January 2013 Chevy High Performance Q&A

Kevin McClelland Jan 28, 2013 0 Comment(s)

Drivetrain Losses

I have a question that’s been bugging me since I tried to figure out how much horsepower I needed to run in the 140s at 10.90. It seems that chassis dyno operators are always quoting a drivetrain loss of anywhere from 15 to 35 percent compared to crankshaft horsepower. Is there a sliding scale for horsepower output? In other words, all things being equal, why would a 1,000hp engine lose 150 horses to the drivetrain and a 100-horse engine loses only 15, assuming a 15 percent loss? Am I missing something or is friction the culprit? Thanks!

Gary Thompson
Pahrump, NV

For all you readers, Gary has a C4 Corvette that he runs in Super Street with a small-block Chevy. This is why he would be asking how much horsepower to run 10.90 e.t., but at a much faster 140 mph. A car running flat out will cover the quarter-mile at 10.90 seconds at around 122-124 mph. The e.t. for a car traveling 140 mph from a standing start is in the mid 9-second range. In the Super classes we run throttle stops to adjust the performance of our vehicles to hit the magic 10.90, 9.90, and 8.90 seconds, respectively.

To your specific question first, all things are not created equal when you’re talking about the driveline to harness 100 ponies and 1,000 hp. To transmit the power produced by a 1,000hp big-block you need much larger parts; e.g., transmissions and differentials. If not, you’d end up scattering the parts all over the strip if you used the clutch, trans, and diff from a Chevette behind that 1,000 hp! Yes, it is frictional differences and rotating mass that cause the greater losses. Also, the more load that you apply to the transmission and rearend gears, the more power is converted into heat and frictional power loss. The lowest percentage of driveline losses will be with a front-wheel-drive vehicle, since they don’t change the direction of rotation 90 degrees as with a hypoid gear arrangement in a standard rearend. Most front-drive transmissions and differentials keep the rotation of their components going the same direction as the crankshaft. These types will commonly come in at around 10 percent loss. Next would be your manual trans–equipped vehicles, at around 12 percent best case, to an average 15. Then you would have your standard automatic non-lockup–equipped vehicles. These start in the 18 percent range and climb from there. In performance vehicles, Powerglides are king at stealing the least power from your engine. Inversely, the TH400s rob the biggest chunk because of the sheer weight of their components and drive direction within the trans. Luckily, the performance aftermarket has built components that will withstand the abuse in a Powerglide since they were originally intended to be behind 250 hp tops. The worst crank-to-tire loss we ever saw on the dyno (first dyno’d on the engine dyno, then in the car on the chassis dyno) was 25 percent. This was a Brand-F car with a four-speed overdrive transmission, P/S, A/C, Alt., six-blade steel fan, and a 9-inch diff. We swapped out the fan and water pump to electric units and gained 6 percent at the rear tires.

Now that we’ve gotten past the loss factor, let’s talk about one more issue on chassis dyno testing race cars. Stall converters basically make it impossible to accurately dyno race cars. You can use them, but the torque number is pretty useless. If you are to test on a chassis dyno, you must bring the vehicle speed up to your stall speed in High gear. You can figure this out by calculating the vehicle speed in High gear at your stall rpm. This may sound crazy, but you may start your test around 100 mph, and take your car to redline, which would be in the low 140s. If you start the test at a much lower speed, your engine will sit in the stall of the torque converter and cook the fluid, and also give you a greatly inflated torque number. Torque converters multiply torque when they are in any type of stall condition below 1:1 engine speed and input shaft speed.

So how much horsepower do you need to push your Corvette to 10.90 at 140 mph? If you were to run 140 at that e.t. and then you ran the car wide open with no throttle stop, you should see between 142 and 143 mph. This would be combined with a 9.40-9.50 e.t. We’re making a couple of assumptions. We know your Vette is probably close to the Super Street minimum weight of 2,800 pounds, with driver. For your Vette to run 143 at 2,800 pounds, it’s going to take approximately 730 hp. (Now all you keyboard hot rodders are going to go online to a site like Wallace Racing, which has all the performance calculators you could ever wish for.) From years of experience, automatic transmission race cars usually eat up 75-100 more horsepower than the e.t. predictors on the market. These prediction formulas are very accurate with a fully vetted manual trans, unlimited race car. They are spot on in applications like NHRA Pro Stock–type cars. Everything we’ve ever had consumed the extra power listed above, perhaps because we don’t have the lowest friction driveline, or our vehicles are less than desirable, aerodynamically.

Good luck finding a few more ponies for your Vette. See ya in Vegas.

Source: wallaceracing.com

Fire in the Hole, Rewind

In the May issue, you advised Betty about her hard start problem. I may have a solution. I had a similar problem with my ’87 Cavalier Z24. It turned out to be the fuse for the fuel pump start circuit. Cranking the pump got no power. Once the engine delivered enough oil pressure and I released the ignition switch, the engine fired.

