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Thread: 2.7 Rebuild Question

  1. #1

    2.7 Rebuild Question

    I want to build what I call an "RE", a 2.7 motor for my '72 E coupe using a later 7R case, the new Mahle P&C's, my E cams and my MFI (rebuilt for the 2.7 specs). I want to keep the E grind vs. going to the S to keep some semblance of "E" and because I like the power curve. This will be a street car that will look very stock but perhaps surprise some folks.

    I have been told that the cylinder pressures created by the 9.5 +/- CR of the new Mahles may be too great with my stock E cam and heads. Is this true?

    If so, can anything be done besides going to a more aggressive cam that will also work with my MFI and lower the cylinder pressures?

    I know that I could use the older 8.5:1 P&C's but would like to eke all the power I can out of the higher CR, now that they're available.

    My builder is very experienced with older 911's and has built some bullet-proof RS motors. I trust him but would just like to hear some other opinions.
    John Mackay
    Washington, DC
    '72 E Coupe

  2. #2
    Moderator Chuck Miller's Avatar
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    jsm,

    When you say "the new Mahle P/C's at 9.5" I assume you mean the 'racing' sets Andial are selling and that Andail says Mahle makes especially for them. Andail part # AD 928.02 90mm 2.7 RS - 9.5 C/R. (they also have them in 10.5 C/R)

    If so, they are REALLY expensive!!! ... very cool, but REALLY expensive... I'm doing homework on the same P/C's right now and Andail quoted me $2706.00... w/ no break...

    If you have a different source or better price for these exact same Mahle 9.5 P/C's, please let me know.

    Thanks

    BTW - All 'standard' Mahle 2.7 RS piston sets are 8.5 C/R #911.103.928.01 ... and can be bought for about $1100.00

    BTWII - I would go with S cams in your conversion.
    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
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  3. #3
    Goldmember ttweed's Avatar
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    Re: 2.7 Rebuild Question

    Originally posted by jsm911E
    I want to build what I call an "RE", a 2.7 motor for my '72 E coupe using a later 7R case, the new Mahle P&C's, my E cams and my MFI (rebuilt for the 2.7 specs). I want to keep the E grind vs. going to the S to keep some semblance of "E" and because I like the power curve.
    I agree w/ Chuck. Go with the "S" cams. You are increasing displacement and compression with these changes and I don't think you will really retain the characteristics of the "E" cams in your 2.4 that you like so much if you use them in the 2.7. The power curve will not be the same. The "S" cams will work great with that setup, and will NOT be as peaky as they are in the smaller motors. You will have great low-end torque, still, especially if you stay with a stock exhaust or SSIs and resist the temptation to put big headers on it.

    Be sure you do all the proper case mods with the 7R and use all Raceware or ARP fasteners and some Aase springs and retainers. It'll be a great motor and you'll love it (except for the lightness in your wallet).

    TT
    Tom Tweed
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  4. #4
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    Have to agree with Chuck and TT,

    Think if the 2.7RS motor as a big brother to the 2.4E...rather than the screaming sister that the 2.4S represents.

    Having drive in front of both the added capacity of the 2.7 means the S cams feel so much less peaky than the 2.4 version.

  5. #5
    Thanks for the feedback guys, very helpful.

    I do plan to keep my SSI's and go with either a single pipe stock or dual pipe sport muffler.

    Afterall, "it's only money". It may be a cliche but very relevant when referring to Porsches and I can't think of a better place to spend car money.
    John Mackay
    Washington, DC
    '72 E Coupe

  6. #6
    Moderator Chuck Miller's Avatar
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    John,

    I would suggest there's some fiddling around finding the sweet spot with the MFI / 'twin pipe sport muffler' combo ... I would stick with the stock muffler ... the closer to the factory (Leistritz or Bischof) the better... As other's have heard from me, every time I've ventured away from the stock muffler on my 2.4-S (twice for a total of about $1200.00) I've lost power somewhere ... usually on the bottom and/or transition mid-range... not to say I didn't gain just a little on the top ... but with a small motor that's not where you want it... Loud doesn't always = smooth and fast.

    Although I know there's other opinions on this ;-)

    Also,

    You didn't say if you were indeed going with the Mahle/Andial
    9.5 90mm 2.7 RS Piston set ... if so, if you don't mind, what was the price your mechanic quoted you ?

    As you can guess I'm ready to do the same thing and want to get as smart as I can on this...

    Thanks John,
    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
    Chuck:

    I have been just trying to sort out the specs of what my Tech will build over the past year (takes time to assemble that kind of $$). I've been sorting through the displacement and induction possibilities which seem to be infinite.

    I want the benefit of the larger displacement but really wanted to find a way to keep the E cams and MFI--the things that made my E an E. He was going to look into the 8.5:1 P&C's with E cams and MFI to see if it could work but he worried about the resultant cylinder pressures with the newer, hi comp P&C's and that milder cam. He knew the S cam would work.

    When I contacted the east coast source noted in Pano Mahle ad, I was quoted about the same price you noted earlier--$2700.

    But that raises another interesting question:

    Is the one point in compression worth the $1400 or so vs. the original RS 8.5:1?

    I'm inclined to think that it is for me for the reason I mentioned but everyone must make their own decision on the $ vs. speed trade-off.

    Any idea what the peak HP will be for this motor? I know that the 8.5:1 RS was 210 DIN. Everything else being equal, what's a point worth in HP on the 2.7 with S cams?

    Thanks,
    John Mackay
    Washington, DC
    '72 E Coupe

  8. #8
    Moderator Chuck Miller's Avatar
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    I want the benefit of the larger displacement but really wanted to find a way to keep the E cams and MFI--the things that made my E an E. He was going to look into the 8.5:1 P&C's with E cams and MFI to see if it could work but he worried about the resultant cylinder pressures with the newer, hi comp P&C's and that milder cam. He knew the S cam would work.

    >> John,
    Those Germans were and are REALLY sharp engineers; they took into consideration the total dynamic of your engine to make your E an E. The displacement vs. compression vs. cam profile vs. cylinder pressure, vs. piston speed, vs. fuel delivery... etc. etc.. In other words, you don't have to venture very far to counteract all the attributes that make your E significant to you ... AND replace them (If done well) with other attributes that'll make your smile just as big or bigger! :-)

    B]When I contacted the east coast source noted in Pano Mahle ad, I was quoted about the same price you noted earlier--$2700

    >> Oh well ... no deals to be had

    Is the one point in compression worth the $1400 or so vs. the original RS 8.5:1?

    >> It all depends ... for me, you bet.

    However I am told a sharp tech can get about 9 to 9.1 using the 8.5 RS set and a little creative machine work.

    Any idea what the peak HP will be for this motor? I know that the 8.5:1 RS was 210 DIN. Everything else being equal, what's a point worth in HP on the 2.7 with S cams?

    >> Hard to say, but all things equal I would say about 15 - 20 HP
    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
    According to my 30 year old mechanical eng'rg text book on Combustion Engine Processes, Chuck's on target.....an increase from 8.5 to 9.5 should give about 5% increase in power.

    BTW: how well do the 9.8 :1 engines tolerate 93 octane fuel vailable today???

  10. #10
    Moderator Chuck Miller's Avatar
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    Bob,

    As far as I've read 9.8 is about the the beginning of twin plug necessity...
    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

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