Sydney was washed out again but luckily we were notified the day before. It saves driving 150 kilometres (93 miles) only to have to turn around and drive back home. Oh well, what to do? Of course! Drive 4 1/2 hours (383 kilometres or 237 miles) in the other direction out to Bodangora Drags, held about 10minutes drive from Wellington, NSW. Once again this is a laid back, country style, park your egos at the front gate type of race meeting put on by the Dubbo mob. Don't get me wrong, ANDRA rules and regulations still apply but the day has a nice relaxed feel to it. I stayed overnight and was up nice and early to head out to the track which is a local airstrip. Well that was the plan. Overnight temps of -0 degrees C meant the car was nicely iced up with even the side windows locked in place by ice. I know it sounds trivial to those who put up with this shit for months at a time, but to a coastal dweller it's a bit different. Anyhow, back to racing. There were mainly street driven cars with a smattering of race only machines. You know the type. "If I can get that small block wedged into that old unregistered grocery-getter in the back yard, I think I'll go racing". Damn quick some of them are too. Good racing all day with me doing 8 runs all together including the final of the B bracket. I couldn't quite reel him in and broke out anyway. 9.18 on a 9.19 dial. Anybody got any ideas on why I'm getting that vapour coming out of the carby trumpets? It's noticeable as I rev it out in second. Doesn't seem to affect it, just curious.
Great vids once again. The late model in that second vid sure put on an impressive burn-out. Rather a shame he couldn't back it up actually racing. Your carb fogging looks to me to be what the kids today call "reversion", which we used to call a "standing wave" (and I of course, still do ). That back wave being initiated by the snapping shut of the intake valve on the moving stream of air coming in. It travels back up the intake pipes 'til, at higher tach it can actually spray gas back out the ends. My early work on this slant with the SU's gave me exactly that problem. I solved it the old way, by simply making the intake system's total length longer than the standing wave can get to, thus capturing it inside the system. This had the result of upping my tach capabilities as well, eliminating a naturally occuring tach limit, so that I wound up needing to pay a bit more attention to not over taching it. Here's my set-up, at just shy of 10" added to the carbs ...... With the cam I had in the previous engine and these stacks she'd easily pass any safe tach level if I simply didn't lift my damn foot. I've pushed 6200 occasionally myself (but I do NOT make that a habit), and've heard it do even more with another driver a couple times. Likely a contribution to it's eventual discorporation at the last Eagle Field meet. The (different) cam in the replacement engine seems to be floating the valves (likely a too aggressive ramping) at 4800, so I'll need to see if I can find something somewhere between that and the one I destroyed. May need to finally spring for a custom cut.
Thanks for the reply Dick. When you worked out the required length was it a case of trying different lengths till you found one that worked or is there a formula?
Mine isn't a specific or tuned length per se, as the wave position in the system varies with the tach (or more accurately, with the airflow). I just made'em longer than the wave can reach. All mine does is keep the wave inside the system, with flowing air on top of/behind it, rather than let it get outside. Bells're supposed to do the job, and they work on short stroke mills, but just aren't enough for a tractor engine. From the vids your bells look like 3.5" to 4", so 6" or 7" stacks might do it (and'd look great to boot ). If you want to go hard core, you can find actual tuned intake length tables around on the 'net. They're for specific ( fairly narrow) rev bands, just like tuned exhaust systems are though. I prefer the broader band style set-up that our long stroke engines afford. Do take note that your mount flanges don't occlude your balance ports though. I made that mistake initially. Mine originally worked fine at 6.5", but I recently added a bit more so as to get the ends up above the body's boundary layer air. I also gave the ends a slight slash cut into the wind. That adds a bit of pressure, but far more importantly it helps direct the flow down the backside of the tubes, and more smoothly into the carbs & across the jets. The same as the concept for the carb mounting angle on the manifold flowing to the valves in the first place. Back in the mechanically injected F/X days, they sometimes put bells on the top ends of their stacks (440 likely has some pictures). Looked really sharp with the belled stacks sticking straight up through the hoods, but it was still the stacks that did the actual job.
As Old6 said the length can vary and charts can be found for specific RPM ranges, but too long won't cause much of a problem (within reason). Longer tubes help with mid range torque and look cool to boot. Got my first experience with the concept by clamping a tall beer can on the Carter W-1 on my '50 Chevy 6 cyl street racer back in high school. As for the belled ends on the tubes if you use an air flow meter they do have a very noticeable effect on air velocity through the tubes. Something this Bernoulli guy figured out a few years back. I have some pictures of the concept in use, but this one of Landy's Dodge is one of my favorites.
Got into a conversation at the track with a gentleman who is a legend in the a- fuel and junior fuel ranks about the positives and negatives of naturally aspirated stack type in injector inlets. He said their is nothing better than the ram air effect created by a hat(scoop) type system. His name is Gene Adams and he is known to be a front runner at every event he shows up at.
Sorry, Kiddies, I hadn't meant to lead us off into injector territory. In as much as we're carbie users, I only picked that reference to illustrate my point about the stacks themselves. Since our carbs aren't enclosed systems like injectors, the benefits we can get from the tunneling effects of simple tube stacks is useful.
Old6, is there any performance gain when running 2/4/or 6 -- 94's or 97's with a 6-10 riser on top of the carbs and frog mouth covers on top for looks. Just think it looks nice on a nostalgia race motors. Is there more HP to be found working with intake tube lengths ? Thinking 6 cylinder inline motors.
