jrubicon

i agree 100%, my GT500 was an entirely different animal in cold weather

btw GREAT THREAD!

racerns

FI engines (turbo or supercharged) will lose power at a lower rate with altitude than NA motors if they are able to maintain their boost pressure. The ZR1 has a pressure control valve for the purpose of maintaining pressue at different atmospheric conditons. You will still have some loss though. Lets say you are have an NA motor at 1 atm (14.7psi) at sea level your total pressure is 14.7 psi. The same NA motor at 5000 ft only has a total pressure of 11.8psi so you have a pressure loss of 19.7%. If you have supercharged engine making 10psi of boost then at sea level you have a total pressure of (14.7+10) 24.7 psi. Then that supercharged engine at 5000 ft is still making 10psi boost but the atmospheric pressure is only 11.8psi so the total pressure is (11.8+10) 21.8 psi but only a pressure loss of 11.8%. Pressure loss is equivalent to hp loss.

If you take a look at most of the 1/4 mile correction calculators that correct to sea level for a given DA, they give separate results for NA and FI engines with the FI results always closer to the actual.

jdmvette

GForceSS

As mentioned in my earlier post, Altitude and barometric pressure effects are similar on NA and SC (turbos can adjust to some degree). Temp affects all FIs much more than NA.

Then there is humidity.

The supercharger can release pressure easily via the valve, however it cannot add boost for high altitude; it is crank rotation limited.

racerns

I think the theory there is that the valve is always relieving while at sea level and closes more as you gain altitude to maintain pressure. I would guess that at some point in altitude gain you would start losing boost pressure.

Espresso

It will be interesting to see how it all shakes out.

Pro Stock John

I always enjoy reading about these track outings, you guys always share info and run well. Racerns = Neil? We met a number of years ago.

I get what Jamie was saying, in fact none of the comments about DA versus traction are really contradictory in the thread. I remember taking a OT car and running 10.008@133mph with a 1.38 and on slicks, and the car started getting loose. The next time I tried to run that 9.9x it got colder and I had to get out of it mid track. There is that fine line.

I'd love to see what a ZR1 would do in this circles hands with long cooldowns, and I'd put a fan on the engine too. The not quite 132mph hot lapping and limited number of runs makes me think that 135mph is possible!

MTVette

Very nice writeup, and amazing performance for a factory stock car.

I calculated the 0-60 mph times for the runs, and the best was 3.13 seconds. The calculation spreadsheet was posted here a while ago. This 0-60 time puts the ZR1 well into the class of 'super-exotics'. Way to go!

racerns

Hey John that is me. I think we met up at the drag strip near Rockford for a mfba get together. That may have been the last time I took my camaro down a drag strip. When was that '01 or '02? I only lived out in IL for a few years.

Pro Stock John

Yeah one of those years.

So what is the rev limiter set at in a ZR1, 66xx?

Tom400CFI

No. That is correct math/theory for turbos w/wastgates up to the point that the turbo is maxed out (though you didn't account for added heat with elevation), but it is dead wrong for "gear" driven supercharged motors. The supercharger absolutely can not maintain "10psi boost" as the DA decreases. And how could it? The only way possible would be to have a CVT driving the 'charger, and have the CVT control module reference Absolute pressure, and "shift" the supercharger drive ratio according to DA. That supercharger drive system does not exist on the ZR1.

I believe that the "blow off valve" in the ZR1's supercharger has been proven to NOT function as a wastegate, as you're implying.



In all the correction calculators I've used and seen (a lot since I live at 7000' and take corrections seriously), I've NEVER seen one that had a separate factor for "FI engines"...and how could one? All FI engines are different. Doesn't matter though, b/c all mechanically driven FI engines have no means by which to "know" or compensate for changing elevation.

racerns

Here is the description of how the the bypass valve controls boost of the superchager from this article http://www.corvetteactioncenter.com/..._pursuit3.html
That sounds a bit like a wastegate to me. In this case is will not open until it sees 10.5 psi. I believe that with the current pulley size if the valve was not allowed to open, at sea level, then you would see higher than 10.5 psi max boost pressure. So in a sense there is some extra boost capacity with the current pulley. The question is how much? I still think this feature will help keep a constant boost pressure in changing atmospheric conditons such as altitued change. The other function would be to protect against overboost.

Here is one that does. http://www.dragtimes.com/da-density-...calculator.php I don't have the source but I remember that when they apply the standard NHRA correction factors to turbo cars the use 50% of the factor. I still think that some of the newer superchargers do have the ability to compensate via a similar setup as the ZR1 but I could be wrong. I have a a friend that is an engineer for Eaton and have sent him a message to see if he can confirm or deny my theory.

Tom400CFI

I've read that before, and....

I think that you're misunderstanding the function and operation of that valve. First, this is not like a "waste gate" at all. It is like (and it is) a "Blow off valve". A waste gate, relieves the driving force (exhaust gas) from driving the turbine; the "dumps" the exhaust around the exhaust turbine, thereby reducing parasitic loss and slowing the compressor speed. The BOV opens during...overboost situations, like after slamming the throttle closed under full boost operation. While the BOV is open, the exhaust COULD and likely is still "working" to drive the turbine. In the supercharged system, a mechanism that could perform a similar function as the waste gate would again, be that CVT drive system. Simply slow down the blower, while simultaneously reducing draw on the engine.

The point of the BOV in this engine as stated in the article, is to improve efficiency. Having the compressor 'over boost', or pump more air than required at or near sea level (where most people live), only to bleed off some of that boost through a BOV....that's incredibly INEFFICIENT. No way would GM design something to operate that way, in these times. Not to mention the added heat such a design would generate!



That's b/c turbo cars function entirely differently than a crank-driven supercharged car. Due to the waste gate design, a turbo'ed car can SPEED UP the drive speed of the compressor as required to maintain roughly the same manifold density. How can a supercharged car "speed up" the compressor? It can't.

EDIT: I looked at that Dragtimes calculator. I don't buy it. First I stand behind what I say about supercharger operation. Second, I've ran 12.89 @111 in my box-stock, base LS2 C6 in Salt Lake City (4200' elevation). Plugging in the data, THAT calculator is claiming that my car (box stock LS2) will run 11.67 @122 at sea level. Not only is that "calculated result" up-in-the-night, but it's not even close to consistent w/any other calculator that I've used. The calculators that I've used have been verified for accuracy, by me, running the "calculated number" at a lower elevation venue. I'll never run 11.67 in my car. That calculator is "Busted".

I think that you're wrong. BUT...

As a mechanic of over 20 years, I absolutely would like to hear this persons input.

Tom400CFI

Does your car have a boost gauge? If so, go pull the vacuum line off that vacuum pot, plug the line, and go rip on it and tells us what peak boost you see, and at what RPM. My guess is that you MIGHT see 11 lbs at or right before the redline.

a1231212

MTVette

And I actually got 3.09 on my 383 LT1 C4!

Joe_G

Heh, here's one area I can beat a ZR1 - my 1.5 60 foots are 2.7 second 0-60 times per my computer (I scan all my runs)!

a1231212

no fair

siffert

That is neither here nor there, you got a heavily modded car with DR's.
Even so, the ZR1 bone stock would blow your doors off by the end of the 1/4.

Joe_G

Sure...but I'd beat him in the first 60 feet!

btw he was on DR's too.