Curious - mpg from a 2.25 or 2.5 petrol with Mega bits

[Snagger]

Hi all,

Im am toying with the idea of buying my wife an 88” as a surprise out here, something I can restore over the next few years and keep relatively standard, just a few simple alterations to make it more comfortable to us as we get older like interior trim, overdrive, maybe parabolics and better seats.  I’d probably go petrol to avoid the punitive political climate and likely charges in the UK and Europe.  So, what mods can be made to increase fuel efficiency to the 2.25 or 2.5 petrols?  I was thinking Megajolt and Megasquirt would be a good start.  ACR do a range of mods to make them 2.8l with fancy cams and manifolds, but I presume that has more effect on performance than fuel economy, and would take some mileage to redeem the expense.

So, what can be done and roughly what mpg should it result in for mixed cycle driving in a mildly hilly region in a clean 88” (no rack, roof bars or towing) with standard running gear, 7.50 tyres, overdrive and minimal extra weight over empty?  If there are any significant difference for soft top vs hard top, or for Lightweights (if I can find a good project base), then please say so.

[rtbarton]

I had an overdrive, vacuum gauge and 750 x 16 tyres on a SIII SWB petrol hardtop.  I managed to achieve 29 mpg on a run between Sheffield and Silloth (Cumbria) by driving to the vac. gauge.  The journey took about an hour longer than normal though 😰

[lo-fi]

An ACR flowed head, megasquirt running spark and fuel and decent exhaust will likely be the sweet spot. Unless you want to go down a massive rabbit hole doing a multi point injection setup, I'd suggest looking at a single point injection throttle body. It fits to the manifold like a carb and contains throttle plate, injector, throttle position sensor, temp sensor and injector. I have a mate running that exact setup on an Essex V6 (I helped him tune it) and gets low 30's mpg on a long run. Multi point injection is better when set up well, but it's a lot of work to get it there on an engine that doesn't have a factory setup. 

[FridgeFreezer]

Of course it depends massively how you tune it and how you drive it, but running closed-loop with a lambda sensor allowing 5-10% correction on cruise will make a big difference purely because it is keeping the mixture very close to stoich over many miles. Start/stop and hard acceleration never helps but MS may well make it more driveable which may well help.

Anecdotally I've found MS to be at least no worse than factory EFI - if I drive the ambulance as if it had a TDi I can approach 20mpg from a 3ton windbreak with a 4.6 V8 in it.

[Eightpot]

 Those old petrols are in such a low state of tune, if you're not trying to run it off watered down parafin in a rain forest and are using modern fuels my first thoughts would be to cover the basics and sharpen the performance-  raise compression ratio a little, blueprint the manifold castings, a little work on the head ports and valve seats, swap the oil bath air filter for something a little more free flowing, an exhaust bigger than a straw, electronic ignition kit, the weber carb conversions seem a little better on fuel if you want to stay mechanical.  I'd be inclined to keep the cam the same as you don't want to turn it into a revvy engine with torque high up the range.

Keeping rolling resistance low makes a good difference as well - the MT tyres I just fitted have made a noticeable difference to the ATs which rolled easier, and making sure brakes don't drag.  Bonnet mounted spare does seem to improve economy on long runs, I've never noticed any difference between hard/soft tops.   

[Bowie69]

All the above, acr head, I would go cam, personally, megasquirt (which does ignition so no need for megajolt), port match manifolds/head and a larger exhaust. 

The biggie here is raising compression ratio to something like 9.5:1 (or whatever you think you can get away with) a lot are 7:1, some 8:1 as stock. 

Start with a five main bearing crank if you can... Actually, scrub that, if you can, start with a 2.5 and dress it up to look like a 2.25, clearly you aren't going for stock here! 

25mpg should be achievable with all that lot. 

Have to confess, I wonder why you are bothered by mpg in a place where petrol is cheaper than water 😛

[Snagger]

1 hour ago, Bowie69 said:

 

Have to confess, I wonder why you are bothered by mpg in a place where petrol is cheaper than water 😛

The idea would be to build it up here, and later bring it home and convert it to RHD.  But petrol has doubled in price over the last five years here, with signs of it continuing to rise like everything else.

 

Thanks for all the replies.  It’d be run on ATs, not mud, so that’d be one saving area.  I didn’t know Megasquirt dealt with both fuel injection and ignition, so that’s a bonus, and yes, the thought occurred of dressing a 2.5, but I’m not too worried about it looking like the 2.25 (given the aesthetic changes of the MS system).  25-30 sounds very respectable, about what I get on the Tdi in the 109 (with all the extras bolted on).  That should be quite manageable for a domestic used frequent driver, and is a lot better than the mid teens I got from the knackered 2.25 originally in my 109, which did 400mpg of oil!

