Help-Impact of RSR Silencer on Throttle Response
 

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?

Here's some discussion from the 911 tech forum:

http://forums.pelicanparts.com/porsche-911-technical-forum/272228-help-impact-rsr-silencer-throttle-response.html#post2474657

Any of you engine guys have any ideas??

What is the diameter of the headers? It sounds like they might be too large.

Can you post the dyno sheets?

Here's the dyno results:

http://forums.pelicanparts.com/showthread.php?s=&threadid=246563&highlight=Dyno+d ay

The headers are 1 3/4" and should be the right size for my 3.0.

Here's some info on a similar problem with with the MFI cars.

http://forums.pelicanparts.com/showthread.php?s=&threadid=182146&perpage=20&highl ight=mfi%20muffler&pagenumber=1

It's all about tube length and resonant frequency scavenging. I think I'll keep the caps on and try running at the Glen later this year with them off to see how it will work with the revs continuously above 4000.

1 3/4 inch headers are too large for an engine with these specs.
Put your head in the sand and ignore my comment or just try a smaller set and you'll be pleasantly surprised.

http://forums.pelicanparts.com/uploa...1142795192.jpg

Boy...Headers too big?

The car runs great with the muffler capped. Would 1 5/8 " headers solve my throttle response problem without robbing any high end power?

You may find that with these cams 1 1/2 headers may be the best choice.
We just sent 3 ralley engines to Europe with simmilar specs and we found 1 1/2 headers were the best choice.

I don't know your carb jetting, port size or fuel etc but header size matters.

RSR 3.0= RSR sprint cams, 43 mm intake ports, 43 mm exhuast ports, huge MFI, megaphones. a totally different engine from what we built.


http://forums.pelicanparts.com/uploa...1142801809.jpg http://forums.pelicanparts.com/uploa...1142801838.jpg http://forums.pelicanparts.com/uploa...1142801857.jpg

Tom:

I'd agree with Henry that those 1.75" headers are too big for a 3.0 and a modest cam like what you are using. They simply kill the low & mid-range HP without a benefit on top. If you were using RSR sprint cams or something more aggressive that gave an effective powerband between 6000 to 8000+, then 1.75" headers can work.

We usually use 1 5/8" headers for engines like yours to preserve some low and mid-range performance without compromising HP to 7400 RPM.

1 3/4" headers are appropriate for 3.2-3.5 litre race motors.

I will also tell you that your engine's jetting requirements are different when the tail pipes are uncorked and thats another factor in why your motor suffers when you make those exhaust changes.

I'd also reiterate once again that 911 engines using moderate to aggressive cams are VERY sensitive to exhaust systems (headers & mufflers) and they play an integral part of an engine's effective power band. :)

Guys

Thanks for all of the input. I am always amazed at the wealth of knowledge and experience on this forum.

My engine runs so well with the muffler capped that I am starting to think I may be using 1 5/8" headers. I know I got a lot of input from this forum and other places before I selected the header size and I wouldn't think I would make a mistake like that. I bought the headers from Scott at Auto Associates who was my primary advisor on the engine configuration. I'll check with him.

Thanks again.

HI just a bit of a highjack on the thread to HENRY, did the three engines go to XS racing by any chance??? (john cropper), if so i was down there last week, exelant job on the engines, look just as good in the car.
regards mike

Hey Tom,

A quick call to the guys in Canton should confirm.

That being said, I know from experience that when you switch from a conventional single-out muffler to a dual-out sport muffler, you have to richen up the MFI a couple clicks to maintain the torque. I leave the theoretical explanation to experts, but it has to do with the overlap of the cams and the back pressure (or lack thereof) from the exhaust system.

Also, I respectfully disagree with the 4000-7000 rpm concept. While it's true that the Porsche dyno charts didn't read below 4000, the engine must be able to idle and operate smoothly below 4000 rpm. A good idle and part-throttle performance down low are very important to being able to modulate the throttle in the corners. Donohue goes into detail in this in The Unfair Advantage.

Well...

I talked to Scott at Auto Associates today. The headers are indeed 1 3/4". He said that they build all of their 3.0 race motors with this size. He also mentioned that my heads have been ported to 43 mm on the exhaust side and that this matches much better with the larger headers.

Like John, he suggested that I try some enrichment on the idle jets for the PMO's to get back some of the low rpm throttle response with the headers open.

I don't know what to say except that the car runs great with the muffler capped. It makes around 285 HP at 7500 RPM and wants to continue to rev past 8000 with the caps off. Scott says that the 1 5/8' would restrict the top end.

Thanks again for all the input.

Man-o-man! I just love it when you put up photos of your engines Henry! Nothing like dreamin'......