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MG MGB Technical - Horsepower boost for $500?

Tax refund time is almost here. I think I will have about $500.00 set aside for the "B". (74.5, fairly stock except for a Weber DGV and Petronix electronic ignition conversion, both installed by PO). What would be the best way to spend $500 for a little more bhp? Is that a pipe dream, can $500 worth of upgrades (if there are any in that price range) be worth it? Thanks
Luke

Get a pair of rebuilt S.U.s. They will give you the most fun for your money!!! I bought a new DGV. As soon as my SUs were rebuilt, I sold the Weber. It was the smartest move I ever made!!! There is much more seat-of-the-pants power and the engine sounds better (in my opinion.) By the time you sell the old carb and manifold, you can buy a new header as well.

The other option would be to put up with the carb and install a new cam and lifters, but on a tired engine, you might be asking for trouble. You could also spend some time ona dyno and have your ignition curve tuned to your engine. That's always worth a few hp too.
Jeff Schlemmer

I second that motion.
carl w french 1

Unless you are a really experienced tuner, you might consider taking the car into an MG expert with a known reputation. Maybe $150 for a first class tune.
D F Sexton

1 3/4" SUs are an option as well. Or you can go to Harlandsharp.com and get some higher ratio roller rockers for about $250. That would give you the effect of a larger cam, but there's a little more wear and tear on the valvetrain.
Jeff Schlemmer

I wonder if you do all the work yourself other than the machining, if you can get your cylinder head ported and polished for $500. Big HP gains are possible there although other areas such the carb may bottleneck the engine's potential even with the revamped head.
Steve Simmons

Seem to recall the Browns up in Oregon were changing between $5-600 for porting. Probably your best bang for the buck, even if you keep the Weber. You need to contact them for actual pricing, though. I'm sure others are available- just these are the guys I know.

Personal opinion here- I think there is little seat of the pants improvement going to HS6's with essentially a stock motor. If you're going to the SU's and are paying for carbs anyway, and the 6' aren't much more, then OK, but I have a hard time believing a 6 is a good investment for a stock motor

If it were me, i'd go for the porting, modify the distributor advance curve, then go for some high-ratio rockers (I've had stockers for my MGC modified to convert to 1.6:1- I think you could get MGB stockers modified to yield 1.55:1). And find someone that can jet the DGV right along the way. In the order listed.

(While I'm not a big fan of DGV's on MGB's, it's what you have, which means you spend zero dollars to replace it. The carb was used on millions of cars, so it's not junk like many sseem to believe. It's just that i'm not sure I believe anyone's really invested the time to get a good set up for the B).

If you want to go with SU's, and are going to spend the money, you may want to look at early 240Z carbs (pre-tuna cans). Hitatchi did nome nice detail on these that make them, IMHO, probably a better thought-out carb. Other HS6 carbs can be taken off old Volvos. Remember to get the most out of an HS6 you need to at least open up the thermal isolator blocks , even if you don't port the manifold.

Points to consider


greg fast

I *thought* I had read some posts of those who had converted to 1-3/4" SU's (without any other upgrades) as having some problems with torque.

Ring a bell with anyone else?

rick
rick ingram

I recall the same, Rick. Can't remember the details of the problem but I think I remember something about the large carbs only working well on larger bores with higher compression where heavy aspiration is necessary.
Steve Simmons

You will loose power on the lower end of the RPM range with HS6's. Reed Peter Burgess's book. The only time HS6's work well is if you've bored your engine out to 1950cc. My 73B came with a DGV and I didn't like it. The car ran really well but just didn't have much power. Luckly the stock SU's were in the trunk. I installed them about a week after I got the car. I wonderful improvement in power. This winter I have also had the head rebuilt. For $375 he rebuilt the head using 3 angle cut valves, did some very light port work, and skimmed off 30 thousands. That will bring the compression ratio up to 8.8 to 1 just like the older B's which had more power. I wish I could share how it drove but we got two feet of snow here. Check back in about 4 or 5 weeks. Most agree stock carbs and head work are the places to start for most bang for your buck. I would stay away from headers. The stock manifold is pretty good.

my 2 cents.

