63 replies to this topic

[SicNNasty]

Hello all.

 

I've had a nagging question regarding twin carburation and the benefit thereof.

 

From what I've read here and there, there is little to no benefit on peak power but that twins tend to deliver better torque lower down on the rev range, giving better drivebility.

 

In my edition of Vizards book there seems to be a misprint. I've got the second edition reprint but on page 119 he refers to Fig 7.2 that I can't seem to find anywhere in the book. Regardless he explains that in his tests there was no difference in the 2500-4000rpm range. And goes on to say that "the power production of twin SUs over a big single, is hardly worth the hassle of having to have that extra carb."

 

I suppose my first question should be, is this book still relevant or is it outdated?

 

His views would lead me to believe that indeed there is no difference in peak power, but it would also disprove the idea that twin carburation offers better torque lower down on the rev range.

 

I can't seem to find any dyno results with similar enough setups showing twin carbs over a bigger single carb. But then the persons that I have seen mentioning this higher torque on the lower range definitely seem to know what they're talking about.

 

Any thoughts of where I might find some evidence of this lower range torque? Or even if someone has first hand experience to share.

 

Thanks!

[KernowCooper]

No its current as the engines and carbs are the same so the statement stands as factual, as a lot here will confirm, you will as I found out a while back get a sharper throttle response very low down on twins, but this is surpassed by 1500-1800rpm onwards depending on the spec of the engine

[ACDodd]

You do get more acceleration on correctly sized twins as a big single will hold the engine back in lower gears at the piston cannot rise fast enough to allow the engine to accelerate at its full potential. This is the benefit of twins over a single carb. There are no power benefits.

The stopwatch does not lie.

AC

[SicNNasty]

The stopwatch does not lie.

AC

 

Indeed it doesn't.

 

Just to be clear on this, is it torque that's allowing for the piston to accelerate faster in the lower gears on twins or is it some other mecanical phenomenon that my nooby brain is omiting to take into account.

 

Sorry for being so pedantic, but I'd hate to be thinking about this all wrong.

 

(Just think, I'd end up explaining it all wrong, people taking my word for it, and before you know twin stripes along the side of your car gives you more peak power!)

[ACDodd]

No, it the pistons in the carbs on twins can open slower and still realise the same airflow. Therefore the engine can 'pickup' quicker in lower gears as the single carb cannot open quick enough to allow the engine to accelerate at its maximum potential. This is the reason a well setup weber carb accelerates very well. Obviously using a thinner oil combination in the su will allow the SU piston to rise quicker, however the result of this is a large increse in steady state fuel mix as the needle needs to be richened to avoid flat spots on acceleration.

 

Twin Carbs over come thiss issue, and fuel more appropriately more of the time.

 

AC

[Skortchio]

Smaller masses are easier to move, so 2 smaller pistons are able to respond faster than a single large piston (as they only have to lift half as far ish) meaning that at the rising speed is slower on a big single - small piston lifts faster = more air/fuel = more go. This also holds true for on/off throttle which is the better response people talk about from twins.

Edited by Skortchio, 05 June 2013 - 11:09 AM.

[Skortchio]

AC types faster then me. I've hurt my wrist, that's my excuse.

[SicNNasty]

Wow, yeah that makes sense. Too many pistons and not enough technnical terminology in my test tube brain! So I really was looking at it completely wrong!!

 

Thanks a lot. That has actually got it a lot clearer in my minds eye.

[carbon]

Fig 7.2 is on page 76 of Vizards book (1st edition, blue cover). It compares the power output of a 1430 :

- single 1.5 SU peak power 93bhp @ 6,200 rpm

- single 1.75 SU peak power 96bhp @ 6,200 rpm

- twin 1.5 SU's peak power 97bhp @ 6,250 rpm

- single 45DCOE Weber peak power 98bhp @ 6,200 rpm

- single 48 IDA Weber peak power 99bhp @ 6,200 rpm

 

The plots also shows that the torque for the twin 1.5 inch SUs was better than the 45DCOE under 5,500 rpm. But not as good as the 48IDA.

[SicNNasty]

Wow, now I feel like a right idiot... Page 77... Oh come on man, who tells you on page 119 to check a graph that's 42 pages away without telling you it's 42 pages away...

 

Yeah okay making excuses because I feel dumb 

[Ethel]

To play devil's aardvark, each of a pair of carbs has half the airflow of a single carb to do the piston lifting but the twins are more than half the diameter, though they have about the same area (2x 1-1/4 vs 1x 1-3/4). An SU also relies on retarding the piston lift to give the equivalent mixture enrichment of an accelerator pump. It's why we have arguments about the best oil to slop in the damper. I'm not disputing twins pick up faster, maybe the fact the jet is closer the cylinders it's supplying makes the difference. It may only equate to a fraction of a second's advantage but that's enough to ensure the competition will always be playing catch up. I guess playing around with inlet manifold lengths would test my prognosis. Though ITB's would be the fuel injection equivalent.

[elanbaby]

Hmmm, how fast do the carb pistons have to rise during acceleration. When it takes, let´s say 12s from 0-60 and changing through 4 gears, then there´s always some seconds for the piston to rise through the rev range.   I agree that Webers sound far sportier when revving an engine when not in gear, revs rise much faster than with SUs.   I also drive an old Lotus Elan and people (and books) seem to agree that with double Webers you get slightly more peak power than with same size double Strombergs (constant depression carbs, similar to SU). But the Strombergs provide more torque throughout the lower rev range. Why? They say that constant depression carbs provide correct size of carb diameter and therefore the air intake speed is kept up high. Which serves to good combustion chamber fillings at lower revs. I am perhaps not saying this right, but you probably understand...      Do Minis really respond different?

Edited by elanbaby, 07 June 2013 - 09:10 AM.

[Ethel]

Constant depression, or variable choke, means the progression is smoother, so the fuelling can closer to ideal more often. There's a theory that the fuel atomisation, or lack of it, from an SU suits the peculiarities of the A Series's Siamese ports.

[SicNNasty]

I'm pedantic at the best of times and you're playing the devils Aardwark... Doesn't seem like a good idea!

 

"each of a pair of carbs has half the airflow of a single carb"

 

I get the math and logic behind this, it just doesn't seem to equate with this 

 

"maybe the fact the jet is closer the cylinders it's supplying makes the difference"

 

 

Again I'm probally missing something, but surely the carb nearest to the cylinder that's drawing air would be submitted to a higher depression than the carb furthest to it. I'm likely missing something to do with the intake manifold for twin SUs, but what is throwing me is the fact than in Vizards book, when he states the airflow capabilities of different carbs, when it comes to the double barrel carbs he only states the airflow capability of a single barrel explaining that each cylinder would only "see" one barrel.

 

I would have thought than in the case of Twin SUs the twin would only be drawn on if the first carb nearest to the cylinder that is drawing air has reached it's limit or is this thinking of it all wrong again?

 

For some reason I'm picturing myself blowing into a recorder and checking if the airflowing out the first hole is the same as the second...

[Ethel]

The point about the closeness of the carb is merely that the shorter distance to travel means any alteration initiated by the carb gets into the cylinder sooner. It's unlikely any cylinder will draw from the carb bolted to the other inlet port, before any charge has made it through the balance tube a different cylinder will be on induction to pull it back. I think it would seriously mess with the fuel mixture if it did