Low end torque

[Physicist]

I read somewhere that the lost of low end torque (from bigger bore exhaust systems) is mainly due to the fact that it allows for cooling of the exhaust gas. The cooling effect makes it dense and therefore heavier which equates to engine working harder to expel it. Something to this extent.

From this logic ---> does it mean that straight through exhaust (shorter length same diameter) increases low end torque?

Any guru's' to help out?

 

[Racebred]

nope. read shaun's excellent essay on this. he posted this sometime back, from the vag forum. maybe he can provide the link again.

 

[Shaun]

Thanks RB. Actually what I wrote was with respect to turbo applications which are much simpler.. also dunno where to dig it up now. The stuff I've posted here more recently (search the few exhaust threads) is more applicable to NA applications. Eric and Rodney also provided good views on the issue.

I have yet to write about what I've learned about how exhaust system flow is dealt with as a separate entity to pressure wave tuning. The dissociation and optimization of each, allows very good results which are easy to test for. Sound damping does not need to come at the expense of power - although it often does.

 

[Shaun]

I read somewhere that the lost of low end torque (from bigger bore exhaust systems) is mainly due to the fact that it allows for cooling of the exhaust gas. The cooling effect makes it dense and therefore heavier which equates to engine working harder to expel it. Something to this extent.

From this logic ---> does it mean that straight through exhaust (shorter length same diameter) increases low end torque?

Any guru's' to help out?

It is not due so much to cooling as it is due to lack of average system velocity.

I don't know your definition of low end, but secondaries as long as 50 inches are for optimizing in the 3000rpm and below range. Secondary length as measured from collector to the first major open space that the gasses come to.. whether it is a big enough cat, resonator box, or muffler.

Also the probabilities that you will have to rework cam timing to optimize for changes made is very high .. almost certain. Valvetronic or DVANOS may help by searching for best torque, though that is something I suspect but cannot confirm. Can someone confirm?

If you have a street BMW meant mainly for commute, please keep it stock. Save your money.

 

[Physicist]

Thanks Shaun. Thats what I thought as well. To keep it stock. I was reading up on the mechanics of it all as its interesting.

 

[Shaun]

oh yes.. open space obviously includes atmosphere as well.. forgot to put that in.

no problem, have fun.

 

[Racebred]

"Valvetronic or DVANOS may help by searching for best torque, though that is something I suspect but cannot confirm. Can someone confirm? "

yes, almost definitely. i'm beginning to think, that by thinkering with dvanos one can almost achieve the gains derived from cams upgrade.

 

[Shaun]

Cool

If the system is already searching for best torque across the RPM and loads, then there can be no more optimization from phasing since best torque already found.

If the system has no torque calculation or measurement (definitely not since Motronic ME 7.X and up is based on torqe request and meeting it), or if the feedback/calculation has lag to it, then there is room for optimization by setting original targets closer to what they should be.

 

[stevenwu]

Re: Low end torque

Hi Shuan,

Your definition of "from collector", does it include the length of the header or measured from the point where 3 merged to 1 piping is ?

I noticed that between M52TU and M54 exhaust system, the merging point is different. M54 merged about 30inches later than M52TU, before the resonance block.

Also from single pipe to twin pipes setup, if their cross-sectional area remain unchanged, is there any perfromance improvenment ?

Please advice.

 

[Shaun]

Re: Low end torque

Hi Shuan,

Your definition of "from collector", does it include the length of the header or measured from the point where 3 merged to 1 piping is ?

Hi Steven.. yes "from collector" is measured from the point the the 3 merge into 1. It does not include the primary pipes.

I noticed that between M52TU and M54 exhaust system, the merging point is different. M54 merged about 30inches later than M52TU, before the resonance block.

You mean the M54 had primaries some 30 inches longer than the M52TU? The average production street engine primary is roughly only 6-10 inches long. To add another 30 inches to that is extremely long and more representative of the old basket-of-snakes V12s of the 60s - and even they weren't that long . Do you have any pictures of what you mean? Now you have me all curious.

Also from single pipe to twin pipes setup, if their cross-sectional area remain unchanged, is there any perfromance improvenment ?

For similar CSA, a twin pipe will have more wall area/friction hence increased resistance to flow. The twin pipe system will also weigh more, given the use of similar material.

The only reason twin pipes or single ovate pipes are used is to gain ground clearance. Also, the larger the pipe diameter, the more difficult it is to make a small-radius mandrel bend in it. For this reason twin small diameter pipes make it easier to wind the exhaust tract through the tight spaces on the underside of a car.

If there are no tight turns that need to be made, then single pipe of the right CSA is absolutely the way to go.



Kind regards

 

[Shaun]

Re: Low end torque

i'm beginning to think, that by thinkering with dvanos one can almost achieve the gains derived from cams upgrade.

While we're here and since I rushed through this thread some months ago... cam phasing like DVANOS can only give a portion of the benefit of an actual cam upgrade especially as the durations get larger and more typical of race cams. Large enough phase range will allow the movement of intake valve closing point to possibly match a large duration cam, but intake valve opening point will not be be the same as a large duration cam. At high RPM especially on high scavenge pentroof 4 valves that most all modern 4 valves run, you a good amount of that effect and the lack of power vs a real cam will be substantial. We're talking about 40-70 crankshaft degrees difference which is a lot.

 

[Shaun]

Re: Low end torque

I noticed that between M52TU and M54 exhaust system, the merging point is different. M54 merged about 30inches later than M52TU, before the resonance block.

