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Thread: 911 2.4E MFI pump on a Carrera RS clone engine w/o changing 3D cam?

  1. #1

    911 2.4E MFI pump on a Carrera RS clone engine w/o changing 3D cam?

    I have heard different opinions on this. Some say changing the cam is required, both because the pump otherwise will be supplying too little fuel, and because the maximum fuel flow will occur at lower revs. Others claim that the difference is virtually negligible, and that all any MFI pump will deliver enough fuel for an RS engine.

    Anyone here with hands-on experience or thoughts on this?

    Einar

  2. #2
    Member 30westrob's Avatar
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    No hands-on experience. I have heard many people say the pump must always match, no exceptions. I also know of several people who have done major engine modifications with good results using normal pump adjustments. 2.4S to 2.7RS appears to work.
    You have nothing to loose by trying. Rob

  3. #3
    Senior Member HughH's Avatar
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    I have a 2.7 rs engine build in my 2.4E in all respects apart from E cams. Ie a genuine RS space cam in the pump as well as all the other 2.7 bits. It took a lot of fine tuning of the pump internals (governor settings etc) to make it work properly. So I would be sceptical that you could use a E space cam in an otherwise RS build without a massive amount of work and probably compromises
    by the way the decision to use E cams was deliberate. It gives more torque than an RS but at the expense of some top end power. But drivability in all places except on the track is better in my opinion for using the E cams.
    Hugh Hodges
    73 911E
    Melbourne Australia

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    Early S Registry #776

  4. #4
    '72 911T 3,0 liter MFI Albert Blue street/DE toy Jeff Higgins's Avatar
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    I'm currently running a 3.0 liter engine with 10.5:1 compression, custom cams ground to my specifications by John Dougherty, ported heads, and on and on. My MFI pump began life on a 2.4 liter "T" specification engine. I spent a great deal of time adapting this pump for use on this engine, and managed to learn a bit about all of this along the way. I have a few threads over on Pelican Parts detailing my experiences - search for "MFI Open Heart Surgery" and you will find them.

    I discovered that any MFI pump is capable of delivering way too much fuel for any engine combination you can imagine. The governor mechanism, including the space cam, flyweights, and flyweight control springs are in place to regulate and reduce fuel delivery as required based upon RPM and throttle position. The pump side of the pump, that containing the plungers and the cam that drives them, is identical in every pump.

    The best advice I can give you is to simply "try it". In doing so, however, you will need a reliable, accurate means of monitoring and recording operating parameters including A/F ratio, RPM, and throttle position through a full range of driving conditions. You will need to be able to evaluate this data to determine if the existing fuel delivery from your pump is "good enough", or if it needs correction at specific operational conditions. This is not for the feint of heart - it takes a great deal of time and effort. And that is just the evaluation phase of the whole thing - if the fuel delivery needs correction under specific operating conditions, that is where the fun really begins.

    If you possess the patience and skills to do this yourself, it will be a very daunting, time consuming, arduous task. If you do not possess the patience and skills, it will be a very expensive proposition to have someone else do it for you. For me, it was an intellectual and engineering challenge that intrigued me, and I was eager to learn. I was equally eager to prove the naysayers wrong, those who said "yer nuts" and "it can't be done".

    The bottom line is that it can be done. And the real differences between pumps are not as great as some would have us believe. I suspect that your "E" cam can be made to work on your "RS" engine. Be aware, too, that A/F ratios on MFI engines vary a good deal more than modern Motronic or EFI engines. The old Russian engineering adage of "perfection is the enemy of good enough" certainly applies. So, set it up, run it, and see. The only real danger is too lean, so monitor your A/F ratios, be safe and conservative with them, and prepare to dig in.
    "God invented whisky so the Irish wouldn't rule the world."

  5. #5
    Thank you for all replies. I realize that finding velocity stacks (throttle bodies) for S/RS is difficult, and that the few items available are very pricey. Is it possible to

    a) obtain new, repro a somewhat decent price?

    b) purchase magnesium ones and have them machined to larger diameter (yes, I know that it increases along the length)

    Einar

  6. #6
    '72 911T 3,0 liter MFI Albert Blue street/DE toy Jeff Higgins's Avatar
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    a) Try TRE Motorsports, Dave Bazaglou: https://tremotorsports.com/engine/?c...+MFI+Solutions

    b) Eurometrix re-bores throttle bodies and stacks to whatever size you want: https://eurometrix.ws

    On my own engine, I began with 2.4 "T" parts all around, from pump to throttle bodies and stacks. I reworked the pump myself and had the throttle bodies bored to "S" spec by a load machinist who had knowledge of the procedure. He also bored my plastic "T" stacks to "S" spec.
    "God invented whisky so the Irish wouldn't rule the world."

  7. #7

  8. #8
    Quote Originally Posted by Jeff Higgins View Post
    ... I reworked the pump myself and had the throttle bodies bored to "S" spec by a load machinist who had knowledge of the procedure. He also bored my plastic "T" stacks to "S" spec.
    Eurometrix will not bore plastic stacks.
    Tre only supply stacks up to 2.2S.

    I find it a bit peculiar that the size that probably by far is the the most sought after, does not seem to be available anywhere

  9. #9
    Member #226 R Gruppe Life Member #147
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    The more rare the car model, the more rare the parts. I’ve been a mechanic 40 years, 90% MFI 911’s. Didn’t have the time to experiment, mixed and matched a few T/E parts with no success. Pretty much a cookbook process to make things work without hassle, in a timely manner. E ports, cams, throttles, pump. S ports, cams, throttles, pump as well as RS. RSR clone pumps by Pacific FI, built From 2.0’s pumps, worked well with HBF, GE 80 or 100 cams, a variety of port sizes, 3.0’s, 3.4’s single or twin plug. Just my experience. G

  10. #10
    '72 911T 3,0 liter MFI Albert Blue street/DE toy Jeff Higgins's Avatar
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    Correct, Eurometrix only bores the mag stacks. Like I said, I had a local machinist bore my plastic stacks. He had made his own reamer that allowed him to do this.

    I'm surprised TRE doesn't supply bigger stacks.

    Let me ask around and see if I can come up with anyone else still supplying, or working on, MFI stuff.
    "God invented whisky so the Irish wouldn't rule the world."

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