Bigger Venturi

H

HardScrabble

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Read somewhere round and about a bit on the DEI plate track edge.

No clue as to the source of the info or its veracity.

Anyways, the claim is that the DEI cars were much larger venturi in their carbs than anyone else. The implication, though not clearly stated, was that they were not within the approved parameters. This was claimed to be the reason that telemetry was showing both Jr and Mikey at running at less than 100% throttle during mmuchof the race. It was purported that this gave them a competitive edge over the other teams.

This is not to be construed as anything more than rumor. The purpose of this post is to ask a question.

Given the presence of the restrictor plate, with its effect on intake flow, would the use of larger venturi give any HP gain? My first reaction is that the carb can outflow the plate as it is, so nothing would be gained. Is there something more subtle at work here?
 
Really don't know what a venturi is, but I'm assuming it has something to do with the opening of the ports on the carb. With the restrictor plate and gasket, I don't see how anything could get around it to make to the venturi.

But then again, I don't know what a venturi is and I ain't no mechanic. ;)
 
While I don't know what type of carb or flow rates they have, this is what I do know.

The primary reason for keeping venturi size small is to maintain airspeed through the boosters. Small venturis porduce good low end on smaller carbs. Therefore big venturis produce good top end on big carbs. The dilemma is to use a different-design booster venturi. Styles on some carbs (I don't know which they use): downleg boosters and annular boosters. Annular boosters are more responsive than downleg boosters. They pick up the vacuum signal quicker and start flowing sooner. While annular boosters offer more bottom-end driveability, they run richer on the top end and may siphon fuel out of the float bowls. The annular booster also decreases the carb’s ultimate airflow potential because it has a larger physical presence within the venturi. For example, a 950-cfm carb with annular boosters may have the same venturi size as a 1,050-cfm with downleg boosters.

Don't know if that heled or not.
 
Getting there,

Understand what you're saying Nate, but can't relate it with plate in place.

The carb all the teams use I believe is a Holley 750 rated at 830 cfm.
 
kinda sounds to me the venturi can change the thrust i guess of the fuel and air mixture, increasing the amount of fuel and air getting into the engine, and increasing horsepower??
 
increased top end horsepower, kinda like putting a bigger whipper on the mixing bowl. Still can only get so much in the bowl, just whipped better I would imagine.
 
Last I knew NASCAR dictated the carb number as well as the venturi and throttle bore sizes. Teams are allowed to tune with jets and power valves (if they use them) and that's about all.
Those items are checked during pre-race inspection.
 
Natedogg,give us a hand on this?On a NON-restrictor plate engine this would definitely be an advantage,correct?Reason would seem to suggest that.

Then is it an advantage with the plate in place?I mean,what excactly would enlarged venturi accomplished in an already restricted motor?

I know I sound confusing.Someone explain this to me like I was a first grader,please!
 
ok, I'm not real up on carbs in general, but First we have to know just what a venturi does.

As air flows through the narrow part of the venturi, the velocity increases and the pressure will drop. If the pressure drops lower than fuel pressure, which is at atmospheric, atmospheric pressure will push fuel into the venturi.

carbsmaincircuit.gif


This will introduce a lower than atmospheric pressure at the discharge nozzle. A booster venturi can be used to further decrease pressure and help fuel distribution and atomization. The fuel is at atmospheric pressure due to the bowl vent.

With the pressure at the discharge nozzle being lower than the fuel, the fuel will be pushed out of the discharge nozzle. Like a shower head. I think there is a whole lot more to it, unless they have something inovative that passes inspection and does what I am aboutto say. The better they can mix the fuel the better the hp and miliage. Its not how much you can stuff down the intake, its the quality of the stuff. WIth a tad extra hp, you can back off and still run all the while saving gas. Sounds simple, but unless they have david copperfield, I can't see such an advandge getting by.
 
Thanks for the info.