There'd likely be some hp gain available if you want to go to the effort of tuning for optimum length for a given tach range. There, you're talking about the whole system as it comes up to the valve, not just to the carb. Tuning the exhaust to match'd complete the package of course. The primary benefit in just adding the basic tubes (with or without covers) would be if the engine were putting standing waves out the top of the carbs presently. That'd capture it and keep it inside the system. A secondary benefit can be in using available laminar flow to your advantage, but that takes a bit more number crunching as well. I'd think that any form of scoop would need to be devised so as to pressurize the bowl to the same level as the throat if you want the carb to function properly. However you could discreetly vent the tube or the back of the scoop to defeat a pressure imbalance if you want the scoops for purely cosmetic reasons. I've seen this done on "communal" scoops as well.
A square ended tube has about a 5% reduction in flow. A bell (radius) end has about a 5% increase in flow. Info thanks to Holley and Hilborn.
I also discovered that just a flat washer style flange on the end will cause a small increase it velocity through the tube with my experimentation. Back in the mid 70s I worked for a company that did air conveying systems for bulk materials (sand, cement, flour for bakeries etc) and we used a large shop-vac and an airspeed gauge to figure out velocities through the tubing (mostly 3" & 4" tubes). Sort of a poor mans flow bench, so being a hot rodder I did a bit of experimenting beyond what was necessary for the jobs we were working on.
A while back I read somewhere that there was also a difference between flared ends like mine and ends that are completely rolled over/around. I was thinking maybe making some new intake runners approximately 7 inches long. Roll some round bar so they look like donuts and weld them on the external edge of the runners. I think they'd be a cross between what I've got and Tom's idea. What do you think? Dick, what width was the slash you put in the front ? Hacksaw or cut-off wheel thickness?
Sorry, I didn't communicate the end cuts well. The slash is the end itself, it's at an angle that I cut it off rather than leave it a square end. So I guess you'd call it 85* rather than 90*. It's just enough to direct the incoming air down the back of the tube a bit more smoothly, but not enough to over pressure the carbs at speed.
Air doesn't like to turn square corners, SO, anything that will help is good. a radius equal to 1/4 the diameter is a good place to start. A complete roll at the end is even better.
Sorry 'bout that chief . Too much beer, not enough brain cells. Instead of a 90 degree cut off the end, you've cut off at 85. If they were left at 90 degrees, you would have a siphoning effect like the old style pump fly-sprays. Too much and you have a ram effect which I'm guessing is not the desired thing for SU style carbs. Did you match the tubes diameter to the carby inner diameter ?
Y'know, you'd think our beer & our brain cells'd get along better. After all they both start with "B", right? Yeah, the 85* seems to balance out about neutral on'em. I leave the bowl vents open, outside the tubes as, and you nailed it; I match the tubes to the carb mouth diameter hoping for a smoother flow into the throat. Not a clue what you win for it though.
Don`t know about su`s, but it would seem to me that if the reversion impulse is happening at the carb inlet , it would interfere with fuel delivery thru the carb . It would seem that longer velocity stacks just mask the problem. Maybe some one could explain.
It definitely DOES interfere with the fuel delivery through the carb, i.e. the fuel curve. When the intake valve closes it creates a compression wave which travels back up the intake runner. The velocity of the charge in the runner reverses its normal direction and blows back out the carb venturi. The venturi restriction syphons additional fuel out of the booster, creating "fuel standoff", the fog of fuel seen above the carb.. If there is no scoop or stack this raw fuel blows away. If there is a stack or scoop this fuel - or at least some of it - gets drawn back down the carb on the next intake event. Needless to say this plays hell with trying to establish the correct air/fuel ratio in the engine. A plenum chamber under the carbs will greatly attenuate (lessen) the amount of reversion seen in any one particlar venturi as it will effectively disperse the reversion pulse throughout the plenum. Of course, now you will be drawing a charge through every venturii on the intake system for each cylinder filling event, thus necessitating the need for significantly smaller carbs than an IR intake will need.
LOL, concurrent typing rears it's hilarious head again. Here's my somewhat less urbane version of that. You're quite correct. The wave disrupts the mix when it over travels the incoming airflow enough to reach up to disturb that flow at the point of fuel entry (the jets in SU's, and the ports in regular carbs) at the aforementioned Bernoulli restriction. This is a given, every time a blockage (valve closure) occurs, and is not avoidable. What a tube does is fight it by collecting a longer column of moving air atop the standing wave to sort of ram the wave back down the system. With enough of that effect it's kept below the access point of the fuel. Obviously, a decently shaped bell atop contributes nicely to the efficiency of the tube as well, but isn't enough by itself for our low taching engines. A large enough plenum (common or individual) does the same by absorbing/dissipating that wave below those points as well. The whole objective is to keep the wave below the mix access point in some way. The tubes're easier to build than plenums are, and can be adjusted after the fact easier as well, if needed. And they do look sexy. Now for the reasoning behind my angled tips ...... In better Carbs the ports're moved closer to the center of the airstream (and better efficiency) by "bridges" and/or annular rings. In slide carbs (SU's and such) you try to mount the units in such a manner that the airstream is directed to the side of the system that the jet is on. This is often aided by angling the base flanges, as in the Lynx models for the Slant 6. My angling of the tube tips is intended to similarly "scoop" the air toward the rear of the tubes, and induce a more controlled (less buffeting) flow for the carb to work with. And of course, they look even sexier.
The Jalopy Journal
