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Thread: Engine Conversion

  1. #1

    Default Engine Conversion

    Sorry, this is a bit of a long post…so please bear with me

    I have a 1985 Series III (ex-MOD) 109 and, before I commit to any further restoration, I would like to upgrade my 5MB, 2.25L engine to see if I can get more top end speed (at lower RPM using an overdrive) without overly compromising the low end torque.

    Having now read many web pages and what seems like a thousand forum threads (many of them very good), I know that there are some basic things that can (and have been) done to a 2.25L engine to this end (ie. cam change, CR increase, carburation changes, exhaust modifications, piston over-boring, etc…) and I also know that there are other numerous other engine conversions options that would get me what I am looking for and more. But, the 2.25L engine is the bird I have in hand and I would like to keep things as stock as possible, so I have decided to do what I can with it.

    However, before I take the above approach, I would like to get thoughts on the idea of converting my 5MB, 2.25L engine into a 2.5L by:

    1. stroking the engine by installing a 2.5L crankshaft, camshaft and pistons, and
    2. possibly upgrading to 2.5L intake/exhaust manifolds and a 2 bbl carburetor (Weber 34ICH installed now)

    I have looked at the specs for both the 2.25L and 2.5L engines and, although I am not a seasoned engine builder, it looks to me like it should be possible since I understand (but am not positive) that:

    1. both engines use the same base block
    2. both engines use the same length connecting rods
    3. the 2.5L pistons are shorter from crown to gudgeon pin to, I presume, accommodate the additional throw (~5/16”) and maintain head clearance
    4. the 2.5L pistons are recessed to, I presume, accommodate the additional throw (~5/16”) and maintain an 8:1 CR

    Now, I’ll be the first to agree that this seems like a lot of trouble to go to to get an additional peak 6HP and 9 lbft (difference in peak power and torque for a stock 2.25L and 2.5L engine), but once done I could then apply additional performance upgrades to what would now essentially be a 2.5L engine, namely:

    1. increase CR to 9:1 by milling the head/block
    2. retard the stock 2.5L cam a bit or install a performance cam (source?)
    3. port (and maybe polish) the head and manifolds
    4. up-size and open the exhaust slightly

    So, my questions are:

    1. Can the proposed base 2.25L to 2.5L conversion be done and what are the primary ‘gotchas’ (either known or anticipated)?
    2. Assuming the proposed base 2.25L to 2.5L conversion could be done, what else could be (or should not be done) to further improve the performance of the resulting 2.5L engine?

    Thanks for reading/replying!

  2. #2
    Join Date
    Apr 2010
    Location
    Rochester, NY
    Posts
    421

    Default

    Your goal of more top end speed won’t be achieved by what you describe... you need a different power plant in there...
    I went the engine build route in my 109 because i wanted to keep a stock appearance.. if that isn’t part of your equation look for a different motor
    1971 SIIa 88" NAS Dauntles V6
    1974 SIII 109" RHD

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