Re: Vacuum leak? Marc has me confused now!
 

The engine had more power than GM thought was necessary to set the record.

To ensure durability, best fuel economy at the target speed and ensure no consistent throttle opening, the engine had a throttle stop.

The durability issue was the biggest reason. In 1990, no one had ever run any production car in an attempt to set the 24-hr speed record. The previous attempts (Ford, Mercedes, Audi) were done with modified or purpose-built vehicles. No one knew if the LT5 would last. In fact, GM was so worried the attempt would fail, the Record Run was done in secrecy with few media present and only limited documentation by internal GM and sponsors.

Re: Vacuum leak? Marc has me confused now!
 

It's probably the check valve (most common) but you'd want to
find the little black hose and trace it up toward the engine.
You'll find a rubber connector about half way up. Open it.
Put you finger over the hose that runs tot he pump.
The pump should run and then stop. Keep it plugged.
The pump should not run again (wait over 60sec).
If all ok then the leak is under the plenum (check valve etc).
If the pump runs again then the leak in in the diaphragm inside
the pump. This happens when a pump has cycled too many times
and has worn out the diaphragm.

Re: Vacuum leak? Marc has me confused now!
 

I'd say, you have a leak but not really a big one or the pump might have a somewhat restricted filter screen.

I just went out to the shop and checked the pump run time on my 95. After the initial run period, my aux. vac. pump runs for about a second every 15-20 seconds. I know my SPT system is in good condition. The SPT rule of thumb is, once the system is evacuated, one second every 15 seconds.

Re: Vacuum leak? Marc has me confused now!
 

Apples n Oranges??

When the LT5 performance is down, the question of the SPT system immediately comes under scrutiny, as it should - especially if a "61" occurs. However, for anyone following these "secondary" threads over the years, there is a tendency to proclaim AH-HA! prematurely! - pointing to the pump cycling as indicative of the problem being the SPT system when it isn't, regardless of whether it too needs to be serviced.

I think for the OP's benefit, the distinction to be made is between what is characteristic of a proper "tight" vacuum circuit vs. what the system will tolerate before causing an issue i.e., the SPTs not opening or throwing a "61" code.

Keep in mind the SPT is a closed circuit. So, if there were no leaks, even at WOT the pump wouldn't run but perhaps just long enough to replenish the initial reservoir vacuum expended to initially pull the actuators. So, barring any leaks, the pump would not run continuously - even at WOT.

Now, if there are some leaks, which typically there are (the many check valves, rubber connections, cracks, etc), the pump is there to maintain the vacuum...and shuts OFF when the necessary vacuum is reached - and will come back ON when before vacuum level is depleted to the point of jeopardizing the SPT system.

Marc's quick n dirty way to sort out the SPT comes from years of hands-on experience: specifically if the vacuum pump shuts off, even for a second, then one can assume there is sufficient vacuum, at least up to the secondary port throttle valve. He also discusses how to observe the actuators to verify their operation; verifying the circuit from the valve to and including the actuators (and vacuum can be monitored during that test as well.

The fine point missed (as I see it) in this/these discussion is NOT that the secondary circuit isn't perhaps in need of service - it could very well be. But! In the quest for locating a performance issue, one may be able to put the SPT system aside for latter attention while the real culprit is ferreted out. Make sense?

Re: Vacuum leak? Marc has me confused now!
 

I don't think this is a performance issue. At least reading the OP there is no indication of loss of power. The thread is about being confused. Getting apparently different or conflicting info.

Given that we're trying to clear it all up and not likely to get a consensus I'm sure the waters are now completely muddy. Oh well.

I see this whole "vacuum" issue as this.
You either have no issue with a leaking system
or you do. If you do you either decide to fix it
or let it go until you get a code 61.
Hey, but that's just me.

Re: Vacuum leak? Marc has me confused now! Th
 

Marc would say that it's operating properly. I was with him when looking at a stock ZR-1 that exhibited the same pattern. As long as the pump shuts off, the system is holding sufficient vacuum.

Re: Vacuum leak? Marc has me confused now!
 

Exactly, Scott. My Z pulls like a freight train and idles smoothly at 800. Just trying to figure out how bad my leak is. Thanks for "boiling it down".

Re: Vacuum leak? Marc has me confused now!
 

From Mr Haibecks site: "Has a broadening effect on the useful power band for the LT5. The throttle response is as good at 3000 rpm as it is at 4000 rpm with a 3.45 gear ratio"

So basically with stock gearing a downshift to 3rd gear may have been necessary to perform a passing manouver, with 4.10's, the same manouver can be performed in 4th gear.

Swapping to a lower gear increases torque multiplication to the wheels, enabling greater vehicle acceleration while engine output remains constant.

I understand what you mean.

xxxxxx
There would have been substantial engine vacuum with the LT5 throttle stopped at 70%. In addition to durability, other reason for the stop could be:
1)fuel economy
2) drivers right leg comfort, much easier to hold a pedal on a stop than holding against throttle spring tension

I do think that durability was certainly a concern, which is further evidenced by the decision to run a higher 3.07 ring/pinion. This would aid driveshaft, transmission, front accessory and engine durability, and again would aid fuel economy.

That record run was a great accomplishment.

Re: Vacuum leak? Marc has me confused now!
 

Another reason they used a 3.07 rear gear was to keep the RPMs under 6500 as the radiator was designed to bypass at higher RPMs due to higher water pressure. They had to keep it cool!!

Re: Vacuum leak? Marc has me confused now!
 

And, too: does not the coolant pump begin cavitation just north of 5500 rpm?

Also, I read a comment (in Heart Of The Beast(?)) regarding the desire of limiting the rpm to place the LT5 in its peak torque region.