Tim
Tim Swanson

Luke,

Pardon as I realize you were specific about bhp but with the promise of spring nearing, consider items that won't take you off line. How 'bout something else for the B? New tires perhaps. Top maybe--Moss is having their annual upholstry sale now. How's your distributor and braking system? Just too many ways to cook-off five bills enhancing your B. Tough spot to be in! Have fun.
Paul Hanley

Paul,
The top is very good. The tires are new along with a set of Minilite style wheels. The exhaust is new and the brakes, rotors, pads, brake line pipes were all replaced last year. Interior is good, no rips, could use a better carpet set however. The DGV carb has proved to be reliable and my only concern with going back to the SU's was less reliability. The major thing It could use is a respray, but I'm not getting that done for $500.
Luke
Luke

SUs in good repair are no less reliable than a Weber. I'm convinced that the reputation that SUs have for unreliability is that people replace 30-year-old, worn out SUs with a brand new Weber and conclude "Weber good, SU bad." When it's really "New good, worn out bad." I run SUs and never touch them....
Rob Edwards

Concur that a well rebuilt SU is reliable.

Changing a head will not take a car out of comission long- A head can be changed in an afternoon. If you want to keep the car on the road, maybe see if you can work a deal to get a ported head delivered and return yours to keep the car on the road.

For these motors, I believe the head is foundational. I think you can spend alot of money on stuff with marginal return, until you get the motor to breath. The head, high lift ratio rockers and you're 75% there. Add a good ignition curve and you're going to get 90% of what you can get out of a street mtor.

I'll restate, while I'm not a fan of the DGV, it's what you've got. Switching to SU's at this point is spending money for only incremental improvement.
greg fast

Luke, the Weber carb has as much a tendency for problems as the SU, just different. Rebuilt SUs will give you considerably more torque.
If you are your own mechanic, find a reputable machine shop who can do a 3-angle valve job, with a back-cut on the valves as per Peter Burgess's book. Its good for a couple horsepower and an inexpensive add-on. Mild porting will help, but don't do it unless you can have the results checked on a flow bench. I only paid $300 to have my head ported and flowbenched, but its aluminum (easier and faster.)

You can also have the flywheel lightened. Removing 6lbs from the flywheel will allow faster aceleration. Add a Triumph TR7 clutch disc while you're in there. That should be well under $500.
Jeff Schlemmer

I adjust the SUs about once per year and they usually don't need it. An Austin Healey owner on the show "A Carography" says the last time he adjusted his SUs was ten ears ago and they are still fine.
Steve Simmons

The great feature of the SU is the variable venturi. It allows a large carb with good driveability and economy. You compare the size of the stock SUs to other engines and it becomes apparent that carb upgrades are only needed for a highly modified engine and even then the effect isn't meaningful under 4500 rpm. A mild modified cam, a cleaned up head and an engine with a good strong 2500 to 5000 rpm power band makes a real peppy and driveable car. Whats good for max output for a race track does not make a good street engine. On the late US cars, changing to earlier specs in carbs, cam, distributor and small dish pistons can transform the car from pokey to peppy.
If you check the archives, a few years back there was a discussion that using one original late model carb gave more low and midrange power on an upgraded engine than the dual carbs.
Barry
Barry Parkinson

Hi Luke
Twin 1.5 su carbs with a freeflow peco hdr freeflow system{mild steel}with the right style of air filter fitted and tuned should do very well.
I think that the 1.5 su is the prefered option over the 1.75 i think it has been discussed in detail on this site before
Also gas flowing the head wll change the whole dynamics of the engine
Richard H

I really think roller rockers are a lot of money for limited gains in a car for the street. I have now just gotten my second Delta regrind cam in the 270 range and it was only a bout $100 from Lawrie Alexander at www.britcars.com.
That and the SU's will make a huge gain.
carl w french 1

