Page 1 of 2 12 LastLast
Results 1 to 10 of 13

Thread: Air:Fuel Mixture, MFI Feedback needed

  1. #1

    Air:Fuel Mixture, MFI Feedback needed

    Hey Folks

    Another MFI question.

    I have '72 RS replica. It came to me as a 2.9 liter with MFI (with the correct RS serial number, the last digit was re-stamped to make it so. I'm hoping that is indication of the correct internals.), a modified duel out muffler. It had lots of grunt but ran very rich and was reluctant to pull much higher than 6k rpm. After a closer look it turned out to have CIS pistons and cams. After doing much reading here I decided to go back to 2.7 liter, 9.5:1 JE pistons, GE60 cams. It runs really well now but still runs rich. Black smoke on initial hard acceleration, only 13 mpg highway(8.5 on the track). My mechanic, who is not unfamiliar with MFI, and is great with early cars, went through all the standard stuff when putting the engine back together. Everything is dialed in, ignition, throttle linkages etc. I finally had it on a dyno the other day. I'll attach the graph. Not having much experience with this stuff I'd really appreciate some feedback on the results. My mechanic was pleased enough with the numbers that he's almost apprehensive about tinkering anymore. I'm doing only enough street driving to get me back and forth to the track for DEs. Mileage isn't really my concern as much as whether or not I'm washing the cylinders and contaminating the oil. Some would say richer is better than too lean (including my mechanic).

    Here's the question (questions): How rich is too rich? The dyno is reading Air:Fuel somewhere between 11:1 and 13:1. Max hp 221, Max Torque 195. How do these numbers compare to a correctly dialed in RS? Am I splitting hairs or does the graph indicate an issue consistent with the smoke and mileage? We've swapped out the cold start solenoid with a proven unit. Could it be the muffler? Again, I'm not sure what's inside that. Should I go back to a single out Dansk sport muffler possibly giving better back pressure?

    I spoke briefly with Gus at Pacific (very kind guy). But after giving him the details he suggested tweaking the main adjustment screw to see what happens. Funny. I thought he'd be the last guy suggesting diving right at that. Won't that just make it too lean at the top end?

    In any case I'd love to hear some opinions. I guess I'm hoping for a fix less involved and costly than actually sending the pump to Gus.

    Thanks in advance for any feedback.

    A.T.

    P.S. I'll post some pics on the "Show Your Ride" thread as soon as I clean the schmutz off the rear bumper.
    Attached Images Attached Images  
    Last edited by A.T.; 12-10-2010 at 08:37 PM. Reason: Add attachment
    A.T.
    Early 911S Registry Member #500

  2. #2
    Im far from an expert but would be very pleased with those numbers. Did you Dyno With and without an Air Filter?
    I would try Different air cleaners and see if you can get anything from that. 220 Horsepower sounds Fun! Are you Advancing timing until Detonation And then Backing off, Or just Going with Reccomended Advance? Ive always timed my Toys with a timing gun first, Then Tried to max my Advance with no detonation.

  3. #3
    Moderator Chuck Miller's Avatar
    Join Date
    Oct 2002
    Location
    Reseda, CA.
    Posts
    12,277

    Question

    Quick question A.T.....

    I don't see any reference if your dyno numbers are from engine dyno or chassis dyno readings...

    - If they are engine readings you seem to be a bit off with the equipment you have...
    - If they are chassis readings I believe you are WAY on the good side of the equation...

    Very basic numbers for the engine you have would be roughly about 225-235 HP at the flywheel .... reading about in the high 190's to about 205 or so at the wheels.

    Again these are VERY basic numbers, but having a motor very similar to yours for over 7 years (but never having it on the dyno) I've done a lot of homework and comparative questioning.

    Cheers
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

    TYP901 #62
    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
    '67 Malibu 327 spt cpe - Period 350 Rebuild

    98 Chevy S-10 Utility
    15 GTI Commuter

  4. #4
    Senior Member 2.5MFI's Avatar
    Join Date
    Sep 2006
    Location
    Bend, Oregon
    Posts
    432
    I think your A/F ratios are very good and stay fairly consistent for your 2.7 MFI engine. A little rich at the beginning but this seems common with MFI. At 4000 rpm the MFI pump is into the high speed spring capsule adjustment. Any adjustment of the main rack as Gus suggested will move the entire A/F ratio from idle to top end. But since your A/F ratio at 6500 rpm is around 13, I would do what Gus said and try about 3 clicks clockwise on the main rack and that should improve the 4000 area but lean out the 6500 just a little. Some adjustment of the high speed (white screws) in the pumps spring capsule would change the fuel flow from 4000 rpm and up, and might help, but it's very time consuming. To really see the effect after any adjustment your going to have to put it back on the dyno. Again, I think your very close, just a little rich at the bottom end.
    Mark Jung
    MFI Werks
    Early 911S Registry #972
    Carrera T w/LWB, MT, RWS, PCCB
    72 T 66 x 100 MFI Twin Plug Coupe
    R Gruppe #686

  5. #5
    Thanks guys.

