Streamlining conundrum: lowering Cd without raising gearing may be problematic

[MetroMPG]

Note: Phil Knox has given me permission to repost items that he originally posted at MaxMPG. Here's another. - Darin

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Hellow all. As I continue data reduction on T-100 roadtest results, Ive run into something I want to throw out now rather than later.

As I combed my library form aero-related literature to help substantiate my findings, I was caught by surprise by a paper submitted to the Society of Automotive Engineers SAE. Heres the crux of it. When you streamline, you reduce the engine load which in itself is not a bad thing, infact its the whole idea.

The catch is that as you continue to lower drag,you continue to lower the load, the throttle body begins to close down, volumetric efficiency begins to suffer and the engines brake specific fuel consumption ( the amount of fuel the engine uses to produce each horsepower in an hour ) actually goes up.

By lowering the numerical ratio of the final drive you can increases the load on the engine and move its brake specific fuel consumption back up its engine "map" into more efficient territory.

Here's the big rub! If you don't do the gear-matching, you can loose up to 40% of the streamlining benefit.

This may explain why the T-100 was still getting 32-mpg at 75-mph. It takes over 20% more horsepower to go 75 than it does to go 70. From the increase from 25 to 32mpg I calculated that the drag coefficient would have to be on the order of Cd 0.25, a 28% reduction from Cd 0.44. It's quite possible that the blister and boattail have reduced the drag although is not showing as might be expected because of this conundrum.Drat!!!

I don't know any powertrain guys. All my phone numbers at GM are no good now.I once could just call up Glen Scharf at the aero lab and bounce numbers off him. If anybody knows any real aerodynamicists we could noodle with I'd like to know. I pulled another 32-mpg this last weekend and I hope the T-100 is not stuck there.

Since Toyota does not offer a range of differential gears as the BIG-THREE, I'd be forced to try and put some enormous tires on the back of the truck and then "lower" it to get the standard body rake back.I'll keep scratching my head on this one.

Since some of you are actively modifying your vehicles,I'd like every one of you see the highest possible returns on your investments. I feel personally responsible for some of your efforts,don't want to screw the pooch and have you going down a deadend road. More soon, Phil.

[Daox]

Makes sense. However, if you pulse & glide you don't have to worry about that as much.

[MetroMPG]

That's true. And if you drive an un-throttled streamlined diesel, you don't have to worry about it as much either.

[dremd]

What rear end gears do you have now?
Us Supra guys swap rear end parts with 4WD guys, I don't know if you have the same rear axle, or not, but there are quite a few gear options for our rear end (but hard to find) if it is the same as the (say 4 runner) rear axle let me know and I'll dig up some links/ vendors for you.

[Frank Lee]

As far as rear end gears, if you can't find suitable gears for the T100 axle, swapping in an entire rear end from another make or brand of vehicle should not be very difficult and in fact may be cheaper than buying a new gearset. Maybe in addition to getting taller gears, you can go a bit narrower for nicer skirts.

[jjackstone]

Raising the gearing is one of the modifications that Craig Vetter's site suggests after streamlining a vehicle.
http://www.craigvetter.com/pages/470...ng.html#anchor

JJ

[Big Dave]

I can relate to Phil’s problem.

Gearing has helped me. It is to the point that I need to re-gear again to fully reap the benefits of my aerodynamic efforts.

Both gas engines and diesels have really good reasons why gearing improves MPG.
Both engines have a frictional drag involved in just the mechanical running of the engine and pumping the fluids around in the engine. It is not an inconsiderable amount of friction. According to curves I obtained, dropping my gear ratio from 3.73 to 3.08 reduced my engine frictional HP by 12 HP at 70 MPH. If I could drop my ratio another 30% I will get another 8 HP frictional reduction.

In addition gas engines suffer another penalty for excessively high(numerically) gearing. Gas engines modulate power output by throttling. Gas engines are married to a very narrow mixture range, so less power means less air flow which means you close the throttle and make the engine less thermodynamically efficient. A gas engine is most efficient at wide-open throttle. That is what makes hybrids efficient. They use the batteries to help a small engine cope with varying road loads and that small engine operates WOT a lot of the time. A true series hybrid where the engine only charges the battery and never sees road load could use a single engine speed: WOT.

If he still has the OEM gearing I do suspect Phil has hit a wall. As stated, there is no aftermarket support for Toyota axles. But Toyota stuff often resembles GM stuff. My advice would be to get an old GM 10-bolt axle. If necessary, narrow it to fit his truck. Narrowing axles is not for the DIY and will not be without some cost ($300-750), but a GM 10 bolt will allow Phil a wide range of axle ratios all the way down to 2.47.

I cannot reduce my axle ratio unless I degrade the carrying/towing capability of the truck. So I either have to substitute a T-56 transmission or put on a Gear Vendor or US Gear overdrive unit. The GV and USG will reduce my engine RPM by 20%. Both are fairly easy if pricey add-ons – about $3500 installed. Problem is that I wind up pushing not one but two overdrive meshes. The T-56 (there is a 650 ft-lb version made for Vipers) gives me a 31% lower engine RPM and I only have to push one overdrive gear mesh. Every gar mesh robs some frictional HP. In these big trannies the gear frictional HP is so large that these manual transmissions have to have external transmission coolers. The optimal drive train for my truck would be a seven-speed Spicer with a straight-through top gear and a 1.56:1 rear axle ratio, but no such axle exists.

[mattW]

Another option would be to swap the motor out to one with 30% (or proportionally) less hp. That would allow the motor to run at higher throttle/load plus it would weigh less and use less fuel at idle compared to higher gears which only change the throttle/load. I guess it would cost more but it would net more gains.

If I haven't already introduced myself my name is matt and I am a lurker on here and GS. I am an aero and EV enthusist from Australia. You can view my gas log here (in annoying American units) and you might want to check out the EV/Aero wiki that I have done a bunch of work on.

[tasdrouille]

If the load becomes to low, timing might also become adversly affected in regards to FE. The attached image is for the hyundai betaII 2.0 cvvt engine, but this should generally apply to most modern engines.

[personx]

I wonder if re-programing the engine computer would help this problem.