The closer you are to 1900 RPM the less fuel required to produce a give amount of HP/torque.Parasitic drag increases with the square of speed.
Calculated HP loss due to aero drag is: 14.1 horsepower at 70mph.
Calculated HP loss due to aero drag is: 17.3 horsepower at 75mph.
With stock gearing the engine need to produce 30ft/lbs of torque just to make up for Parasitic drag @ 70mph
With the .681 gearing the RPMs will be lower at 70mph so you'll need an extra 3ft/lbs of torque. Just to account for the normal drag at 70mph.
So the question can the engine decrease its friction losses from a 250rpm decrease and save more than 3ft/lbs of torque? if so, then the math says the gear change is worth the effort. Cost effective - depends on if we elect another oil guy as president again.
I think the 7.17 with the taller final drive would be a good combo. I think going with the .68 would be too big a gap between fourth and fifth. The 7.17 with the taller drive may give me about 8-10 more mph at a given RPM at highway speeds, which may be worth it. I don't think my car with RC3 will have any trouble pulling the higher ratio.I've done the .717 5th gear and it does not drop the rpms enough to worry about lugging the engine. it drops the rpms by roughly 50-75rpms at 55mph and may be 125-150 rpms at 75mph.
I did this mainly because I was able to sell my .756 5th for $250 and bought the .717 for $330 so for $80 I was able to to the thoughts out of my mind. Would I do this gear ratio swap again...nope. Because the gain was not worth the time. Maybe a .68 but then I bet there would be a lugging of the motor at 50-60mph. oh well my 2 cents. I may not have the math to back it up but I have played with a couple gears sets before.
Yes I did. Big time. I don't know how. The 54 hp figure is correct.JB,
You slipped up in your math. 1900 RPMs x 150 ft-lbs = 54.3 hp.
Not if the graph is to be believed. The red line furthest to the left shows less fuel is used at 1400 rpm (200~210 g) than at 1900 rpm (220~230) for the same power.The closer you are to 1900 RPM the less fuel required to produce a give amount of HP/torque.
Jon, won't an intake cleaning take care of the hubris? Or perhaps some PowerService?I had also forgotten the problem of "hubris". I'll try not to forget again.
I have a different graph that is easier to understand that is HP per fuel quantity per HP and 1900 is were the A3 TDI produces HP the most efficiently, so I used 1900. I also used 1900 because we were discussing figuers in the 2400-3000 range and as the engine gets closer to 1900 RPM (descending from 2000+ RPM)it produces HP more efficiently.
If you can find a drive-in freezer, you can also cool the transmission below freezing to help install the gear!Same thing goes for reinstalling, heat it to 100C and it slide right onto the shaft.
This whole excercise of this thread in which we are guessing which ratios are the best for fuel economy are related to distance traveled per unit of fuel. I truly couldn't care less about how many hp per gallon. I care about how many miles per gallon, and 1900 is not where the TDI engine produces distance most efficiently....1900 is w(h)ere the A3 TDI produces HP the most efficiently, so I used 1900.
Exactly! And we know that air resistance at 60 MPH is constant, lets say it requires 20 HP for an A4 Jetta to maintain 60 MPH. The engine in our cars burns less fuel at 1900 RPM to produce 20 HP that it burns at 2900 RPM to make 20 HP.
When did I say anything at all what-so-ever about using the max torque of the engine at 1900??? You are misunderstanding my posts, go and re-read them before you continue to spout off with stupid acusations of WOT with the handbrake applied to maintain 1900 RPM.If I wanted the engine to make all of its potential peak torque at 1900, I'd drive with my right foot on the floor and use the parking brake to regulate my road speed.
So if stock final drive is 3.156 can I use the table on the left to estimate the RPM drop? Looks correct, as it matches my engine speed (accounting for speedo error) at 60 MPH. If so, 2600 RPM at actual 80 MPH would be great.
Quick rewind to David594's post on 08 Oct.When did I say anything at all what-so-ever about using the max torque of the engine at 1900??? You are misunderstanding my posts, go and re-read them before you continue to spout off with stupid acusations of WOT with the handbrake applied to maintain 1900 RPM.
This was followed by Jackbombay on 09 OctThe main advantage in terms of economy should be the fact that you are keeping the car closer to its max torque. And the max torque should also be the where the car is nearest its maximum mechanical effeciency. This is what your gains would be from, not friction losses.
It was in your paraphrase of David that you made his post appear to say that driving at 1900 rpm is most efficient.Lug Nut, I think you misunderstood the poster you quoted. He, IMO, is saying that driving at peak torque RPM, regardless of right foot position will make HP more efficiently than any other RPM, thus the gains from this mod are more than lower friction, but also more efficient production of any given amount of HP due to operating closer to the peak torque output RPM.