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MG MGB Technical - SU conversion - I'm confused HS or HIF

I'm looking into an eventual SU conversion to my '79 B. I've been reading the archives. Some folks like the 'HS' and some like the 'HIF'. There's some issues with air cleaner clearance, too.

So, could someone give the skinny on this. Is the install easier on one or the other? Do I need a particular year intake to handle the booster brake hose?

I'm looking for the easier route here, if there is one.

Thanks in adavnace for your input.
Dave
David Steverson

David-
The remote floatbowl of the rear SU HS4 carburetor will give clearance problems with your Original Equipment servo-boosted master cylinder. The SU HIF4 will not have this problem. As for the intake manifold fitting for the distributor's vacuum advance hose, any of the North American Market intake manifolds either have it or can be readily modified to accept it. When seeking improvements in airflow capacity, things become considerably more complicated when trying to fit aircleaners on a Rubber Bumper car that has been modified to use dual carburetors. Unfortunately, the way the servo-boosted master cylinder projects from the bulkhead forces most conventional owners into the use of conical airfilters when installing dual carburetors. The problem with the conical airfilters is that their shallowness creates induction pulse problems, their small internal volume that will not allow the fitting of a set of stub stacks, and their small surface area. The K&N airfilters all use the same filtering medium, so the smaller the surface area of the filter, the less the airflow potential will be. Conversely, the bigger the surface area, the greater the airflow potential. This is why the 6" X 3 1/4" deep filters are preferred by those who go for serious power increases with a B Series engine. Induction pulse problems aside, the airflow capacity of the little conical or pancake filters is more appropriate to a mildly power-enhanced 1275cc A Series engine, such as is fitted to the MG midget or the Austin Healey Sprite. In addition to this problem, the remote floatbowls of the SU HS4 carburetor will interfere with the master cylinder, thus such a conversion requires the use of a set of SU HIF4 carburetors.
Retrofitting the earlier non-boosted master cylinder is the common solution, but this is not a bolt on affair as its mounting flange is turned 90° so the mounting holes of the pedal box won't line up, and the appropriate earlier pedal box assembly is radically different, even having a different mounting hole pattern at its base that requires drilling a new pattern in the body of the car. This is just one of the reasons that it's unusual to see a Rubber Bumper model with an uprated B Series engine: It's much more work. When somebody wants to go for really dramatic power increases, he swiftly comes to think that he'll need to retrofit the earlier brake master cylinder and pedal box assembly so that he can mount airfilters that have a decent airflow capacity onto the carburetors like the Chrome Bumper model owners do. "After all," he reasons, "it's not that difficult, really, it just requires some persistence and time, plus another master cylinder and the earlier pedal box assembly. If my boosting servo and master cylinder are in good shape, then I can always sell them as a unit to help cover the cost of the earlier master cylinder and pedal box assembly because the servo is getting harder and harder to find." And, to the conventional, orthodox thinker, this reasoning holds true. However, read on-
Fabrication of a plenum chamber to go on the carburetors and running a large diameter breather duct hose to a remote aircleaner housing would enable the retention of the existing boosted brake system. From the aircleaner housing the intake hose can be run to the air passages neatly provided beneath the bumper in the vented front valance of the 1972 through 1974 1/2 models of the chrome bumper cars. You'll need to do some scavenging in the junkyards to find the right box (more work) and then figure out a mounting system for it (still more work), but the larger, more commodious engine compartment of the later Rubber Bumper models should make it a relatively easy task. To equal the airflow capacity of a pair of 6" diameter 3 1/4" deep round airfilters you'll need an airfilter housing box with a filter that has an area of about 122 square inches (11" X 11").
I would suggest a Maniflow fabricated steel Hy-Flow intake manifold (British Automotive Part# SUB4-2; Maniflow Part# L137), the improvements become even more impressive. It has high flow potential coupled with good port velocity, enabling it to take advantage of the inertial effects of the fuel/air charge to better fill the cylinders at high engine speeds with the added benefit of maintaining excellent fuel suspension within the incoming fuel/air charge. In addition, because of the slower heat transference of steel as compared to aluminum, it conducts less heat from the head into the incoming fuel/air charge, thus making for greater fuel/air charge density and hence more power potential. Polishing and chrome plating the exterior of this manifold will enable it to reflect away radiant heat emitting from the exhaust manifold, keeping it cooler. If you choose to use this intake manifold with SU HIF4 carburetors you will need to use either the early version of the UK/European Market SU HIF4 carburetors with the vacuum takeoff fitting on the carburetor body for provision for a ported advance mechanism, or, if you use the North American Market SU HIF4 which lacks provision for a vacuum takeoff, Advanced Performance Technology also offers the option of welding in a nipple on the crossover tube which would allow the use of a manifold-advance distributor. If you wish to run an anti-run-on valve and do not have the carbon canister of the 18GK and later engines, you will need to use the thinner Advanced Performance Technology?s phenolic carburetor spacers (APT Part# MFA338) which come suitably modified to provide fittings for a vacuum line, as well as the later exhaust manifold (Casting# 3911) as both have a mounting flange thickness of 7/16". This phenolic spacer with the vacuum takeoff incorporated into its design is a Maniflow item intended to be used with the Maniflow intake manifold which has no provision for vacuum takeoff on its crossover balance tube. A companion unported phenolic spacer of the same thickness is also available from Advanced Performance Technology, although a second spacer with a vacuum takeoff may be substituted to allow the use of a vacuum-assisted servo for a power brake system. Because both the angle of this intake manifold is higher (20°) than that of the Original Equipment intake manifold in order to enhance its flow characteristics, and variances in production tolerances of the bodyshell of the car, in a few cases longer and larger diameter aircleaners will not allow the installation of an underhood insulation pad, hence the thinner design of the Advanced Performance Technology?s spacers.
Why not stay with the OE intake manifold? Due to the sudden change of cross section that occurs in the area of the balance tube intersection, the airflow within them is markedly disrupted into a vortex effect. The resulting turbulence causes the fuel/air mixture to condense somewhat and also impedes airflow by causing the mass of the fuel/air charge which has been vectored into the upper section of the intake manifold to swirl 180° towards its bottom. When the fuel/air charge reaches the turn into the throat of the port, its inertia then causes it to careen into the opposite wall of the throat of the port instead of flowing along its contour as it should, thus impeding its own flow past the intake valve into the cylinder. While smoothing the cast surface of the inside the manifold and blending the change of cross section, as well as making a .250" radius at both the leading and trailing edges of the balance tube can reduce this vortex effect, such efforts can't take the place of the better design of the Maniflow intake manifold.
Steve S.

