Exhaust scavenging on a turbo header 2

[GBeaty]

I have seen many site detailing the process of exhaust scavenging on NA cars, but never a turbo one. I would assume it works the same, but there are a few things I'm curious about.

The pressure in the exhaust manifold gets considerably higher than atmospheric. Doesn't this change the resonance of the header as the pressure wave's speed changes?

Also, what happens when the wastegate opens? Do the exhaust scavenging pressure-waves continue to help, or do things change?

How about intake manifold runner length, is that effected by turbocharging at all?

Thanks for any help,
Grant Beaty

 

[buzz41]

I've noticed that as well. I'm sure the tuning is there though on all out applications.
It might simply be too complicated in most installations to do both resonant exhaust and turbo, too. Also, the turbo might be a damping load on resonance - so a little less effective.
Pressure itself should not effect wave speed or tuning, just temp. - 1% change for 2% change in absolute temp.

I just thought of another angle to this. One of the effects of manifold tuning is to complete the scavenging at valve overlap time. With a good turbo setup, there is considerably higher intake boost than exaust pressure, so the blow through will take care of the scavenging aspect of resonant manifolding. Again, maybe less advantage,then, for resonant manifolds.

 

[MaxRaceSoftware]

a quick look at all the SuperCharger and TurboCharger classes in drag racing ...and most all have
"Zoomie" type individual headers with no collector
and the Turbos have just enough pipe to direct exhaust away from driver

probably the most PSI you could see from Intake Runner
Ram-Effect would be approx. 6 psi near 140% percent Ve ...
the Blower or Turbo will be greater than this !!!

you could use intake runner length to help out
but the blower or turbo is going to dominate

and at OVERLAP , the blow-across effects should just about
cancel out any need for exhaust wave tuning scavenging


Larry Meaux (meauxracing@mindspring.com)
Meaux Racing Heads - MaxRace Software
ET_Analyst for DragRacers

http://www.mindspring.com/~meauxracing/

Support Israel - Genesis 12:3

 

[GregLocock]

buzz got it in one. A correctly selected automotive turbo takes almost all of the pressure waves out of the exhaust, so as each cylinder's pulse comes past it speeds up by 50-100%. Therefore it acts as a low pass filter.

It might be worth tuning the manifold pipe lengths ahead of the turbo, but I have a feeling that the turbo will be supressing any reflections back up the manifold from itself (it is effectively a 'sink' for the pulses).

I'd guess equal length runners of minimal length would be the optimum.




Cheers

Greg Locock

 

[hpfreak]

A turbo is an energy converter, it uses the heat and pressure from the exhaust to spin the the intake compressor wheel. Turbos are placed as close to the ports as possible to take advantage of the high temperature of the ex gas before it cools. The boost produced creates a lot more torque than runner tuning. So it is more important to place a turbo close to the cylinder head to get the high ex temps, than to have long ex runners to get a small advantage in tuning.

As for the intake, the tuning effects are still there and there are gains to be had. The pressure waves are still available in the system, they are just nominally higher due to the boost. Most applications end up with short runners simply due to packaging.

 

[evelrod]

The cast iron LOG type manifolds appear to give the best performance, throttle response curves in turbo applications. I have used a tubing header, properly insulated,etc. but it still had far worse lag charictics on throttle pick up out of the corners(I wanted to try a 'plenum' just before the turbo intake to even out the ex pulses, but did not have the time. It might have helped.). Went back to simple, ugly, standard (ported, to be sure) manifold---problem solved. Exhaust from the turbo was a 3 inch dia tube to a Supertrap six inch sprintcar type silencer. It could have been shorter but, it had to exit the body shell behind the driver. We had looked into the intake tuning deal and found with the widely varrying pressure in the intake manifold, while possible, 'tuning' is not practical(at least not for us with intercooler and hoses etc.). As hpfreak says above, it most probably is a matter of 'packaging'.

