what gear for 7% grade

[serialk11r]

I'm pretty sure the OP didn't say 2500rpm, someone mentioned that was the best BSFC and then someone else got confused.

800rpm is definitely not good for the engine and 1000 probably isn't either. Most cars' owners manuals tell you explicitly to not put heavy load on the engine at low rpm (with manual transmission). Even if you had sufficient oil flow to protect the bearings (which is debatable, on a typical engine I would guess it's not entirely adequate), the cylinder walls are not under hydrodynamic lubrication even with piston squirters and high load at low engine speed will squeeze the oil film out.

[digital rules]

How about a light load at 1000 RPM? Down to 1000 at times when in stop & crawl traffic with light pressure on the accelterator. The engine doesn't seem to mind. I can almost stop & still use 2nd gear as well, but not sure if this is OK either?

Thanks!!

[YeahPete]

Holy cow a 4 mile 7% grade. I guess I would approach it by as you said gain as much speed as possible and attempt to maintain 5th gear. But if you lose speed its time to admit defeat and downshift. We can't really win them all. 22mph in 5th gear is definately too low. I don't know what the speed limit in this section is but you can't just go over 10 mph below the speed limit and bottleneck traffic. Its not the right thing to do and you are not the only person on the road.

When your starting your car from a stop its just better to use the gears. First is for starting out. Second gear is good as long as your are still moving (rolling stop signs). I have been known to skip 3rd and 4th gears every now and then.

I feel I use more gas if I am trying to power up a hill in 5th gear with the accelerator floored and losing speed than to just shift to 4th and get over the hill quicker and not flooring it. I don't know for sure but I think I read that somewhere but don't remember where :P. Can someone prove me wrong please.

Cars maximum effeciency at 50-80% throttle? So if your flooring it to get up a hill would it make sense to downshift if your loosing too much speed?

I personally like to be going 10 mph over the limit at the top of the hill to give me a nice long coast. Doing so has netted me an improvement from 36mpg(constant 60mph speed) to 46mpg (pulse and glide (65mph - 55mph) on the hilly state road.

[user removed]

A 7% grade downhill coasting will get you way over the speed limit. Coming back from Va Tech there is a similar grade fro close to that distance. Behind a big rig, I was using 5th and DFCO to slow down from 85 MPH. The rig was sucking me into his rear end. I think I could have got close to 90 in clean air. Dropping 7 feet vertical at 65 MPH (100fet per second) at 75 (speed limit 70) you aredropping vertically about 9 feet a second, about 36-40 HP in energy to add to your speed.

Climbing a 7% grade in WVa my Scion XB wasrunning 4 k RPM and down shifted two gears.

regards
Mech

[Ecky]

Perhaps I don't understand grades, having spent most of my life in Florida, but so long as I'm able to accelerate in a gear it should be safe to run in that gear?

I recently moved to Tallahassee and had to learn driving on hills, and found I could accelerate up some steep grades in 5th from as little as 15 or 20mph (850-1100rpm). I typically don't shift before 2000rpm, but I see my wife accelerating up some grades I'd eyeball at 30-40% in 5th below 25mph. Should I make sure she downshifts?

[cbaber]

The owners manual for my Honda says drive in the highest gear that still allows the car to accelerate, which is the general rule I follow. If you can still accelerate then the motor is not being overworked, and you should be safe. If you are losing speed at high engine loads, it's time to downshift.

As far as MPG strategy, the most fuel efficient gear is the highest one. If you are going any speed, say 30 mph, higher gears will be more fuel efficient than lower gears because of the lower RPMs. The higher load required in the higher gears doesn't matter, as long as you are not going high enough to enrich the mixture (~90% load, WOT).

Adding to the confusion is BSFC. Keep in mind that the ideal RPM and load range set by the BSFC chart might be the most power efficient, but it is not necessarily the most fuel efficient. People that do P&G accelerate based on the BSFC chart because they want to maximize the efficiency of getting back to the top end speed so they can glide.

People that use the DWL (driving with load) technique need to use the lowest possible load for each situation, because lower loads = higher MPG's. We are not concerned about the BSFC while cruising at a steady state, only when accelerating. So going up a hill on a highway you need to find the highest gear you can maintain your minimum speed in.

[Cobb]

Sounds like the engines in hondas have more balls or torque to them then Suzuki? I know Japanese made engines are funny about spark plugs and love to rev. I have experience with Suzuki sidekicks 2&4 doors, swifts and gem metros with 3 and 4 cylinders.

So, for the suzukis it seems they are rather gutless til about 3 grand, then the power band starts about 5. In my zukis I use a 5-10 degree advance on the cam to drop that.

The Honda insight I have has the IMA assist and vtec that kicks in at 2700 rpms. It does rev rather low than some 6 or 8 cylinder engines in vehicles I have driven. Then again it has the ima for the lower speeds and lower end.

Yeah, I agree. If you can accelerate in that gear, its fine. I try to stay around where the max torque is in the rpm band vs the BSFC.

[cbaber]

Originally Posted by Cobb

Sounds like the engines in hondas have more balls or torque to them then Suzuki? I know Japanese made engines are funny about spark plugs and love to rev. I have experience with Suzuki sidekicks 2&4 doors, swifts and gem metros with 3 and 4 cylinders.

So, for the suzukis it seems they are rather gutless til about 3 grand, then the power band starts about 5. In my zukis I use a 5-10 degree advance on the cam to drop that.

The Honda insight I have has the IMA assist and vtec that kicks in at 2700 rpms. It does rev rather low than some 6 or 8 cylinder engines in vehicles I have driven. Then again it has the ima for the lower speeds and lower end.

Yeah, I agree. If you can accelerate in that gear, its fine. I try to stay around where the max torque is in the rpm band vs the BSFC.

My car isn't exactly happy below 2500 RPM, but I've learned to sacrifice speed and power for fuel economy. I try to shift below 2500 RPM, but I accelerate with about 80% load so I can get to top gear as quickly as possible.

Many Honda's come with shorter gear transmissions which encourage higher RPM driving to compliment the VTEC system. But for the VX, HX, HF, CX, and some DX/LX models, the longer geared transmissions make it pretty easy to drive like a grandma. It might not be exciting or fast, but thats where the better fuel economy is.

[Cobb]

cbaber, when does your vtec kick in?

Just for notes, 55 mph was 1700 rpms, 70 mph was 2300 rpms. Not bad for a 1.3 engine. My sidekick runs 3 thousand rpms at 60 and it just climbs from there.

[redpoint5]

My TSX 6-speed runs 3000 RPM at 70 MPH and the VTec kicks in at 6,000 RPM.

I'm envious of everyone that has more usable and efficient gears and VTec engagement.