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MG MGB GT V8 Factory Originals Technical - High HP from 18B

The latest issue of Grassroots Motorsports has a mini 2 page feature on a 70 split bumper B owned by Pierce motorsports which has been modified with their crossflow aluminum head, Mallory dual point distributor, twin Weber 45DCOEs, higher compression pistons, mild cam, headers, re-inforced bottom end mated to a Sierra five-speed (the maligned merkur xr4ti in the US). Anyway, flywheel horsepower is about 165 at 8,500 rpm - zoinks Scooby!! This particular car also had a nice set ot 15" Panasports with knockoffs, shocks out back, panhard rod, etc.. Nice, clean looking car that can dust a Miata.

The number $15,000 was mentioned - which I assume is the total investment - car and mods. I am happy to see this kind of effort lavished on the four banger -another decent option for added ummph under the hood.
That HP number is still at the very low end of V8 or even V6 output, but it may have purist appeal. Just wondering if anyone knows more about this setup, durability, torque, drivability etc..
Brian Corrigan

That's quite some motor.What it can never have is the low down grunt of the V8, but I bet it's a blast to drive on the open road. Wonder what it's like in traffic? You obviously have to rev it to get that power. 8500 rpm - didn't know the old B series could take that. As far as I'm aware, it is reasonably easy to get 130 bhp, but from there on it gets exponentially more expensive to add each extra 10 bhp.
Mike Howlett

It is that way with all engines, it comes into effect when you start getting past the 1:1 ratio of HP to cubic inches. All motors experience it, which is why the old saying goes, "there is no replacement for displacement". I While I am a V8 guy, I too am glad to see that people are still spending time and efort to get the full performance out of the old block..

One thing that makes the V8 so appealing to peolpe is the width of the powerband. with a high revving 4cyl putting out 130hp, that is peak hp and you are probably only over 100hp for a couple hundred RPM. While a V8 @200hp you are in that 100+ range for much longer, plus add in the torque factor..
Larry Embrey

That power is quite achievable, but not at those revs, peak will be at much lower rpm. It may run to 8500, briefly, and perhaps then infrequently, but if it were to routinely reach those rpms then I would expect much more power. If not the set up is wrong.

Roger Parker

I doubt you could replicate the engine for 10k, let alone the trany and other mods. No way it would include the car. For example, the head and carbs are already over 5k by the time you get them on the car. The V8 or V6 is the only way to go for afordable and long lived high performance. IMHO


I have to agree with you about the Crossflow conversion being more expensive than a V8 conversion. I'm currently in the planning stages of building an engine with a Derrington Crossflow head, so I know what's involved. It's not a simple bolt-on swap if you want it to last. To do it right so that it'll last is expensive. The cost of the head casting, followed by having it reworked by Peter Burgess so its potential can be realized, special valve guides, valves, stem seals, camshaft, tappets, pushrods, valve springs, high pressure oil pump, the best of bearings, blueprinting and balancing, carburetors and their soft-mount kit, intake and big bore exhaust manifolds (plus muffler), aircleaners, etc. all push the cost of such a conversion up to rarified levels. However, your $10K estimate must be in Canadian Dollars, not US dollars. If it is, then I'd say that you're pretty close to being right on the money. Of course, you'll need to remove all of the gunk and corrosion from the block and recore the radiator to aluminum as well (more $$$). Then there's converting the ignition to a pointless breaker system and using a remote oilfilter, too (more $$$). Only fanatics whose object is to have the ultimate B Series engine would be willing to do it. If it's raw, brute power that somone is after, I have to agree that the V8 is cheaper, less finicky, and better.
Steve S.

Just read the article mentioned above. Nice car. I did note however that the exhaust is still 3 branch!? Now, my dear old mum always told me that "what goes in must come out" so do we not have a major bottleneck in the siamesed exhaust ports?

I also am no expert, but I have a hard time believing that an engine that is happy at 8000 rpm is going to have any power at 3000 rpm. The Heck with the head, I want to know what kind of CAM that thing runs? I did not see any mention of Variable Cam Timing in the article.

Mum also said "don't believe everything you read". ;-)


I have a 67 B with the Derrington (HRG) cross flow head, 1 3/4" SUs, bigger cam, .040 over on the bore, and balanced crank, rods, etc. I had to make an intake manifold, since I couldn't find a new one at the time. Intake port design is very good. Strangely enough I noticed the largest power gain in the mid range, which is a good thing on the street, but upper end power is also better. Its fun to drive, but not the same as my V8! One thing thats fun is seeing the look on people's faces, when their looking under the hood and tring to figure out whats different. Carbs on the other side of the engine!

bill jacobson

Steve S.,
Have you given any thought to fabricating an intake manifold linking cylinders one with four and two with three? Sounds complex, but it appears that the pulses through the S.U.'s could be evened out this way and the longer intake runs could improve low and mid range torque.
George B.

It's an intriguing idea, but why go to all that trouble when you can simply run a pair of Weber DCOEs?
Steve S.

The S.U.'s would have better low end repsonse and better part throttle fuel economy, I believe.
George B.

I see your point. That may be so theoretically, but low-end torque and part-throttle fuel economy isn't the purpose of a Crossflow head. It's the mid-range performance advantage that it's all about. Properly reworked by an expert (such as Peter Burgess) you can have 40% more mid-range power output over that of a stock head. How does 154 ft-lbs of torque @ 3,000 rpm sound? Highway cruising would be effortless. Passing on the open road would require no downshifting. Hitting the throttle while coming out of a curve on a winding mountain road would produce really fun results. A 135 hp top-end would not be surprising, and these figures pertain to an engine with a very mild camshaft, so it shouldn't be tempermental. The Derrington Crossflow head is for four-banger addicts who want a performance package that can't be had any other way, and hang the cost! Part-throttle fuel economy? You gotta be kidding! Who cares?
Steve S.

"The S.U.'s would have better low end repsonse and better part throttle fuel economy, I believe."

I have access to a set of twin choke SUs which should do the job too.

This thread was discussed between 12/03/2002 and 27/03/2002

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