glass slipper

"1. The LT5 thermostat not only controls the temperature of the engine but limits the max pressure on the radiator inlet. When the radiator pressure is over a certain limit (about 25 psi at the radiator inlet), the bypass valve (spring disc on the thermostat) will open further and balance the amount of coolant flow to the radiator and engine."

The above is a direct quote from the post by the GM engineer you referenced. It's very clear the bypass valve continues to allow flow to the radiator but "balance" an amount of flow back to the engine to limit pressure to 25 PSI. Tom is correct, the drawing is correct, and common sense is correct...no engineer would design a cooling system to bypass all coolant flow to the radiator and send it back to the engine at 5800 RPM on an engine meant to live life beyond 6000 RPM.

Having said that, I think it was pretty crappy of GM to circumvent the Lotus edict of 25 GPM/100 HP by effectively limiting flow through the radiator to 60 GPM. If any GM engineers who worked on the LT5 project are reading this, I'm giving you a big fat raspberry!

zr1fred

I agree that there should be no problems with autocrossing a ZR-1. A lot of good information here but most not relevant to the OP's question. I think that most people are confusing the potential problem of by-pass and ignoring the real danger of cavitation
Marc Haibeck
[]

"I am aware of three causes of overheating for the LT5 engine: 1. Loss of coolant. 2. An air locked water pump after servicing. This can happen when several quarts of coolant are drained for service. When the system is refilled it may be observed that a large portion of the drained coolant can’t be reinstalled. When the engine is started the water pump may not circulate the coolant. This can be observed in two ways. The vapor inlet, below the cap on the black surge tank, will not be flowing coolant when the engine is started. If this is ignored it can be observed that the engine is getting very hot but the coolant gauge is only at 25% of scale. Normally the gauge goes to 50% or more of the scale after about 5 minutes of run time. 50% of the scale is 200 degrees F. 3. Operation of the engine over 6000 rpm for more than one minute. Water pump cavitation occurs at 6000 rpm. When cavitation occurs, the coolant flow through the engine drops off sharply. "

mgg

Just my two cents. I auto-x for almost 15 years with a c4 L98. Many of my friends ran their ZR-1s and I never ever witnessed an overheating problem with any of then in any condition. I own a 90 ZR-1 now and would not hesitate to auto-x it and worry about overheating. I raced a C4 L98 and used a modified ZR-1 oilcooler which worked. much better than the other highly respected oil coolers. I ran many races in the summer in brutal conditions at Summit Point, Watkins Glen and VIR without issues. I am not arguing that this problem cannot exist but I have never witnessed in my car or any of my friends ZR-1s. I would certainly give it a try and find out for yourself. As stated the extra weight could prove a little problematic but it will be a lot of fun. Depending on the course you might just surprise a few people. Have fun.

zr1fred

Here is a video demonstrating/explaining cavitation...
https://search.yahoo.com/search?fr=m...4&p=cavitation

Tyler Townsley

When the bypass opens the pump PULLS the water from the path of least resistance which is down the bypass ckt. At 5800 rpm there is a 1 in coolant passage straight to the pump with no resistance, the path trough the radiator however has considerable flow resistance so I believe that's how the water will flow. Marc and I have had several discussions about the 'cavitation' in the pump. I believe this is caused by a couple of things: 1. The water becoming superheated by the bypassing of the radiator when the bypass opens. 2. The head pressure of the system caused by an undersized radiator connected to an oversized pump.

The balance referred to was that there was a path for the water to flow through the system at 100 gpm and that does not include it going through the radiator that can only flow 60 gpm.

This has been a good discussion and hopefully there is enough here for you to come to your own conclusions.

Tyler

Tom400CFI

Sorry Tyler, but you need to go back to physics class. Centrifugal pumps don't "pull". Additionally, this is a closed system...a loop.

