hillbillyracer
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| posted on 23/4/08 at 10:20 PM |
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Slightly OT. Are my throttle bodies too big?
One day I'll actually start building a Locost, but for the moment among other things I'm playing about with a Mini. I'm going to
have a go at putting Megasquirt on it & bought a pair of throttle bodies from a Ducati 748 on Ebay. The listing did'nt give the bore
diameter but I looked at the spec for the bike & at 90ishBHP they would be a fair match for a 1275 Mini. However, they're 45mm! Now a Mini
making use of a 45 weber I would expect to be making 120bhp or more & even then it'd have chokes somewhat less than 45mm. I dont know the
spec of the engine but it has a HIF44 SU at the moment I'd be suprised if I make more than 90-95BHP but head mods etc may see more later.
Now I understand that I dont need air speed to draw the fuel but how much will them being oversized matter? Will it just make it a bit jumpy at light
throttle opening because only a small amount of throttle will let a lot of air through (that could be sorted with some well thought out linkage) or
will it have more effect on things?
Would I be better using 2 bodies from a 4 cyl bike with bores of about 40mm?
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phoenix70
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| posted on 24/4/08 at 12:44 AM |
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My thoughts, and I stand to be corrected. If the TB's are too big then the MS is going to have to inject large amounts of fuel to keep the
mixture right with the amount of air that is going in, so you may find that they are too big.
Maybe wroing but I'm sure someone else will correct me, you may get away with choking them a little to reduce the air flow.
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Paul TigerB6
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| posted on 24/4/08 at 07:06 AM |
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Seems quite a strange combination to me and can only imagine its quite a complex way to fit throttle bodies - taking a pair of them from a V-twin
engine. Not sure how you are planning to fit these via a manifold but it seems like you must be using each TB to run a pair of cylinders somehow??
Simpler answer surely (???) would be to get a set of 4 TB's from a small 4 cylinder bike and then a pretty much standard type manifold from the
likes of Bogg Bros. It might be worth talking to them anyway as they may have done the conversion before so can advise on suitable throttle bodies but
i cant think of any from smaller than a 600cc bike - R6 / CBR600 / ZX6R etc. Think this conversion would be much simpler with bike carbs but sure it
must be do-able. Have a look at Bogg's though - linky.....
HERE for Bogg Brothers Mini manifold
[Edited on 24/4/08 by Paul TigerB6]
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lsdweb
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| posted on 24/4/08 at 07:29 AM |
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hillbillyracer - there's a fair bit of info out there about injection on a five port head - it's not easy because you have a lot of fuel
'robbing' going on - have a search - Dave Walker's written a lot on this I think - in fact, it may be in his book - I'll have
a look later.
Paul - the Mini has a 5 port head - with only two inlet ports, so each cylinder shares its fuel / air with another - 1 & 2 and 3 & 4, so, in
theory, two throttle bodies would work. But, because of the sharing set-up, one of the cylinders robs the incoming air/fuel. One of the most popular
Minin mods is a single twin-choke carb (Webber etc) with each choke feding two cylinders.
Rover had huge difficulties in injecting the Mini and they went for single point initially.
Regards
Wyn
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Paul TigerB6
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| posted on 24/4/08 at 08:00 AM |
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Gotcha - well you learn something new every day. Cheers Wyn.
Paul
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Duncan_P
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| posted on 24/4/08 at 09:04 AM |
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There is also a theoretical chance of 'back flow' (where the air flow reverses at low rpm/wide throttle opening) with oversized throttle
bodies. But this is just a theoretical chance, and I believe a lot less likely with a standardish engine so i wouldn't worry about it.
I think your main concern is as you say a drivability one of a little throttle pedal movement causing a comparatively large throttle opening.
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martyn_16v
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| posted on 24/4/08 at 10:36 AM |
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quote:
Originally posted by phoenix70
My thoughts, and I stand to be corrected. If the TB's are too big then the MS is going to have to inject large amounts of fuel to keep the
mixture right with the amount of air that is going in, so you may find that they are too big.