[Tanuki]

Best I ever got out of my 88" SIIA was about 24MPG - that was driving at as-near-a-constant-55MPH-as-possible on motorways early in the morning when there was little other traffic about. The only 'mods' it had were an ex-Rover-2000 SU-and-aircleaner on a homemade elbow-adapter, Sparkrite electronic ignition and a straight-through exhaust.

If I was doing it again I'd start by doing all I could to get the compression-ratio as high as possible, given that modern petrol's so much better.

Mega-ing it with single-point injection, balancing/blueprinting, a decent low-restriction exhaust.

I've heard it said that fitting the Diesel camshaft to the petrol engine is worth doing too. I'd reckon you should be able to get a reliable/efficient 110-120BHP, as well as making the engine more free-revving and smoother (thereby obviating the need to mess about changing the gearing). Probably 25-28MPG but the problem I've always found is that improving vehicles mean I end up travelling faster which offsets any efficiency-based fuel-saving.

Also, consider using modern thin synthetic engine oil - 5W30 or similar - they offer a significant friction/drag-reduction over old gloopy 20W-50.

[Gazzar]

Interesting thread. My next job on the lightweight is to build up a 2.5 petrol with modern engine management.

ACR will feature prominently.  I'm told the engine already has a Power plus head, but I'm looking at a high torque cam, single-point injection and electronic spark, high flow manifold and exhaust. 

I don't know what CR the engine is at the moment. So I'll cross that when I get to it.

Air box will be an oil bath, as I really prefer them for naturally aspirated engines. 

[jessejazza]

2 hours ago, Tanuki said:

Best I ever got out of my 88" SIIA was about 24MPG - that was driving at as-near-a-constant-55MPH-as-possible on motorways early in the morning when there was little other traffic about. The only 'mods' it had were an ex-Rover-2000 SU-and-aircleaner on a homemade elbow-adapter, Sparkrite electronic ignition and a straight-through exhaust.

If I was doing it again I'd start by doing all I could to get the compression-ratio as high as possible, given that modern petrol's so much better.

Mega-ing it with single-point injection, balancing/blueprinting, a decent low-restriction exhaust.

I've heard it said that fitting the Diesel camshaft to the petrol engine is worth doing too. I'd reckon you should be able to get a reliable/efficient 110-120BHP, as well as making the engine more free-revving and smoother (thereby obviating the need to mess about changing the gearing). Probably 25-28MPG but the problem I've always found is that improving vehicles mean I end up travelling faster which offsets any efficiency-based fuel-saving.

Also, consider using modern thin synthetic engine oil - 5W30 or similar - they offer a significant friction/drag-reduction over old gloopy 20W-50.

I would research the use of synthetic oils carefully. The previous owner of my Herald ran it on synthetic and ended up getting a new engine. I've not looked into it I just stay with 15W/20W for all classics.

[Bowie69]

Of course the other option is just fitting a disco mpi engine, which should  bolt up to the series transmission. 

There's even a turbo version... 

 

[Daan]

As mentioned, the compression ratio is what will improve efficency. I built an engine using a 2.5TD block, 2.5 petrol pistons, 2.25 con rods and a 2.25 head with 2.5l valves.

I got the compression ratio up to 9:1, achieved partly by using the 2.25 head on a 2.5 and and partly by skimming (2 mm I seem to remember) the head. 

The power was good, MPG, well it was on LPG, so I never really looked.

Daan

[smallfry]

Apart from general wear, its the tired distributor and carburettor that are the weak spots, so Megasquirt or similar would be great.

Used to have a Lightweight soft top with a five bearing 2.25. Did about 15mpg if you were careful. After a home made manifold with a 2inch SU, some porting, paying particular attention to the exhaust port and valves and Lumenition, got around 24mpg normally.

Also fitted with free wheeling hubs, RR diffs (3.54) and 750/16 Goodrich Trac edge (remember them ?) If I was really careful, could get 27-28,but you wouldn't normally drive like that !

Relatively refayned and quiet too.

Oh,and dont forget to inform your insurance company 😄

[FridgeFreezer]

13 hours ago, smallfry said:

Apart from general wear, its the tired distributor and carburettor that are the weak spots, so Megasquirt or similar would be great.

Even in good fettle, the old Lucas points ignition is a way off modern coil packs...

 

[Nonimouse]

A friend has a 2.25 with a stage 2 ACR head and a supercharger, it goes well and is relatively economic

[lo-fi]

3 hours ago, Nonimouse said:

A friend has a 2.25 with a stage 2 ACR head and a supercharger, it goes well and is relatively economic

Pics or it didn't happen!

Got to see that one, definitely need some details! 

[Gazzar]

15 minutes ago, lo-fi said:

Pics or it didn't happen!

Got to see that one, definitely need some details! 

Agreed, though I'd not be able to use the oil bath if I went that route. 

 

[Nonimouse]

On 12/19/2019 at 3:51 PM, Gazzar said:

Agreed, though I'd not be able to use the oil bath if I went that route. 

 

I'll try and get some pictures - next time I see him