This is a basic hydraulic system, with a pressure relief valve. That's all it is. LOOK AT IT. The spring in the "bottom" of the t-stat is your pressure relief spring. The pump is moving water and as it backs up on the inlet side of the radiator, pressure builds. Pressure is resistance to flow and that is what we have here. When the pressure is sufficient, the spring begins to compress....but it's not an "over center switch"...it's a spring, so it's always "pushing back" maintaining resistance to flow (pressure) to the inlet side of the radiator. You make it sounds as if there is a mechaism that "switches" at 58whatever hundred RPM, opening a sewer pipe for the bypass and that is absolutely untrue! LOOK at what you erroneously stated here...LOOK at it, then THINK:Really? Is that true? "NO RESISTANCE"?? No, it's not true at all! How is pushing a spring loaded valve open, against that spring, "no resistance"?? It's not. It is that spring, that resistance to flow, that guarantees there will always be flow through the radiator. Look at it. Think.

Water still flows through the radiator. Only way to stop that is w/another valve that physically closes off the radiator. SHOW US THAT VALVE! It doesn't exist.

Stop regurgitating what someone else theorized (guessed) what might be happening, and LOOK at the system. How does it work? A portion of the water is relieved through a very basic pressure relief valve, back to pump. It is really that simple.

.

mike100

ok captain semantics. I said 'mostly'.. and just like an electrical circuit, the lion's share of the flow would follow the path of least resistance (the bypass) which doesn't really transfer any heat.

The subject of cavitation in bypass is also interesting to me...I know people have done gear changes to run in the Silver State Classic and the like. I do not know if anybody successfully ever ran 100 gpm flow with an aluminum radiator before or not- I would think the hoses would still be stressed.

glass slipper

I'm a mechanical engineer who specializes in hydraulics and power transmission with the focus of trouble shooting failed hydraulic circuits. Many principles of engineering are used in my troubleshooting "bag of tricks" including the 1st law of Thermodynamics/Conservation of Energy, Conservation of Momentum, Bernoulli's Principle, and a few important formulas for flow and pressure.

First, the bypass relief valve is a single stage relief valve that opens/allows flow proportional to pressure. In other words, it doesn't go full open at 25 PSI. The temperature regulating side of the thermostat doesn't open all the way at 190°F, it has a cracking temp and a full open temp...the bypass relief valve is the same way, it has a cracking pressure with minimal flow and a wide open pressure with max flow with the relationship being linear.

Second, your statement that there is no flow through the radiator at 5800 RPM would indicate a pressure difference of zero between the inlet and outlet of the radiator...that simply isn't true. In nature, all physical properties "flow" from a higher state to a lower state. A ball 10' above the ground has a certain amount of kinetic energy and the ground represents zero kinetic energy. If you let the ball go, it "flows" from a higher state of kinetic energy to a lower state of kinetic energy which means it falls to the ground. Pressure, electricity, heat, or any other physical property you can name ALWAYS flows from a higher state of energy to a lower state of energy. It is physically impossible for water not to be flowing through the radiator when the bypass valve is opening.

Third, as Tom said, the pump does not "pull" water through the bypass valve. The only thing the pump does on the suction side is create a low pressure area in the loop to create flow by making a lower energy area for the higher energy fluid to flow to, the pump doesn't care where the fluid comes from.

I have a lot of respect for what you've done in the ZR-1 world and what you bring to this board, the above is meant to help you understand what's going on in the cooling circuit as you've helped us understand more about our beloved beast.

glass slipper

While the cooling water flow circuit has an analogous electrical circuit, it isn't like the electrical circuit you're imagining. The bypass isn't the path of least resistance since 60 GPM goes through the radiator and 40 GPM goes through the bypass at high RPM. Not only does the lion's share of flow not go through the bypass, it's not even a majority of the flow.

PS Many people have gone to high flow radiators which prevents the bypass from opening because the radiator can now handle the flow which in turn prevents pressure from building on the inlet side of the radiator to the point where the bypass valve begins to actuate.

Tom400CFI

Glass slipper "gets it".


You must not have read the rest of my posts. The "lions share of flow", or the majority, will always be through the radiator...not the Bypass.

93Rubie

Depends on how serious you are. If its just for fun you can run or do anything you want. If you want to be faster, car more fun, and better tire wear you need to modify things a bit.

