Switching from SSI to Headers on MFI "E"
 

Car: 1971 911E, 2.2 liter, MFI, 2249#, A GA O V ZA, 15x6, 225/55 Hoosiers
Application: "Stock" Class PCA Club Racing
Current exhaust: SSI with factory 2-2 sport muffler.

Question #1
I just got an LM-1 Data Logger and need some way to mount the sensor bung.

I know it is no small feat to weld Stainless Steel, would you recommend removing the SSI and welding, or can it be done on the car? (James Achard, I would be particularly interested in your opinon on this given your welding expertise)

Question #2
In the alternative, as this is a full-race application, I would consider switching to headers. This is generally not recommended for a street application as it tends to result in flat spots, etc. however some have reported success in using headers and megaphones, provided one can tune (using the LM-1) for maximum power up high.

Advantages of headers & megaphones that I see: lighter weight, removal of associated heater ducting, small power increase over SSI/Sport combo.

Disadvantages: rust (I will have any components jet-hot coated before installation) and noise (will have to install a 911R-style muffler for Lime Rock and Summit Point, which are 1/2 of my 2005 race schedule.

Thoughts?

James Achard? Mike Piera? Grady Clay? Steve Weiner? Ed LoPresti? Dan Jacobs? Henry Schmidt?

Re: Switching from SSI to Headers on MFI "E"
 

Sorry John, you're stuck with me for the opening response.

I was under the impression that for racing applications, the MFI was running sufficiently rich to over come any flat-spot, and that most of the time the revs were kept high enough above that flat-spot range anyway. Still, none of what I've said here should be taken seriously without appropriate scrutiny by those with real experience.

Hey, Jim, thanks for joining the thread. My enumeration of persons I believe to have specific firsthand knowledge of my exact situation is in no way intended to deter anyone from joining in.

Wow, can you tell I've been spending too much time OT?

So if I understand you correctly, what causes the flat spots is areas of leanness as you move up the RPM scale. The SSI/Sport combo, because it has higher back pressure than the essentially open header/megaphone combo, tends to retain more mixture in the cylinder at overlap, thus keeping the mixture rich and avoiding flat spots. Have I understood it?

I think what you're saying makes sense based on what I've read about the problem with using other than the factory exhaust systems. But, I'm quickly finding myself in over my head. You were right to list those folks. They have the right background to answer this question accurately. I just didn't want your excellent question to languish waiting for a response. :D

John, what's your budget? There is that hot new exhaust that extracts a lot of HP out there. Not sure about your application but on mine (2 liter, high comp, PMO's) its supossed to be unreal.

Quoted at $3.5K though.

On your choice of SSI vs headers, perhaps you should consider any weight differences.

John, we just recently dyno'd a 2.4 S motor with SSI's and a sport muffler versus stepped headers and open exhaust.

Peak horsepower was only 2HP more with the headers. The only real difference was that the headers/open exhaust stayed up there higher after peak.

Tyson, that is a very interesting revelation.

I suppose the real way to test this is on the dyno, back to back on the same day. I would be interested in an HP curve that slopes off less dramatically after peaking, as I tend to spend a lot of time on the far side of the power peak (this is a 911E engine) for racecraft purposes, e.g, passing somebody.

Well, the headers didn't cause the motor to LOSE power, that's a plus.

And Herr Schmidt, I would add your name to my list above. I was, just the other night, singing your praises WRT how you came up with a suspension engineering solution that turned out to be identical to something the factory did on the 935. . .that you didn't know until after the fact!

Juan, is that a Burns system?

John_cramer;
Here's a thread that you might find interesting if you missed it the first time. Keep in mind that louder does not necessarily mean better or more HP. It just means that more dB are coming out the exhaust. BTW, I don't think that it is the "header" that will make a difference in your case since the early factory heat exchangers and SSI's all appear to have comparable performance. The SSI's might have a better collector design though. It's the muffler that appears to be the big factor and I believe that this is due to the very short secondary pipe on 911's. If the engine were in the front and you had a 4 or 5 foot length of pipe after the collectors, I doubt that the engine would be as sensitive to muffler design as it is.

