620 Land Speed Record

[G-Duax]

Trick to Weber jets is to buy all small ones, and drill them yourself.

[distributorguy]

Trust me - I spent HOURS both at the bench and at my tailgate drilling and soldering jets trying to come up with the best setup.  Most of my jets are stamped 160, but bagged as drilled.  I can do a main jet and air corrector change on both carbs in under 3 minutes.  The problem is that what works in the morning doesn't work at lunchtime, and really doesn't work in the heat of the afternoon, so you're always chasing your tail, never knowing if the line you're waiting in will let you run in an hour or 5 hours.  See the dilemma?  Adjusting a jet while in line isn't as stressful as ripping apart 2 carbs, since just as you think you have another 30 minutes, low and behold the guys in front of you will pull out of line and you're expected to fire up and go.  You ask why I don't drive the truck?  Because I'm dealing with this stuff.  

 

Every hour I walked the line to count how many vehicles were ahead of us.  Ranged as high as 60+.  There were always 2 lines side by side, taking turns - at least until Wednesday when everyone left.  

[G-Duax]

Back a ways when I was racing AMA flat track, when everyone was struggling to get their Mikuni VMs to run right, I never had a problem, because I was using Tillotson & Walbro snowmobile carbs on my bikes, with adjustable jets.

 

If you could find 4 matching HD Tillotsons, they are easily adapted to any DCOE manifold, and if needed, they had accelerator pumps on them from when they were used on Harley bikes back in the '70s, and all that stuff is still available.

 

I have taken them, and bored the venturi out, including the little booster venturi, and replaced with annular ring venturi.

With 8 jets feeding fuel into the bore, instead of just 1, you get better atomization, and the carb flows more air.

 

A 44mm HD with annular ring, will out flow a 50mm Mikuni due to less junk in the bore.

 

Here is one, with pump section (that you don't need):

 

Here is one without the pump section, just the metering diaphragm:

 

 

But why am I trying to tell a guy in Minnesota about sled carbs........

[distributorguy]

LOL!   The first carb I ever rebuilt at age 13 was a Walbro.  Yamaha 340 motor in an Arctic cat Puma.  Piece together what you can afford.  Have you heard of Dennis Kirk?  That's where we drove to pick up parts.  10 minutes from home.  

 

I wish there was a simple way to post pics here.  I have a pair of pristine HS8 SUs sitting on the bench, but they're torn down.  Larger than my 50mm Webers, and a relatively unobstructed bore, especially compared to your Tillotson pics.  I know you modify them, but there is little left to modify in the HS8s.  I'm guessing they'll be good for 350+ cfm each.  They have odd jet adjusters though, and I need to assemble one to see how it works - very different from the 1 3/4" and 1 1/2" setup.  I think I can make them adjustable with a topside screw away from the exhaust manifold.  

[G-Duax]

Yup, have bought a lot of stuff from Dennis Kirk over the years.

[G-Duax]

One thing I don't like about 2 SUs is that if you use the Datsun  style manifold (1 carb on cylinders 1&2, second carb on cylinders 3&4), you are not getting evenly timed intake pulses at the carbs.

Now if you were to fabricate a custom intake, with cylinders 1&4, and 2&3 paired up, you would have evenly timed pulses.

[Str8jacket]

I had a single weber manifold with the standard pairing 1-2 and 3-4, and it never ran well. Hunted down a Lynx mani with the 1-4 2-3 arrangement and was a different engine, idled better ran 100% better when on the pedal. I think the pairing issue is why a lot of people say you cant run a single DCOE.

 

The other thing is the exhaust, IR with carbs are less tollerant to any issues there. Are you running big tuned headers or are you stuck with factory?

[distributorguy]

That would work, but there's not enough space for it.  Unless I convert to RHD?  I was planning to minimize the crossover or make it go away altogether to help reduce the extra pulses that redirect airflow in the manifold.   Its not like the Weber manifold is any different.  With SUs you'll actually get less pulsing, sort of.  There's potential for 2 &3 to go a little more lean, depending on how well fuel siphons from the jets.  Since we rarely get under 5K rpm, I'm not so sure the pulsing is going to have a dramatic "break" between 1 & 3 or 4 & 1 to cause an issue?  We're talking milliseconds.  Can it matter?  Of course, but to what extent?  

 

 

My header is computer designed and its huge.  No restriction, good scavenging.  I may redesign it to run out the back instead of the wheel well to make more room and quiet it down.  Its ungodly loud.  