Koni Shocks
Soften front bar to reduce understeer-easy ways to do this.
Good tires-Might be problematic with current crop of 200TW tires only going to 255/40/17 as the widest 17" available. That being said my 93 Z07 is quite fast on 4 of those.
Alignment
Brake pads/fluid flush

Other odds and ends that are B Street legal....

mike100

alright..my bad...I think another way to look at this is the C4 radiator is a 300 hp radiator at 60 gpm. Add 100 hp and it will overheat at sustained rpm since about the same amount of h20 will be going thru the heat exchange. I'm not sure how well the system does with more pressure across the radiator (even if you had an aftermarket welded tank unit). Have people blown hoses off with the bypass altered?

Tom400CFI

I can agree with that. Very good way to put it, IMO. Well said.

Tyler Townsley

I guess we agree to disagree.

As for the rpm this happens try this. Find a stretch of road you can run the car at 5600 rpm in second see how long it takes to spike the water temp. (Actually it should not spike). Try the same test at 5800 rpm and see what happens. Try it again at 6000 and again at 6200 rpm. This will establish your baseline for how long you can hold a gear in an autocross event without overheating the engine.

Remember one thing. When the bypass opens water goes right back to the block without cooling, so water at the pump p/u rises as the water continues to circulate. You have a 1 inch pipe of water going right back into the block that gets no cooling and a 1 in pipe of water going through the radiator getting x cooling. These streams mix and go back into the water pump. At some point the radiator flow reaches a point it cannot remove any more heat, the radiator outlet temp is say 160 deg whereas the hot pipe (bypass) keeps increasing as the water outlet (bypass inlet) after exiting the engine keeps getting hotter. So in reality it does not matter what flows through the radiator the opening of a alternate water flow path to the water pump of water that is not cooled will cause a quick a non liner increase in water temp.

Several years ago I did some testing to find out how to get the most cooling with the present system. I started out by buying a aftermarket radiator that would flow 100+gpm (Ron Davis). I then tried the following.
1. Took out the thermostat. It ran hot.
2. Plugged the bypass with no thermostat, ran hot.
(concluded from the above the water was moving so fast through the radiator it was not being cooled.)
3. I plugged the bypass and cut the thermostat spring so it was always open. This resulted in the coolest temps as I could run the interstate at 70 mph on a 95 deg day and the water temps were 165 175. The only problem was when the outside temp was less than 65 or so it ran so cool it would not go into closed loop and there was not much heat in the heater, had to carry around a piece of cardboard to put in front of the radiator.

I also found that as soon as I got in town the temps went back up to pretty close to what one sees on a hot day in traffic. I added 3 computer fans to the inside fenders on both sides to vent the under hood air out through the wheel wells, helped some but not as much as I thought it would.

I am done.

Tyler

Tom400CFI

Are you disagreeing b/c you simply want to stick to what you think you know? Or did you actually LOOK at what I've presented and THINK about it....and you still disagree?



That is right. I completely agree with this part...but now I'm confused; above you've claimed that "by 5800 rpm it is routing all water around the radiator right straight back into the block"...but here you're stating that cooled water and bypassed water mix (which is true). What are you actually saying?


In reality is certainly DOES matter how much flows through the radiator! Absolutely. The greater the %age of water going through the radiator, the more cooling capacity you have. Depending on many variables (ambient temps, for example) the capacity could be enough to prevent overheating...or even a rise in temps. The World Record Run proved that...right? If ALL the water were going through the bypass in that run it would have over heated in ~30 seconds...but it ran WOT for 25 hours.



The reasoning for this should be self evident, if you LOOK at the bypass valve. The spring loaded pressure relieve valve, portion of the T-stat (the bottom of the stat) is what provides the resistance to flow through the bypass -as I've stated numerous times now. You removed that, when you removed the t-stat! So in this case, w/the stat (and consequently, the pressure relief valve) removed, the bypass circuit was literally the "1" pipe w/no resistance" that you have been talking about, and it definitely would flow most of the water though the bypass in that case...and run hot.


LORE. You can't run water though a cooling system too fast and cause overheating. That is not how heat transfer works. Unfortunately, your conclusion was erroneous here. How did you block the bypass? I'm guessing that you think you blocked it, but didn't. I highly encourage you to read the content in this link: LORE about "too fast" coolant flow



This is also self evident; in traffic you get no air flow through the radiator, until the fans come on. The engine will simply heat up to the fan on temp. Simple as that. To improve cooling in this situation, you need more fan, earlier/at a lower temp. That's it.