For the sake of clarity this is the full context of what I read from a Mike Daly column:

"Rusty Wallace was forced to backpedal on his Chevrolet-is-cheating comments at Daytona, but in Speedway Scene's Rockingham issue Mike Snow quotes an unnamed crew chief acknowledging that Dale Junior's carburetor contained overly large venturi openings, but that this was ignored, and that literally no one in the garage believed Ty Norris' protestation of innocence in the wake of Wallace's Daytona blast. The unnamed crew chief further cited telemetry available on NOL showing that Junior and Mikey Waltrip were running at only 70% throttle versus others running 100%.

"It is here that I would like to make a request of readers - if anyone was able to read such telemetry on NOL during the Daytona 500, I would like to hear from you to try and verify this assertion. I do have my own suspicions - suffice it to say that, with more than enough trouble looming, NASCAR had better hope DEI gets a LOT in the way of a serious fight for the win from Dodge, Ford, and Pontiac come Talladega in April."

As boB mentioned the carbs and the mods allowed are regulated by NASCAR, they are not off the shelf items, though you can purchase a very good carb from any number performance suppliers. As with any other regulated area of the ccar, there are grey areas in which the teams can and do work.

I immediately assumed, which may be foolish, when I read this that the "booster venturi" was the item in question. These can be replaced by "performance units" and are fairly easy to modify if you know what you are doing. It is just as possible now that I think about it to modify the main venturi area of the carb. Smoothing and polishing this area was pretty much standard back in the day when I fooled around with some of this stuff.

As a matter of fact the the excellent drawing that NateDogg posted shows what was one of the standard mods every performance carb carried back then and stil do. If you follow the "Bowl Vent" passage up to its top you notice the opening in this drawing is angled. This opening, at least back in my day, came from the factory cut square to the vent tube. The first thing anyone did was to cut this at the angle you see. The purpose is that as air is drawn towards the venturi it passes this vent tube at pretty decent velocity. By angling this opening towards that flow you actually pressurize the fuel bowl slightly.

Anyway, I have to question this whole story somewhat. The unnamed crew chief would have to get a pretty darn close at DEI's carb to know if the venturi opening had been enlarged, something the teams tend to guard against. Second the whole thing strikes me as odd because of the Rusty Wallace deal. His eam got caught with an unapproved carb mod, so for now someone to say DEI did not get caught for the same type infraction seems too convenient. I do not think that Rusty or any member of his team are at the heart of this story by the way.
 
First thing here is that until I saw the parts in question myself, I would tend to question anyone's comments or remarks.
NASCAR makes it a habit to be quite thorough in regards to this type of thing.
(My son can attest to how thorought NASCAR can be when there is a question about a carburetor. He has some first hand experience with their inspectors on this subject.)

Not trying to knock Mike Daly, but he has been known to write a whole lot about issues when he has but very little information on which to base his work.

My last rulebook (it's a few years old now) states that the only carb allowed will be the Holley 4150 series with a 1-9/16 inch maximun venturi bore and 1-11/16 inch throttle bores. Boosters may be reworked by light polishing only, no reshaping. Only Holley replacement parts for the 4150 carb may be used. The venturis may be hand polished only and may not exceed the maximun diameter. Venturis must be completely round and may not be raised or lowered in the carb body.
Alterations to allow extra air to be picked up below the opening of the venturi, such as altered gaskets, base plates, and drilling holes into the carburetor will not be permitted.
For all NASCAR Winston Cup Series Events which require a restrictor plate, the only carburetor permitted will be the Holley 4150 Series with a 1- 11/16 Throttle bore and a minimum 1 -3/8 inch venturi.
Carburetors may be reworked as specified in the NASCAR Winston Cup Series Specifications.
There are some other specifications listed in this section, but they really are not relative to what we're talking about here.
(These specs may have changed, but I'm sure they are still very similar. As I said, my copy of the rule book is somewhat outdated.)

Think about this one for awhile. Rusty was caught with a carburetor which had SMALLER throttle bores than the rule book specifies.

As far as the DEI cars running at 70% throttle, did anyone actually see the telemetry that is mentioned, and if that is the case, you can bet that NASCAR's inspectors saw it as well. But once again, give some thought to how that might be possible?
Might it have nothing at all to do with the carb, but other things which that team has found to work to their advantage which are completely within the rules?
 
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