I have had time to look into some 502 build option specs. In particular the following two web sites have dyno test data for different build configurations:http://www.fourwheeler.com/how-to/engine/131-9809-gm-performance-parts-502-torque-motor-buildup/http://www.chevrolet.com/performance/crate-engines
The Four Wheeler link shows results from several builds, including a high RPM, High HP version. The following graph shows torque curves for these builds plus the Chevy crate motor specs:
Out of these options, the 2 torque curves that look like they have the most potential are the Chevy Crate ZZ502 and the Four Wheeler 502 Version 3A build. For comparison, the ZZ502 would be a “Torque” motor build and the Version 3A would be a “Horsepower” build. The interesting thing that I note is that the “Horsepower” build actually makes a little more torque than the “Torque” build, plus it makes that torque at a lot higher RPM, so it makes a lot more power.
If anyone has any other input on torque curves for this engine, please add them to the thread.
I applied 3 different gear ratio options to these two engine options, the 2.12 Nola has, a 2.68 and a 2.88 ratio. I also used a 90% efficiency reduction factor on the gear box. This is just a guess on my part and I am open to any input on what a good figure to use here would be. Some of the discussions I have seen on this subject said that gear box efficiency varies a lot and is hard to predict, but that should be part of the discussion.
This 21 foot boat application needs to throw the largest diameter prop possible. So this will need an 82” (or even bigger) prop with a maximum RPM in the range of 2,300 to 2,500, but you can run them slower, particularly with more blades, as others have noted here.
The first thing that this makes clear is that the 502 does not make the kind of torque or power that everyone has said this application needs. So it is going to be tough to make this 21 footer run dry. Looking at these curves, it looks like the 2.12 ratio box on hand is not enough gear to turn a big prop.
From my view, the 2.88 ratio and a high Hp build looks like the best, or probably only way to go, especially given the desire to run dry (because it is an airboat after all, it needs to run dry). That puts the motor operating above 4,000 RPM for most of the time, so make sure the heads and valve train gets the attention that it needs.
One note of caution, none of these sources provided torque data in the lower RPM region and the HP motor curve show the torque going flat below 4,500 RPM. If that is the case, then it is as close to perfect for the application as possible, but I wonder if these are really valid figures. Since this boat will be marginal at best on ground, it is going to need as much torque as possible off the line to get started on ground.
The end result is a big prop, turning at or below 2,000 RPM and a motor that operates in the 4,500+ RPM range. Talk to the motor builder and see what they have to say about maintaining good torque (~ 500 ft-lbs) below 4,500 RPM. If you have to sacrifice some top end torque to keep the low end flat, that would be a tradeoff to consider.
One last point, do note that the “Torque” curve is a factory crate spec engine while the “HP” curve is an aftermarket tuned unit. If this motor can be built to make substantially more torque down low, the above should be re-accessed.
How about some input to solve this riddle.
I grew up thinking I-10 was the Mason Dixon line.
1986 Airboat Engineering Inc., 14' Marsh Master. Refreshed narrow deck, SV O-540, 72” NGQ. A Bob Stossel original.