IDK why "you're done"...you stand a chance to learn something here. I'd think that you would be interested in doing so...given your obvious interest in possessing ZR-1 knowledge. This is a good discussion that can clear up some LORE about the mighty ZR-1.

Tyler Townsley

OK I can continue this discussion but you need to stop concluding things that are not true.
1. When I blocked the bypass I put a 1 in stopper in it. It was blocked.
2. When I cut the thermostat spring that is the spring I cut, not the bypass spring.
http://thepandatrap.com/tyler/misc/Thermo.jpg
3.I Blocked bypass removed thermo it ran hot. For some reason the radiator was not removing the same amount of heat it removed with a thermo in the system.
4. I had several conversations concerning the bypass function with LOTUS engineers associated with LT 5 project over the years and the 5200-5800 rpm threshold for bypass opening start and full open is a fact. When I asked if that meant water flow through the radiator stopped they said yes.
5. In 1995 a Dr in Tampa was using his ZR-1 for SCCA races and blew 2 engines and we all were puzzled why such a robust engine was failing so quickly. When he posted on the old ZR-1 net his problems I decided to find out why. The answer was the bypass system, he was keeping the rpms above the 5800 rpm threshold too long and fried the engines. Chev designed a cooling system based on the assumption that owners would not be running the motors over 5800 rpm for an extended period so they put in a bypass to protect the radiator.
6. Any time someone posts about running a ZR-1 into the red zone I have always urged that they understand how the bypass effects the engine temps.
7. I talked to Tommy Morrison about the record run and asked if they did anything to address the way the bypass worked and he did not even know how the system worked. I asked him about the engine temps during the run and he said they were near the redline, they also limited the rpm to 5700 which is just below full bypass (5800) and the air temps were near freezing with snow flurries. Had it been a 90 degree day the motors would have overheated, This is consistent with what I have concluded over the years, the stock system cannot sustain high rpms for too long and does not support your contention that there is sufficient flow through the radiator to cool the motor above the bypass opening.

That is my perspective.

Lets talk about your link.

A radiator can remove x btus which is dependent on design, flow of water and air speed and temp. How much heat can be removed depends on how long that column of water is exposed to the radiator surface. If the engineer knows the exit temp from the motor at 5500 rpm and a pump flow of 45 gpm is going to be 220 degrees and he wants to cool water to 185 degrees then he uses a radiator that will extract the necessary heat from the water and a thermo that regulates the flow to achieve that. Its the thermostat regulates the flow rate through the radiator. I may have a 85 gpm pump but the thermo restricts the absolute flow rate to that which results in the desired heat being removed from the water(say 55 gpm). If I remove the thermo and the flow rate is that of the pump 85 gpm and the system overheats then tell me how if its not he water circulation through the radiator too quickly.

The engine puts y btus into the water stream, the radiator either slows or speeds up the water flow through the radiator to remove sufficient btus to see a water temp at pump inlet of 185 deg. In most cars fans are added to force more air through cooling fins thus adding to btu extraction.

I do not buy the assumption in that link its far more complex than that.

Tyler

Tom400CFI

Sounds like we BOTH need to take that advice.



Was the radiator getting hot? It's cool that you ran a test, but your test was incomplete/didn't prove anything. There was an error in your testing method. If the bypass was truly plugged, and the path through the radiator was truly open, no way would your car have run hot. There is something else going on in your test, that hasn't been accounted for. It would be interesting to re-test or explore this more deeply.

As for the Dr that overheated his ZR-1 that proves nothing either. It's anecdotal evidence, and we don't know all the circumstances -at least not from your post. And who in the world would run theri car around an auto-x track above 6000 RPM?? You'd have to be in 1st gear the entire time. Oh yeah....those guys that think they have some "high RPM screamer" engine. Those guys...I've seen them at events. That = terrible driving and slow times. You posted about one guy conditions of which were unknown, who had heating issues. Two people have already posted on this thread that they've seen no overheating issues in autox, with years...DECADES of events.


I think that you misunderstood what they were saying. LOOK at the system. LOOK at how it works. Is is possible That they said something LIKE: "At xxxx RPM, the system bypasses coolant back to pump"....and you interpreted that to mean at xxxx RPM ALL the coolant is going back to pump? I don't care about the RPM threashold at all. I "get it" that at some RPM, the bypass valve begins to open and at some higher RPM, it's open as far as it will open. I don't care what RPM that occurs at...it doesn't matter. What matters is that ALL the coolant does not flow through the bypass at any time. Doesn't happen. Can't physically happen unless there is a specific valve to close the radiator loop. In order for that to happen, there would need to be another valve, closing off the radiator loop of the circuit. Show us that valve.



Whoa! Where did I post that was my contention? I never said any such thing. I agree with you, that in hot temps, extended near redline operation would likely cause overheating. In fact, way back in post #18 I said that;
"This could be a slight concern at a big track"
Then in post #33 I agreed with Mike100's post #32 where he stated that:
So I don't believe I've disputed anywhere that it's possibleto overheat a ZR-1 by driving it like a moron, in hot temps. What I DID say was that I don't believe a ZR-1 will overheat in any autox I've ever seen...and I've seen a lot. I guess if you want to drive around the course at redline in 1st gear on a 100 degree day, you could overheat it....but you could overheat my stock LT1 using the same, retarded technique. Anyone can break anything, if they try hard enough.





FAST FLOWING COOLANT:

NO! How much heat, depends on the temperature differential between the radiator core and the air contacting the core. The greater the temp differential, the greater the heat transfer.

No. That is not the function of the thermostat. Not at all.



That was specifically addressed in the link I provided. As for why YOUR car overheated during your test, I commented on that above already. I said: "There is something else going on in your test, that hasn't been accounted for." That isn't meant to be an insult, it's easy to create a test and overlook things.


It's really not. You're making this WAY more complex (in your heat) than it is. T-stats, bypass valves...LOOK at the system! It's not complicated. I'm friends with a cooling systems engineer at GM. We've talked about this LORE and he readily agrees that it's just that; LORE. Now lets look at this logically:
1. The faster the coolant flows, the more turbulence there is, and the more "pressure" there is. THis creates better heat extraction from the engine, AND better heat expulsion from the radiator.
2. If slower coolant flow = better cooling, why did Lotus spec a 100 GPM pump??
3. Why do all or most high performance pumps flow more GPM?
4. If the radiator was a heater (like in your house) under which circumstances would it provide more heat: A. When you pump hot liquid into it, let it sit, and then after all the heat has been pulled from it, and it's room temp, you pump more hot liquid into it? OR B. constantly flow hot liquid through it?
Obviously, B would warm your body better/more. Same pricnciple applies with airflow through the radiator; Above, you said that
With fans, we're speeding up airflow through the radiator, right? So how come we aren't pulling "air through the radiator so fast it doesn't pick up any heat"?? I've never heard that one before, but it's the same principle. Because, what we are REALLY doing is constatnly providing cool (ambient) air temps around all the cooling fins, which is the biggest temp differential that we can provide...and that = the most heat transfer.
Likewise, when we saturate the radiator core w//hot, fast moving coolant, the radiator core is as HOT as it can be all the time, for the given coolant temp, and the radiator core temp to ambient air temp is again, as great as it can be -maximum heat transfer will occur.
SLOWING COOLANT IN THE SYSTEM, RETARDS HEAT TRANSFER...which is cooling. Coolant moving too fast is LORE. -Something has happened in the past that went un-explained, and the cause was blamed on a guess.

Questions for you:
1. Where is the valve that closes off the radiator portion of the circuit?
2. You said in post #34 about coolant "mixing" from the BP circuit and the radiator circuit, but continue to say that "all coolant flows though the BP circuit. (?) Which is it??
3. Have you LOOKED at the pics I've posted, read and really THOUGHT about the comments I've made about the BP being nothing more than a pressure relief valve? It doesn't seem like you have.
4. If I go remove my t-stat from my car, and make a video of it running at what ever temps it runs at (will probably be around 100*F) will that help show you that the T-stat doesn't "slow coolant to provide better cooling"? I too have removed T-stats from cars and they typically ran in the low 100's *F....even with "fast flowing coolant".
5. Would you be interested in re-running your test and video'ing it?


