Engine Conversion

[tynker]

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:


stroking the engine by installing a 2.5L crankshaft, camshaft and pistons, and
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:


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


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

So, my questions are:


Can the proposed base 2.25L to 2.5L conversion be done and what are the primary ‘gotchas’ (either known or anticipated)?
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!

[rejeep]

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

[Moritz]

Agree. My Series 3 rebuild involved stroking, including a 2.5 camshaft and pistons, and adding a Weber 2 bbl and a Pierce manifold. Also put in the high ratio transfer box. Honestly, probably as the truck was unavailable for so long for the rebuild and restoration, I was unable to detect any real difference in top end speed/power. Maybe the engine was a little quieter at speed. Maybe. Not that there wasn't a difference, just my butt dyno wasn't calibrated enough to detect it after being off line so long.

[tynker]

Thanks for the feedback thus far. These are the things that I need to hear before I start spreading Land Rover bits all over the garage!

[tynker]

Agree. My Series 3 rebuild involved stroking, including a 2.5 camshaft and pistons, and adding a Weber 2 bbl and a Pierce manifold. Also put in the high ratio transfer box. Honestly, probably as the truck was unavailable for so long for the rebuild and restoration, I was unable to detect any real difference in top end speed/power. Maybe the engine was a little quieter at speed. Maybe. Not that there wasn't a difference, just my butt dyno wasn't calibrated enough to detect it after being off line so long.

What did you do about an exhaust manifold? Cap the existing heat riser box or install headers or ? Do you like the Pierce manifold? What Weber carburetor did you use - 32/36 DGV? If so, how has it performed?

[Moritz]

It's the stock exhaust manifold with a cap on the riser box. Since I drove her for years through Wyoming winters with a "tuned" exhaust manifold without a heat riser box I wasn't worried. On that the Pierce manifold will probably be fine. The carb is as you surmised a 32/36 DGEV. Honestly, due to a timing chain tensioner issue, can't say much about performance. Had only couple hundred miles break in on it until the rattle drove me to tearing the front end off to discover the problem. That said, and given it had been a long long time since restoration began, didn't feel like a noticeable improvement in performance. Could be my butt dyno lost some of it's memory. Same with the high ratio transfer case. Still a dog, but my lovable dog. Should have things back together after Christmas and looking forward to being back on the road (too damn pretty to be back on the trail!)