The engine will only 'suck' in as much air as it needs at the time, so having bigger throttle bodies won't affect how much goes in
(until the old ones became a restriction at least). The trouble is, a small volume of air being pulled through a big hole doesn't need to move
that fast. At low demands, the air can be moving so slowly through the throttles that the fuel being sprayed into the airflow doesn't get
carried into the combustion chamber effectively, it just falls back out of suspension and onto the port walls. This makes it a pig to idle and drive
gently.
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BenB
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| posted on 24/4/08 at 01:45 PM |
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Big throttle bodies are less of a problem than carbs with too large chokes.
On throttle bodies you're not relying on the venturi effect to suck the fuel into the airflow. You do, however, need fast enough air flow to
keep the fuel in the air stream. If you have ridiculously large throttle bodies though as already mentioned the flow can be that slow that the fuel
can "fall" out of the air stream and pool in the inlet manifold making the engine run like pants.....
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paulf
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| posted on 24/4/08 at 04:22 PM |
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Have you reserched into fitting twin throttle bodys to an A series? As mentioned the biggest problem is making the cylinders share the fuel
evenly.There is some info on the megasquirt forums but dont think it has been overcome fully yet, you would really need sequential injection to make
it work properly but there are ways around it .
The throttle bodys may cause a driveabiity problem as a small opening may supply enough air for full throttle running so throttle resolution will be
poor,bike TBs also have a different closed angle to most cars and this will accentuate the problem.
Paul
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rusty nuts
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| posted on 24/4/08 at 06:16 PM |
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Don't know if it will help but when deciding what TBs to use on my car I looked at a guide for tuning webers possibly from Dave Andrews? it gave
the approximate choke size for given engine size and power . I then found a set with the bore I wanted .
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lsdweb
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| posted on 24/4/08 at 06:50 PM |
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Here's the link to Dave Walker's article - on a midget but still a Siamese A Series!
Link
I'd be wary of trying this!
Regards
Wyn
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hillbillyracer
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| posted on 24/4/08 at 07:05 PM |
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Thanks for the replies fellas. At 45mm just about the same as the HIF44 it's currently got on & they're supposedly good for 100bhp so
using one may be a fair possibilty which would as you say get around the charge robbing issue. Mabye I'd be better off using seperate injectors
though? The way I was going to try & reduce the charge robbing was to make the manifolds as long as possible (the clocks are on the steering
column so theres plenty room for a big air box). I suppose I could use one & then change to two if I tune it further, but a wilder cam could make
charge robbing worse!
I'm only just starting out on this & was only really having a look about to see what was available when I saw these & thought,
I'll just have a go at them, they seem cheap & jumped the gun really. Mabye I should buy another set to make four & put them on the
Locost! Of course I'd need 200bhp or more to be making use of them & that'd just be silly, would'nt it?
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hillbillyracer
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| posted on 24/4/08 at 07:21 PM |
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Cheers for the link LSDweb, Dave Walker usually talks sense & I'm better off for having read that. I did know about the charge robbing but
had'nt reaslised it was such a problem. The way I read that is that using two seperate bodies is only practicle on lower tuned engines which
dont benifit from them & higher tuned engines that would make good use of them cant due to the charge robbing!
It looks like the way to do it will be to use one of these bodies with the injectors close to the butterfly, mabye even on the manifold the car has
on now. I doubt whether the one injector will flow enough, mabye I'll make up a sandwich section & mount the other or both injectors on
that.
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MikeRJ
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| posted on 25/4/08 at 04:44 PM |
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The Siamese Port Injection Problem. This explains the issue in great detail.
There are (or were) a couple of web pages showing DIY twin throttle body conversions on an A Series and both were abandoned due to charge robbing.
However a solution now exists in the form of the Canems ECU which has been developed specifically for
siamesed port fuel injection. It's £395 for a ready built ECU which is somewhat more than a Megasquirt, but less than pretty much any other
commercialy available fuel and spark ECU.
That said, looking at the unit and the screenshots of the software I suspect it may well be megasquirt based, which is no bad thing I suppose.
EDIT: Megasquirt Development for siamesed port operation
[Edited on 25/4/08 by MikeRJ]
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hillbillyracer
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| posted on 25/4/08 at 07:53 PM |
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Thanks Mike, there are folk out there set on making it work! I think for me with my experience I may just try the single throttle body for now though.
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