[G-Duax]

But a Weber manifold is way different, each cylinder has it's own throat.

And I believe a paired manifold for a single throttle bore, that isn't pulling a full 360 degrees apart, will effect cylinder filling much more at 8000 rpm, than it will at 4000 rpm.

[Str8jacket]

What length runners does your program say for a tunnel ram style, 3rd harmonic? I wonder if you would be better off making a common plenum for both SUs to sit and have the 4 runners equal? Is there room to have the runners in a big sweeping 180* over the tappet cover and the plenum on the RHS? Plenty room for carbs, away from exhaust? You could bang one together out of exhaust tube ghetto rough just to try it. Big arcs on 6in radius couldnt be worse than the bends in a downdrought mani. 2lt plenum volume and the self compensation of the SUs might give you more top end

[distributorguy]

What length runners does your program say for a tunnel ram style, 3rd harmonic? I wonder if you would be better off making a common plenum for both SUs to sit and have the 4 runners equal? Is there room to have the runners in a big sweeping 180* over the tappet cover and the plenum on the RHS? Plenty room for carbs, away from exhaust? You could bang one together out of exhaust tube ghetto rough just to try it. Big arcs on 6in radius couldnt be worse than the bends in a downdrought mani. 2lt plenum volume and the self compensation of the SUs might give you more top end

 

That's not a bad idea - and an easy way to correct the firing order.   We run a huge hood scoop, so there's lots of room over the valve cover.  I'll have to check the rule book,  but I don't recall seeing any kind of rule regarding moving the carbs if you retain the stock style head.  For our short runs, using steel likely isn't going to be an issue, since I can also coat it with a thermal dispersant.  It would be a LOT of extra work, but building siamesed runners could potentially alleviate pulse issues and help prevent fuel from dropping out of suspension?  No computer program for the intake side.  I'm guessing from flowbench experience here.  

[G-Duax]

 building siamesed runners could potentially alleviate pulse issues and help prevent fuel from dropping out of suspension?  No computer program for the intake side.  I'm guessing from flowbench experience here.  

 

Plenty of intake tract formulas on line.

Fuel dropping out of suspension is more of a atomization problem, but will still be less of a problem if you correct the issue with the cylinder drawing order being out of whack.

 

I still don't like the idea of using 2 large SUs, why not 4 smaller ones?

You should always try to maintain a constant cross section from carb mouth to intake valve.

Having a really large carb, feeding 2 smaller bores throws that right out the window.

[Stoffregen Motorsports]

The two inherent attributes of the dual PHH/DCOE carbs that I think make the difference are: the individual runners and the accelerator pumps. You can't add accel pumps to SU's, but I do like the idea of the four SU's (one per port), but that could get cramped really quickly.

 

I still don't think that dual SU's will compare with dual PHH/DCOE carbs as far as power goes, but it sounds like your main complaint is tuning. Yes, the SU's are easier to tune, but once you break through that tuning barrier with the Webers (again, I would switch to Solex 50's), you will be better off.

 

If you could find some time to do acceleration runs on a roller dyno, you could get all the tuning issues to a point where you just need to make simple jet changes at the track.

[Str8jacket]

I would think that from an all out high rpm HP point of view that you would be better off not having individual runner carbs. They are definitely the go when you want the best response and more area / torque under the curve. But from all the research ive done (not experience so it could be all shit!) That for absolute HP perspective a tunnel ram or single plane type mani with a central plenum feeding well placed runners that wont steal from each other is the way to go.

 

As horrible as it sounds a down draught mani with a central plenum and a double pumper holley like a 600 with annular boosters located so that each runner is fed from a venturi but still an open plenum to allow a strong signal would be an option to look at. Just my thoughts.

 

I do know if you have a common plenum the carb size doesnt need to be as big as if it is IR.

[G-Duax]

You might rethink that if you look at non-turbo F1 engines....

[Str8jacket]

You might rethink that if you look at non-turbo F1 engines....

They are the shiznit, but is their design criteria the same as what is being asked of the engine in this application?

 

IR is awesome but they are less tolerant of any conditions that arent in the ideal range. Things like an inlet length and cam that is causing rich afr in the mid range that you then tune out to get perfect, then once out of the rev range where reversion is and into high rpm all of a sudden its going lean, a longer header or a longer inlet may tu ne it out but it is a lot of work. One thjng i have learnt in building the manifolds for my project that it is easy to be too short on inlet length but very hard to go too long. A plenum style mani may not be the F1 ducks nuts but if it gives the most HP at WOT with less dicking around then thats a win

[Stoffregen Motorsports]

A log manifold or open plenum manifold is an easy way to get pretty good results, but to achieve truly perfect tuning, individual runners are preferred.

