Dominic Sorresso

Also a motor is going to use only so much air and fuel and a given rpm and vacuum. Whether you're driving w one or two injectors operating, the fuel metered will be the same for both. As wfot stated, there are 4 VE tables in the LT-5 calibration along w 4 Spark Advance tables. Upper/lower rpm and Open/Closed port throttle. The switch over is supposed
to be seamless. And the Port Throttles (secondaries) open and close at varying TPS% depending on rpm. The higher the RPM, the lower the TPS% required for Port Throttles to open. My secondaries turn on(port throttle not there) once TPS% leaves idle setting. So I idle w just primary injectors but everything else operates w both.

Tom400CFI

Book says the original design had the small TB specifically to feed the independent plenum, "spiderweb" (mini runners) and "air shrouded injection" -like you said. The book doesn't say where that small TB was located. Without more details, I guess it's hard (for me anyway) to decipher with certainty, where the small TB was originally located and if it moved or not, when "kept" for production. It's not unreasonable to assume that it was in the same location, for the "air shrouded Injection" (very similar to the LT1's IAC manifolding and "throttle"), and they simply eliminated that part of the internal casting in the plenum/runners for the "ASI", and kept the small TB.

It makes good sense, what you say about the TPS vs. MAP ratio, and how a small TB could help that situation -especially at the low end of the throttle range where changes are super sensitive. Still, there are plenty of large TB, MAP motors that meet/met EPA criteria w/o any such complexity, so part of me wonders how much was necessary in the end and if it was kept more for driver throttle control. Cracking open the two large throttle plates off idle would create a very "jumpy" or overly sensitive throttle tip in -not a very "refined" feel, in such a high tech, refined engine.

I don't see it specifically stated in the book, what the purpose of the small TB was.....in the end. I mean, it does say "for idle, partial throttle and cruising conditions" when talking about the small TB, manifolding and "ASI", but the "ASI" turned out to be a total failure and was abandoned...the only remaining feature being the small TB.



Where did you read that? Port EFI doesn't need velocity through the TB for low end tq....any velocity created there, lost in two ways;
1. as soon as that air passes the TB and enters the relatevly large space called the plenum, it's going to slow right down.
2. When measuring "low end tq", we're typically at WOT...which means all three blades are wide open. Where is the velocity, created by the small blade?

Dominic Sorresso

Tom,

You'll find answers to some of your questions here:

http://www.zr1netregistry.com/Inform...ZR1Trivia.aspx

There's an answer for the Octopus air webbing on the Phase 1 motor which was eventually dropped.

We know that too much cam, too big a cylinder head, too big a throttle bore affects low end performance. Things get "flabby" because the pressure differential between atmosphere and the motor internally is small. The small primary acts as a restrictor raising the vacuum, in turn increasing the speed of the airflow through motor. A motor requires X amount of air at a given rpm based on its displacement. I can get that amount of air the motor either by increasing the size of the opening or increasing the speed of the air going through the opening. The greater the differential pressure (lower MAP), the higher the air velocity. Both you and I are familiar w TBI systems and that higher air velocity through the TB is desired so you shear off the fuel being sprayed by the injector onto the walls of the throttle opening.
In the case of SPFI, the faster air travels past the injector nozzle better it enhances the already atomized fuel and the amount of fuel the air carries. When people complain of "drone" from an exhaust, its due to when the motor is operating in the higher kPa when differential vacuum is lower. Then you get a "flabby" sound coming from the exhaust creating a resonance.
Looking at an LT-5 induction system, its an L-98 TPI on steroids. The air has a LONG path to the chamber with a 90d turn in the plenum, and almost semi S 180d from top of runner, thru the injector housing and into the chamber.
Put a vinyl covered binder in front of the airhorn of a stock LT-5. It'll rip the vinyl off the binder.
IIRC, the ZR-1 needed 7hp to maintain a stable cruising speed on flatroad. So once at cruise, the main TBs are barely cracked open.

Tom400CFI

Read that link and found absolutely nothing about the TB assy.

It does? At WOT?? How does it do that?

Dom, any throttle device, on ANY engine, "acts as a restrictor, raising the vacuum". That is what "throttles" do. Doesn't matter if there are 1 big'n, 2 mediums, 3 different sized ones, 4 smallish ones, 4 progressive ones, doesn't matter if they're butterflies, round slides, flat slides, "D" slides, iris'...what ever. It's a fairly precision device, that "acts as a restrictor, raising the vacuum".



Yes, Dom, I know how motors work. and I agree that higher air speeds at the point of the injector, in a port injected motor, are important. THAT, has nothing to do w/the throttle body. Any velocity gained in the TB (which again, how is velocity "gained" when you have two 53mm or whatever blades wide open?) is immediately killed, in the plenum! I said that above, but you post on as if you think the velocity in the TB is maintained through the plenum, down the runners, to the valve....it's not. Even if it was, for any given engine load, you've have to have the same total area, opened by throttle blades to meet that requirement. Doesn't really matter if the opening is shaped like a dime, or two 53mm Crescents; it's still the same area, and the velocity though it will be the same. Then immediately "die", in the plenum.


