SU carb porn

[dougie]

Right you are..........................if we were talking about a stock motor, this is the racing forum.

The fact that the DHMC and the BMC Works Dept. both transitioned to triple 2" SU HD8's in 1960 and later three 45 DCOE Webers on their<span style="font-weight: bold"> race</span> prepared cars is evidence enough for me.

Still not convince. Recall the legendary Doane Spencer who build the Hollywood Sports Cars Austin-Healey 3000 that won 19 of 20 SCCA races in 1961.
Using three 2" SU HD8's his motor developed 205 b.h.p. at 6000 rpm. Doanes's final race version used a milder cam giving 195 b.h.p. for a wider range of useful torque.

BTY, I asked Chris Vandergriff if he retained any of his dad's old race tuning data from HSC at the Portland Historics last year, unfortunately that was before computers and nothing was kept.

Dougie

 

[billspohn]

The fact that the DHMC and the BMC Works Dept. both transitioned to triple 2" SU HD8's in 1960 and later three 45 DCOE Webers on their<span style="font-weight: bold"> race</span> prepared cars is evidence enough for me.

Actually that isn't evidence of anything in particular except that they had a bunch of 2" carbs available and someone in the company thought they were the right choice, and that they worked passably well. heck, they specified the same carbs (twin in that case) for the stock Rover 2000TC, the most over carbed engine I can think of out of Britain.

I'm talking about dyno results comparing the triple 1 3/4" with triple 2", and ideally also with triple 45 DCOE. Those power curves would be really interesting to look at - do you know if any such exist?

I have to wonder if the situation isn't similar to MGB experience - 2" on race engines are OK but offer little benefit except for full throttle use, and lap times aren't always very different from well set up 1 3/4" cars with their better low range flexibility. Always willing to learn, though, if anyone has some actual power curves.

2" triples certainly look nice....but then I am an old Jag guy, too.

 

[Roger]

The thing about SUs is that it's hard to get 'em too big. If your throttle diameter is "too" big, you don't get the depression over the choke, and that's what counts in the end. OK, so you're running with the pistons never fully at the top of their travel, but so what? It's not as though the SU has a noice smooth passage anyway, so what's a bit more turbulence?
I even ran an Austin Seven (750 sidevalve) on 2 HS2s for a while, and it worked fine once I'd sorted out needles, springs, etc. And a 1098 Mini on 2 1.5" which should have been too big.

 

[dougie]

Bill...................you really gotta just be jerking my chain. I'll play along though, here's more clear evidence that three 2" SU's or Weber's have the highest performance level for the Healey 3000 6-cylinder.

First of all let me prequalify the individuals who's data I'm quoting, they're not marketing, sales, or "someone in the company who thought they were the right choice" type of people. They're not company "tools". I'm not talking about the Rover's problems or MGB's. My application is specific to a properly ported, camed and correctly built Austin-Healey 6-cylinder race motor.

Eddie Maher, Morris Engine's development engineer's best recorded performance using the 1 3/4" SU's was 169 bhp at 5,500 rpm. His Sebring Race engines of 1965 produced 200 bhp at 5750 rpm with three Webers. Joe Huffaker's Healey race motors claimed 215 bhp at 6000 rpm with Webers and 204 bhp at 6500 rpm on 2" SU HD8's.

The only controversy that ever existed with race tuners on SU's was whether three 2" SU's had better performance over the wider torque range of two 2" SU's. After improved manifolds and cam grinds were developed to accommodate the three 2" SU's and Webers those in the know never looked back.

The majority of the U.S. race tracks we (vintage racers) run on are larger and have longer straights then the 1.5 mile Mission track you maybe accustom too.

In this case bigger is better.

 

[billspohn]

Thanks, Doug, that was all I wanted - some actual numbers rather than the all too common "It's bigger so it MUST be better".

As for Mission, I have never run the track. On the infrequent occasions when I get the chance to race in recent years I have always headed south to SIR or Portland, although some of the fun is gone from the latter because for some reason they insist on running the chicane.

Am I correct in assuming that your (Maher's) figures for the 1 3/4" are for triple carbs?

 

[dougie]

Yes, Eddie Maher's figures are recorded from his triple carb. preparation of the 56'-57' Bonneville
6-cylinder endurance car.

I do believe that with a bigger bore and modern pistons one could achieve higher numbers than his with triple 1 3/4" HD6 SU's.

As from the chicane in Portland, it's usually the big bore guys that get in trouble without it. It does make for some interesting dices.

Dougie

 

[GregW]

here's more clear evidence that three 2" SU's or Weber's have the <span style="font-weight: bold">highest performance level</span> for the Healey 3000 6-cylinder.

Did they ever compare triple Mikunis? :laugh:

 

[billspohn]

Yes, Eddie Maher's figures are recorded from his triple carb. preparation of the 56'-57' Bonneville
6-cylinder endurance car.

I do believe that with a bigger bore and modern pistons one could achieve higher numbers than his with triple 1 3/4" HD6 SU's.

As from the chicane in Portland, it's usually the big bore guys that get in trouble without it. It does make for some interesting dices.

Dougie

Ah - so the figures were with a 100-6 motor? We really need a well set up 300 motor with those carbs on it to see if they are a significant bottleneck, but I doubt anyone will ever do that as they will just opt for what the factory did. It would be nice to have hard data, though - same size engine, same compression, cam etc., just the carb size different. I'm betting there might be a bit less difference than we might think.

Chicane - I run either a locker or a Quaife and low speed tight corners aren't as much fun as with an open diff. OTOH, I can top end lots of stuff and passing going into a braking area is far safer than diving inside someone going into a corner, especially if the person being passed exhibits the all to frequently low level of awareness of what is going on around them.

While the length of the straight may be less safe for CART or Group 7 cars, for anything production based it is safer. Hard to convince the organisers of that (I've tried). So when at PIR, I generally do tail sliding turns through the chicane to keep whatever is behind me, behind me. I'm sure that is about as much fun for them as it is for me.