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SU Carb Interchange

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lesingepsycho

Jedi Warrior
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Ok, so I'm sittin here looking at the new Moss catalog and on one of the very first pages they show an air horn for the "Supercharger HIF44". They say it works with the "first generation M45 and second generation MP45 Sprite/Midget supercharger kits". Then on the supercharger pages of the catalog it says the kit comes with an HIF4. I was wondering about the interchange (if any) between these two carburetors HIF4-HIF44. Specifically I'm wondering about the dashpots themselves and <span style="font-style: italic">more specifically </span> I'm wondering if my HIF44 fuel injection adapter would work on somebody's supercharger setup.

JACK
 
The HIF44 is going to be larger than the HIF4. The earlier one will be the HIF4 which follows the size/numbering scheme of the earlier HS series carbs. An HIF4 will therefore have a nominal throat diameter close to 1.5" The HIF44 follows the later metric numbering scheme. Therefore, the HIF44 has a 44mm throat, just a bit under 1.75"

The terminology used for SU carb parts varies from person to person and book to book. By dashpot are you referring to the cast vacuum chamber or the "screw in" damper?
 
Thanks for the response Doug!
I knew that the throttle bore diameters were different between the HIF4, HIF6, HIF38, HIF44, etc. but mass production often means that many individual parts get reused even between slightly different models. That's what I'm hoping for anyways.
By dashpot I'm talking about the cast vacuum chamber that attaches to the top of the HIF44 body with three screws. This is the piece that the screw in damper rod threads into and which houses the piston that carries the metering needle.
On my fuel injection setup, you remove the three screws and take away everything above the carb body itself; dashpot, piston, damper. Then the aluminum adapter bolts straight onto the carb body and holds the injector at a slight angle to shoot down into the carb body. If that adapter will fit other similarly designed models of carburetor then I can offer it for sale to more people, maybe even including the supercharger crowd.

Thanks again,
JACK
 
Jack, I am getting ready to start an EFI project on my Sprite engined Westfield and I have a few questions. Are you using sequential injection? How close are your AFR's between the front or rear cylinders and the centers? What size injector are you running and are you having any trouble getting a smooth idle and a reasonable duty cycle?

I have an NA 1380 and so far it looks like I will be using a set of RC51 TB's since they have two injectors each and there is a good selection of injectors. I am currently running MegaJolt and am building my o2 controllers now while I acquire the supplies to build the manifold. I have to taper the bore from 2 inches down to 1.5 inches at the head and a friend suggested I buy a couple of alloy baseball bats with the right taper to use for the tubes. What do you think? I am having trouble finding tapered tube.

Sorry for all the questions........
 
Why not use some thick wall tubing/ chuck it in the lathe and use a boring bar to create your own taper. This way you can control the length and taper angle/curve.
 
That sounds like a lot of work to use those TB's when there are so many other options available. Also having that much throttle is going to make low-end and off-idle tuning rather difficult. Injectors come in basically 3 sizes, so whatever set up you use, there is a world of injectors available to you. My 1500 uses one injector that fires four times per cycle and it works great. So there is really no need for 4 injectors on an a-series.

Since you already have crank triggered spark, you are half-way to a pretty sweet sequential injection set-up. You can use your old dizzy as your camshaft timing pick up. That combined with MS3 and you could build yourself a sweet runner/plenum manifold which will flow much better than those motorcycle TB's.
 
Trevor Jessie said:
Why not use some thick wall tubing/ chuck it in the lathe and use a boring bar to create your own taper. This way you can control the length and taper angle/curve.
Click to expand...
Trevor Jessie said:
Why not use some thick wall tubing/ chuck it in the lathe and use a boring bar to create your own taper. This way you can control the length and taper angle/curve.
Click to expand...

I don't have a lathe.
 
Local machinIst?
 
Westfield_XI said:
I have a few questions. Are you using sequential injection?
Click to expand...

No, I'm using a single injector throttle body system. So as far as the internal combustion engine is concerned, it knows no difference between a carb and throttle body so long as the AFR is right when it gets to the cylinder. Sequential systems have been discussed many times and I can see no easy way to use sequential injection on a siamese ported motor without facing charge robbing. 1-3-4-2 is problematic with siamesed ports, plain and simple. The throttle body however is just like a precisely controlled wet-manifold single carb. The air/fuel is mixed WELL ahead of the intake ports and so is going to be shared just like with a carb system. Mine works just like Moriss'; One injector fires 4 times per revolution. Even at 12.5 psi, 6500 RPM the injector was keeping up and never leaned out.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]How close are your AFR's between the front or rear cylinders and the centers? [/QUOTE]

I had no particular way to monitor indiviual cylinder AFRs but I was happy with the overall AFR and again, there should be no difference as compared to a similar carb setup.
The air/fuel is premixed and "omni-present" in the runners so to speak.

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]What size injector are you running and are you having any trouble getting a smooth idle and a reasonable duty cycle?[/QUOTE]

The injector is from an 88-89 GMC pickup 2.8L TBI system. It's rated at 33lb/hr and I don't have the duty cycle in front of me but later on I can look at my datalogs and get back to you about the duty cycles I was seeing. I'm already behind for an appointment today!

<div class="ubbcode-block"><div class="ubbcode-header">Quote:] I have to taper the bore from 2 inches down to 1.5 inches at the head and a friend suggested I buy a couple of alloy baseball bats with the right taper to use for the tubes. What do you think? I am having trouble finding tapered tube.[/QUOTE]

I LOVE IT!!! I love rat-rod, backyard, shade-tree engineering. I say give it a shot!

