Charge properties, supercharged engine???
Discussion
I was looking at the design of the good old A series with it's twin intake ports. For those who aren't familiar, these twin ports are often fed by a single carb. The SU produces quite coarse droplets of fuel which tend to centrifuge out a high revs and cause the outer cylinders to run rich/inners to run lean. This is all well and good and a remedy is to fit either twin SU's or a Weber (split). My question is, would this effect be worse under boost conditions or not.
People (including myself) running FI minis generally run an MG Metro Turbo carb. I don't know of anyone who blows through a set of twin carbs so I would assume that it isn't necessary. However, not wanting to take anyones word for it, I was just curious about what happens to the more fuel in a higher density medium, I assume that it is more likely to aggregate/centrifuge but I could be wrong.
Anyone any ideas?
I figure, as I am running a hugely inefficient supercharger, having the inner cylinders slightly richer relatively will allow me to run more advance, hence get more power.
:) Stu
People (including myself) running FI minis generally run an MG Metro Turbo carb. I don't know of anyone who blows through a set of twin carbs so I would assume that it isn't necessary. However, not wanting to take anyones word for it, I was just curious about what happens to the more fuel in a higher density medium, I assume that it is more likely to aggregate/centrifuge but I could be wrong.
Anyone any ideas?
I figure, as I am running a hugely inefficient supercharger, having the inner cylinders slightly richer relatively will allow me to run more advance, hence get more power.
:) Stu
deltafox said:
Question.
Why do you wish to run a carb?
Surely the "problem" of fuel drop-out and poor distribution could easily be eliminated by running an engine management system?
And the engine would be better safeguarded under boost conditions.
Just a thought...
Its an A Series.... those siamesed inlets are a real pain when it comes to setting up any EFI. IIRC the Rover injected 1.3 Coopers had some very complicated timing gear running in the MEMS unit to work out when to inject.
.....and the SBC guys over in america run loads of power, offten nearly as much as the dierect port injection guys even when under FI!
if twin cab/s get rid of the problem on N/A units then its def worth a try on FI. as for the fueling, dont forget that running rich can be just as detrimental to power as running lean, its just rich is safer than lean. if you realy want to cool the intake track then try allky injection (will help with fueling to). the US guys dont seem to have a bad thing to say about it!
Chris.
if twin cab/s get rid of the problem on N/A units then its def worth a try on FI. as for the fueling, dont forget that running rich can be just as detrimental to power as running lean, its just rich is safer than lean. if you realy want to cool the intake track then try allky injection (will help with fueling to). the US guys dont seem to have a bad thing to say about it!
Chris.
350matt said:
how about putting the carb in front of the blower?
That way you'll get a very wellk mixed air/ fuel charge, also seal the blower at bit better so increasing efficiency and drop the air temp a bit as the fuel evaportaing off the blower will pull heat out
Matt
You can't use an intercooler with a suck through set up AFAIK?
Id never actually heard of the SU producing large droplets of fuel. If anything I was under the impression the SU was a damn good carburettor, especially in that area.
And certainly in a blow through turbo'd application, a single 1 3/4" can provide for over 200bhp, so why bother with using 2 carbs ?
And certainly in a blow through turbo'd application, a single 1 3/4" can provide for over 200bhp, so why bother with using 2 carbs ?
you're right, the SU produces fine fuel droplets hence it's great fuel economy.
the distribution problem comes from the fuel droplet being significantly heavier than the air.
the first cylinder of each siamese pair gets more air than fuel because as the inlet opens the fuel lags behind the air as the air gets moving in the port, then the second cylinder of the pair gets it's turn to suck, the fuel has already been accelerated by the action of the first cylinder in the pair so the second cylinder gets a proper fuel-air. Or at least it would be, but because the first cylinder runs weak you have to richen the mixture, so the second cylinder runs rich if the first gets the correct mixture.
if you see what i mean
the distribution problem comes from the fuel droplet being significantly heavier than the air.
the first cylinder of each siamese pair gets more air than fuel because as the inlet opens the fuel lags behind the air as the air gets moving in the port, then the second cylinder of the pair gets it's turn to suck, the fuel has already been accelerated by the action of the first cylinder in the pair so the second cylinder gets a proper fuel-air. Or at least it would be, but because the first cylinder runs weak you have to richen the mixture, so the second cylinder runs rich if the first gets the correct mixture.
if you see what i mean
stevieturbo said:
Id never actually heard of the SU producing large droplets of fuel. If anything I was under the impression the SU was a damn good carburettor, especially in that area.