I actually had a very similar question -- I'm glad someone started a thread about it. I've got about the same amount of cash available for an HP bump, but my B is a '79, also with a Weber DGV carb, and the cylinders already have been bored 30 over. I'm already planning on lowering it this summer, but I'd sure like a boost off the line to to complement the extra kick around the corners -- any different recommendations for me, or would the rebuilt SUs be just as nice?
Chris St. Pierre

Chris, rebuilt SU's and a properly curved distributor will give any MGB a serious performance boost. In my case, the change is just as effective as purchasing a new aluminum head, having it ported, and raising the compression of the engine to 10:1.
Jeff Schlemmer

Alternatively.... save the $500, sell cat/dog or sundry relatives and buy a supercharger....for a stock engine it makes a great difference (but yes it costs more than 500)
Michael Beswick

I second Jeff's first motion.
Get rid of that *&^%$#@ S/Z carburetor.
Twin HS4's are the best!
The engine you have is rated at 62.5 HP, right?

The Pertonix is good for about 2 HP.

The SU"s will get it up to at least 80 HP

And it cou;ldn't hurt to open up the exhaust system a bit.
Safety Fast
Dwight


Dwight McCullough

The car's a 74 1/2, which would mean it came with SU HIF4 carbs. A PO converted to a Weber DGV.

All information contained in the first post.
greg fast

Luke: In this order:
-New timing chain
-Head off re-grind valves
-HS4 or HIF4s

or
-New timing chain
Polish and port
Tubular exhaust manifold

add later
Carbs & cam + 1" sway bar

RMW

How does a new timing chain create more-than-stock horsepower?
Jeff Schlemmer

I considered a header and larger bore exhaust but many comments in previous threads state that the stock manifold is very good and a header wouldn't help very much. I know this wouldn't be cheap but if I'm going to switch carbs would Weber DCOE sidedrafts be better than the SU's? And to Michael, would bolting on a supercharger put undo stress on the engine, does it need any internal strenghtening? And also, I couldn't get 10 cents for the dog.
Luke

IMO, changing the cam to a milder, higher lift variety such as a Piper 270 and doing a "Peter Burgess job" to the head would do way more than just changing out the carbs. The SUs work just fine unless you're boring out the engine to 1950cc.

The next $ to hp would be the aforementioned supercharger setup from Moss. Although in the $3,000 range, its straight bolt-on kit gives you 40% to 50% more hp (in the right rpm range), it is everyday drivable and can be bolted on in a w/e by a shadetree mechanic. But, yes, the bottom end has to be in pretty good shape to have it installed. The weakest link can snap the motor and after spending $3K on a supercharger, why not just have the complete engine re-done anyway?

Luigi
Luigi

Before I even consider spending $500, I tried to figure what bhp the car is putting out now and what amount of bhp I should expect to gain. Being a 74.5 is it producing horsepower in the 90 range or more in the 60 or 70 range. I know there is that info out there but it gets confusing as they usually list 1974 and then 1975. Would a 74.5 be more on line with the 74. I believe the 74.5 didn't have that much difference in the engine than the 74. I'm confused.
Anyway, car facts-
74.5 MGB
Weber DGV conversion
Petronix electronic ignition conversion
Longflo air cleaner
Lucas sport coil
Stock exhaust manifold
Air pump pollution control equipment disconnected
Luke

74.5

Same engine as 74
Raised ride height
Think it was still chrome bumper
greg fast

Luke ,

AT the very beginning you might by the very relevant book by Peter Burgess at let's say 25$ "How to power tune MGB 4 cyl.". And then you will find out your way to go with, sure .
Regards.
Renou

Luke, you might want to read this article by Steve S of Virginia who writes about viewing the engine as a system.
http://www.studdsey.fsnet.co.uk/mgbtunings.htm
Regards,
Dennis
D F Sexton

Thanks Renou, good point. I don't want to start something and find when I'm done I've gone way overboard and end up with an MG shell sitting on a modern car chassis sporting a giant V8. (Not that's a bad thing, just not what I want to do). D.F, that's a great article, when I get to work tomorrow I will print it out (why use my ink and paper, and the boss will think I'm working on some great project). I guess the $500 will be saved while I drive, wash, and polish the car, also known as enjoying it. I will do some minor cosmetic improvements and maybe switch back to the SU's as the manufactorer intended. To all that replied to this thread, thanks for the input.
Luke

Luigi--

Why wouldn't the SUs work fine with an overbored engine? That's what I've got, currently with a Weber DGV. What would you recommend instead of the SUs?
Chris St. Pierre

The '74.5 had the same rated HP as the (US) 72-74 B: 78.5 SAE net. The 68-71 had 84 HP SAE net, 92 gross.
The 74.5 has raised suspension and rubber bumpers.

Luke & Chris: the very best bang for buck is in cylinder head mods, as Greg (an acomplished racer) noted. Get in touch with Mike and Sean Brown (don't have the email address handy but someone here will provide it). Send them your head and for about the available cash they will give you a 25-30% power boost throughout the rev range. Absolutely the best thing you can do and I promise you will be very happy with the improvement.

Keep the Weber DGV since you already have it. It's a fine carb that is used on 2 liter sports racers and Formula Fords and actually shows slightly more power on a dyno than dual SU'S when PROPERLY JETTED AND ADJUSTED. There is an outfit called Top End Performance that will supply an uprated jetting kit for around $50 to match the head improvements.

And Renou is right: the best $25 you can spend is for Peter Burgess' book. It will tell you all you need to know about B performance enhancements.
Terry

Luke - what is the engine number ?
RMW

RMW,
The engine number is 18V836ZL4314
Luke

Luke ,

As suggested by DF Sexton, Steve S's article is very relevant too in terms of tips and tricks permitting a near standard form but very efficient output engine; I
recently reprinted it for reading, it's fabulous. Plus P.Burgess head mods and you will catch a mustang.
HTH.
Renou

Luke,

I notice that your engine is low compression.

I think you have two choices - save up for a supercharger ( you need the LC for that)
- or buy a set of 60 thou over high compression pistons and have the block bored to suit.

I would favour the latter - it will give you an immediate and noticeable power increase and forms the base for future improvements.

IMHO you would be wasting money at this time with head porting - the engine will not make best use of it.

While the engine is out, if the camshaft is worn or you have enough money, swap it for a Piper 270.

HC pistons should cost 150 bucks - I can do them for that, incl postage. Boring in the UK costs around 100 bucks, shop around till you find a comparable quote. That should leave you 250 bucks for a new cam, followers, chain and all the gaskets.

Chris at Octarine Services

Luke
LC - I go with Chris above - in the bad old days we achieved the same thing by skimming the head (and using 100 octane) - the chain is important because we all give away power with bad timing - the cam speaks for itself - you have the best head and so long as it is in good condition - it's worth spending some money at some time - with higher lift cam and clean head/bigger valves you will have better breathing - richer needles on 1.5" SUs will give you more HP - a well made tubular manifold is good for more (so is a side-draft webber).


Probably the best advice is to draw up a plan of incremental improvements - if you need tech data from this side (which I doubt) I am not sure if Downton are still functioning but Janspeed are still in Salisbury and pre-date (Peter B) - Peter and perhaps Chris here would give directions.

An MG is for life - you don't need to rush at it.