Steve! Welcome back!
Paul Hanley

Steve,
I really appreciate your input. That's what I needed to know, although it's not as straight forward as I had hoped.
You must be some typist, as that would have taken me days to write :)
Dave
David Steverson

Steve S., it's a pleasure to have you back.

Re: "Fabrication of a plenum chamber to go on the carburetors" TWM Induction makes this, and they've dropped the price to $250 (used to be over $300). But I'm not willing to pay that much.

1) Granted the chamber has to be several inches deep to allow for the backwash pulses of the carburetors. What other measurements should a person keep in mind when designing one of these?

2) I'm thinking the only way to get the air from the valence to carburetor is to tunnel up through the panel in front of the radiator, then back through the radiator surround. Suppose I don't want to cut through the sheet metal floor in front of the radiator. Do you see any problems just grabbing air from the space in front of the radiator surround?

Matt Kulka

Matt,
I am not certain of how much space you are left with with the twin carb set-up but I believe you may still run into interference problems with the TWN air plenum chamber and the brake booster. Barry Kindig and Mike Brown designed such a chamber for his twin carb setup and the thickness of the chamber at the carb face was less than 2.5" I believe. Perhaps they will pipe in and give the exact figure.
Frank

What about the air filter shown on this SU conversion?

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2485175725&category=31857

Dave
David Steverson

Not to contradict Steve, but the float bowl of the rear HS4 does not interfere with the factory brake booster. Having done a number of conversions to dual SU's (usually with HS4 carbs) this is not an issue. Air filter fitup for the rear carb is the problem.

You can check out the two different tech sessions that our local club held. Both cases used the HS4 carbs.

http://chicagolandmgclub.com/photos/carbswap04/

http://chicagolandmgclub.com/photos/carbswap_zs2su/

The second one shows some of the typical trials and tribulations you can run into!

Tom
Tom Sotomayor

Tom is right, HS-4's fit booster-equipped cars just fine.

Steve is right that in theory the conical K&N filters are not a good solution. Mike Brown has confirmed to me in the past that the K&Ns do not fair well on a flow bench. However, I've made a lot of chassis dyno pulls (700+) and found in the course of this testing, theory does not always match reality. I consistently have seen the conical K&N filters make about 2HP more on nearly stock engines than any other filter. I would guess a highly modified engine buzzing above 6000 RPM would lose some power.