And certainly in a blow through turbo'd application, a single 1 3/4" can provide for over 200bhp, so why bother with using 2 carbs ?
Which was my point, a turbo guy can just turn up the boost, I have a crappy M45 blower and the biggest intercooler I can get is pretty small (considering the blower is around the front of the engine) The thought of twins were that it allowed better fuel distribution between pairs of cylinders and I wondered if this effect was amplified by a denser charge. IIRC, the old lucas fuel injection (8 port) was crap as the mist was too fine, this didn't allow charge cooling to happen and advance was limited by this to a degree that Webers were better. The SU also produces coarser droplets than a Weber but not as coarse as a Reece Fish or an Amal.
You know me Stevie, I don't like taking anyones word for it!
I like the idea of getting the absolute maximum out of my engine. With 60% adiabatic efficiency max at 0.5 bar, it aint good and inlet temperatures can get ridiculous. Since my this engine has exploded, I'd like the next one to be on the verge and produce more welly. Every trick in the book needed....
easiest way around a series distribution problems is to make a 7 port head by dividing the siamesed port and running a 4 branch inlet manifold with one injector per port. if you go for management you can program water injection if inlet temps are critical.
if you're hell-bent on using carbs then there will still be mixture quality variations unless you run one carb per cyl on the 7 port head or run twin carbs a fair distance from the head face but then you get packaging problems, low air speed fuel drop out problems and might not totally cure the distribution problem anyway.
7 port / management is the best way imo.
if you're convinced the weber has smaller droplets (i'm sure it hasn't though) why not use one of those?
if you're hell-bent on using carbs then there will still be mixture quality variations unless you run one carb per cyl on the 7 port head or run twin carbs a fair distance from the head face but then you get packaging problems, low air speed fuel drop out problems and might not totally cure the distribution problem anyway.
7 port / management is the best way imo.
if you're convinced the weber has smaller droplets (i'm sure it hasn't though) why not use one of those?
Steve, Bini blowers are £100 on ebay, which is virtually free as blowers go. I've got a cash intensive lifestyle and too many projects, hence I'm on a bit of a budget. Also, I refuse to spend a fortune on a Mini as they are just old saloon cars.
The 7 port head from a conversion of a 5 port seriously restricts flow. I can't remember how much but I have some flowbench graphs around. I'm fully with you with fuel injection but port dividing is not good.
We shall see about the twin carbs, I'm convinced it's a brilliant idea, not sure to what degree though. I suppose I should just take off the silencer, lower the suspension onto the bumpstops and be happy with what I've got. (with a disconnected speedo)
The 7 port head from a conversion of a 5 port seriously restricts flow. I can't remember how much but I have some flowbench graphs around. I'm fully with you with fuel injection but port dividing is not good.
We shall see about the twin carbs, I'm convinced it's a brilliant idea, not sure to what degree though. I suppose I should just take off the silencer, lower the suspension onto the bumpstops and be happy with what I've got. (with a disconnected speedo)
joospeed said:
and are you really sure the mixture distribution problem is such a big problem anyway? I know people make great hp on the std turbo sealed hif.
you really do seem to be making a mountain out of a molehill here
I've poked around with some figures and reckon that the disparity between outer pairs is more significant under boost, rectifying the situation SHOULD free up more power, perhaps enough to merit doing it. My curiousity was to see if anyone agreed with me and could explain themselves.
Blowing through a side-draught brings it's own problems with sealing the carb, I think this is significant and the old turbo technics 'carb in a box' idea takes up too much space and is pretty unsatisfactory.
I just looked at the Turbo inlet manifold, shaped to keep everything in suspension and saw a massive pressure drop, whilst everyone with turbo engines is jumping around doing angled valve seats, they are ignoring this crapness. Sure, if you can just increase the pressure diff, it will overcome it......anyway.
I suppose I'll have to do a back to back test on this one.
The denser mixture will provide the fuel droplets with more buoyancy, so making them less prone to being centrifuged out.