Roger
RMW

Chris,
I have had the "B" since July 04, I have been reading these threads since about August when I found them. I trust your judgement and knowledge. The car, a 74.5 MGB Tourer, shows about 64,000 on the odometer. I am sure it is higher as it appears the speedo might have been changed. (It has a different style chrome bezel than the other instrments, not to mention that when I travel a mile it will only register about 2/3 of a mile, no wonder I thought I was getting lousy gas mileage.) Anyway the point of this is I will probably be needing a rebuild soon and at that point I hope to have the means to make the performance upgrades. I will buy the Burgess book and devise a plan. In the meantime, if this New England weather will ever warm up I will enjoy the car for this summer. I hope to add another to my fleet in the future and when the "B" is down I won't have severe MGB withdrawal.
Thanks all,
John
PS. Chris, I have your website bookmarked and will contact you when the time comes.
Luke

Sorry, Chris. What I meant to say is that the 1-3/4" SUs won't really do you much until you do a little headwork and bore out to 1860cc to 1950cc. Up until then the original 1-1/2" SUs should more than suffice.

Or then again, I may be wrong but my '69 w/ a HC 1860 is running great throughout the 2500 to 4000 rev range with the 1-1/2" SUs, 270 Piper cam, K & N filters and richer needles.

Oh, and I second or third that item about getting Peter's book.
Luigi

Luke, Where in New England are you located? There are some great gearheads in the area who can help you as well. There are also some very good local Moss discount vendors.
carl w french 1

I'm in Northern RI. Where are the discount Moss dealers? Also any good MGB mechanics in the area? I can do little things on the car but doing things like pulling heads intimidates me.
Luke

I don't have much experience with the tuners.

However, I have had LBC's since 1973 and totally agree with cylinder head mods. A little of my own history:

While in college, I had a 69 Sprite and rebuilt the engine. While I had the head off, I put a grinding stone on the drill and went at it. Had absolutely zero porting knowledge at the time. Really worked over the exhaust valve bosses as visually they were blocking the flow big time. With only this haphazard amateur port job, the car all of sudden went like stink and would bury the tach in 4th gear.

Later I got David Vizard's little yellow book on the 1275 cc midget engines. An extremely good read. has absolutely great information on combustion chamber improvements and repeatedly advises always using the long center branch header design. Note that the stock exhast arrangement (in a dual SU setup) is really much the same as the Peco header. I think the Peco header's chief advantage is the larger I.D.

Then I bought Peter Burgess's book. The icing on the cake as far as tuning knowledge goes. At one point in the book Peter mentions the importance of relieving the blockage from the exhaust valve bosses and recommends pretty extreme grinding there. See first paragraph above.

I don't know if the 74-1/2 has the bigger valve head (1.625 intakes) but a '73 head does. I have been frustrated for years with this head as it just doesn't seem to have the 'pop' that older MG's have (even with a cam and a Mikuni twin choke sidedraft).

I am beginning to think after all these years that the potential of the 1.625 head simply can't be realized because of the exhaust restrictions in the valve guide bosses. Yeah, I know that this area is supposed to be less affected by losses because of the forced pumping action of the exhaust stroke. But I think the actual pumping losses really add up at high rpm - exactly where my engine falls flat.

I am going to really get after completing my spare big valve head and am following Burgess's recommendations. After the experience I had with the 1275 head I will really pay attentionto the exhaust valve bosses.

Simply food for thought. Head work does give a pretty big bang for buck.
Richard Smith

Hi Richard,
Your point of vue is really interesting. Everybody anywhere is according to a strong improvement in head porting too. Very recently was a shoot on a french web site of a B ported head cast iron with nearly no exhaust boss seeable through the port. Your experience confirms
P. Burgess point of view about head work ROI .
what you call "icing on the cake" here is told "cherry on the cake".
Regards.

usefull links:

http://membres.lycos.fr/mgcontact/fileupload/uploads/combustionchamber.jpg
http://membres.lycos.fr/mgcontact/fileupload/uploads/1110044918_exhaustport.jpg
Renou

Those are great pictures Renou, it would be really helpfull if it was a before and after picture.
Luke

Luke,

I'll try tp have take a shoot "before". Please leave an E-mail adress to reply you, if possible in the afternoon time for European people, To morrow I have to go leav home during the next three days for my job .
Regards.
Renou

There's the email address.
Luke

This thread was discussed between 04/03/2005 and 20/03/2005

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