Relative pros and cons, carbs Vs injection?

[BJD]

Anybody got any good links for info on injected engines converted to carbs? I'm interested in the relative pros and cons, potential performance, and value for money. Its for a four pot car (BMW M42 1.8 16v). Power at present after minor mods and live dyno remap is a disappointing 153 flywheel. I can't use the M44 bottom end for a relatively cheap and easy power hike to 1.9 as bore/stroke has to be standard, but top end mods are relatively free.

I was initially thinking of something like twin 40 Webers, but having seen the prices these now go for I'd like to look at the possibility of using bike carbs as a cheap (ish) alternative. I'm really after food for thought for now, anything I do will likely be after this season.

I'm thinking that cams and head flowing is probably a must for any significant power increase, but where to from there?

Cheers m'dears
Brian

[Casper]

Whilst having no hard information here are my thoughts.

Webers are hard to get exactly right and more important keep them there. it is fairly easy to get them close to right the rest is difficult and those that can do it are getting harder to find.

Injection is far more "mappable". You can use an aftermarket ECU, a megasquirt comes to mind especially the latest iteration and you should get a far more tractable motor with a more usable power band and if you don't fire the tuner. In short the carby's MIGHT give you more peak power in one spot but usable power as wide as possible is the name of the game.

Many have used bike carby's on cars and they do well but tuning them to do well is another story.

If you are not producing the power you expect what exactly did you expect and why? if other motors of a similar spec produce more then find the reason for the difference. If it is an on track comparison with other cars and you are just starting racing then you cannot compare yourself with someone who has been racing longer. Everyone who starts out says the same, their car does not have the power off the corners, it is almost a commandment of new race car drivers.

[BJD]

Hi, I'm not exactly new to track (four years racing rars, six years previous to that racing bikes) so was thinking of power output rather than looking at laptimes, they are down to me rather than the car. I race in a mixed series so don't have any direct comparison. I was thinking that people may have some experience of carbs from using the M42 engine in something like 2002 conversions, or perhaps kit cars?

I had read (on the internet somewhere so it must be true :-) ) that when BMW introduced the 318is they gave it an artificially soft tuning chip so as to maintain a big power difference from the flagship 325i, so was expecting a bit more than I have now (153 brake tuned 318is Vs 175 brake standard 325i). Perhaps I was too unrealistic in my thinking, so now I'm planning the next stage of tuning for next season. I'm also trying to get a feel of what potential increase there is to be had from an upgrade. As manifolds etc will be bespoke rather than off the shelf, is any potential benefit worth the work/development costs involved.

Cheers
Brian

[tristancliffe]

I've recently gone the other way, converting a 2 litre toyota engine from 48 Dell'Ortos to 38mm throttle bodies. The increase in power was marginal (a few hp), and the gains in torque were equally small, but the engine is able to pull cleanly from a much lower RPM.

Converting to carbs from injection, you'll gain a lot in simplicity (carbs are really simple, even Webers, and require so little maintenance it's untrue), but you'll lose power, torque and driveability.

Our change is worth about half a second to two seconds per lap (depending on the track), so expect to lose that in the reverse conversion.

[BJD]

Ta, although I'd be converting from road set up single tb on a standard manifold rather than individual throttle bodies, they are a no no under the regs but would be an obvious choice if allowed for all of the reasons you have mentioned.

[phoenix]

The stock engine is supposed to give around 138 BHP and 129 lb ft of torque, so even with 153 BHP you are not doing too badly - and I suspect you would struggle to match what you have acheived so far by just bolting on some carbs.

The M42 engine has an all steel forged bottom end and a shortish stroke of 81mm, which is potentially good for you as it means it can rev safely. It also has variable length intake manifold controlled by the ECU, which means if you throw it away and chuck carbs on you could be losing out on a whole lot of torque and BHP. With the size of valves fitted as standard (30.05mm I believe) you have a potential 200+ BHP/145+ lb/ft inside the motor just waiting to get out (Apparently K series engines can produce 220 BHP on 29mm inlets). With 33mm inlet valves, if the head will take them, will get you well over 200 BHP at about 8000 RPM (a no-expense spared race engine should see 250 BHP with 33mm inlets). But it will take more than a set of carbs to extract any further increase I would think.

The standard compression ratio is very modest at 10:1 and the cams are no doubt very mild. I would suggest that rather than spend money on carbs and inlet manifolds you get a new set of pistons to increase CR, a new set of cams - reprofiled ones from Kent would probably do the job - and then a re-map.

That should take the power up quite a bit as the cams will allow the engine to breath at higher revs and the higher CR will improve your torque too, but don't expect too much; I would suspect anything north of 180 BHP is going to require far more detailed and costly work to the head, exhaust manifolding etc, even though the engine has the potential to give a lot more.

Having said all of the above, there must be BMW engine tuning specialists who know this engine inside out, so I would speak to them next.

[BJD]

Thanks, very useful info (tho the variable inlets are only on the later E36 version). I've been on google quite a lot for tuning info, the issue I'm coming up against is that most tuners (and for some reason there aren't that many specialists for this engine, Metric Mechanics in the states is the only one I can find, the rest seem to be resellers for camshafts etc) look at bore/stroke mods as well, so its difficult to find any top end tuning info done in isolation, and I haven't found anything at all on an M42 on carbs. The bottom end has had a rebuild and lightened/balanced, so good for any future tuning. If I get anywhere remotely close to 180 I'd be delighted, and it looks like this could be achieved with the standard valve size which is good news. The head has just had new valves/3 angle valve job, so looks like this won't be wasted.

