davepl

I'm not going to beat on the issue more, but please re-read what I said. There are not two heat sources, it's the same heat source split into two stages.

And I think pretty much every non-NASCAR auto sport in the world runs forced induction motors. Most are turbo, of course.

You have very strongly held opinions about the C7Z and supercharging given the picture of the C5 next to your name. Do you have a lot of experience with the C7Z? I have no idea if that pic is just one of your favorites and you own one of these or not (nor shall I presume to guess).

(To be fair I do not - my TPW is Monday. So I can only speak to the physics, not the emotion.)

OnPoint

Anybody seen any posted IAT 2 temps on a stock Z that were alarming, or even concerning?

davepl

And had I been smarter, I would have flipped to the next chapter:

Chapter 4: The Balance of Heat

He goes through the math of a 1075F compression chamber, 90F ambient temps, 13:1 compression "standard day" and goes from there. More than you could ever want to know, complete with charts and graphs and formulas.

RedLS6

This statement is not actually correct - the theoretical supercharged engine will have more air mass in each cylinder as compared to the N/A engine, given equal flywheel power. The supercharged engine will also have a higher cylinder mean effective pressure, operate at a higher BSFC, and more heat will be rejected to the water jacket.

The power required to drive the blower is not trivial, and is not equal to the compression losses inside the N/A cylinder.

RC000E

I understand in principle you can't heat the same compressed gas twice (must edit for FURTHER clarification...I understand that whether you compress it all in a cylinder, or part in the supercharger, then that gas further in the cylinder is all the same in the end in terms of heat), that's not what I'm disagreeing with. What I'm saying is, you have two independent heat sources, being cooled by the same system. The supercharger has friction internally, bearing heat, etc...it's also heating air variably based upon where in its efficiency range it's located at a given rpm...that outlet heat is cooled and heat transferred to the cooling system. The engine in itself is also a source of heat, for which the coolant system must absorb energy, etc. Given the fact there is also work being done by the engine to turn the supercharger, etc...this is yet another source of heat that is "waste energy" so to speak.

I'm on a Corvette forum, and is a simple discussion...I wasn't here to split engineering hairs and principles...I didn't feel the discussion required this much scrutiny.

As far as my having "experience" with a C7Z or what I own or haven't owned...I fail to see the point to the question. I've built custom forced induction systems for various applications for 15 years...it doesn't really matter what I own does it?

If you question my love for this car though...my anger is merely out of disappointment...that's it. I attend countless events for this car...I've loved this cars since I was a child and I've never waivered...but the Z06 stood for something that this is not...that's all there is too it...

Me at Detroit...private viewing to see the debut of the car. I arrived on a bus with about 70 people all aged at about that number...

davepl

Ah, but I'm on a Corvette forum too and I don't think it's a simple discussion. It's kind of interesting, at least to me. But you can't make a statement about supercharging spoiling the car without considering the engineering hairs and principles - or at least not a meaningful one.

I don't mean to imply I'm doing you any favors, but no point in you being very disappointed with the car for the wrong reasons (if they're the wrong reasons). If the reality is inescapable that they overheat during hard racing, that's disappointing to some folks. But don't write it off for being supercharged, that's all I'm getting at.

For the reasons I detailed in my explanation, we'll have to agree to disagree. Same mass. Same compression. Same heat. Same system.

700cc of air is ingested by the blower. It is compressed (let's say) to double density at 350cc by the blower. It is compressed to 100cc by the piston.

Or, 700cc of air is ingested by the larger NA engine. It is compressed to 100cc by the piston.

Other than adiabatic inefficiencies in the supercharger, I see no difference. Since we (or at least I) don't know the adiabatic efficiency of compressing a gas in a cylinder, we can't then compare that to the compressor efficiency on the LT4. I will go out on a limb and grant that the cylinder compression is probably going to be more efficient than the supercharger even with intercooling. How much, I don't know. 15%?

But even if you think of it as two systems, there's no -requirement- that the intercooler dump its heat load in front of the radiator. I don't know where they are on the Z, but on my 510hp Range Rover they're separate and NOT in the coolant airflow path. So in that case I could make a (weak) case for the supercharged engine being more heat efficient because the intercooler gets to dump heat that would have been made otherwise on the compression stroke -outside- the engine cooling system. It's like a little extra cooling system on its own - but likely adds as much heat through friction and compressor inefficiency anyway.

