jeopardy on at 7, definitely, Kmart sucks...

 

Researching another issue I had a forum member send me his dyno .drf files for comparision. Our mods are very similar, he has a FAST but I've got thinner gaskets and more compression.

I've got 3.90 gears and he has stock gears.

Here's our dyno sheet based on RPM.



Looks like the same car doesn't it! I promise it's not. Our cars are just that similar.

Now here's our same cars, but the x-axis is MPH instead of RPM. I've got 3.90 gears vs 3.42 gears. Notice that I've got 24 more HP at 55 mph (185-161) and 46 (331-285) at 90 mph (I can get those exact figures on the Winpep software). That's why gears FEEL so much faster SOTP. You've got a lot more HP avail at any given road speed. Particularly on the highway!

 

At 140 MPH, the 3.42 car has ~430 HP more than you...seems to contradict your statement in bold.

 

I think my statement is accurate - I could have clarified "in the same gear" but I think that's understood that the analysis was done in the same gear. I further think it's obvious that cars in different gears would be at different places in their HP/TQ curve, once both were back in the same gear my statment would stand. I think it's also universally reported that putting lower gears in the car makes it feel "peppier" and this graph happens to show why pretty well.

Of course a higher geared car has a higher terminal speed at redine than a lower geared car on the dyno...on the track where aerodynamics are the limiting factor, I don't know which one would be faster. I've never been able to test the top speed in the real world - I've tried to get there on the back straight of Sebring but couldn't make it!

I did do the following analysis. Chevy rates the stock car with a 186 top speed. Per my favorite gear ratio calculator website, http://www.xse.com/leres/ss/calculator.html with 3.42 gears and a MZ6 trans, 186 occurs at 5675 rpm in 5th. Interestingly, with 3.90 gears, 186.8 mph is at stock redline of 6500, so the cars should have the same top speed. Of course if the 3.90 car owner increased the redline to 7000, if the car would pull it the top speed would be 201.1. The 3.42 car is not redline limited.

You're the math expert on the board - perhaps you could figure out how much HP is required to go 186 MPH and this be able to prove mathmatically if the 3.90 equipped car would be able to pull to 7000 rpm and thus 200 mph? It would be interesting to know if my car theoretically could go 200 mph.

 

Yes, it was obvious your analysis was done in the same gear since both dyno runs were done in 4th gear and I agree it was understood. But here's the thing about any hypothesis...if you can find any case where it's not true, then the hypothesis is not accurate. And at 140 MPH, your hypothesis "at any given road speed" is not true therefore your statement is inaccurate. Even if you had said "in any given gear at any given speed", it would still be inaccurate as you stated above in reference to the cars being at different places on their respective HP/TQ curves. If you had said "in the same gear at the same RPM", then the statement would be correct but then the two cars would be at different speeds. I guess what I'm saying is you're comparing apples and oranges. When the 3.90 car is at 5000 RPM at a certain speed, the 3.42 car is going to be at 4385 RPM and what you're saying is the equivalent of saying the 3.90 car has more HP at 5000 RPM than the 3.42 car has at 4385 RPM...kind of a no brainer when the HP curves are almost exactly the same and peak HP occurs close to redline. But then again, if I'm in the 3.42 car, I'm going to be in 3rd gear putting out more HP then the 3.90 car in 4th gear. The advantages of the 3.90 gear at the drag strip are 1) better multipliplication of torque for a better launch, 2) gets up on top of the HP curve in 1st gear quicker, 3) you come across the line close to redline in 4th gear (if geared properly) increasing the area under the HP curve put to the ground vs the 3.42 gear car just getting into 4th and still climbing back up the curve. On an autocross or roadracing course, gears don't automatically give you an advantage...there may be some courses where the 3.42 gear is the best and others where the 4.10 gear is the best or any number of other ratios. Serious/professional roadracers have computer programs that will determine the best ratio based on number of shifts and highest average HP put to the ground during a lap. On a race from a roll of ~40 MPH, gears give you no advantage as clearly shown by this graph from post #37:



The two cars just see-saw back and forth depending on which one is up on top of HP curve/in the lower gear as you said above.

The 186 MPH top end Chevy states occurs with the MN6/3.42 at 5916 RPM (by the calculator from your link) which puts it close to its' HP peak at 6000 RPM. The HP curve is actually pretty flat at that point so we can say it's putting out 400 HP therefore 400 HP is required to go 186 MPH. At 6500 RPM, it's putting out about 390 HP but 3.90 gears will eat up a little more HP due to the gear tooth pitch angle increasing sliding velocity/friction at the gear tooth interface. A stock LS2 MZ6 3.90 C6 might get close to 186 MPH/6500 RPM, but I don't think it'll quite make it. Of course your car is modded meaning it'll make 186 MPH easily and may pull close to 200 MPH. We know the Z06 requires 505 HP to go 198 MPH but the Cd is higher...your peak rwhp number is close to the LS7 but it doesn't occur at your new 7000 RPM redline meaning you'll definitely get to 195 MPH @ 6800 RPM, you might get 198 MPH @ 6900 RPM and 201 MPH @7000 RPM is possible on a good day (cold temps, high barometric pressure). Of course a nice tail wind would certainly get you to 200 MPH. I can't see your dyno graphs at work so I'm going off memory, I'll look at it again when I get home.

 

Ahh...don't bother with it. I didn't mean to start a scientific argument over how precise my statement was and I was concerned my comments this morning would bait you into one. I shouldn't have responded to your post but I was afraid folks other than you would misconstrue what I was trying to say originally based on your response. Probably shouldn't have posted a response now either.

Here is my original point restated. At any given cruising speed, once I'm in the gear I stay in for that speed, be it 40, 50, 60, or 70, I've always thought my car felt "peppier" once I put the 3.90 gears in it and I thought this graph indicated why pretty nicely.

"Peppier" is tough to precisely define. But like ****, I know it when I see it. To be more precise in this instance, feel it.

I wasn't trying to start an argument with my statement and don't wish to pursue one any further. I feel like I'm at work making legal arguments (not that I"m a lawyer but I do argue with a lot of them! )

 

No problems, if you're happy, that's all that matters. I'd be pretty happy with 1/4 mile times in Z06 territory too.

No arguments though, just a friendly discussion...at least you know your car has a very good chance of going 200 MPH.

 

Excellent - you're right...and I appreciate the kind words too.

I think I've been dealing with too many lawyers! Certainly enough for today. Sorry about that.