Bottom line to all of this is cost, application and, packaging. The turbo/supercharger gives such a boost in performance for relatively little cost ($$$ & HP) it is suprising to me that they are not more widely used in street/emission controlled vehicles.


Rod

 

[GBeaty]

Thanks everyone for their input.

One clarification: I'm asking this because of my observation of the lack of performance difference between a tubular header vs. a cast log manifold. The equal-length headers offer smooth transitions and collectors, while the cast iron manifold makes poor (90 degrees or greater) angles with the exhaust ports. Yet so far, there isn't a noticable performance difference (although boost pressures higher than 20psi haven't been tested, and this isn't exactly scientific testing).

But most street engines are negative pressure, that is the exhaust pressure between the head and turbo is greating than the intake boost pressure, so there can be reversion during overlap.

Rod, I think one of the main problems in turbos on street cars is heating up the cat during a cold start (the turbo removes a lot of heat from the exhaust).

 

[Whittey]

I believe (and I lent out my book, so I can't confirm any time soon) that Hugh McInnes said headers in a turbo environment were as important as, if not more important than in n/a.... He didn't go into detail though, so whether this was from flow (which porting would solve) or from pulse tuning, I have no idea.


-=Whittey=-

 

[bobtail]

We've seen gains of 14bhp by changing from stock cast manifold to a tubular jobbie on a 1.9 TDI golf

 

[Whittey]

Bobtail, on that tubular header was it a tuned tubular header or do you believe the benefit was mainly from the (assumed) increased flow?


-=Whittey=-

 

[SammyDaFish]

hmm.... why did this thread die? I'm interested to know more. Is there a true bennifit to an equil length header in a turbo application? What about twin entry turbos? Some manufacturers have used them to help spoolup on small displacement engines (Toyota 3SG an CT-26 for example). Is there a need for a reversoion damn in a turbo application?

 

[Warpspeed]

There are a whole lot of issues to resolve, there is no one correct solution.

On a small turbo street application where exhaust back pressure is considerably higher than boost pressure, the best approach seems to be the shortest length/minimum volume manifold that does not actually restrict flow.This is going to give best turbo response, but this is for a tractable street setup, not a max power application.

Exhaust reversion is going to always be a problem because of the very high static exhaust manifold pressure, and there is nothing you can do in the way of manifold design that is going to really help much.

Now consider a more healthy engine, with a much larger exhaust turbine, where exhaust back pressure might be about the same as boost pressure. Here there might be some gains to be had by thinking a bit more about the exhaust manifold design.

One factor to consider is exhaust cam duration and how many cylinders feed (each?) turbo.

For instance on a six cylinder engine with three cylinders feeding one turbo, the firing interval would be 240 degrees between each cylinder. If exhaust cam duration is 240 degrees or less, each exhaust valve will completely shut before another opens. In this case you can combine all three cylinders very close to the head and build a short minimum volume exhaust manifold, again for best turbo response.

The identical setup with a 290 degree duration exhaust cam will have periods where two exhaust valves are open together. One cylinder will be right at the beginning of the high pressure blow-down phase, while its neighbor will be at the sensitive valve overlap period. One cylinder will blow straight into the other.

You can fix this by using long individual runners from each port to the turbo flange. The pulse has to travel two runner lengths before it can blow into an adjacent cylinder. So above any reasonable mid range RPM, you can effectively isolate the exhaust ports.

On a big cam, big turbo engine, low RPM turbo response is not an issue, so the extra pipe volume does not hurt.

Pipe tuning on an n/a engine works by having the end of the pipe discharging into a low pressure. This creates a negative return wave that can be timed to coincide with valve overlap.

A turbo manifold terminates in a restriction, because the turbine scroll is always the most restrictive part of the whole exhaust system. So there can be no return negative wave to tune.

Most of the individual runner exhaust branches that you will see, are usually about twelve to fifteen inches long, which is sufficient to separate cylinders, but far too short for exhaust pipe tuning in the usual sense.