Maybe here's a far simpler example to convey what I'm trying to: driving the supercharger takes work, right? I bet it could be 100hp on the LT4. Most people think of that as lost work, that we must burn another 100hp worth of fuel, and hence make a lot more heat, to do that work.

In reality however that work would have been done on the compression stroke of the larger NA motor.

The blower will have belt friction, bearings, and so on. So it'll have some loss - but maybe 10hp, not the 100hp people think of. And it's probably this last point that I should have started with!

davepl

Probably a little. It has to contend with the frictional losses of the blower, but that might only be 1-2%, as opposed to pumping losses that could be 25%. But I argued (without proving) that the pumping losses would have be incurred by the piston doing the work anyway.

Since I'm a data guy, here it is:

Typical BSFC @ Peak Torque:

4-stroke naturally aspirated -> 0.47
4-stroke supercharged -> 0.50

With intercooling I've seen as low as 0.42 (blown Mopar 340 of all things), but not on the same dyno so not fair to include.

So up to 6% I guess. But the real discussion was heat, and as long as the intercooler can shed that 6% of heat plus heat due to compressor inefficiency, it all nets out the same.

As a random data point, the BSFC from my stock-rebuild BBC 402 from my '70 GMC was 0.50 at peak torque and 0.57 at peak hp, but I'm running 12.5:1 AFR so likely far richer than what the LT4 would.

RedLS6

Again, No!

Look at an Otto cycle PV diagram. Look at the compression stroke on the PV. Now look at the exhaust stroke. The work done is the area inside the PV curve. What happens to this area, if the compression work is increased? Hint - the exhaust stroke line moves up, and some (much) of this compression work can be (is) recovered.

75 horsepower for an LT4 blower is probably about right at full boost. In fact, now that we know the inlet and outlet temps of this blower, the mass airflow rate, and we have a good guess as to its efficiency based on the other TVS curves, we can calculate pretty closely the power required to drive it by doing an inlet/outlet enthalpy calculation using adiabatic air compression modified by the actual efficiency. It's actually an easy calculation. This power is 100% non-recoverable. It manifests itself as added heat in the blower outlet air, and as added heat sent to the water jacket, and as added heat blown out of the exhaust.

MavsAK

Could be the 200hp difference in the Stingray, vs the ZO6.

The cooling system in the base car has a hell of a lot less work...than the Identical System, in a 650 horsepower car has to put up with.

If you can't figure out that more hp = more heat...maybe cars aren't for you.

GM should remove the grill cover from the C7 ZO6, and upgrade the radiator to a double row unit. That'd solve a whole lot of issues.
GM also has a long, storied history of undercooling corvettes, and performance vehicles. (because they use the same rads in economy cars which cuts overhead).

the stock cooling system is barely adequate for 460 horses.
In what reality do you think 650 horses isn't going to generate loads more heat??

Another problem is let's face it..the ZO6 has an insane compression ratio for a SCed engine. CR generates tons of heat as it is. 10:1 on a SCed engine is crazy. DI or not. I suspect that we'll probably be seeing bigger blower, and lower CR C7Zs in the future. (Something more manageable like 9.5 to 1 probably) Lugging around all that drag induced by downforce also isn't helping things.

So really the problem is five fold, namely coming back to 1 an inadequate radiator. Single row rads, should Never Ever be put in ANY serious performance car. Especially since double rows can be had for under a grand....Retail.
Two, weeny charger that's spinning a ton of rpms (which means frankly...it beats the living **** out of the air which generates more heat than a bigger blower, and Roots SCs suck *** anyway, especially when it comes to heat generation...and nothing can ever be done to bring it anywhere close to what a centri setup can manage heat wise), and three, an insane CR for a boosted engine.
Three... the engine has to work harder, and CAN work harder for longer with the downforce. You can be on the throttle longer, with more downforce..on the otherside though..it also produces more drag.
Four, ....it has a grill insert with tons of bars in the way instead of a big larger mesh on grillwork. There's no way in hell the stock grill is a good idea on a ZO6. There's too many obstructions there.

Fix the rad, chop up the grill, and you're pretty much good to go.

Solo40oz

Haha - I love it. I prefer an object oriented approach, so maybe all of the C7 Z06 threads can inherit from the C6 ZR1 parent

Darius

My 1st track day in my Z06 was bad, car was running very hot, 230deg coolant. Close to 300 oil. Motor dropped a valve and was ruined. My 1st track day with my Viper TA, temp never got above 203. Ran perfect.

OnPoint

You dropped a valve in a C7 Z06?