Finally, stub stacks can be fitted inside the conical filters for HS-4s and will provide about 1HP. Stub stacks will not fit inside conical filters for HS-6 carbs.

And all you wanted to know was HIF vs HS-4...

--Carl
Carl Heideman

There has been a great deal of discussion of the relative merits and vices of the SU HS4 carburetor and those of its successor, the SU HIF4 carburetor. Advocates of the SU HS4 point out the greater ease with which the jet can be changed with the carburetor in place on the engine and the metering advantage of its concentrically mounted needle and jet. Some feel that its remote float bowl design gives it a "Vintage" appearance. However, the SU HS4 is not without its vices. It requires the removal of its aircleaner boxes to enable the use of a special short wrench to effect mixture adjustment, which results in a richer mixture when the aircleaner boxes are refitted. It also has a tendency to leak gasoline from its floatbowl junction. In addition, it has a tendency to run rich or lean under conditions of rapid acceleration and deceleration, during hard cornering, and when on a steep road. The SU HIF4 largely addressed these problems by having its float bowl integral with its body, thus allowing the float to surround the jet and hence more consistently meter fuel under high angles of tilt and under conditions of heavy cornering stresses.
Although more expensive to purchase and more time consuming to set up than the SU HS4, the SU HIF4 is easier to adjust and has superior performance potential due to its higher maximum flow rate which gives somewhat better performance at high engine speeds. This can be improved by retrofitting the throttle disks from the earlier pre-1968 SU HS4. These earlier throttle discs lack the flow-obstructing poppet valve of the later versions and also greatly improve engine braking, but you will have to file a notch in the bottom to facilitate airflow to the jet. Under no circumstances should you attempt to modify the jet bridge to enhance airflow. Its square profile is intended to cause the incoming air charge to lift slightly and thus keep the atomized fuel in suspension, which it does quite effectively. Replacing both its piston with its biased needle and its dashpot with the earlier piston with its concentric needle and a mating dashpot from the pre-1969 SU HS4 (a simple "drop in" parts swap) improves its long term performance further. This is due to the fact that the spring loaded biased needle wears the Internal Diameter of the jet, requiring the jet to be replaced every 20,000 miles. These modifications will enable the SU HIF4 to meter fuel within a hair's breadth of a well setup Weber DCOE carburetor at a fraction of the cost. Just be sure to refit the phenolic spacers and heatshield when you install them or the fuel will percolate in the floatbowls, causing the engine to run lean and all but refuse to restart after being parked for a while when hot. Should you decide to reuse your old heatshield, be sure that its insulating pads on the side facing the intake manifold are in good condition. If they are not, new insulating material can be obtained at any Speed Shop frequented by the local Hot Rod set. Be aware that the heatshields used with the SU HS4 carburetors (Victoria British Part # 10-35) and SU HIF4 carburetors (Victoria British Part # 3-5742) are not interchangeable.
During routine adjustment its mixture can be modified from above with nothing more than a simple screwdriver, hence removal of the aircleaner boxes is not necessary. Its thermosensitive mixture control makes for easier cold weather starting. Those who have converted their cars from the SU HS4 to the SU HIF4 usually report a 1 to 2 mpg increase in fuel economy. Unfortunately, rejetting requires that it be removed from the intake manifold and its thermosensitive mixture adjustment control can cause it to run lean if underhood temperatures rise badly in heavy traffic on hot summer days. Consequently, Jet Hot coating of the exhaust manifold is a worthwhile investment.
If you are refitting a post-1974 single carbureted engine with dual SU carburetors, be aware that the two carburetor types, SU HS4 and SU HIF4, use different intake and exhaust manifolds. The SU HS4 intake manifold can be readily modified for provision for distributor vacuum takeoff and has a mounting flange thickness of 9/16". In fact, the intake manifold of the SU HS4-equipped 18GK engine already has this modification. The HIF4 intake manifold also has provision for distributor vacuum takeoff and has a mounting flange thickness of 7/16". There are also two different exhaust manifolds with mounting flange thicknesses that are respectively paired with these intake manifolds. Should you elect to install a header rather than an Original Equipment exhaust manifold, be sure to check the thickness of its flanges before you make your purchase, otherwise you'll be likely to find yourself fabricating custom half-moon shims!
Steve S.

