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MGB Convert away from Z/S a real boost?

Boggsy64

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I only have driven my B with Zenith / Stromberg and cat converter etc. I am wondering if converting to another setup (duals, or downdraft, or whatever ?) and improving the exhaust flow (removing the cat converter and getting better header) is really going to deliver that much more of a rush? As I am going to rebuild the engine I am wondering if the rush, the feel, the acceleration, the joy is really worth the conversion cost?
 

LouNH

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Converted mine to dual SU HS4s and while the car might not be a measurable difference with a stopwatch, it was a real seat in the pants difference. Felt like a whole new car. Some people claim they have been able to get the Zenith to perform well but i have never been able to get performance out of it. The exhaust manifold alone for the SU's flows a lot better than the cat manifold. I wouldn't bother with headers, the metal is thin, they tend to crack and are noisy. They don't really offer an improvement over the stock SU exhaust manifold.
The SU's were incredibly easy to rebuild and you never have to worry about a tear in the diaphragm. You can cruise ebay pick up a set of SUs and exhaust manifold at a decent price. The rebuild kit for SU's is around 150.00 for both carbs. Also if you're going the SU route, HS4s idle better than HIF's.
 
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Boggsy64

Boggsy64

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Just a glance at the "all in one" intake and exhaust manifold and you can tell it is not good. I put a weber carb and a header on a rabbit years ago when I rebuilt the engine and WOW I could smoke the tires afterward. I agree an SU exhaust manifold would be a good way to go. What about the Weber downdraft carb? And yes Bob I would love to get a comparison!
 

drooartz

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Only potential issue I could see would be if you're in an area that requires a smog check on your car (here it's by county, but if you're in a county that checks it is everything after '67).

Besides, twin SUs just look great on a British sports car.
 

DrEntropy

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Putting a set of SU's alone will be a reliability improvement but with a header it gets even better... as long as the head is also dealt with. i.e.: match the ports (to intake and header), do some port clean-up, cc the chambers.

A number of publications cover this. Hunt down Clive Trickey's books. His treatment of Mini engines applies equally well to the MG 1800's.

Some proper planning ahead of throwing money into a bunch of "bolt-on HP" bits can make the difference between real gain and a so-so result.

I wish the Hooker headers were still available for the MGB, Those things were proper.
 
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Boggsy64

Boggsy64

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It seems many people really like the SU's. I have heard from others that SU's are difficult to keep in sync and tune, and the Zenith is much easier. The downdraft Weber looks clean and easy to install (and I am guessing simple to tune). I am looking to get most "go fast" in the best way. P.S. I found an original Z/S manifold today on Ebay that someone modified to separate the inlet and still keep the exhaust and Cat converter in place. They had used this modified manifold to then mount a Weber downdraft carb and keep their exhaust system intact. Looked pretty good and only $75 dollars, a novel approach.
 

JPSmit

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The Zenith Strombergs are a beast unto themselves - and different from earlier carbs. Assuming the MGB carb is like the Midget one, it is more complicated and the mixture can't be adjusted. Nor can the air intake be tampered with (k&n filters etc) My personal (untested) theory is that as long as you have all the smog stuff, the ZS will run correctly but without the smog stuff it is almost impossible to get right. I had a ZS on Ms. Triss, rebuilt it, added manual choke and could never get it right. I swapped in an SU and love it. Of all the carbs mentioned, SU's are far and away the simplest. The "problem" is that people mess with them and/or that the shafts get worn. There are lots of 30-50 year old carbs that need a re-shaft and rebuild. Then, Once you have it dialed in, top up the dashpots occasionally and you will have years of trouble free motoring,
 

Flash_Harry

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Our '74 B-GT engine was rebuilt - 0.040 overbore, VP-11 cam, 9.1 compression ratio, ports cleaned really well, balanced, etc., and HIF4 SU carbs rebuilt by Joe Curto. It goes pretty well, and even in 5th gear will pull hills easily. It wasn't cheap, but it's fast enough to get me in trouble if I'm not careful. Plus, my thinking is that if I want to go someplace in a REAL hurry, I'll drive our 2008 Mini Cooper...
 
