt, off somewhere in hyperspace on the 993 turbo forum....

Geoffrey

"Without a doubt, off somewhere in hyperspace on the 993 turbo forum.... "

Don't forget the dyno sheets...

NineMeister

Ooooo, you pull a low punch. Just had a mk1 GT3 with a Manthey 400 kit fitted that has made 404hp & 440Nm on our non-Maha dyno, so had I better call him up tomorrow and tell him that his cars are underperforming?

crg53

I have a question for you guys. Does Ceramic coating the exhaust system help exhaust gas velocity, I know that is the case on 2 stroke engines, but will it also work on 4 strokes?

NineMeister

Sorry for the OT comment, back to the matter in hand:

If we ignore resonant effects for a moment, during the exhaust cycle of the engine the two most significant events are the initial blowdown of the exhaust gas from the cylinder and then the scavenging of the cylinder during overlap. During the initial blowdown phase, which begins with the exhaust valve opening point, the exhaust gas exits the cylinder at an extremely high pressure and initiates the movement of the gas down the header pipe. Smaller header pipes hold back the outflow of exhaust gas and as a result increase the pressure within the cylinder - which in turn pushes down harder on the piston and hence produces marginally more effort on the crank - hence more torque. At the end of the exhaust cycle at overlap, the exhaust gas slug makes its way up the pipe at high velocity which reduces the pressure within the header/cylinder to below atmospheric pressure. It is this low pressure that scavenges the remaining exhaust gas from the cylinder and initiates the flow in the intake tract simply because the inlet valves and exhaust valves are both open at the same time. Again, a smaller header pipe increases velocity at lower rpms, reducing exhaust header pressure which thus increases the pressure differential between the intake and exhaust. The increased pressure differential is what does the work scavenging the cylinder where the higher intake pressure pushes its way past the opening intake valve and into the cylinder, so shoving the remaining exhaust residue past the closing exhaust valve.



Turbocharged engines can be considered as no more than a conventional atmo motor with an increased air density on both sides of the cylinder. In practice exhaust back pressure is approximately cancelled out by the boost pressure, so the engine works in a fairly conventional way. Now, if Pseigel had mentioned the supercharged engine it would have been more interesting, because in this case you have increased air density on one side of the cylinder only, hence the requirements of the intake and exhaust side of the engine are very different.

NineMeister

Maintaining heat within the header is always a good idea because this maintains the energy (and hence velocity) of the gas in the pipe. On an aircooled engine the secondary effect is that it also limits the heat transfer (via radiation) back into the engine itself which, in theory, would allow the engine to run cooler. Ceramic coatings (either internal or external) does reduce heat transfer through the wall of the pipe which is good for velocity as you say. Another method would be to simply lag the headers and collector with thermal wrap (which in my experience is more efficient) but it makes for a lot less attractive an installation.

Be aware that external coatings or lagging also increases the operating temperature of the pipe, so as a result will potentially also reduce the life of the headers if the material will not take the additional heat loading. Internal pipe coatings reduce the material temperature, so in theory this should increase the life of the header.

Mike Murphy

Well said, Colin! It is also interesting to talk about resonant effect, but as you alluded to earlier, it can be complicated enough without them. It's also interesting to watch the effect of good scavenging at work. As you know, if you turn the lights off and watch flames shoot out the exhaust, you can see just how much scavenging an engine can produce during camshaft overlap.