djjjr42, I think I may see the missing piece of the puzzle. See if this makes sense to you.

1. The vacuum advance port is just upstream of the throttle plate, I think we all agree on that.

2. At small throttle plate openings the plate sweeps past the port and it's exposed to <about> the same pressure as the manifold, assuming dynamic flow effects are minimal.

3. The engine is always trying to pull in the same volume of air for a given rpm. As the throttle is opened, the pressure in the manifold increases towards atmospheric, the pressure that the vacuum port sees will be rising along with the manifold minus the pressure drop due to Bernoulli effect. All we have to do is figure out if the Bernoulli effect is significant.

Attached is a screenshot from a CFD model that I ran to show the degree of the Bernoulli effect. The vacuum is on the right side. The throttle plate is at 45 degrees. Notice that the pressure in the location under the center of the throttle plate is nearly the same as that of the air towards the plenum. Essentially, the pressure signal from the port just under the center of the throttle plate WILL BE the same as the plenum.

 

Pretty much what you'd expect at WOT, which 45* is very close to since most carbs don't actually pitch the carb throttle blades the full 90* like they're supposed to...do the same model but pull the walls in a lot closer and use just a tip in throttle at 15*...



I will await your photo tutorial showing how to make the rats nest work with an aftermarket carb!

And, in fact, I have been thinking about that rat's nest and all the space it occupies that it shouldn't have to...I'm looking to create room for two Accel E-coils in the oil tube area and can't honestly see why all the snaking tubes have to be spaced so far apart and in such a haphazard manner.



Screw the environment, I want to let my rotary breathe!!

Ah, maihn....we all gotta try to do some small part, especially when rotaries, by their side port nature, are such eco-terror engines. "A single tear drop raises the Ocean"

 

what does the throttle bump do?
haha i was just wondering because in my car its not hooked up to anything.

 

Yeh I agree, we need to pay a carbon tax for our rotary pleasures.

 

No, not a tax...we need to keep running some form of air pump and cats on ALL rotaries...even with a big street port, a stock Nikki running through such a system knocked out 135 HP and passed TX State emissions...I wish to thank Ari Yallon and Chris for pulling it off with their custom SC header and exhaust system.

 

Here's 15 degrees.

 

Ray!!! I had no idea you had ever removed anything, dealer stickers off the glass included!

What is keeping your silver one from idling correctly? Maybe we can help...

To answer your question, I guess that really depends on your perspective. If your idea of performance is gas mileage and being environmentally friendly (whether the state makes you comply or not), then by all means leave your rats nest on. However, if you are looking to unleash the power that is hidden in your rotary by the restrictions of the stock setup, an aftermarket carb and exhaust are the way to go. My 7 is not a daily driver, its a car strictly purchased for fun. My state does not require vehicles to pass emission tests anymore, and since I don't drive it that much, gas mileage is really of no concern. What I am concerned with is how to make that thing go faster when I do drive it, so I can have the most fun with it! I have installed a full RB exhaust system, removed the rats nest & air pump, and installed the weber dcoe 45. Just those modifications alone are supposed to add about 50hp to the car, a 50% increase over the 100hp @stock.

I did the emissions removal and installed the RB system all at once. I can tell you that I did notice a significant increase in power from those 2 mods alone, not to mention a more pleasant exhaust note. You must remember that all that stuff on our 7's - the multiple cats, the air pump, all those freakin' hoses and solenoids...are there for pretty much one reason, to keep emissions within regulations enforced in the US at the time the car was produced. The air pump adds drag to the rotational force of the engine, the cats are very restrictive for the exhaust, and all those vacuum hoses and solenoids are well, just overkill! Not to mention a PITA when you are trying to troubleshoot a problem. When I say removing that stuff allows your rotary to breathe, I mean exactly that. With an aftermarket carb and exhaust you really are opening up a lot of restricted hp that is otherwise lost to those stock devices.

All that being said, I love my Weber! Granted, I have just installed it and am awaiting some new jets & venturis to further tune it, but that baby really did add some hp that I could feel on the old butt dyno! The sound it makes now when I get on the throttle (coupled with the RB exhaust) is to die for! It's sooo addictive!!! It cranks and idles fine, even under cold conditions (you're near me so you know about the recent cold snap). I did not even have to give up the manual choke, though it only idles with the choke at about 2k instead of 4k. I have not had a chance to see what kind of mpg I am getting, but as I said earlier I really don't care much about that. The thing is, now that it will run faster it will only be tempting me to stay on the gas! However, I have read that these carbs really don't suck too much gas while cruising, but of course fuel consumption always varies by the driver. I fall into the lead foot category, personally.

