Twin 1 1/2 v Weber

[wantafaster1]

I'm wondering if anyone can tell me, based on experience, are twin HS4s good enough on a full race 1380, ie 649, 36/30 valve sizes, tunnel ported head. Its a good fit professionally built motor, I've only ever used a weber on it because I couldn't at the time find a rolling road with SU experience.

Now, intended use is tarmac sprint and maybe single venue stage rallies. I don't believe in revving things to 9000 or anything like that for the sake of it, from what I remember this did its best work from 4.5 to 6.5K . Don't know exactly if I'd describe better driveability as what I'm looking but better behaviour if that makes sense?

BTW only used 34 chokes in the weber, obviously 1 1/2 is bigger than this, or should I just use bigger chokes in the DCOE?

Thanks in advance.

[Vegard]

Have you dynoed your engine and found 34 chokes to be optimal?
The reason for asking is that Miglias run that spec apart from the cc, and they use 38, often 40....,

[wantafaster1]

There weren't any rolling roads at the time that did not involve a ferry trip. The chokes came in the weber. In fact this was built before we had internet...........

[Smiffy]

A 45 weber does outperform HS4's, if fitted with correctly sized chokes, If your only revving it to 6500rpms why use a 649 ?

[wantafaster1]

I would have revved it higher than that, maybe 7500 but maybe only briefly to save a gearchange between corners, 7000 as change up point. I'm about to resurrect this after many years and I'm wondering could I make it better without altering the tune.

You raised a good point, a 649 has always been described to me as the cam that gives nothing away. I would like to have tried a 544 but used a piper 285 and Kent 286, not impressed with either. I have used a 649 on the road and found it better than I imagined but tickover awful as you know.

I apologize for not saying that the Weber is on a shorter inlet, again I won't be cutting the bulk head on the new car either. Nor am I looking more power, I was originally wondering could i use a pair of Hs4 carbs without losing any power. Also, some people discover a misfire in long corners I understand, why is this?

[wantafaster1]

Oh yes, the engine builder uses two 1 3/4 on his own rally car these days.

[Spider]

Assuming both set ups were done on a dyno to optimise each, the Weber will give slightly better peak toque and HP. however, the SU's will be considerably better than the Weber at the low to mid-range (up to peak toque) engine speeds.

[Smiffy]

Assuming both set ups were done on a dyno to optimise each, the Weber will give slightly better peak toque and HP. however, the SU's will be considerably better than the Weber at the low to mid-range (up to peak toque) engine speeds.

You've got that the wrong way around, a properly calibrated weber gives more peak torque than the SU's.

[ivor badger 2]

The biggest advantage on a competition car between a Weber (with 40 chokes) and 1 3/4 SUs. Is that if you have to tweak the carb or service it, you don't have to piss about trying to balance the needles and reset the carbs.

[Spider]

Assuming both set ups were done on a dyno to optimise each, the Weber will give slightly better peak toque and HP. however, the SU's will be considerably better than the Weber at the low to mid-range (up to peak toque) engine speeds.

You've got that the wrong way around, a properly calibrated weber gives more peak torque than the SU's.

??? Try reading my post again.

Are you always an argumentative dickhead or does your mum dress you that way each day?

[InimiaD]

I used twin 1 3/4 HS6's on my old rally motor and they suited the stage, and point n squirt type events better due to the superior low down torque.(IMHO)
If I remember correctly i also modified the needles from the swing type to fixed by glueing them in with araldite.
The carbs were modified slightly, mainly in the butterfly and piston areas and i can't remember having fuel starvation problems.

[Smiffy]

Assuming both set ups were done on a dyno to optimise each, the Weber will give slightly better peak toque and HP. however, the SU's will be considerably better than the Weber at the low to mid-range (up to peak toque) engine speeds.

You've got that the wrong way around, a properly calibrated weber gives more peak torque than the SU's.

??? Try reading my post again.

