Advice please

[richphy]

I am considering buying a standard Sprint engine to put in my Sprint while I save up some cash to spend on upgrading my current engine which has Weber 48s. If I bolt a pair of two inch SUs onto a standard Sprint engine with no other mods, what kind of bhp increase can I expect? Any ideas please?

[Carledo]

2" SUs with no other mods will probably make it worse. If the intake throats/valves/cam can't handle the extra fuel, you'll just be wasting it!
I used to come across this all the time back in the days before fuel injection. All you get is lousy fuel consumption and an engine riddled with flat spots!

Steve

[Mad Mart]

I put some 2" SUs on a standard engine once. It was pretty bad on low revs (pulling away was awful). Once I hit around 3000 rpm it took off. Obviously MPG reduced. I wouldn't recommend doing it for normal road use.

What cam are you running with the 48s?

[Bumpa]

This is an old chestnut often talked about on the MGB forum. The carbs on your car are sized according to what the engine can use. Think of the engine as an air pump. It is sucking a certain volume of air in on each intake stroke. To get efficient fuel vapourisation the air needs to be speeded up as it passes through the carb. That's what the venturi is for. If you fit a larger size carb the air doesn't speed up enough, fuel atomisation is poorer and the engine doesn't run as well, particularly at low revs. Before you fit larger carbs you have to get the engine to demand more air and that usually involves cylinder head work and different camshaft profiles. It is usually a falacy that bigger carbs on their own give a power boost.

[Carledo]

For many years, I ran Ford Zephyrs and Zodiacs of the MkII variety with the 2553cc straight six motor. I was also the official technical adviser to the club for years.
On these cars, the Holy Grail of fuelling was the Aquaplane triple SU manifold (they cost megabucks nowadays) I used to see many cars in my shop where some young hopeful had slapped one on complete with 3x2" or 1.75" SUs then wondered why it ran like a bag of spanners and did 5mpg!
I ran one myself for many years, an Auto Zodiac Farnham estate, with a 104bhp MKIII engine wearing only the Aquaplane and a bespoke 6 branch extractor manifold (the original exhaust manifold is a joke, a piece of pipe with holes in, bolted to the side of the engine with a 4" radius 180 degree bend at the front!) But I ran mine with 3x 1.125" SUs and this worked a treat, perfect idle, massively improved throttle response and up to 35mpg on a run (from a 2.5 litre auto!!!) But I much doubt it had many more horsepower than the original setup, it just used them better!

Steve

[richphy]

Thanks for all the responses everyone, there were some pretty detailed answers.
As for my engine with 48 Webers, I dont know what cam it has as I bought it second hand and the seller did not know. But I had it rolling road tested and it got 130bhp at the wheels. It was lumpy at idle but it really took off from about 3,500rpm. Im guessing the cam is one or two up from standard.

[gmsclassics]

Yes, my experience with 2" SUs on even a mildly tuned engine was the same as Mart's. Not that easy to start either at times. Better on standard, much more driveable.

I've only ever had my race car on a rolling road once. It was supposedly a reasonably conservative one and with the 45 Dellortos I use, that gave about 150 at the rear wheels. I use an STR139 cam so I suspect with your 48s (and I'm jealous!) your current cam will be quite mild. Also to make the most of the carb, and potential cam, you will need to have the ports opened up and flowed/polished or you still won't get enough air/gas in/out. What cam you decide to use will depend on what you will be using the car for, mine is impossible on anything other than the track, but then in order to use the power/speed it is capable of requires major suspension and brake modifications to avoid it being potentially lethal.

There are a few videos on YouTube - search for 'Dolomite Sprint No 20'. Just to put this in perspective if you watch a lap at Hampton Downs, this is at an average lap speed of just over 70mph.

Geoff

[richphy]

Thanks for the response Geoff and Ill have a look on YouTube. Ive just bought a standard Sprint engine to use while I work out what to do with the non-running one with 48s.Cheers.

[Dolly-Nut]

I have had a standard Sprint lump on 2" SUs. Setup correctly the mpg was over 30 iirc. Idle was poor below 1000rpm but it had slightly more poke at higher rpm. The manifold was opened to 2" as well.
Webers/Dellortos give more of a power gain over the whole rev range in my experience, but at the expense of your fuel level!