Good luck! I love the questions. Keep'em comin!

JACK
 
You should find this setup works very well, I have a set on another car and the performance is unprecedented!!
 
BlueMax said:
You should find this setup works very well, I have a set on another car and the performance is unprecedented!!
Click to expand...

What is it?
 
What your looking at is a TWM throttle body for 1 ¾ setup. With a 1380 you’ll perform just fine, you shouldn’t have any issues. You’ll have to purchase your intake manifold from Mini Spares in the UK and you’ll have to provide your own Pectel ECU and O2 sensor. Wire it up, set your fuel pressure and FA ratio accordingly threw the programmable Pectel ECU.
 
That looks great! However I suspect it is beyond my budget. Anyway, I like to tinker and enjoy building as much of my car myself as possible. I built my MJ unit from the DIY kit and the same with the MS2 I will use on this project. I found a place that sells compatible o2 controllers as a DIY kit and I plan on building a pair of them, see 14point7.com for more info. The Honda TB's can be had for 150$ or so for the pair, complete, and look to be able to fit my packaging needs. I have been running a pair of heavily modded SUs from APT which worked fine, but kept boiling over when idling due to drivers side footwell interference with the rear float chamber requiring such a short intake manifold that I couldn't fit a heat shield. I tried ceramic coating the header, wrapping the header in sheets of aluminized heat barrier from Pegasus Racing, but it still vaporizes the fuel in traffic. The simple answer is to just put a Weber on it, but if I wanted simple I never would have built my own kit! Everything is complicated by having such a low hood line that I had to shorten the oil filler spigot on the pressed steel valve cover to stop it rubbing.

Am I right in thinking that you guys with the single TB are supercharged? If you are, that would really mix the charge and be just like a carb. With a NA engine an injector will open and then close and send a slug of vaporized fuel down the intake as opposed to the continuous stream from a carb jet. If that slug of fuel is not timed to arrive when the correct valve is open in a Siamese port head than some fuel will be drawn into the wrong cylinder due to valve overlap. To get around this the injector pulse is made very short to arrive and be completed before the other valve opens requiring a fairly high flow injector to keep a reasonable duty cycle. I am just beginning to learn the quirks of injecting this engine, but these folks seem to have a handle on it AFAIK: turbominis.co.UK there is a very clear explanation of the problems with the A-series in the EFI section under the Build Manual sticky. Very interesting read.

But what the heck am I saying, you guys are already up and running and I am still soldering PCBs and welding exhaust bungs....
 
Yes, TWM throttle bodies can stretch the pocket book quite a bit; however your time and speculation can stretch a pocket book as well. What you’ve gained is that you’ve learned that this road doesn’t work. I’ve found that when you go down a proven road you certainly know where you’ll end up. Perhaps you have deeper pockets to develop prototypes, plus the tooling for development, and a complement of C&C lathes and milling machines. Then a flow bench to see if all your design work pays off, I don’t know I’m just saying? Here' a photo of the setup on my other car!
 
If you are injecting close to the port, the siamese head can be very problematic. However, if you are running a wet manifold it is much more forgiving. But running a wet FI manifold will leave you with much less power than some well tuned SUs. Running a wet manifold with forced induction will put you in a power range well beyond the SU's. Ideally, port injection with forced induction, but it is very difficult to sort out (both in terms of the injectors and the runner design.

With a siamese head I'm not sure that you are going to gain any power with a homebrew port injection that is naturally aspirated. But I agree, that it is fun to experiment with.
 
BlueMax said:
Yes, TWM throttle bodies can stretch the pocket book quite a bit; however your time and speculation can stretch a pocket book as well. What you’ve gained is that you’ve learned that this road doesn’t work. I’ve found that when you go down a proven road you certainly know where you’ll end up. Perhaps you have deeper pockets to develop prototypes, plus the tooling for development, and a complement of C&C lathes and milling machines. Then a flow bench to see if all your design work pays off, I don’t know I’m just saying? Here' a photo of the setup on my other car!
Click to expand...

I guess it all depends what you are going for: I have a low compression big bore engine that runs on cheap 87 octane and makes just under 100 HP on an engine dyno. I don't think I would need to go to the extent you are speaking of to equal that with a home built EFI system. I am not trying to beat the SUs, just get close to them and be able to say that I built it myself.
 
I had some bats for this purpose a while back. Someone told me that type of al doesn't like to be welded. I don't know first hand. P/U a bat from Goodwill and see if it'll take a weld first.
 
<div class="ubbcode-block"><div class="ubbcode-header">Quote:]Sequential systems have been discussed many times and I can see no easy way to use sequential injection on a siamese ported motor without facing charge robbing.[/QUOTE]

I think you maybe confusing sequential with multi-point. A sequential injection system is timed so that the injector fires as the valve is opening.

Also, charge robbing is always a factor on siamese ported motors regardless of fuel delivery set up. The best you can hope for is to keep the charge robbing at a functional minimum.
 
Interesting. That looks almost exactly like the setup I have for sale on ebay right now except that 959 British Pounds is equal to $1559.53 so at $750 I suppose my kit is reasonably priced!

I checked my datalogs and it looks like the DutyCycle is usually around 10% at idle, cruises around 25%-50% depending on speed and TPS, most hard accelerations are in the 70% range and the max my injetor ever saw was on a first gear, off the line, run up to 6000RPM or so with 11 psi and it hit 92%DutyCycle.

JACK
 
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