    Villeza, I tested it with the factory air filter in place. There is a great thread on the forum here where someone dyno tested an engine with and without air filters and with K+N filters and compared stock mufflers with twin out sport mufflers. It showed the factory set up air filter to be the best power. Although these tests showed the dual out sport muffler to produce a little more horse power there still seem to be a lot of different opinions. Some people suggest that the single out muffler makes for better back pressure and subsequently more power. So many variables.

    Chuck, it was a chassis dyno. So, 220 hp at the wheels. I gather that is a good thing. Do you run a twin out muffler or single?

    Mark, I think the time issue is probably part of my mechanic's reluctance as well. Here's what another friend suggested.

    "The dyno run clearly shows that somebody who knows MFI pumps needs to take the rear cover off and adjust the small screws internally, which change the slope of the line, rather than just lowering or lifting the line, which is what the main mixture screw does. It will probably take a few iterative trials to get it quite right."

    Obviously I'd want to make certain that all the other variables are checked off again before going there. I hope to collect as much info here as possible and then to sit down with my mechanic and discuss the approach.
    A.T.
    Early 911S Registry Member #500

  6. #6
    Moderator Chuck Miller's Avatar
    Join Date
    Oct 2002
    Location
    Reseda, CA.
    Posts
    12,277

    Thumbs up

    Chuck, it was a chassis dyno. So, 220 hp at the wheels. I gather that is a good thing. Do you run a twin out muffler or single?
    WOW A.T. !!!!!!!!

    That's about 255 at the flywheel!!!!!!!!!

    Is there something you're not telling us........

    That's truly BIG HP for a 9.5/2.7 with cams and nothing else....... good'on'ya

    I've tried 2 different 2 in single out sport mufflers, the first an A.J Sport, and the second a Monty MFI single........... they both gave me slightly more on the 'top' but both took away a little on the bottom and transition..... had to tweak the pump too much to run right on the bottom... so back on went the stocker.

    Again... all this is 'seat of the pants dyno' of over 20 years of driving the same car.

    Cheers
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

    TYP901 #62
    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
    '67 Malibu 327 spt cpe - Period 350 Rebuild

    98 Chevy S-10 Utility
    15 GTI Commuter

  7. #7
    Hey Chuck

    Thanks. I don't think there was any special voodoo performed during the reassembly. Titanium valve spring retainers was as fancy as it got.

    When you were tweaking your mfi did you get any more adventurous than adjusting the main rack? Were you turning any other screws? Were you able to make mixture adjustment at different ranges in the rpm?

    I just remembered I had this amazing link to an index of MFI info. I think I have some reading to do.

    http://forums.pelicanparts.com/porsc...rce-index.html
    A.T.
    Early 911S Registry Member #500

  8. #8
    Moderator Chuck Miller's Avatar
    Join Date
    Oct 2002
    Location
    Reseda, CA.
    Posts
    12,277
    When you were tweaking your mfi did you get any more adventurous than adjusting the main rack? Were you turning any other screws? Were you able to make mixture adjustment at different ranges in the rpm?
    I did not A.T., I just fiddled with the two adjustments keeping VERY good track of where I started...

    The Pelican thread is pretty amazing......

    Each of these motors are a little unique as to where the 'sweet spots' are......

    - I pretty much have the pump where it wants/likes to be... It runs strong, gets (relatively) good mileage under low RPM conditions, and comes down from a track session to a steady 950 idle..... not saying there's not more in to be gained by having it comprehensively checked and adjusted, however it seems to be pretty happy where it is.

    - I kept my 2.4S points distributor in the rebuild and the sweet spot for advance went from about 35' to just under 37' ...... just a fraction under that and I start to loose the 'snap'....... not saying there's not more to be gained with a trigger ignition and a scope... but it seems pretty happy where it is.

    ... kind'a all part of the deal
    Chuck Miller
    Creative Advisor/Message Board Moderator - Early 911S Registry #109
    R Gruppe #88

    TYP901 #62
    '73S cpe #1099 - Matched # 2.7/9.5 RS spec rebuild
    '67 Malibu 327 spt cpe - Period 350 Rebuild

    98 Chevy S-10 Utility
    15 GTI Commuter

  9. #9
    '72 911T 3,0 liter MFI Albert Blue street/DE toy Jeff Higgins's Avatar
    Join Date
    Jul 2006
    Location
    Lynnwood, WA
    Posts
    190
    It's amazing how much more we can see these days with regards to engine tuning. Our modern equipment has sure opened our eyes with regards to what is happening with regards to A/F ratios, ignition timing, and the like. This relatively new found ability to get precise numbers at all rpm's, loads, throttle positions, and whatnot has succeeded in kind of making the old style MFI look bad. Especially compared to modern EFI systems.

