Individual K&N filters on MFI

[ThomasWilkinson]

Dave,

I am running ITG (Induction Technologies Group) oiled foam air cleaners and an early "rally" sport twin pipe muffler set up on my 2.7RS spec. motor.
The sport muffler was added first after a stock single tip unit. Then the air cleaners second. Seat of the pants I can't say I noticed any dramatic changes positive or negative. What I did get were some very nice sounds and visuals.
I've been told by a very reputable local source that the stock air box makes torque on the dyno. But I believe he was talking low/midrange rpm.
The shop that did both my motors (2.7RS/2.5 Webers) uses K&N and ITG filters on most of their MFI motors so I'm going to take the stance that they believe in them. I've also seen a great deal of sport mufflers come out of the same shop (primarily on smaller [2.0-2.7L] motors]. If either change was resulting in a loss of horsepower, they would not be installing them. As noted by another poster, Bruce Andersons' book shows a stock 2.4S MFI dyno comparison using a stock, and twin pipe sport muffler. The later picked up a good 10-12+hp from 4500rpm up.

TW
R Gruppe #130

[RSwannabe]

TW,

Your impressions are very similar to my own, but I still want to quantify this and put it to rest if possible.

Preparations for the Dyno testing are continuing. I am still looking for a stock early muffler locally, but not having any luck so far. Scott, is the offer to borrow yours still open? If so, check how much it would cost to mail it up to me (zip 98033).

The testing is tentatively scheduled for December 11th as part of a group of 911's hitting the dyno.

Brooke

[Cornpanzer]

The way I look at the matter is that if the difference in airflow of the seperate filters and loss of backpressure with the sport exhaust caused a loss of HP, Porsche would have never campaigned race cars with sport exhaust and open stacks. May be skewed thinking....but makes sense to me

[RSwannabe]

Dave,

I have often thought the same thing, but those cars also had hotter cams, bigger ports, and ran to 8,000+ rpm. Not necessarily the same beast.

In searching around for a stock muffler, I've been talking to a lot of guys at local shops and telling them about the test I want to do. Its funny how well respected mechanics will have completely contradictory opinions on this. One swears that the sport muffler will kill the low end and that K&N's are not worth a damn. Another says that sport mufflers add 10 hp and if someone says different they must be smoking crack. And these are both very reputable 911 wrenchs with lots of experience.

Let's get some definitive data! I found a stock muffler I can borrow, so all parts have been located and I am ready for testing. I should hopefully be on the dyno on the 11th (contingent on the group deal getting pulled together).

Brooke

[LeafGreen]

....Just some more thoughts on the subject. Assuming your engine is tuned at its best with the stock air filter and exhaust and you've got some good baseline runs, you should notice a change after the exhaust swap on the second run (assuming the exhaust is radically different). Rather than stopping here and accepting the answer (loss or gain in HP/torque) you should find out why.

For example: Was the HP loss, or gain, at high RPM a result of the mixture leaning out? Or richening up? Did a mixture adjustment increase power in this area without affecting the low end and overall drivability? Perhaps the engine was not in optimum tune during the baseline run and the sport muffler "fixed it", giving the impression that this exhaust will definitely provide a power increase.

Hats off to you guys for not going on hear say! Like Brooke says "lets get some data"!

[RSwannabe]

Leaf,

After talking with a number of people, I am going aim to keep the A/F ratio roughly the same for all 4 configurations by adjusting the main rack. I won't be fine tuning specific portions of the rpm range, but I'll try too maintain a 12.5:1 or 13:1 ratio to keep things even. Hoipefully that will give us the best ballpark estimate of the effects of different configurations.

I just got off the phone with Denny Akers, a mechanic I really respect and who has a lot of racing experinece in the old cars. His prediction is that the sport muffler will yield 10 - 20 hp gain in the high revs, but with a loss down low. He is of the opinion that the gain up top is not worth the loss of torque in the lower revs. He did not have a good guess as to the effect of the K&N's.

Brooke

[LeafGreen]

Brook,

I think adjusting the rack to try and keep the mixture square is a good effort in maximizing results within a fixed timeframe (by the way, more power is made when the mixture is as lean as possible rather than on the rich side). I have a good personal experience at the drag strip about leaning out the carb on my 383 and improving my ET and MPH with each jet change. I didn't finish, but I ended up with the stock jets in the OEM 440 Carter AVS - and the car wasn't slowing down.

I would agree with your mechanic that a small top end gain in HP is not worth the sacrifice in power in the RPM range where we spend most of our time (the most average HP you can make in the usable RPM range will win races, not peak HP). I also agree, that in essence what he is saying, is the reduced back pressure of the sport muffler will aid in scavenging, thereby adding top end HP.

What I want to know is:
1) How the mixture is affected to get that HP gain.
2) The mufflers effect on the mixture throughout the RPM range.

Boy....This is good stuff!

[ThomasWilkinson]

A comment, if I may, regarding top end power gains.
Depends on the driver and how he uses the car.
One could argue that many early S drivers do not operate their engine in the range where it really makes power frequently enough to justify the loss in low and midrange power that these "peaky" motors sacrifice. A T or E may in fact be a much better car in most situations. But those addicted to the top end shriek are willing to make the sacrifice. Adding more top end via a sport muffler or other means just extends the "high".
A friend of mine has a 2.6 (92x66) MFI motor with K&Ns, twin pipe, and cams a notch up from S (WEB custom grind). Sounds nasty and rocks hard from 5500-7500. But I wouldn't want to commute in it. Not the intent or use.
If you want a daily driver, buy an SC with gobs of torque at 2500-4000rpm.
I'm eager to here the results of this comparison. Just felt like adding some relative .02.

Tom Wilkinson
R Gruppe #130
1970 914-6 2.5LS
1973 911 2.7RS spec.

[Cornpanzer]

Hmmm, I get irritated if people dont take my "S" to 7200 on test drives. Who says you cant redline on daily commutes
Looking forward to the results as well!

[RSwannabe]

Looks like the dyno testing will take place Decmber 18th (see "Dyno Day" thread on Pelican Pacific Northwest forum). I'll keep everyone posted as things progress.

Brooke