I think I get what you mean.. you're talking about the merge point for the two secondaries correct? I wouldn't worry too much about it. Production street cars might not even utilize wave tuning to any meaningful degree given how precisely everything has to go together on both intake and exhaust tracts as well as the engine build itself, in order for the benefits to be seen. The stack up of production tolerances and other overriding priorities like time, cost, space, can well relegate wave tuning. If anything the stock systems are already tuned for best low end power which is what the car in standard application needs anyway.

 

[Tanzy]

Re: Low end torque

I think the Prof is referring to the resonator box where the exhaust from both headers first meet before splitting again to the rear muffler box. That is a device that helps reduce high pitch noise from the exhaust. There's nothing inside and I assume it works by noise cancellation; clashing the exhaust pulses from the two pipes.

 

[Shaun]

Re: Low end torque

Earlier when I put down twin pipes, I don't mean to put down twin pipes from engines that have two banks like any V, or Boxer type engines. On those engines types, trying to merge the two pipes into one of equal total CSA usually means a substantial bend in the pipes which can mean more resistance to flow vs the wall friction of twin pipes. The sharper the required bend(s) to merge, the more restriction there is. The smaller the distance to atmosphere, and the wider the engine, the sharper the required bends. Mid and rear engine cars have very low exhaust to atmosphere lengths so usually have twin pipes. Front V or Boxer engined cars have the winding to accomplish so somtimes stay twin. On a high RPM race car you often find single exhausts per bank with a straight and short shot to atmosphere, ruleset permitting. This is the ideal zero compromise setup.

 

[Racebred]

Re: Low end torque

While we're here and since I rushed through this thread some months ago... cam phasing like DVANOS can only give a portion of the benefit of an actual cam upgrade especially as the durations get larger and more typical of race cams. Large enough phase range will allow the movement of intake valve closing point to possibly match a large duration cam, but intake valve opening point will not be be the same as a large duration cam. At high RPM especially on high scavenge pentroof 4 valves that most all modern 4 valves run, you a good amount of that effect and the lack of power vs a real cam will be substantial. We're talking about 40-70 crankshaft degrees difference which is a lot.

hmmmm so basically while DVANOS can mobilise their tiny actuators to modify lift and duration, it's only up to a certain extent right? Now, should we change cams that gives you a different area under the curve, with stock ECU, what do you think the effect will be? Will there be closed loop checks to ensure that the valves behave more like stock? Or will the values remain?

 

[Shaun]

Re: Low end torque

hmmmm so basically while DVANOS can mobilise their tiny actuators to modify lift and duration, it's only up to a certain extent right?

Yes the limits are set by the lobe.

Now, should we change cams that gives you a different area under the curve, with stock ECU, what do you think the effect will be?

Not something I would pioneer. The problem is not the matching of the physical bits, but not knowing how the software was written and how badly it reacts to seeing values other than expected. Modern ECUs have a good number of sensors and a huge range of values that they can calculate and check against an expected range. If important ones are out of spec it can do react any number of ways depending on how the engineers have programmed it to.

Will there be closed loop checks to ensure that the valves behave more like stock? Or will the values remain?

It is too costly and complex to track valves with the technology we have now. An example of what the ECU can do is compare engine torque to throttle position at a given RPM. If it falls outside of the low limit (torque) then it knows there is some sort of trapping problem (late intake valve at low tract velocity closing lowering CR, too much overlap for RPM). Low limit if it exists, is unknown. Reaction.. unknown.

Now.. who will take the first step and find out if there are any limits (and in all likelyhood there are), and where they lie?

 

[stevenwu]

Re: Low end torque

Thanks Shaun.

I just noticed something that is very interesting and wish to share with all of us.

M52TU engine and M54 engine both having 2.5L but M52TU has poor throttle response, lower torque values on both low (5000rpm) have the following reasons.

a) Restricted intake filter box, smaller CSA at the intake.
b) Restricted exhaust box, excluding middle piping which do not contribute much on high end after switching between single and twin piping.

I think most of us already knew about the 2 points above. However, very sad to say that 'they' further detuned it at the intake runner side, programmed it to be exactly opposite.

In other word, when it suppose to be long runner at low rpm, it switched it to short runner. Switched to short runner length at high rpm (>4000rpm). The switching of the resonance valve is 180 deg out of phase when comparing to M54 profile. This way, it will cause it to react poorer torque values at both ends and maintain same torque values at mid range, 245Nm@3500rpm.

Just wish to share so for those who wish to 'tune up' all the 323, 523 etc engine, just re-programmed this valve to M54 setting will probably help a lot.

Thanks.

 

[Shaun]

Re: Low end torque

Interesting.. I wonder why they would do that to kill both ends ?

 

[stevenwu]

Re: Low end torque

I believed the reason is to 'de-tune' it to have lower hp for marketing purposes with the same capacity engine.

 

[reardriven]

Re: Low end torque

The only reason twin pipes or single ovate pipes are used is to gain ground clearance. Also, the larger the pipe diameter, the more difficult it is to make a small-radius mandrel bend in it. For this reason twin small diameter pipes make it easier to wind the exhaust tract through the tight spaces on the underside of a car.

I've read somewhere in Bimmerfest that there is this vacuum actuated valve that closes/opens one of the twin pipes before the tail muffler which serves as some sort of a noise-control mechanism and some Americans have gone to disconnect the vacuum tube and plugged it with golf-tees so that the valve remains open all the time.

Does this apply to our Euro spec cars?