torque off idle and desktop dyno

[mobythevan]

I am playing around with desktop dyno again looking at 350 engine builds and what interests me is the torque off idle up to 2000rpm, because the dyno only shows 2000rpm and up. In particular I was throwing together parts like vortec heads and smaller cams like the XE256H with dual plane manifold and carb. I realize the vortec heads are 64cc and may present dynamic compression ratios that are too high unless dished pistons are used, but I am more interested to know what the torque curve would look like all the way from idle or 1000rpm up. Any suggestions?

[A. G. Olphart]

I don't believe there is any way that the program can be made to show anything below the estimate for 2000 RPM; here is a quote from the Q&A section of the version 3.10 manual:

 

"Question: I have tried many different engine combinations using the same engine displacements and have noticed that several of the power curves begin at nearly the same horsepower and torque values at 2000rpm. Why are they so similar at this engine speed?

 

Answer: Since the Dyno2000 uses a simulation technique that iterates toward an answer—this technique is not the same as used in Iterative Testing—the first power point must be developed based on educated “guesses†about mass flow and other variables. The next point, at 2500rpm, is calculated from the starting point, plus the data obtained from the completed simulation, so accuracy is higher. By 3000rpm, the power points are based on simulation calculations with virtually no remaining influence from the initial estimations."

 

The lack of low speed numbers doesn't bother me nearly as much as the total lack of any octane requirement figures.

 

Basically, it is fun to play with.

[Turbo Meister]

I recently obtained the DynoSim Engine Simulator and DragSim Drag Strip Vehicle Simulator. The engine file actually contains a 2.4, 2.6, and 2.8 liter Datsun engine that can be modified. The drag file contains a Datsun 240zx ??? that will accept any engine and drivetrain from the engine dyno files. So long as you have specific engine component parts you can develop fairly accurate hp and torque curves on the program's dyno charts.

My twin turbo 383 chevy shows as developing 815 ft lbs of torque. However, my 72 240z is more than 1/2 second slower in the quarter mile than the drag chart indicates it should be with that power output. Well, my rear suspension needs adjustment and maybe drag slicks would bring me closer to the projected e.t.

Anyway, these programs are helpful because the itirators give you suggestions for performance improvement and the colorful charts are fun to print out.

Hanns

[Michael]

I’ve had similar frustrations with Desktop Dyno....

 

It begins its calculation only at 2000 rpm, and for various reasons is pretty unreliable at or below 3000 rpm. So if you’re more interested in a high-torque street motor than a race motor, the entire DD simulation is questionable for your rpm range of interest.

 

It accounts reasonably well (perhaps) for changes in cam duration, but not cam lift. As in, increase the cam lift enormously (say, add 0.100” to the lift!), and my 454 picks up maybe 5 hp at the peak!

 

It implies that going to a larger carb, larger diameter headers, etc., will raise the entire torque curve, instead of mostly just helping at the top end – an implication that’s completely false. This is because DD assumes “perfect atomization in the carb”, for example – regardless of jet size, venturi size, vacuum signal, etc.

 

It is remarkably insensitive to changes in cylinder head flow rates, especially to increasing the flow rate. Try doubling the flow rate of your existing heads – the torque curve should change greatly, right? This isn’t a physical scenario, but it’s useful for sanity check of the code. Well, on my engine combo, DD shows almost no improvement.

 

The cam calculation based on duration at 0.050” is inaccurate. DD really needs duration numbers as-advertised (at 0.006” lift).

 

If I built my engine according to Desktop Dyno recommendations, I’d end up with a large-duration but low-lift solid-roller cam, a pair of 1200 cfm Dominators on a tunnel-ram manifold, open headers, a compression ratio of 16:1 and small-port heads with tiny valves. It would spin up to 11,000 rpm, still make great torque at 2000 rpm and run on 87-octane pump gas. Yeah.