JG853

I am not sure how much racing you watch. I am a big Super Dirt Late Model Fan - All motor. Nascar is now like the old IROC series, where every basically has the same car. There are a lot of series that have NA engines. And they produce a ton of power.

I have to say that I do think the Z06 has gotten away from it's roots. While the new C7 Z06 is definitely setting some fast times at the drag strip in the A8 configuration, the road course results are disappointing. Track cars typically are NA or use turbos. Supercharging is not the ideal set-up for the road courses in my opinion.

davepl

Nope. Go do the math yourself, it's all there:

http://en.wikipedia.org/wiki/Otto_cy...exhaust_stroke

Anyway, I lack the inclination to argue physics vs conventional wisdom, and will leave it to you guys to argue it out.

davepl

I watch zero NASCAR, but I never said they don't produce power (ancient power, but power). But they only do it because they're constrained by the rules. They don't use restrictor plates because it's a good idea either.

You mean drum brakes?

JoesC5

Are you aware that a 1963 Z06 with it's drum brakes and 360 HP could out perform a 1949 Crosley Hot Shot with it's 4 wheel disc brakes and 26.5 HP?

RedLS6

I'm arguing the physics end of it. I don't need Wikipedia, I'm an engineer and have worked through this enough times; you're pulling info out of thin air. It's incorrect.

It's simple enough, and we don't need PV diagrams to figure it out. The supercharger takes about 75 horsepower directly off the crank at full boost and full engine power. This means that our 650hp C7 Z06's are actually making more like 725 horsepower inside the cylinders, but only 650 is making it out to the standard SAE dyno test due to the added crank load.

If the C7 Z06 were a normally aspirated 650hp, the 75 horsepower parasitic load (frictional blower loss + power required to compress the air to 9psi) would not exist. In this N/A motor, there would be some pumping losses, but nowhere near 75hp extra magically appearing on the compression stroke because the supercharger is no longer there.

From your argument, any added work done on the compression stroke (such as a higher compression ratio on the N/A motor) would raise point 2 on the PV diagram; this represents work done by the crank. Point 3 is subsequently raised; The area under line 3-4 is raised; this represents work done on the crank by the combustion process; The net effect is the compressive losses are returned to the crank by higher energy on the combustion stroke.

However, there simply is no pumping loss equivalent to a supercharger, in a N/A motor, which creates extra compression work. You're pulling this out of thin air.

There are multiple heat generation sources; the combustion energy is the primary one; but there's also the heat added to the air by the blower, which must be removed by the limited frontal airflow capacity. There is no magical cooling solution in the tune; the issue is heat rejection to the engines water jacket and oil system, and subsequent issues with removing it given the frontal airflow and heat exchanger capacity.

davepl

An engineer! Well, I had no idea. Normally I wouldn't take a "because I said so" argument, but with those credentials, I must apologize. No math or data is required with that kind of authority.

Paulchristian

well...team viper has the SRT, TA, and now the ACR. Seems to me they are doing a lot of tweaking already in this young Gen V product cycle...already two higher performance variants on top of the SRT.

The vette only has the C7 Z51 and C7Z...I am sure we will see something in the near future that will go a long way to correct the issues we are seeing.

Keep in mind, its only 2015 (first year for the Z)...GM is not going to put everything out there right away, new products/tweaks, upgrades will be introduced just as sales are starting to decline. Something special is coming, rest assure...

RedLS6

Ok. Let's go back to this example.

Take your two cylinders again......one is 700cc N/A, and the other is 350cc Supercharged.

Suppose they are both 10:1 compression ratio as well.

The 700cc cylinder fills itself with 700cc of air, and compresses it down to 70cc via the 10:1 ratio. The overall compression is 10 to 1.

The 350cc cylinder has a supercharger which first inhales 700cc, and does a 2:1 pre-compression down to 350cc. The cylinder compresses the air again at 10:1, down to 35cc. The overall compression is 20 to 1.

As you pointed out, there is some difference in adiabatic efficiency between compressing the gas in a cylinder versus a rotating blower. However, more work has been done on the air in the 350cc cylinder due to two-stage compression architecture which produces 20-1 overall compression. This extra work is done driving the supercharger and heating the incoming air. I don't believe one can equivocate the work done between these two radically different compression ratios. The intercooler serves as a necessary inter-stage heat removal device, as 20-1 compression with no inter-stage heat removal would not work from a detonation standpoint.

Maybe we can get back to arguing the physics behind it.