Do I take it that it is not the HS4 which physically interferes with the servo, but the air filters.

Just looking at that monster of an air filter in the picture linked below, and there seems to be no clash between carb and servo, except when air filters enter the equation.

Or is my understanding wrong.

http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&rd=1&item=2485175725&category=31857#ebayphotohosting
Jack

Ignore my question, I should have read up and seen the answere from Chicago.

However, I sure would like a more definitive recommendation as to which turn to take, HS4 heaven or HIF4 bliss.
Jack

Regarding the Ebay car: I've seen that tubing before, when I was installing sewer lines in cabins in southern Oregon. That's 2-1/2" or 3" ABS pipe. It has a 45 degree elbow near the radiator, and probably a simple end-cap on the firewall end. I'm not sure how he connected the air filter, but that's probably buried in the Home Depot plumbing department too.

You can see it's flattened where the bolts come through from the carburetor. I wonder how he got it flat enough to mate to the carburetor faces. And in the heat of the engine compartment, I wonder if it stays flat.

That's a clever idea. I was actually thinking of rain gutter downspout, because it already has a flat face, and the metal would hold up to the exhaust manifold heat without distorting. However, it might not insulate very well. (My focus is not so much single to twin carb conversion as cool air induction for my twin carb non-brake-booster car.)

Finally, my take on the HS vs. HIF: It seems like one of those things people get passionate about. Nobody dislikes the exhaust system they've put together, the tires they ended up with, or their choice of HS vs. HIF. That means either that both solutions are reasonably good, or all of the people who went with the lesser choice have conspired not to admit it. In this case, I believe it's the former.

In your shoes, I'd keep an eye on flea markets and E-bay for a set of rebuildable HS's or HIF's. When you find a good buy, get them no matter which style. Then send them to a competent rebuilder (Joe Curto comes to mind). I doubt you'll ever regret picking one solution over the other.

Afterthought: What a great color on that Ebay car.

Matt Kulka

Steve,
you write that when changing from throttle disks with poppet valves to disks without, a notch should be filed in the bottom to feed air to the jet.
I did the above conversion on my '69 B, should I have filed notches?
Thanks,
Rufus
R Pool

Greetings, FYI; I e-mailed the e-bay seller of the MG with the "custom airbox" and asked where it came from. Turns out that it was ABS Pipe. Here is his reply:

I made the air intake out of black ABS plastics pipe. I cut one section of pipe in half and got it real hot in the oven and then glued it with pipe glue to another pipe. I had to do this twice, so it was three layers thick. Then I planed that side flat and cut the appropriate holes. The filter connects using a sewer pipe fitting or a pipe size adapter. The SUs are the way to go if you want good reliable power.

There was no mention about the effects of heat on the air box. Apparently, it is tolerating the engine bay temps.
I still like the looks of the air box that Sean & Mike built for Barry K. Has anyone heard how that is working out?
Patrick Keenan

I have two sets of HIF4 AUD550 SUs and the throttle valves do not have notches. Moss specifies plates w/o notchs for the AUD550/630. Rufus, your 69 plates should not need notches. FWIW, Clifton
Clifton Gordon

Dave
I put a HIF44 with a Austin Marina manifold and conical K&N filter,on my '79 B with very good results All I had to do was repalce the needle and spring and now get a lot more power out of it without the balancing problems of twin SUs.This has been a better conversion IMHO than a weber setup and I got everything off Ebay for under $200!
This is another alternative to the other conversions that I don't see much of but was very easy to do (less than 3 hours)
Pat
patrick bailey

Steve, the ONLY time I have to remove the K&N setup on my HS4's is to balance the airflow or to clean the filters. No need to remove the filters to adjust the mixture.
Ken Lessig

Glad you chimed in Ken as I was wondering how I managed to adjust the jet position myself without removing the filter boxes... Oh, yeah, I put my hand back there.

I didn't need no stinkin' wrench either.

You can also cut the phenolic spacer in half to provide a little more clearance for the filter.

The Maniflow intake sounds nice but I expect David can get satisfactory results with pretty much any combo. Anything must be better than the single zenith-stromberg.

While he's at it he can get himself the HS-6 kit from Burlen Fuel Systems and a tri-y header from 'bout anyone and have a good go of it for pretty reasonable.

Have we pretty much mushed it to death yet, Dave?

lol

Mike!
mike!

This thread was discussed between 21/07/2004 and 26/07/2004

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