bassplayer27360

I've decided to prepare my '91 ZR1 for Autocross. Any tips or advice for setting the car up would be appreciated. Thanks!

Tom400CFI

More info would help. What class?

You doing this for FUN, or to WIN?

bassplayer27360

I'm thinking SCCA Solo B Stock Class. I'd like to WIN, but at 63 years of age I don't think I'm as fast as I once was. I was very competitive in my 20's, 30's and 40's with several Porsche's, BMW's, Alfa's and even a Chevrolet Citation X11.

Tom400CFI

To win in B stock, you'll need tires, shocks, like-new suspension bushings etc, and probably something other than a ZR-1 (it's heavy).

To have FUN, just go there on your street tires, set your shocks to "sport" and rip it up!

bassplayer27360

That's probably what I'll do. Thanks!

Tyler Townsley

Before doing an autocross you need to understand the workings of the radiator bypass system. You can do a search on this forum or go to the ZR 1 forum and search there.

Tyler

Tom400CFI

First I've ever heard of this feature. How did this thing run for 24 hours at WOT with such a feature?

mike100

They put 3.08's in the rear end.

Tyler Townsley

They got lucky. They did not know about the bypass but the target rpm of (~5200) and cold day allowed enough water through to keep the car cool.

Tyler

FASTAZU

It bypasses at around 6000-6500 RPM's to much water pressure for the plastic side tanks on the radiator.

Tom400CFI

Lot of conflicting info here. 5200 RPM, 6500....

I don't think the RPM matters too much. When it by passes, it's bypassing enough water to limit peak pressures....but there is still a LOT of water going through that radiator. Probably not going to over heat, unless you run it at redline for extended periods, in extreme conditions. Can't imagine how this could be an issue for a 30 -60 second auto x

mike100

to clarify- the endurance record run- they decided to run at 5200 rpm which is well below the bypass rpm of over 6000 and something rpm. If they didn't know about the bypass, they may have been doing it more for fuel consumption and general engine longevity.

Some autocross courses could last 90 seconds, but I don't think the car would overheat. as coolant will mostly be flowing about 1/2 the time dpending on gear selection and how tight the course is.

Tom400CFI

Yep... I get it now.



Coolant doesn't stop flowing, ever. Some coolant will always be flowing through the radiator.

glass slipper

Your "imagination" is correct, I've had no issues for over 22 years of autocrossing and some courses were more than 60 seconds. The engine simply doesn't spend a significant amount of time in the bypass "zone" on the course. For a while my daughter was running it too so we were hot lapping it. In the Florida summer, many runs were started at ~225°F and ended at ~240°F...there are no mods to the car.

The bypass feature was a GM design because Lotus wouldn't back down from their 25 GPM/100 HP standard water pump spec and GM refused to have a ZR-1 specific radiator...another GM screw-up on what could've been a much better engine had they left Lotus alone. If that engine had been allowed to "be all it could be", today we could have a 6.2L "LT5" with 650 HP without a supercharger. Direct injection and VVT would maybe get more than 650 HP and the engine would be the same weight/size (outer dimensions) as the LT4 in the C7Z06 with a far superior HP curve and better MPG. Sorry, I let my imagination run loose with a little "what if"...

Tyler Townsley

The stock ZR-1 cooling system has a bypass system that is designed to route the water around the radiator starting at 5200 and by 5800 rpm it is routing all water around the radiator right straight back into the block. The record run was capped at 5700 rpm, again they got lucky as they had some water going through the radiator and it was a cold day.

Oh yeah another stupid design is the oil cooler, it has a thermostat that does not open until the oil temp is 250 deg. If you see that temp it is probably too late to be effective. Once you go full water bypass it gets very hot very quickly.

Graham will be at BG ask him, he helped design the Lotus side of the system.

Do your research then you can adjust your driving style to the known limitations.

Tyler

Tom400CFI

No way, the system does not bypass the radiator 100%. What would be the benefit of that?? The purpose is to save the radiator from over pressure due to the high pump volume. I know you know that. So they put what is essentially a pressure relieve valve in the circuit to bypass flow beyond what the radiator is capable of handling. Beyond...not all of it. Based on the believable numbers that you posted from THIS POST, It would seem that at max RPM about up to 40% of the coolant is bypassing the radiator (if the figures you posted are correct). Not enough to overheat an engine in any kind of AutoX that I've ever been part of. The radiator is never closed off by this bypass system...it's always wide open, so there is nothing to stop or slow the water from circulating through the radiator. It's simply that at some point (the ~60 gpm point, according to your post), the radiator becomes a limiting factor to coolant flow, the rising pressure on the inlet side begins to force the "pressure relief valve" open, porting some coolant back to the inlet side of the water pump. A this point, coolant flow through the radiator remains static as RPM increases, while coolant flow through the bypass portion increases. This could be a slight concern at a big track (although I still don't see driving around a real track at a constant 5500+ or whatever RPM)...but not at a 60 (or even 90) second autox.


I think that you're creating a concern where there isn't one. How many well maintained C4Z's have over heated at an autoX? Glass Slipper hasn't had any issue in 22 years...

Tyler Townsley

You can think, guess, speculate or whatever but what I described is how it works. This has been verified by the engineers involved in the design of the system. It may not have an effect in 30-60 sec autox but anyone driving the car needs to be aware of this limitation.

Recommend reading the following post: http://www.zr1.net/forum/showthread....adiator+bypass note at the end of the thread is a post by the GM Engineer who designed the system email him.
Tyler

Tom400CFI

Well man, I'm not "thinking" or "guessing".

This isn't that complicated. Look at this picture (I know that you've seen it before)



You see the thing w/the green arrow pointed at it? That is your "pressure relief valve". See the spring behind it? Pushing against it? What do you suppose opens that valve/spring? Water pressure. Where does that water pressure come from? Resistance to flow through the radiator.

Now, what device "closes" the flow to the radiator? Look at this diagram.....where is that device?


The upper radiator hose (the one going TO the radiator) is wide open, all the time....so what device closes off flow to the radiator? None.

Now let's apply some common sense: the point of the bypass is to limit pressure to the radiator, right? So there is zero need to STOP flow to the radiator, only to limit it. The way that I've described it does just that, while still maintaining a good amount of coolant flow through the radiator. The way that you're describing it, would over heat an engine in ~30 seconds. How many track-goers here have over heated their engines that quickly at a track day? I'm guessing (NOW I'm "guessing") none.


As for the link you posted....another thread arguing about thermostats?? No where in that thread did it rebut what I'm saying. Just LOOK at the thing. It's self evident how it works.


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z06scentair

Tom,

You should research this.

https://history.gmheritagecenter.com...R-1_Prototypes

One would gather Tyler has a fairly good understanding of the LT5.

He restored and ownes one the cars pictured in the above article.

Tom400CFI

I clicked on the link....what does that link have to do w/this basic hydraulic system?

Show me the valve that closes off flow to the radiator.

EDIT: THIS ARTICLE from the same web site if far more relevant as in the 3rd paragraph it talks about ZR-1 cooling criteria and performance:
this new Corvette program developing the engine cooling system to handle the hot Arizona ambient temperatures as the engine ran through its entire range of operating parameters. It had to run cool at 600 rpm idling all the way to Wide Open Throttle at 7000 rpm during maximum speed testing on our five mile circular track

What does ^that^ prove? Nothing, but it's way more relevant to the discussion than the link you posted. No car is going to meet that test criteria with the radiator completely eliminated from the cooling loop.

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