I reckon you'd do better to go suck-through without the intercooler. It's easy to work out how much charge cooling you will get from the latent heat of vaporisation of the fuel; you say the intercooler is small and crap, so you may not lose too much by not having it. You'll get rid of a large volume of pipework between the blower and the engine, which can only be good, and you won't have problems with drop-out.
I reckon you'd do better to go suck-through without the intercooler. It's easy to work out how much charge cooling you will get from the latent heat of vaporisation of the fuel; you say the intercooler is small and crap, so you may not lose too much by not having it. You'll get rid of a large volume of pipework between the blower and the engine, which can only be good, and you won't have problems with drop-out.
Turbo inlet manifold is shaped the way it is, because it needs to clear the exhaust manifold.
Certainly removing restrictions will be of benefit, but when you can get 200bhp easily enough from a turbocharged A-series, do you need to be that fussy, or go to the expense or hassle changing or making new parts ?
I dont honestle beleive mixture distribution is an issue.
Have you actually tried monitoring all cylinders via wideband individually ? Can you do this, or even take control of mixtures so that you could adjust them anyway ?
As for sealing a sidedraught. They seal well enough as they are. A friend used them a few times on turbocharged Pintos years ago, and never had any cause for concern. Just make sure it has either plastic floats, or get the brass ones filled with foam so they dont crush under boost.
Certainly removing restrictions will be of benefit, but when you can get 200bhp easily enough from a turbocharged A-series, do you need to be that fussy, or go to the expense or hassle changing or making new parts ?
I dont honestle beleive mixture distribution is an issue.
Have you actually tried monitoring all cylinders via wideband individually ? Can you do this, or even take control of mixtures so that you could adjust them anyway ?
As for sealing a sidedraught. They seal well enough as they are. A friend used them a few times on turbocharged Pintos years ago, and never had any cause for concern. Just make sure it has either plastic floats, or get the brass ones filled with foam so they dont crush under boost.
That's interesting about the sidedraughts sealing, I thought you had to use an Esprit Turbo carb (dellorto). That's the word on the street anyway
I've liked the idea of superchargers ever since I talked to a BMC development chappie about one with a chain driven aircraft cabin blower in the passenger footwell. Someone also had a 115hp 850 mini with a shorrocks blower in that dept. Engine lasted about a month so they had 2 on rotation
I'm heavily into my theory and reckon that there's definately truth in the leaning of the centre cylinders. I wouldn't bother rigging up sensors in the exhaust, I think I would just assume that my maths was right and try it and see what happened. Getting a calibrated sensor or running some sort of gas analyser would probably be ambiguous as well as total overkill.
Probably sensible to just go with a turbo but I'm testing my own credibilty out here, seeing whether I am actually all bull or not.
Cheers for the info.
I've liked the idea of superchargers ever since I talked to a BMC development chappie about one with a chain driven aircraft cabin blower in the passenger footwell. Someone also had a 115hp 850 mini with a shorrocks blower in that dept. Engine lasted about a month so they had 2 on rotation
I'm heavily into my theory and reckon that there's definately truth in the leaning of the centre cylinders. I wouldn't bother rigging up sensors in the exhaust, I think I would just assume that my maths was right and try it and see what happened. Getting a calibrated sensor or running some sort of gas analyser would probably be ambiguous as well as total overkill.
Probably sensible to just go with a turbo but I'm testing my own credibilty out here, seeing whether I am actually all bull or not.
Cheers for the info.
I like the idea of upstream injection but there are some disadvantages:
The evaporative cooling is worth about 25 deg centrigrade drop in the charge temperature so you can get useful density increases, but remember the more of this cooling you 'spend' in the intake manifold the less you have left to cool inside the cylinder.
The more volume of intake manifold you have full of charge, the more spectacular the back fires will be. For a pull-through system it might be prudent to have a blow-off panel to avoid bursting anything.
The evaporative cooling is worth about 25 deg centrigrade drop in the charge temperature so you can get useful density increases, but remember the more of this cooling you 'spend' in the intake manifold the less you have left to cool inside the cylinder.
The more volume of intake manifold you have full of charge, the more spectacular the back fires will be. For a pull-through system it might be prudent to have a blow-off panel to avoid bursting anything.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff