Help - Impact of RSR Silencer on Throttle Response

tom1394racing · 03-18-2006, 02:09 PM

The past couple of weekends I made some early morning runs in my RSR clone with the caps off the ralley muffler. I found that the throttle response below 4000 RPM is somewhat sluggish with hesitation when I run with the caps off. Below 4000 RPM the engine runs much better with the caps on.

Now for reference, the motor is a 10.5:1 3.0 with 46mm PMO's and GE-60 cam. I am running RSR headers into a ralley style muffler. The muffler can be used with the discharge caps off which makes it run like straight headers or with the caps on which makes it discharge out the side like the standard 2 in 1 out muffler.

I dyno'd the engine with and without the caps and the data shows about 15 hp extra with the caps off above 4,000 RPM. I don't have any dyno data below 4000.

My question is....What causes this? I recall some discussion that reduced back pressure can cause reversion in some engines with carbs or MFI. Is this the reason my low RPM throttle response goes to hell when I remove the muffler caps? What is reversion? Does it have something to do with the engine running too lean at low RPM with reduced back pressure?

I am more curious than worried about this. On the street, I will run with the caps on and the throttle response is electric from 2500 to 7500. At some tracks I may try to run with the caps off but there I will be keeping the revs between 4000 and 7500 and should not expect problems.

Can anyone shed some light on this?

pmajka · 03-18-2006, 02:21 PM

I am gonna take a stab at this.

When you have the caps off, you are losing back pressure,

this will give you high rpm HP , but you will lose bottom end torque, this is due to the loss of scavange.

High torque motors have very short runner headers or exhaust manifolds, before the exhaust valve closes, there is a very short moment when it starts the intake cycle and creates a momentary vacum helping to pull the exhaust from the next cylinder. "thus Scavange".

I have had this with my truck when i put headers on and had to drive it to "Exhaust Guy" with no cat or pipes or muffler.

Very loud , no bottom end tq, but gobs of Highend HP.

randywebb · 03-18-2006, 02:49 PM

Quote:

Originally posted by tom1394racing
Can anyone shed some light on this?

tom1394racing · 03-18-2006, 03:41 PM

Randy

Be serious.....I know you the answer!!

randywebb · 03-18-2006, 03:53 PM

It isn't really "back pressure" per se. What is (likely) going on is wave superposition, called scavenging by most car people. The resistance of the exhaust (at certain rpms, aka frequencies) allows the low pressure of the standing wave from one cylinder to coincide in time and space with the exhaust cycle of another, adjacent cylinder. The low pressure (rarefaction) works to help suck the nasty ol' exhaust gases out of that cylinder. This sill not work at every rpm, only in a more or less narrow band of rpms. That's about the best I can do with words... Maybe we can get Daryl to do up a nice animation of this...

Bill V. has posted on this before and so have I - not sure if in same thread or not tho. Search on "superposition" and see if that doesn't pull it up. It seems that Phil Smith was first to discover this in motors and its discussed in his engine book -old but still worth getting via InterLibrary Loan.

Today engine desiners will be using numerical methods to figure out the exhaust resistance - not the analytical eqns. above.

jluetjen · jluetjen's Garage

I took a shot at trying to understand and explane the whole harmonics thing in this thread.

tom1394racing · 03-18-2006, 04:40 PM

OK...I get it.

Exhaust systems and intake systems have resonant frequencies just like organ pipes do. The resonant frequency depends on the length to diameter ratio of the tubing. When the resonant frequency of the intake or exhaust system matches the pumping frequency of the engine, you get the scavenging effect where the intake or exhaust tubing actually helps to pump the intake charge or exhaust charge. In this matching condition, the resistance of the intake or exhaust system is greatly reduced.

In high performance motors with headers and low resistance intake systems, the sensitivity to the scavenging effect is much greater because of the low resistance in the intake and exhaust tubing, When I remove my caps on my muffler, I am moving the resonant frequency of the exhaust systems away from the lower RPM band to a higher rpm band. This change in the resonant frequency of the exhaust system is causing the degradation of my throttle response.

What do you guys think??

randywebb · 10:41 AM

close - not just the resonant freq. for that one pipe but how the standing wave affects the adj. cyl.

Did you read John's prev. post? It will always be hard w/ just words - that's why blackboards were invented....

tom1394racing · 03-19-2006, 04:22 AM

I just spent the last 1/2 hour reading through John's post. Very interesting. It takes me back to the acoustics of music course I took as an elective in grad school.

John's post deals mostly with intake systems but the same principles apply to the exhaust system. The resonant frequencies of either the intake or exhaust systems depend on the length of the tubing system (not length to diameter ratio as I previously mis-stated). As the effective length of the exhaust tubing is reduced, the resonant frequencies where the scavenging (pumping) effect occur, move up in the RPM band.

When I remove the caps on my ralley silencer I must be shortening the effectively tubing length and moving the scavenging effect of the exhaust system into a higher rev band. Perhaps this is causing the poor low rpm throttle response.

However, I always thought that the resonant frequency (scavenging RPM) was set primarily by the length of the header tubing and that the muffler had only a minor impact. However, I recall hearing or reading somewhere that the type of muffler used in some of the early MFI cars had a significant impact on the low RPM throttle response. This has to be what's happening with my car but I'm still not sure it is simply the movement of the scavenging frequency to a higher RPM band.

Any other ideas out there??
Can someone point me to a reference on the impact of mufflers on throttle response?

tom1394racing · 03-19-2006, 06:38 AM

Here's a link to the MFI flat spot issue:

MFI flat spot fixed - it was the freakin' "sport" muffler...

Once again, John seems to have the explanation covered. I guess this problem can occur on carbed engines also.

Bill Verburg · Bill Verburg's Garage

Part of the problem when discussing this is that there are 2 different phenomena going on at the same time in the exhaust.

one is kinetic, i.e the physical transport of spent combustion products to atmosphere, this transport consists of a series of slugs of hot gas the move down the pipe, mix w/ other slugs in the collectors and then continue to the egress.

the second is acoustic, a series of waves which are reflected at the end point(s) and transition points(s) and can help or hinder the kinetic aspect.

There are many variables involved but the main ones are;

pipe length and diameter
exhaust volume and temperature
transition # and configuration
end configuration
rev range at which various effects are desired

The exhaust can also effect the intake because it can enhance volumetric efficiency or hurt it. These effects are transitory and dynamic being limited to relatively narrow rpm ranges on both counts.

Unfortunately the MFI system used on early cars seems to be particularly sensitive to exhaust mods, probably because it is strictly constrained by its design, later electronic types being increasingly adaptive.

Unfortunately one is constrained by what is available and known to work effectively in an engine configured in a particular manner, best recommendation for those engines seems to be to stick to stock mufflers

I went through an experimentation phase w/ my old '72S, never found anything that worked better than the stock muffler.