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Old 12-06-2004, 10:19 AM
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Question: Rear-End Gears on a Chassis Dyno

Ok, I have a question. Yesterday I ran my $3500 project camaro on a dyno for the first time. You can see my website on the car at: http://home.earthlink.net/~noegoman1...%20Project.htm

Basically, it fell flat on its face. I am currently correcting the problems pointed out by the printout. It was running lean, and the timing was not correct at the high rpms.

Something I still know little about is rear-ends. On a chassis dyno, does having taller gears affect the output of torque (and thus power) ?? I have the Holly Systemax II, and I only have 3.23 gears - would that significantly lower the performance on the dyno? -Ed

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Old 12-06-2004, 02:00 PM
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Rear gears are just one piece of an entire system. If you have installed a 'hot' camshaft, higher compression, bigger flowing heads, big carb, short headers and a high stall torque converter, then your engine is designed for higher RPM. The more radical your modifications, the higher the optimal operating speed. In fact a 'hot' engine can easily perform worse that a stock engine at the low end speed range.

Not knowing anything about your combination, My guess is you need a lower rear end ratio to get your engine up into it's optimal RPM range. I suggest you run a Desktop Dyno calculation to see where your particular range is. For the 'typical' (if there is such a thing) street/strip hop up, a 3.54 - 3.73 rear gear range is usually just right.

UPDATE: I thought your link was going to be a picture of your car not a description. From what you have done to the engine, I think you need 3.73 gears. Didn't see what stall speed torque converter you used but you should have something in the range of 2200rpm to let your engine stretch its legs.

Last edited by willys36@aol.com; 12-06-2004 at 02:05 PM.
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Old 12-06-2004, 02:38 PM
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Does gear ratio effect dyno at 6500rpm

Thank you for the reply, and I agree that I should put in 3.73 or a little higher gears in my car.

The question that I still have is this: if my car had 2.15 gears, would the dyno result (at 6500 rpm) be the same if the car had 4.11 gears? Does the final gear ratio have an effect on the torque output to the wheels?

I know that a car would do very poorly at low RPM with small gears, but I am trying to figure out all the reasons that my engine did poorly on the dyno. I am wondering if small gears is one of the problems that I need to fix -Ed
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Old 12-06-2004, 05:37 PM
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The lower gear (higher number) would increase the torque output to the rear wheels. However, since horsepower is the time rate of doing work, and the number of revelutions of the rear wheel is decreased with the lower gear, the horsepower would be the same (disregarding small variances due to differences in frictional losses between the two gear sets)
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Old 12-06-2004, 05:47 PM
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Thank You. -Ed
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Old 12-06-2004, 11:19 PM
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I was wondering this too. So the torque numbers from the dyno will vary with gear ratio? Or does the tire size/gear ratio numbers plug into the dyno and it calculates the correction??
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Old 12-07-2004, 06:31 AM
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If memory serves, the old chassis dyno I used only had mph and hp. Not familiar with the new computer units. I would think that there would be a torque correction ability for the gear ratio / rear tire circumference to standardized the engine torque. However, the torque at the rear wheels is a good thing to know as well.

1 hp = 33,000 ft-lb per minute (by definition)
or 550 ft-lb per second

In a dynomometer, the rotating action of the crankshaft or the drum the wheel is riding on is acted on by some type of braking mechanism. A fluid dyno is what I am most familiar with. In this case, it is a fluid coupling. The drum is one "fan" and the other "fan" is the load unit. The load unit has an arm which is attched to a load cell, and spring loaded variable resitor if you will, which records force in pounds. The length of the arm in feet(R) times the force in pounds (P) results in torque (T), ft-lb. Horsepower = PRN/5252 where N = rpm and the 1/5252 is the value of 2pi/33,000.

Looking at this from the gearset point of view, the reason for using gear reduction is to multiply torque. The torque on the axle shaft is 4.11 times the torque on the driveshaft when using a 4.11 gear. Likewise, using a shorter tire increases the force applied to the road. T=PR is also P=T/R. If you decrease the tire radius, the force applied increases.