.

mike100

I was going to post about this as there is some thermodynamic entropy thing going on with the heat transfer that can be explained by a clever train station analogy that I read once (I can't find it right now though). The idea was that it is a waste of TIME to fill each train car to capacity at a busy station and that you should just bring in another train when the cars are 85% full. You will move more passengers in a given time span this way given the idea that the trains are all back-to-back. The same idea for the heat transfer in water. You just move more of it faster rather than wait for the very last bit of heat xfer to occur. As I understand it the initial heat transfer has more delta-T (just like charging and discharging a capacitor electrically) than waiting for the complete equilibrium to occur. Unless you move it so fast as to not hardly have any heat transfer, higher flow rates were considered beneficial- I guess in real life there is an actual sweet spot for any given radiator and water pump combination.

glass slipper

OK, I ran your test above or as Tom says, I drove it like a "moron". I couldn't do it in 2nd so I ran it in 1st. I drove it at 6500 RPM with the ambient temperature over 90°F so this was going to be an extreme test. I could only hold this for a little over one minute due to traffic but the temps did get up to ~220°F, returning back to 6th gear saw the engine temps reduce back to ~203°F. There is no doubt I lost some cooling capacity as a certain amount of water was bypassing the radiator. I will make a few comments concerning some factors that might have affected my test. First and foremost, my '93 is about as original as you can get. When I say original, I mean an overwhelming majority of the car still consists of the parts it was built with at Bowling Green...the car even has the original windshield wiper blades on it and they still work as good as new (which wasn't very good like all C4s ). I still have the original hoses/belt/radiator cap/thermostat, I changed the water pump once, I've never removed the radiator to clean out the leaves/debris, and my spoiler underneath is a victim of one too many encounters with parking curbs. All of the above would tend to skew the test negatively and I may have been able to run for 2 minutes without excessive coolant temps. On the other hand, a speed of ~50 MPH doesn't demand much HP from the engine and the heat rejected to the radiator is a function of the HP the engine is putting out...in other words, this wasn't much of a stress test for a "normal" cooling system and "normally" wouldn't cause an increase in coolant temps. The fact that I did see a noticeable increase in coolant temps is clear proof the bypass system will negatively impact coolant temps at sustained high RPM operations. I think your synopsis above is more accurate, the hot coolant bypassing the radiator mixes with the coolant from the radiator and begins the upper spiral of coolant temps...once it begins, each "pass" increases the temps more and more because the coolant going through the radiator can't be cooled enough to mix with the bypassed coolant and obtain a satisfactory "average".

If you can imagine the radiator as an orifice that'll only flow so much coolant no matter what the pressure is, you can see why the bypass opening will not stop all flow through the radiator. Once the flow reaches the max through the radiator, the bypass valve opens to prevent excessive pressure but the flow rate through the radiator remains at its max. We're not disagreeing with your statement that sustained high RPM operations will cause overheating and if you look back through all of our posts, you won't find one instance of disagreement on that point. What we disagree with is the statement you made earlier that all flow through the radiator stops when the bypass valve opens. In your post above, you backed off of that statement and are saying the same thing as us.

In the end, it appears we are all in agreement on everything. I hope this discussion hasn't ruffled any feathers as I don't believe that was anybody's intentions here. Again, I respect your knowledge and all of the great things you've brought to our community, I also respect the knowledge Tom and Mike bring to this forum.

PS Sorry to the OP for the hijack but hopefully we all got a better understanding of how our beast works and its limitations.

Tom400CFI

YES!! That is exactly right! When people bring up the 'ol "coolant too fast" lore, what they're neglecting to consider is TIME, and delta T. The more the water cools in the radiator, the closer it's temp gets to ambient..the slower heat transfer occurs. Also, the longer the water is sitting in the radiator having it's heat taken away....the longer water is sitting in the block, getting (too) hot. Well said!