    A two point variation across the range on an MFI motor is not unusual in the least. At full throttle, they all seem to lean out on top, while starting out rich at the bottom and mid range. This can be somewhat affected by adjusting the high speed screws in the little capsule in the governor. Not much, though, and certainly not enough to completely eliminate this tendency. I think what I would do, in this case, is start out following Gus' advice and lean out the main rack screw a bit. 13.5:1 on the very top end, where the motor really isn't going to spend a great deal of time anyway, is just fine. Only if you cannot find a happy compromise with that would I get into the capsule. And, as Chuck says, keep damn good notes...

    The big black cloud when you first open it up is normal as well - remember, with MFI, the fuel delivery is completely independent of intake air requirements. In other words, the amount of fuel being delivered is in no way affected by the amount of air the motor is drawing into itself. You mash the gas, and fuel delivery goes to "full" whether the motor can use it just yet or not. It's a very "dumb" system. Even carbs are "smarter", with the amount of fuel being lifted out of the float bowls and passed through the jets being tied to intake vacuum. When you first mash the gas with carbs, vacuum goes to zero and delivery through the jets likewise goes to zero (that's why they have accelerator pumps, so they don't just hopelessly bog down). MFI isn't that "smart" - you tell it "full delivery", and that's what you get - immediately, and unlike accelerator pumps in carbs, continuously. Yes, fuel delivery is rpm dependent through the action of the flyweights in the governor assembly, but that is to cut per stroke delivery as rpm's increase and volumetric efficiency declines. The motor needs more total fuel delivery as rpm's increase, just because it is spinning faster, but it needs less per stroke delivery. The greatest per stroke delivery requirement is at the rpm at which the motor produces peak torque, not peak horsepower. Anyway, that's where the big black cloud comes from - intake vacuum (and therefor volume of air) goes to zero when you first punch it, but fuel delivery goes to full. Until the motor starts sucking enough air to be able to use all of that fuel, it runs exceedingly rich. Nothing we can do about it.

    I think another thing I would do is to get some part load, part throttle "cruising" A/F numbers. Either back to the dyno, or out on the road with an LM1 or other such A/F meter. If it is cruising at freeway speed in fifth gear between 13:1 and 14:1, or even just a bit leaner (which is perfectly safe with no load to speak of), I would say you are "there". In that case, I would leave the main rack alone and maybe try the high speed screws in the capsule. Just don't expect much from them (maybe half a point at best) and don't move them very far from where they are now. After a couple of turns trying to richen up that top end, its adjustment will also start to affect the mid range, richening it as well. The three springs in there are somewhat inter-dependent - move any one range too much, and it will affect the others by overwhelming their influence.
    "God invented whisky so the Irish wouldn't rule the world."

  10. #10
    Hey Jeff

    Thank you very much for the feedback. I'm sure you are inundated with similar questions after all the digging you've done and knowledge you've gained. Thanks for sharing.

    A.T.
    A.T.
    Early 911S Registry Member #500

Similar Threads

  1. Feedback needed on new repro rear lid grille ?
    By RS-GT in forum General Info
    Replies: 9
    Last Post: 09-20-2013, 03:30 AM
  2. !00 ltr Fuel Sender Needed
    By johnchodgson in forum For Sale: 911 Parts
    Replies: 2
    Last Post: 12-05-2011, 08:38 AM
  3. Replies: 3
    Last Post: 01-29-2009, 03:55 PM
  4. Air mixture adjustment for 72S
    By Stephen T in forum Technical Info
    Replies: 2
    Last Post: 05-17-2006, 06:37 AM
  5. 70 911 Fuel tank needed
    By rich roush in forum For Sale: 911 Parts
    Replies: 7
    Last Post: 04-29-2006, 11:17 AM

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •  

Message Board Disclaimer and Terms of Use
This is a public forum. Messages posted here can be viewed by the public. The Early 911S Registry is not responsible for messages posted in its online forums, and any message will express the views of the author and not the Early 911S Registry. Use of online forums shall constitute the agreement of the user not to post anything of religious or political content, false and defamatory, inaccurate, abusive, vulgar, hateful, harassing, obscene, profane, sexually oriented, threatening, invasive of a person's privacy, or otherwise to violate the law and the further agreement of the user to be solely responsible for and hold the Early 911S Registry harmless in the event of any claim based on their message. Any viewer who finds a message objectionable should contact us immediately by email. The Early 911S Registry has the ability to remove objectionable messages and we will make every effort to do so, within a reasonable time frame, if we determine that removal is necessary.