Are you always an argumentative dickhead or does your mum dress you that way each day?

must admit to reading your post incorrectly, but my engine had more mid range torque with the weber.
As for been a dickhead

[ianh1968]

There are two schools of thought here:

1) Person 'X' says something that you disagree with,
so you tell them that you are right and they are wrong.

You know for a fact that you are definately far more
"experienced" than they are.

Obviously, Person 'X' is a complete idiot because they
disagree with you.

2) Person 'X' says something that you disagree with,
so you outline your case intelligently and try to work out
why your experiences might be different.

You may learn something new from Person 'X', and
Person 'X' may learn something from you.

DISCUSS!

[Pete]

It's obviously much easier to start a playground style spat ! Please stick to the subject if poss and lose the abuse.

I was often advised in speed event paddocks that on a typical large bore higher capacity motor a Weber offered more low down grunt with reference to acceleration out of corners.

[wantafaster1]

Maybe I should ask my question in a different way.

We have a saying from bike racing, " The stopwatch tells no lies", so in that sense bugger the rolling road figures, would the car be slower on twin SUs or about the same?

[Smiffy]

Though I never took mine down the strip, on the dyno & driving it the weber is quicker. BUT it must be set up correctly and by that I mean through the whole rev range not just he top end. This is the hard bit coz most of the rolling roads I've been to, are only bothered about the top end figures.

[wantafaster1]

I agree Smiffy, my mates escort went to a rolling road and came back with an extra 12bhp, it was around 240 brake about 8000 rpm. Unfortunately they gave away 20 lower down, and I'm buggered if I was able to get across the point that in order to get to 8000 he needed to accelerate through the part of the rev range where it had lost out. By the same token I always went a main jet size richer with the racing bike rather than the one the dyno operator picked as the one with the best power. Several times I proved I got a faster lap time, hence the stopwatch tells no lies.

I can see me sticking to the weber here if I can't find out, or better drive a car so equipped. I've been watching sets of carbs on ebay but I'm staggered by the price difference for seemingly same things, and I reckon on any of then I'd be adding on more money for a Maniflow inlet.

[Smiffy]

By the same token I always went a main jet size richer with the racing bike

Yep, plus it really hurts when it seizes and throws you off.

[Cables]

I've ran a similar setup to yours with a 649 cam, ran it on the road with twin HS4's, and with a weber 45. It seems to pull better with a weber, however if I had continued to use my engine in my road car I would have liked to try a pair of HS6's as I feel it would have been a better comparison to a weber 45

[In the shed]

It's about area under the curve. If you plot Hp vs Rpm, it's all very well having a big spike and nothing down below, for a road car, you'd ideally want something a bit more spread out.

If you consider that in a bit more detail....gear ratios, final drive, likely used rev range, you can be operating in a tighter parameter and choose a cam whose rev range has "a greater area under that section of curve". A weber is a better instrument for producing a better flow of mixture than the SU, this would involve huge amounts of fluid flow calculations and modelling, but the fact is that results have shown them to be better, which is why they are used in race cars and not SUs. It's not to say that you could get "the right sort of results" by using an SU.

So, the next question would be, how does the performance of each carb alter from "the ideal". If you were to put a trumpet on the head and use the engine as an air pump and measure how effective it is and call that "the base setting", by fitting an SU or a Weber, how would these characteristics change under various conditions? The result being that the particular camshaft and head combination and it's theoretical power curve would probably be distorted. Could it be the case that an SU makes more out of the lower down range at the expense of top end power, whilst the reverse is true for the Weber? (Again, dyno readings tell the story here).

I imagine it is something to do with the balance pipe. When you consider that each inlet port goes sucksuck-stopstop-sucksuck-stopstop, the balance pipe is going to act as a cushion and improve low range running, to a degree.

Quite clearly, you could improve things by having a bloody great big SU inlet manifold!!!

I am going down the weber route, because it is proven to be better. If you are worrying about road manners, or whether to cut holes in your dash, perhaps it's an idea to put a tartan rug over the back seats, take off the alloy wheels, silly cam and drive everywhere like a christian.