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Bob Claffie

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I had a Z/S on a Spitfire, hated it and never could get it to perform correctly. Car was killed before I had a chance to change it for something else. Had a '72 MGB with HIF's on it and was very pleased when I swapped those for HS4 style. As far as headers are concerned I have never been a fan of them on a street car, really work best at HIGH revs. Other changes to the exhaust will make it more pleasing to the ear but performance gain is marginal. Why get rid of all the smog stuff ? The pump and other stuff probably only accounts for 1/2 horsepower. My son and I had twin MGB's '71 &'72, one desmogged and the other not. Both motors rebuilt to stock specification. Performance equal from a seat of the pants perspective. . Bob
 
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Firm

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My 78 B has the original Zenith, rebuilt and running the original style air cleaner. It is desmoged and it runs great! In fact, it still has the automatic choke on the Z/S and even that works very well. It took some fiddling, but I am really pleased with the way it's running.

My mom has an 80 B with a weber side draft, headers and a free flow exhaust. It is her year-round daily driver so it's used in all conditions. The car is definitely a little quicker than my stock car, but I find the "bark of the weber" a little annoying, not to mention the headers add to the noise. It sounds great at high rpms, but at low rpms it sounds a little rough. I also find that the power delivery is not as smooth. The weber is not without it's problems either, it seems to be suseptible to heat rising up from the headers and causing vapour lock if a heat sheild is not installed and the car is run in very hot weather. We also had an issue with the carb body cracking where it mounts to the manifold - likely a result of expansion/contraction or vibration (car is driven hard and a lot!).

A few years ago we had another 80 B-LE which had a weber downdraft and headers. Very similar to the sidedraft, with a little less punch. It seemed a little less finicky though.

I've not driven a later B with SUs, so I can't really comment on that option. All in all regardless of the setup it's most important to have everything dialed in correctly. A stock B running 100% feels so much better than a hopped up B running at 95%, and a hopped up B running at 100%, while notably quicker, comes with it's own set of compromises.
 

billspohn

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Putting a set of SU's alone will be a reliability improvement but with a header it gets even better... as long as the head is also dealt with. i.e.: match the ports (to intake and header), do some port clean-up, cc the chambers.

A number of publications cover this. Hunt down Clive Trickey's books. His treatment of Mini engines applies equally well to the MG 1800's.

Some proper planning ahead of throwing money into a bunch of "bolt-on HP" bits can make the difference between real gain and a so-so result.

I wish the Hooker headers were still available for the MGB, Those things were proper.


Actually a recent test from Classic Motorsports Magazine discloses that the header doesn't make that much difference after all. The late style (integral with the single carb intake) is worst - it was 5 bhp down by 5000 rpm, but only 2 bhp down at 3500 compared to the earlier MGB dual exit manifold.

Thy also compared a stock cast iron early manifold on a tuned MGB engine to a tubular header and the Maniflow header bettered the stock manifold only by 3 bhp at 3500 and 1 bhp at 6,200.

If you want significant improvements in those engines you need to go inside them.
 

DrEntropy

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Bill said:
If you want significant improvements in those engines you need to go inside them.

That's why I mentioned the head work and Trickey's books, Bill. There is rarely a "bolt-on" increase without doing some real internal and head work. Port matching and chamber modification make a real difference. There was a lot of finish work not done from the factory. Mass production doesn't lend itself to the "fiddly bits" being dealt with.
 

Charley

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If you want significant improvements in those engines you need to go inside them.

Hi Bill,

That's why when I built the engine in my '69 roadster I used the HC pistons, lightest stock valve gear & the '67 grind cam. With the twin SUs, stock header and a free flow exhaust system the little engine pulls well to ~6K and will chug around ranch roads in 3rd & 3rd over just fine. My '70 with the more radical cam, etc, wasn't near as nice for sight seeing.

You really have to think about how you plan to use the car.

CR
 

hottvr

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Is it a good idea to replace the distributor to a 25d on a rubber bumper car after putting SU's on?
 

DrEntropy

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You will benefit from the 25D vacuum advance, IMO.

You still "up there"?
 
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