 

That was my training. Technology changes and what's accurate yesterday may not be the truth today. And my above statement is also a very blanket statement. Each engine should be tuned to it's own unique characteristics.

[distributorguy]

For us, anything under 3500 rpm is idle speed.  That makes our setup unique versus any other types of racing.  Similar to drag racing I guess, but we spend a LOT longer time at full throttle.  That's why I'm betting that a variable venturi carb will offer more power than a fixed throat.  How many people use SUs at Bonneville?  No one with the exception of a few bikes.  That's why I haven't spent any time building these carbs yet. Volume-wise they will work.  In fact, if the firing order were corrected, they may actually work better having 2 than 4.  It comes down to having a manifold that's effective and functional.  I think pulling twice as many pulses through the carb will actually stabilize the dashpots quicker and result in better fuel metering.  Manifold volume versus length versus velocity issues are all a guess at this point.  Assuming enough velocity to maintain fuel suspension (unlike a Weber DGV manifold in most cases), and a good transition at the cylinder split, I'm not afraid of dual carbs on 4 runners.  Yes, there's potential to screw it up.  Its no different than dyno tuning Webers in MN, then going to the salt flats and thinking the tune will be anywhere near correct.  Every jet will be wrong.  If I learned anything last year, its that we can't tune here to run there.  

[wayno]

I don't know if anyone asked this and it might be against the rules, to me it seems like motorcycle carbs would work better, they generally operate at higher RPMs than car/truck engines and are made that way to start with, also the ones I have seen are tucked in tight against the head.

[Stoffregen Motorsports]

I was thinking the same thing. Though I don't have any experience with motorcycle carbs on car engines, they might be a good combination of the attributes you're looking for without the fuss of the Weber jets.

 

Also, what you were saying about velocity, the increased signal from pulling two cylinders through one bore definitely helps with fuel management.

 

The manifold will be the lynch pin. Maybe a standard layout (1-2 and 3-4) with a balance tube (just like most stock SU manifolds) will be enough. You could play around with the size and location of the balance tube.

[G-Duax]

motorcycle carbs.... the ones I have seen are tucked in tight against the head.

 

Mostly for fitment convenience.

My Toyota set up (actually ITBs, but same fitment problems) needs the mouth to be about 9" out from the head to be tuned to 3rd wave. This has cause all sorts of problems with the brake master, which I'm working around in a real Rube Goldberg fashion.....

 

Doubt a balance tube would correct the 1-2/3-4 timing problem.

 

 

[Stoffregen Motorsports]

You might have to explain to some of these younger guys who Rube Goldberg was.

 

ITB's is definitely the best solution. Nice manifold btw.

[G-Duax]

Keep them guessing.....

 

Thanks, but not as good as I would like.

That damn first gen 4-valve had a real funky intake port design, where the front two angled toward the rear, center two came strait out, and the rear two angled forward. Without doing full CNC S-shaped port runners, it would have been very difficult to get the runners so their center lines would be spot on with the port center lines.

[distributorguy]

If you want Rube Goldberg, you should have seen the inspectors looking at my solid throttle linkage to the dual Webers.  7 pivot points, bent linkage rods, machined aluminum brackets, a linkage rod support on the firewall, etc...   I didn't want any cables/housings to potentially fail when both drivers tried to smash the throttle pedal through the floor.    

 

Duax - nice manifold!  The problem finding bike or snowmobile carbs is getting 4 or even 2 that aren't totally rotten.  I have some nice bike ITBs for when we get past this production level stuff.  Again, the SUs I have are exceptional, and plenty big.  Given the engine displacement, they should flow through 10K rpm just like the Webers.  I don't need accelerator pumps.  You change out the dashpot springs to take care of that function, as well as dashpot oil so its actually more tunable than you would suspect.  I saw plenty of guys at Bonny battling ITBs bolted directly to the head, trying to make power.  Longer runners seem to help stabilize mixture.  

 

I'd bet I can make a functional but ugly manifold to correct the firing order and get the carbs on the cold side of the engine.  If only I didn't have so many distributors piled up and needing repair...  4+ weeks backlog built up right now...  

[G-Duax]

Better than not having enough work.......

There are vintage sled web sites that have NOS carbs. The big ones go for around $100 a pop.

 

Do you want links that have the formulas for runner length, etc.?