Ray if you ever get a chance to ride in or drive a modified 7, you might just change your tune. Since you have 2 7's, why not try removing the nest on the one that's not being driven much and see how you like it? The one with the header would be an ideal candidate since it already has no cats- which by the way throws your emissions out the window already. Is that the one you are having trouble with getting it to idle?

 

Purple, I have no idea what I'm looking at here, but those are some damn pretty colors!
Seriously, in retrospect I truly think that we all had some pieces of the puzzle, it was just finding a nice way to fit them all together that was the hardest part. I was just really offended by the cocky attitude that revhead took right off the bat, being so assumptive, critical, and acting all high and mighty to other members. It had nothing to do with me, as I wasn't in the discussion at that point. But I digress...we all know what assuming does! I do apologize for allowing myself to be sucked into his little personal attacks and stooping to his level. He was right about one thing though, that I should have taken my own advice. Although I don't think he was recommending the advice in my signature, that is what I should have followed!

I appreciate you putting effort into seeing this thing through to the bitter end. Your thoroughness is admirable. I do believe that we are lacking some critical pieces of information that are essential in helping us to determine the final answer. That would be how far this vacuum advance port is both from the base of the venturi and from the throttle plate. The close proximity to one or the other would be very influential, or so I would think.

One thing that I do feel that I have a grasp on is that venturi vaccum and manifold vacuum are moving in opposite directions at the same time. Wouldn't it be funny as hell if it turned out that they both had an influence on vac advance, at different RPMs or something?!

 

Hey, who said I don't do my small part!? I only drive the 7 on the weekends!

 

What you're looking at is a tube with an angled plate in it, a simple model of a throttle body. Low pressure is at the right, flow is from left to right.

If you think about the way the venturi effect works, velocity increases with decreasing volume, like the nozzle of a garden hose. The opposite is also true, where flow slows down as volume increases. Pressure goes with the opposite of flow velocity, so as the area increases and flow slows down, pressure goes up.

The vacuum port pick-up is typically just at the upstream side of the throttle plate. The plate opens towards the port, so the port would be at the bottom of the pictures that I showed, just to the left of the center of the plate. Now look at what's happening with area on the bottom side of the plate, as the flow gets past the edge of the throttle plate, the area increases, slowing flow, increasing pressure. As you can see, it's a minor effect in any case.

Hopefully, this gives everyone a pretty good idea of what's going on at the throttle plate and in the vacuum advance port. As you can see, the pressure in the plenum is also <near> the pressure in the vacuum advance port. It's actually so close that I can plug the MAP sensor from my Haltech into either port on my S5 engine and it will run the same way (I have a zero throttle map independent of the MAP signal for idle).

What else is left here guys?

 

I can see my humor needs work, I knew what your model was representing. Just so that I'm clear, did you do this test with your MAP sensor at idle only?

Also, I am still not convinced that venturi vacuum has no effect. It will be increasing proportionally to engine output - as the pressure in the manifold increases with the throttle opening, it is decreasing behind the venturi at the same time, forming an ever- strengthening vacuum there which *may* be an influential factor on the vacuum advance port.
My logic is hitting a dead end when I try to imagine that manifold vacuum is influencing the vac advance, because as that throttle plate is opening more and more, the vaccum is dissipating. Which would mean that the most advance (strongest vacuum) if it was indeed coming from the manifold would be coming just as you crack open the throttle, and then retarding the more that the throttle opens. This doesn't coincide with the info I've read stating that vac advance operates above and beyond mechanical or static advance, and at mid-high RPMs.

 

I drove the car extensively with the MAP on the vac advance port. No difference at all.

I'm not saying the throttle plate has no effect, it certainly does, but it is negligible in comparison to the other forces at play, so to speak.

 

Hmmm, vacuum advance should increase as the throttle is opened until that advance is maxxed out (does not fall off). Then the mechanical advance builds on top of that as the rpms continue to increase. The sum of both vacuum advance and mechanical advance is referred to as "total advance".

If the vacuum source for the vac advance was the manifold, then the vacuum advance would fall off as rpms increased. Which would mean that you could only ever be dealing with vacuum advance or mechanical advance, but never both at the same time. That would mean that there was no such thing as "total advance", but we all know that is not the case.