Cheers
Brian

[drjim]

Have you spoken to Fritzs bits, they do quite a few M42's. I know they were building an "S42" a while ago, but haven't heard much recently.

Jim

[greenamex2]

Jumping from standard injection (single throttle body) to carbs on a Toyota 4a-ge will get you from 124BHP to around 135-140BHP.

Going from carbs to throttle bodies on a road modified 4a-ge power will go from around 160BHP to 165-170BHP but with much better driveability (especially round the mid range).

Those are my actual experiences on a big port 4a-ge.

When you get above the 170BHP mark then injection becomes ever more important in order to maintain any for of driveability. At the top race engine spec you will be losing 10-15BHP plus on carbs (48mm DCOE's) over injection (48mm bodies) plus the carbed engine will be pretty undriveable.

However it is worth pointing out then the big port 4a-ge has some design 'features' that tend to over exagerate figures compared to a better designed engine (such as the later small port 4a-ge).

[tzei]

Cams for M42: http://www.catcams.be/800x600/mainframe.htm?english
Schrick has only one pair:
http://www.avl-schrick.com/dat/MK/Sc...202009%20E.pdf

Prop knew all ready but there you go anyways.

[Notso Swift]

I thought I posted on this, must have lost it when I submitted

Anyway
If the system is optimised there will be Extremely minor difference in power between Carbs and Injections (either single or multi TB) How can there be?
IF there is the right amount of fuel and the right amount of air the power will be the same. (minor changes will be from things like drag of the air, but we are talking 1% or less)
Now, that is a VERY big "if" because it is rare that any system is optimised, especially in our world of limited budgets.

(BTW my first response was before yours Denis, but I changed it a little from what has been said since)

A Carb getting more power than a Single TB means that you are not getting enough air through the single. Like wise with Carbs vs ITB's (remembing that Dennis' venturi may be 44mm in a 48 DCOE, and ITB's will be 48 straight through, so even though the look the same they do not peak flow the same)
But these are only Peak power or power at one point.
Where injection comes into its own is being able vary the fuel dose more precisely (and ignition in most systems) and therefore getting better Average power, exactly as Denis has said, I remember when I started racing a Mini with a wild cam on a 45 and we would set the idle over 2000, just to get it around the pits, with injection in a DOHC 16v with bigger cams I could would idle at 1200, coming on power was even more pronounced. In theory a Carb could be set up as perfectly as a good injection tune, however, the practice (especially now there are less good tuners around) this just doesn't happen, there are too many compromises

Separate to all that is response, which will always favour shorter induction lengths (like Multi carbs and ITB's)

[Notso Swift]

I thought I posted on this, must have lost it when I submitted

Anyway
If the system is optimised there will be Extremely minor difference in power between Carbs and Injections (either single or multi TB) How can there be?
IF there is the right amount of fuel and the right amount of air the power will be the same. (minor changes will be from things like drag of the air, but we are talking 1% or less)
Now, that is a VERY big "if" because it is rare that any system is optimised, especially in our world of limited budgets.

A Carb getting more power than a Single TB means that you are not getting enough air through the single. Like wise with Carbs vs ITB's (remembing that Dennis' venturi may be 44mm in a 48 DCOE, and ITB's will be 48 straight through, so even though the look the same they do not peak flow the same)
But these are only Peak power or power at one point.
Where injection comes into its own is being able vary the fuel dose more precisely (and ignition in most systems) and therefore getting better Average power, exactly as Denis has said, I remember when I started racing a Mini with a wild cam on a 45 and we would set the idle over 2000, just to get it around the pits, with injection in a DOHC 16v with bigger cams I could would idle at 1200, coming on power was even more pronounced. In theory a Carb could be set up as perfectly as a good injection tune, however, the practice (especially now there are less good tuners around) this just doesn't happen, there are too many compromises

Separate to all that is response, which will always favour shorter induction lengths (like Multi carbs and ITB's)

(BTW my first response was before yours Denis, but I changed it a little from what has been said since)

[greenamex2]

Originally Posted by Notso Swift

A Carb getting more power than a Single TB means that you are not getting enough air through the single. Like wise with Carbs vs ITB's (remembing that Dennis' venturi may be 44mm in a 48 DCOE, and ITB's will be 48 straight through, so even though the look the same they do not peak flow the same)
But these are only Peak power or power at one point.
Where injection comes into its own is being able vary the fuel dose more precisely (and ignition in most systems) and therefore getting better Average power, exactly as Denis has said, I remember when I started racing a Mini with a wild cam on a 45 and we would set the idle over 2000, just to get it around the pits, with injection in a DOHC 16v with bigger cams I could would idle at 1200, coming on power was even more pronounced. In theory a Carb could be set up as perfectly as a good injection tune, however, the practice (especially now there are less good tuners around) this just doesn't happen, there are too many compromises

Separate to all that is response, which will always favour shorter induction lengths (like Multi carbs and ITB's)

(BTW my first response was before yours Denis, but I changed it a little from what has been said since)

In my case, my engine needs a 48mm 'hole' and I can't get that with carbs.
And even if I could the gas speed at low RPM is so slow that a carb hasn't got a hope of working properly. With injection you can play with injector positioning/numbers of injectors etc that you just can't do with carbs.
And then you factor in other things like firing angle, wall wetting etc that aren't possible with carbs either.