BTW, since you race in the NE, how many times have you raced in the rain? Have you considered the tradeoff in performance from losing maybe 2lbs of heater box weight in the back versus the value of a hot-air defroster when racing in the rain. I once had a rain race in my ITB car and I was really happy that I still had the heater core hooked up (per the rules) since otherwise I would have been driving around with fogged windshield. So in my case I'm planning on keeping the heaterboxes and somehow hooking them up to the defroster vents in my car for just that purpose.

John,

No doubt, louder is not necessarily better, just louder. I remember reading about the Kadenacy effect (for 2-stroke Kart motors) in Junior High and being fascinated with wave theory, no doubt a result of playing the saxophone (in which activity I was frequently admonished that louder is NOT better, but try telling that to a pre-adolescent. Or for that matter, denizens of the OT board. ) But I digress enormously.

It's probably a good idea to start with the LM-1 installed in a bung in the SSI's with the sport muffler, then get everything dialed in on the dyno, then remove the muffler and bolt up a pair of megaphones, then dial-in again and compare.

I have raced in the rain about three times, out of about 15 races. Which is enough to make me reconsider the heat issue. Especially in light of that video of the guys crashing in the rain.

To say nothing of the additional cost and time involved in replacing SSI's with headers.

John,
My experience with small displacement cars is limited. However on the 2.0 and 2.2 cars that I've worked with I also found that SSIs made very similar power compared to the comonly available inexpensive headers out there (Georges,Burns etc.) I think if you had Burns design a system for you it would make more power for sure however the price would be quite high. If I were you I'de weld a bung to the SSIs. It's not hard to do at all
Dan Jacobs

Tyson.

Was there a significant difference in torque between the two systems?

John,

My experience with my old 2.0S motor revealed that the limited sense of torque was greatly reduced with anything more than the factory muffler. I spoke with Mr. Bursch, who was quiet honest with me (and earned my respect) by stated the factory exhaust system on the early merchanical injected cars is quite well design and hard to beat.

Tinker

John, with the headers, it only lost 5HP between peak (6500) and redline (7300) Horsepower and torque were very similar otherwise, with the exception of a little more mid range with the headers.


But keep in mind that this is SSI's and a sport muffler VS stepped headers and essentially open exhaust. I suspect that with megaphones bolted ontp the SSI's that the differences would beven smaller still.


Also, with your E cams, the headers will be even less of a benefit. I think you should stick with the SSI's.

If you want a really great power band, then regrind your E cams into the mod-Solex grind that Elgin offers. Better top-end than the E cams without sacrificing bottom or mid-range. Better there too actually. I loved my mod-Solex cams.

Tyson, thanks for the advice.

Unfortunately a regrind is a no-can-do, I have to stick with the late "E" profile for stock class purposes.

Having considered everyone's input I have decided to stick with the SSI's. I already own them, which is a plus.

Now I just gotta get someone (James?) to weld a pair of bungs into the SSI's. This will entail taking them off, which is a somewhat hazardous evolution.

John, are you welding in bungs for oxygen sensors? If so, it can be done on the car.

Tyson,

Yes, I plan to have someone weld in bungs for the 02 sensor.

I have to put the bungs at at least the 9:00 position so they don't fill up with water, and also so they don't get torn off--

I'm concerned that the average muffler shop guy will bugger up my SSI's with a welder, I need to find somebody good to handle it.

Or I might have the bungs welded into the muffler just aft of the coupling- the muffler is cheap compared to the SSI's.

Any thoughts?

If it's just for Air/fuel ratio monitoring, then put them in the muffler inlets.

Hi,

My biggest problem with going to headers was the lack of MFI thermostat heat. I built a thumb screw for it, so I can richen it when cold and lean it when ready to hit tha track.

But it took hours with the LM-1 to adjust it right, I ended up NOT screwing the thermostat screw in all the way when warm as everyone suggested, as it would be too lean at the high end that way. I still don't have it right but it's working OK now.

Bigger headers killed my torque below 5200 but the top end is awesome now.

Call me if you have any specific questions, late afternoon is best when my employees are gone.