There may be some evidence to suggest the branches may be tuned to a harmonic frequency, as is done with intake runners, but I have never heard or read of anyone claiming to have successfully done this.

As the runners feed into a restriction, the return wave would be a positive pressure wave, so it is difficult to see how this could be tuned in any way to offer an advantage.

On a real race engine the turbo will be enormous, and the power band very limited indeed. The restriction through the turbine will be quite a bit less than boost pressure. These engines often DO use tuned full length exhaust runners two to three feet long, and it apparently can work very well.

So you really need to think through your application.

 

[SammyDaFish]

Warpspeed, great post ...... that makes a lot of sense, and I've heard mixed responses on the streets about weather or not there is a negative pressure wave on a turbo car. I'm running an inline 6, single turbo. It's a street, mild drag car, estimated output is about 700hp. What I'm trying to do is get a broad power band though, therefore I'm looking at what I can do exhaust wise to increase low speed torque. There are still a few questions I have. You say

"Exhaust reversion is going to always be a problem because of the very high static exhaust manifold pressure, and there is nothing you can do in the way of manifold design that is going to really help much."

So, that leave me with this question. If fighting reversion is such an uphill battle, is it better to port match the head with the header for increased flow, or should a reversion damn be left at the head?

What about log style manifolds? Other than their easy construction, is there any benefit from them? What purpose would leaving a surge just before the turbo serve?

What about twin entry turbos, or headers with the collectors paired in alternating firing order? Would there be any benefit on an engine that didn't have a huge duration cam?

Sorry for all the questions, but I'm eager to learn more. Thanks.

 

[Warpspeed]

We are all still learning Sammy.........

There seem to be two schools of thought here.

The first, is a six equal length runner manifold going straight into a large single entry scroll. For extreme top end power, this would be best with a long(ish) duration cam and large a/r housing. It will be very peaky though.

If you want more engine range, and a bit less top end, a six into two exhaust manifold with a dual entry split pulse exhaust housing is the way I would go for a dual purpose street/strip car.

But it is all still a compromise. How badly do you want to win Sammy ?

 

[SammyDaFish]

Efficiency is how I want to win. Sure, I could just put a bigger turbo on it and crank up the boost to win dyno competitions, but I want a street car that makes 600 wheel horepower and gets 25 miles to the gallon (as long as your right foot stays light ) Light weight, presision tunning and ceramic coating everything is how I plan to do it. I almost have the design completed for my intake manifold, now exhaust is the next thing to tackle. The big issue there is that there are so many myths about turbos, it's hard to figure out what's fact and what's not. That's what got me here.... people who actually seem to know what they're talking about

 

[patprimmer]

My recomendations:-
1) Keep the exhaust system simple and easy flow.
When you consider all the variables re min max exhaust pressure dureing pulseing cycle, effects on out of phase pulse cycle, blow down pressure during TDC overlap, inlet manifold pressure, the pulse tuneing for a turbo engine is just to complicated for mere mortals without the budget to build and test a few engines before you get it right.
2) Size the turbo for maximum efficiency at the power you are looking for, rather than response.
3) Use a small shot of nitrous oxide that cuts of as the turbo comes onto boost, say at 3psi.

The nitrous then spools up the turbo real quick, so you actually use very little nitrous, and then only at full throttle. You also open up your choices on other compromises once the lag problem no longer exists


Regards
pat

 

[68roadrunner]

I love this stuff... in this area (or just about any non-linear acoustics area there is nice, easy, simple thoery and then you have reality. The combination of effects from tubing geometry to exit conditions, inconsistent combustion processes, etc. all add up to one terribly complex set of phenomena, one that cannot easily be understodd, modelled (in simple fashion), or explained. All I really know is that I watch guys run some HUGE HP numbers at the drag strip in the Outlaw 10.5 categories (2000 HP from asingle turbo 400 ci motor - 4.7 sec in 1/8 mile) and they don't know the first thing about acoustical theory.