Barring mechanical condition changes and atmospheric changes, an engine will produce a given amount of horsepower at a given engine rpm. Horsepower must eventually be converted to heat. Fricitonal losses in the drivetrain, increase in the tire and pavement temperature, increase in the air and vehicle skin temperature. Therefore, the horsepower at the rear wheel is constant at a given engine speed, mech. condition, atmospheric cond. It is the engine horsepower less the drivetrain frictional losses. Using the hp formula, if N decreases (a lower gear) PR must increase (torque).
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Old 12-07-2004, 07:53 AM
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Dynojet Compensation

First off, thank you all for helping me out on this issue. I am most certainly going to change my rear-end gears soon.

I was reading last night on a website for DynoJet, the company that made the dyno that I ran on. Unfortunately for me, the program that is run with the dyno compensates for gear ratio and tire size by comparing wheel speed to engine speed. This is good for the dyno, because it standardizes all cars. It is bad for me, because it means that my engine actually ran fairly poorly.

So, my next question is, how does one properly tune a car???? I have searched everywhere for a book that focuses on tuning a chevy carbureted engine. I can't find anything. Is there something out there that pulls together Timing, Fuel Jetting, and other elements, and shows how to systematically and correctly tune a car? Simply put, my engine should be able to produce 350 ft/lbs of torque or better at the rear wheels. It produced only 270. I need to fix this!

My dyno sheet points out that I am running at about 13.5:1 for the air/fuel ratio. That is a little bit lean, but would correcting that to 13:1 have a major impact on the dyno pull?

Next, the engine was knocking while on the dyno. I had set the timing to 35 degrees, but it turns out that I not correct for a 10.5:1 compression. Would a little bit of spark knock make a big difference on the dyno pull (I assume it would). -Ed
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Old 12-07-2004, 08:04 AM
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Car Craft magazine does dyno pulls on almost everything they lay their hands on. Ususally with some on dyno tuning tips too. In the last one they were pulling an old mustang and had it fattened up to 11.5:1 on their best pull.
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Old 12-07-2004, 08:23 AM
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Re: Dynojet Compensation

Quote:
Originally posted by NoEgoMan100
So, my next question is, how does one properly tune a car???? I have searched everywhere for a book that focuses on tuning a Chevy carbureted engine. I can't find anything. Is there something out there that pulls together Timing, Fuel Jetting, and other elements, and shows how to systematically and correctly tune a car? Simply put, my engine should be able to produce 350 ft/lbs of torque or better at the rear wheels. It produced only 270. I need to fix this!

My dyno sheet points out that I am running at about 13.5:1 for the air/fuel ratio. That is a little bit lean, but would correcting that to 13:1 have a major impact on the dyno pull?

Next, the engine was knocking while on the dyno. I had set the timing to 35 degrees, but it turns out that I not correct for a 10.5:1 compression. Would a little bit of spark knock make a big difference on the dyno pull (I assume it would). -Ed
Well, here is how I do it. First I set the initial timing with with a vacuum gauge. Twist the distributor until you have maximum manifold vacuum. Test the car for knock and back down the timing if necessary just enough to eliminate the knock. Set the idle screws the same way incidentally. Get an adjustable vacuum advance pot and adjust it for max advance to just eliminate knock. Centrifugal advance is best set on an ignition machine but can be done seat of the pants by changing out springs, again put in the lightest ones that just avoid knock. Above all, run an MSD ignition box. Those suckers really work.

To tune jets on the carb, I use an Edelbrock O2 sensor (see photo below). Only costs about $130 and can be used to tune multiple cars. There are more expensive ones out there with fancy digital readout but the cheapie Edelbrock works fine. I weld a collar in the exhaust just south of the header flange and temporarily install the sensor. After I set the jets, I remove the instrument and plug the collar with a big hex bolt.

All of these steps are done in a sequence over and over until they are all optimized. Take notes while you are doing it! Usually can zero in on the proper tune within three rounds.

Last edited by willys36@aol.com; 12-07-2004 at 08:31 AM.
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