M10 to M40...

[GypoE30]

Hey all...

I was just pondering as to whether an M40 would be better suited for my 85 318...

I understand the M40 is more efficient due to more valves?(correct me if im wrong)

Tell me what you think, and if not a conversion what should i do to make the engine as effecient as possible..(i use premium unleaded with additive)

[Zayyan]

Buy a diesel hatchback?

I never thought I'd be in a position to say this to anybody but don't be so tight!

M40s are more economical yes but they tick like bitches after a while if they're not very very well-looked-after (would go as far as saying "spoilt").

However the 1.6 M40 is a damn good engine if not ticking - torquier at low revs than the 1.8 (my 1.6 M40 is in a car that's lighter than the car with the 1.8 M40 by an amount that means they have a very similar power to weight ratio so I can confirm this), and I can get economy of over 40mpg if I'm careful.

It's also better (and safer) fun than a quicker car in my opinion as it's reasonably slow and so needs to be pushed to get the best out of it.

All in all if you want an M40 I'd recommend the 1.6; with 102bhp it'd only be 3bhp down on your 105bhp M10 318i so you won't notice a difference in performance, just economy.

BUT...
You don't want an M40, it's not worth the hassle (and money!) just to save a few extra pennies on fuel.

If you can find an M42 engine from a 318is, this will give you 136bhp and similar economy to the 316i as it has a 16-valve head.

This would probably be the best conversion as it would be reasonably insurance-friendly, would offer good economy and respectable performance as well.

Plus engine aside it won't cost much more than an M40 to fit.

Another option would be an LPG conversion if you plan to keep the car for a long time....you could even put a 2.5 in there and still save money on fuel

[Brianmoooore]

M40's got 8 valves, just like the M10.
M42's the one with 16 valves.

[DanThe]

Buy a diesel hatchback?

....you could even put a 2.5 in there and still save money on fuel

As above.. My first E30 was a 316, M10 with a carb, and it drank the fuel like a bastard!
I later dropped a 2.5 and all the running gear in and it was more economical with a performance transformation that made you think it was a different car altogether

[tim_s]

torquier at low revs than the 1.8 (my 1.6 M40 is in a car that's lighter than the car with the 1.8 M40 by an amount that means they have a very similar power to weight ratio so I can confirm this), and I can get economy of over 40mpg if I'm careful.

I was intrigued at this as the 1.8 M40 has the same engine + longer stroke than the 1.6 and thought that this made no sense unless the car was way lighter, so i checked it out in my bmw manual.

m40 1.6:
105 ft lb at 4250 rpm
102 bhp at 5500 rpm, 94 bhp/tonne
1080kg

m40 1.8
122 ft lb at 4250rpm
115bhp at 5500rpm, 105.5 bhp/tonne
1090kg

to get a better power-weight ratio out of a 316i, you'd need it to weigh 921kgs. not likely. the difference in torque is even more marked. i'd personally get the longer stroke 1.8 over the 1.6, gives much more torque and power.
but get an 318is, if you drive it like a loony you'll get about 30mpg, like a granny you'll get 40mpg. and yeah m10 carbs drink fuel like god knows what. been there!

[Zayyan]

to get a better power-weight ratio out of a 316i, you'd need it to weigh 921kgs. not likely.

But when comparing a 2-door M40 316i and a convertible M40 318i which is about 10% more powerful and 10% heavier it starts making sense huh?

As I said in this comparison the 1.6 definitely feels like it has more torque at low revs. Obviously it won't have more overall as it's 200cc down on the 318i

But it's the low down that matters....not needing high revs = saving of fuel. Simple really

[Zayyan]

Oh and before anybody is a smart and says it's nearly 13% more powerful I have a cat which munches about 3bhp leaving me with 112ish bhp.

Which makes it 10bhp or so more powerful than the 102bhp 316i, which is close enough to 10% more.

If you go and drive a 316i and 318i in identical body style you'll notice that if you put it in 5th gear in town at 30mph (a good tip for saving fuel ) with the revs at 1,500rpm and you slow down a little, the 316i will easily pick up the speed again from 1,250ish rpm, but if the 318i falls below 1,400rpm or so it will not be at all happy to accelerate and you'll have to change down.

[hammoj28]

Hmmm. Iv heard and been told that the m10 is a really good engine plus its carb so i guess you can play about with it much more and many people have got big power out if them. But thats not really important because you want economy. So, I found that my m40 316i was better and more fun to play about in than my 318i. Both exactly the same car even same colour just with the different engines. However when overtaking at higher speeds the 318i had a vast difference. You drop a gear and it just kept pulling compared with the 316i. The 316i was nowhere near the capabilities of the 318i at higher revs. Still found the 316i more fun though! Was a perky lil thing at that. I reckon on low down when accelerating it would beat a 318i.

The m42 is definately better though and if you go to all that effort you might aswel stick in an m42. Iv noticed though with my m42 if you put your foot down it drinks way more fuel than a 316i would when your giving it laldy! But it can be economical when you want and is definately more fun than the m40's. Go for an m42 id say.

Btw I found that a full stainless without a cat gave a big difference on my 316i!

[tim_s]

makes more sense, but still find it hard to believe

cos the engines are identical otherwise (same bore, block, head/cam/inlet/injectors), the longer the stroke means the more air per cycle therefore more torque. especially at low revs, where the limits of the induction and fuel systems shouldn't be a massive factor.

i thought maybe the fuel system/induction/cam or head might be different on the 1.6 to account for this, but just looked them up and all engine components i could be arsed to check are the same. the CR of the 1.6 is slightly higher, not enough to make a difference though.

so without seeing a dyno i still can't believe this, the area underneath the torque curve on the 1.8 should be much greater, maybe 20%+ greater. and given that the max power and torque are higher (and the difference in max torque is huge, almost 20 ft lbs.) and that they have the same head/cam/fuel/air, i see no reason (esp seeing the engines are otherwise the same) to suppose that the engine characteristics will differ so greatly in other areas of the power/torque curve.
but what you're saying isn't necessarily that the 316 has more torque lower down, what you're saying is that on part load at 1400rpm in 5th in a car a few hundred kilos lighter the 316 will pick up more confortably than a 1.8 in a heavier car. this may be true i guess, mb down to ecu map, extra car weight, condition of fuel systems etc.

i must admit i've never driven a 316i or a 318i, but i'm guessing the 1.8 = more torquey throughout, a bit more powerful, 1.6 = a bit more revvy.

and by the way, the cat will probably help the torque at the rpms we're talking, even though it will lower the max power slightly.

[GypoE30]

thanks for all the input guys...really appreciate it

those figures given are for a carby engine?...mine is an injected one...

Zayyan: sorry if i looked a bit tight...it was only cause i have been looking at M40 engines for a while due to the fact that the 1990 318i i used to drive had one while the 85 i have now is an M10...im only looking to increase effeciency ...im not really worried about economy cause on a normal 55L tank i can make about 400-500Kms...which is tonnes for me since im a just a student...

i personally couldnt compare them cause the M40 was an auto while the M10 has a manual box...

The main reason i went for the 318i is cause its a beast of a car in comparison to similarly sized vehicles...as well as its reliability, style and the fact that its a Beemer...(nuff said )