None of ^that^, though, has anything to do w/the need, or purpose for a small blade. It explains nothing...it's just "ZR-1 trivia"...most of which is identical to none ZR-1 C4's too. (like sucking a vinyl cover off a binder (?).

The only reasonable explanation I've seen posted for the small TB is the one that GlassSlipper posted about the TPS/MAP ratio. That, and off idle throttle sensitivity are the only reasons why there should be a need for the little port.


.

Dominic Sorresso

"Yes, Dom, I know how motors work. and I agree that higher air speeds at the point of the injector, in a port injected motor, are important. THAT, has nothing to do w/the throttle body. Any velocity gained in the TB (which again, how is velocity "gained" when you have two 53mm or whatever blades wide open?) is immediately killed, in the plenum! I said that above, but you post on as if you think the velocity in the TB is maintained through the plenum, down the runners, to the valve....it's not. Even if it was, for any given engine load, you've have to have the same total area, opened by throttle blades to meet that requirement. Doesn't really matter if the opening is shaped like a dime, or two 53mm Crescents; it's still the same area, and the velocity though it will be the same. Then immediately "die", in the plenum."
- Tom 400CFI

A. I wasn't being condescending to your knowledge of how motors work. You're not the only one reading the thread and you won't be the last. Excuse me for adding some additional detail that you consider unnecessary. Same goes for the other "trivia"

B. The differential pressure represented by MAP is consistent throughout the induction system. It doesn't go from 45kPa to 80kPa and then back again. It would need to do that in order to speed up, slow down, and speed up again as you suggest. If it's 45kPa, then it's 45kPa from throttle body to the valve and chamber. At any given timeframe there's a cylinder requiring air producing that pressure differential. That's what is being measured. Since you know motors Tom, you know that the plenum is immediately behind the throttle body. Where do you suppose the vacuum at the throttle body is coming from??

C. As glass slipper posted the smaller TB allows for a finer increment of throttle opening for low end performance.
My comment about using 7hp for cruising purposes wasn't trivial. Rather I cited it to illustrate that with the need for only 7hp, the motor could easily cruise steady state using just the small throttle blade opening. What I posted supports, not contradicts what glass slipper posted.

D. If you read the information I linked from the Registry, then you read about the Octopus webbing. It wasn't a sub-plenum. It was an air shroud "harness" meant to pipe extra air around the primary injector to aid in cold start emissions. That was the purpose. It was in the Phase 1 motor and dropped at that point. The small port is obviously not a remnant of that experiment.

E. We both agree that higher air velocity around the injector is important, and you're correct in that you can create the same size restriction whether using twin 58mm blades or a single dime size blade. You lose the fine incrementation w the large blades. The small port was what was available in the 80's without the use of electronic throttle control found in the latter C6s that even eliminate the IAC.
So NO, the throttle blade port does not need to be small to maintain velocity. In the case of the LT-5 in 1986, and without the engine control systems of today, that's how they did it.

Peace.

Tom400CFI

Sorry that I misunderstood your tone/intent.

No. If you measured the pressure at the edge of the throttle blade, cracked, it would be lower. That is why the velocity would be so high. Why old cars pick up "vacuum advance" from that area. Pressure would be higher in the plenum than runners albeit very very slightly. Pressure would be lower in the runners (where velocity is higher), and even lower at the valve, where velocity is (or should be)the highest.


No. What is being measured is an average of all the cylinders. While there is some fluctuation in pressure as cylinder cycle, it is slight -especially in closed throttle modes.


The plenum. Which ultimately gets it's draw from the cylinders. Dom, imagine that the plenum was a 55 gallon drum w/the same TB at the far end from the runner openings. Do you think the 55gallon volume of that space would act as a reservoir for the vacuum fluctuations of each cylinder? It would. The draw on the TB would be a virtually seamless draw -during constant mode of operation. The plenum does that. Not to the same extent, but the plenum is a "shock absorber" of sorts, between the TB, and each individual runner. Evidence of this is the sound of air being drawn into the TB -especially at low throttle openings. Do you hear each individual cylinder? No, you do not. You hear a "hisssss" of air being drawn in at a continuous rate.

IDK HOW, you can think that air will move through a ~1cm hole, then enter a large chamber (plenum), and still maintain the same speed. It's not going to.



I have agreed w/GS's comments about finer throttle control several times in this thread. In fact, *I* feel THAT is the only legitimate reason or need for the small TB. Said that in several posts. So I agree there. I don't recall YOU saying that anywhere, until now.



I read it. I said that I did...didn't I? I still have not read anywhere that the TB for that system was entirely different or separate. It may have been...though how would you practically linkage it? Doesn't matter either way. Really has nothing to do w/the thread. I previously thought the small TB may have been a "left over" from that system...maybe it wasn't...maybe it was. There is no writing that I am aware of that clearly states one way or another.


I agree, and have said that many times now.


Bingo. I completely agree. *I* think the primary purpose of the little TB is for....
Not for fuel economy, emissions, but that it was a mechanical way to create a similar effect the C6^ creates using a curved throttle bore, and e-controls....minus the huge lag the C6 has!