There are two circuits for the electric fuel pump: one for starting and one for running. When cranking the fuel pump, power was routed through a different circuit to bypass the oil pressure switch. She may have a similar problem with her pump. The run circuit works but the start circuit has no power.

John McGibney
Via email

You’re spot on, as the fuel pump must be energized during the crank event. Usually this is taken care of by the ECM, which sends out a signal to the fuel pump relay based on the crank voltage being fed to the ECM from the ignition. We experienced this once, where the crank voltage was omitted. I would never have thought to look if it weren’t for the message board, Norotors.com.

Great tip, checking all fuses when running into these types of problems. Yes, some GM vehicles have a crank fuel fuse, and others just send a signal directly from the computer to the fuel pump relay.

Source: norotors.com

60-Foot a Little Slow

I have a ’63 Vette with a 98-inch wheelbase, mild steel tube chassis at 2,450 pounds, with a four-link suspension, antiroll, rear wishbone, and struts. The engine is a 565-cid, producing 1,007 hp, with a Powerglide transmission, a 9-inch 5,600-stall converter, 4.57:1 rear gears, 50-inch wheelie bars, and 16x33 Hoosiers slicks running 9 to 91/2 pounds of air. I launch the car off a transbrake at 4,400 rpm and shift the car at 7,000.

I don’t know the instant center of the rear suspension, but the antiroll and chassis were set up by Ken Kier. I’m located on the East Coast (Maryland) and race from New York to Florida.

My problem is it doesn’t seem to 60-foot very well, the miles per hour doesn’t match the e.t. Could the rear coilovers cause this? I have single-adjustable QA1s. I’m considering double-adjustables and 66-inch wheelie bars.

Here are some runs in varying weather conditions:

April, 68 degrees: 60-foot: 1.265; eighth-mile: 5.491 at 128.96 mph; quarter-mile: 8.501 at 164.17 mph

July, 90 degrees: 60-foot: 1.295; eighth-mile: 5.567 at 128.18 mph; quarter-mile: 8.604 at 163.02 mph

October, 60 degrees: 60-foot: 1.256; eighth-mile: 5.483 at 130.20 mph; quarter-mile: 8.479 at 165.80 mph Thanks for any help you can give.

Kevin A. Banks
Via email

Very cool Vette. You’re making some good steam with your big-block. It’s just that some of it is being eaten up as you’re going down track.

First, let’s take a look at your runs. I wish you’d sent the density altitude of the three different runs. We’d have to wager that when you ran in April and October they were within about 1,000 feet of each other. The run in July was at least 2,000 feet higher than the other two runs. The drop-off in performance in July is directly related to the lack of horsepower because of high temps, humidity, and possibly low barometer. Also, when the track temps climb, as with the July runs, you expect to lose some 60-foot time because of wheel slippage. The variations in the 60-foot times and the performance changes down track don’t look out of line for a gasoline-fueled race car.

We took a look at your pass on “Pass Time”, on YouTube. It offered some insight into how your car is set up. Now, as for your 60-foot performance, you didn’t mention if you’re at 2,450 pounds with driver or not. On “Pass Time,” you said it’s 2,600 pounds with driver. With that weight, the tall 33-inch tires, and your relatively low launch rpm, the mid 1.20s isn’t out of the question. We’d expect your car to 60-foot in the low 1.20s if you turned up the steam on the starting line by raising your launch rpm. Now, what is the name of the game? We’re all about the win lights. If you pour the coals to the car at the hit you may kill your consistency. Unless you’re spinning the tires at the launch, you should be able to get the car moving quicker. Have you tried lowering your tire pressure as a test? Have you raised your launch rpm to see if it’s sensitive? From the video, it looks as if the instant center is a ways out on the car. It barely lifts the left front tire. This is why we would love to see the launch at, say, 5,000 rpm.

You asked about installing a set of double-adjustable shocks. These shocks will give you much more flexibility to adjust your car’s suspension. Again, it seams to work well with the power you’re giving the car now, based on the video. Once you hop the power up, you may need to adjust the compression and rebound independently to optimize the grip you have available.

Finally, yes your e.t. and mph are a little off, given the power you’re producing. This is back to the cars that eat 100 more horsepower than is predicted. Your car, at 2,650 pounds, needs 917 hp to go 165 mph. The eighth-mile and quarter-mile e.t.’s correlate perfectly. If you work on your 60s, you’ll be right in there with picking up your quarter-mile times. You should see a 2:1 correlation between the 60-foot and quarter-mile e.t.; a 0.01-second improvement in 60s should help the quarter by 0.02.

If you really want to try something, borrow a buddy’s 8-inch converter and give it a try. The 9-inch converters are great for efficiency, since they are usually in the 5 percent range, but they lack in torque multiplication. An 8-inch will lose efficiency at the top end to the tune of 9 percent, but it’ll really wake up the car on the starting line. It would be worth it to answer the question. Good luck on your quest for performance, but always remember the name of the game, win lights!


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