The only thing that makes sense to me is that the vacuum source is from the venturi effect, which would continue to increase as the rpms went up. Nothing else would bring us to the point where we would have a "total advance", or the sum of vacuum and mechanical advance. Does this make sense to anyone?

 

That's contrary to everything I'm trying to show here.

The vacuum advance and mechanical advance certainly can be acting at the same time. You can drive at 5000 rpm with the throttle nearly closed, right? Don't get load and speed confused.

 

Heres my contribution to this thread.
Since I found out it has turned into a pissing contest. :-)

Ok. Now that I have finished reading this monumental book and all parties have clamed down. You may continue.

When you guys get done playing I will go in and clean up. If I see anymore type of personal attacks I will not close the thread, I will delete it and let you guys figure out what conclusion you came to.

 

Forgot to add.
I would like to see everyone that is involved work together to make a comprehensible conclusion to this thread so I can archive it and all can benefit.
Thanks.

 

Glad to see this thread re-opened.

Purple, I think I'm following some of what you're saying now and understand why we aren't on the same page just yet. We're getting close though! With your model and explanation it seems that you are attempting to apply the Bernouli principle to the areas at the edges of the throttle body around the plate, when it should be applied at the base of the venturi. To make this model more precise and helpful in bringing us to an agreeable conclusion, you would have to have at least some portion of the model smaller (on the left side of the throttle plate if flow is from l-r), indicating that it is coming from a smaller diameter chamber. Why? Because that's exactly what the venturi does, and also what makes it's vacuum rise proportionally to the power output of the engine. Take my Weber DCOE 45 for instance, the main venturi are choke tubes, size 40 (right now at least, other sizes en route) and they fit in the chambers just outside the reach of the throttle plate, and on the opposite side of the plate in respect to the manifold. Then their are also auxiliary venturis where the metered fuel comes in. The amount of fuel coming out of the bowl increases with total flow and the driving force behind that increase is the vacuum being generated at the base of the auxiliary venturi. Fuel is literally sucked out of the bowl by this vacuum. Does that make sense?

 

Darn, it was reopened lol Doc, I don't care what you do with this thread. I personally didn't find it to be very helpful.

I went with the red diagram, which worked ok on the '84 automatic (I didn't try it the other way because it wasn't my car and didn't have time to experiement with it).

I didn't receive any help during the intake swap with rat's nest removal. I had to figure it all out as I went, with only the diagram as a guide.

If you want to simply delete this festering pile of a thread, I wouldn't miss it.

 

Here's 15 degrees.

D'Oh!! lol...my bad, I meant 15* from full closed, which would be 75* for your program...


If I see anymore type of personal attacks I will not close the thread, I will delete it and let you guys figure out what conclusion you came to.

I think that should be what happens to every post that has a member cursing or attacking another member for whatever reason in this 1stgen formerly-"tech" section...delete the whole post, good information and bad, so that the person has to retype it all in a proper manner that will survive scrutiny if they want to make a contribution to a thread...there's the DoO for the flame attacks...if you're tuff enuff...

 

I'm pretty familiar with the way a carb works. The flow goes through the venturi, picks up fuel, then opens back up, then goes through the throttle, agreed?

I will run another CFD analysis showing that, but I can tell you now that it's not going to change the relationship between the advance port and the plenum.

 

That is 15 degrees from closed.

 

The majority of street driving is done on the idle and transition circuit such as light throttle cruising. When you open the throttle to accelerate you are increasing load are you not? Obviously the amount of fuel being drawn through the main circuit also depends on rpm.

 

Like I said I can open up the throttle in my driveway and not increase the load at all.

 

Venturis are very sensitive to the angle on the expansion side. This model is pretty rough, but you get the idea.

 

That looks great, but I think the venturis should be flat on the backside, not angled as you have them here if I'm looking at this correctly. I'm not sure if that would change what happens though, but it may. Would you do a series of progressions with this model, say in 5 degree increments all the way until the plate is flat? I think that showing what goes on in the full range of motion would be, if nothing else, pretty cool to see.

As of now it looks like the pressure is significantly lower at where the throttle opens up and would pass over the vac advance port which is just left of the plate, is that correct? If so then would that not indicate the strongest vacuum signal over the port and thus the highest advance when the throttle is just opening?