Reducing RPM- Gearing help needed!
 

Hi,

My iS project is currently fitted with a 5 speed Manual Gearbox

5th is 1:1 ratio, and Final Diff ratio is 3.45.

this translates as 3289rpm at 70mph-a speed of 21.27mph/1000rpm

The car has
max HP 138bhp at 6000rpm

max torque 129ft/lb at 4500rpm.

I've been looking at ways to re-time the Cams to give better low down torque, as the car is mainly used on motorways, running approximately 70mph

I dont need it to rev to 6500 for power, as its not a fast car, and doesnt get driven as such.

I've been hunting round my Unit and the web, and have some interesting options available to me to raise the gearing, as follows:

Parts I can use (all of them are "direct bolt on replacements")

e30 318i gearbox- gives 0.8 5th

e36 325 diff- 3.15 ratio final drive.

e36 328i diff- 2.93 final drive.

Info listed as
5th Gear Final Drive MPH/1000rpm RPM@70MPH %change from stock (reduction in revs)

Stock 1:1 3.45 21.27 3289


1 1:1 3.15 23.3 3003 8.7%


2 1:1 2.93 25.05 2793 15%


3 1:0.8 3.45 26.60 2631 20%


4 1:0.8 3.15 29.13 2403 27%


5 1:0.8 2.93 31.32 2235 32%


So which option would you choose and why? I'm thinking the last 2 may be too long for the engine to be able to turn them efficiently...

If any mod knows how to get the table to appear more clearly- cna you please let me know or fix it?

Thanks

My thought is to get a BSFC chart for your engine, and try to get the rpm in the middle of the low-consumption "island" when you're at your cruising speed. This will give you a little leeway for accelerating and passing.

EDIT: See attachment. IMO, for the Saturn shown, you'd want to cruise at 2500 rpm (blue dot). If you speed up or slow down between 1500 and 3500 rpm (green line), you're still in the sweet spot. Likewise if you need more or less torque (between about 93 and 145 Nm) for climbing or descending hills or accelerating (red line), you're still in the sweet spot. You're still pretty good down to about 75 Nm torque because you're still in the 275gm zone. However, if you've modded your car so that you only need about 47 Nm of torque at cruise (I don't know how likely that is, but would guess it's probably rare), then you might want to gear it for 3500 rpm to keep you in the 300gm zone. Somebody please correct me if I'm wrong.

The BSHC charts are made at full load. Normally cruising speed is in that island from the factory.
I think combo #4 would be the way to go.

After doing this 5 gear would only be for cruising. It would be nearly useless for going up hills or passing.

I plan to do something like that for my car. Where I run really tall gearing when I swap the engine and transmission. I will build a high compression 6.7L LSx serries engine bolted to a 6 speed with a top gear of 1:0.5 and use the original 3.08 diff gears for testing. At 70 mph it should be turning about 1400RPM.
I plan on swapping the rear diff and will chose gearing based on my tests with the 3.08. I figure I will keep the 3.08 or go to 3.23

No they're not. See the chart I posted above. It covers the entire operating range of the engine.

equation: MPH = [ 60 / (G×A) ]×[ RPM / rpm ]

where:
MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile
60 = conversion constant, minutes-per-hour
G = gear ratio, XX.X:1, etc.
A = axle ratio, YY.Y:1, etc.

Yeah I've got a good gearing calculator site that takes those factors into account- really handy!

cheers for outlining the calc though-alway good to see what's behind the figures:D



I cant find a BSFC map for the 318iS m42 engine online, so I'm looking to see wht folks think would be the best setup..

Web site attachments are blocked on the other computer I use, I could not see it.

Now the question is how much torque is needed?

Engine torque required to drive the car with 1:1 and 3.45 will be less than engine torqure need to drive 1:0.8 and 3.15.
After regearing you will require more torque at a much lower RPM, it should be do-able.

It loooks like There may be the option to re time the Cams to increase lower RPM torque..

That needs further investigation:D

To gain power at low RPM on a DOCH you want to advance the intake cam and retard the exhaust. This improves low speed driveablilty and fuel economy but hurts high engine speed performance.
Of the 4 valve opening and closing events, the closing of the intake valve has the greatest effect on engine performance. The other 3 open/close events pale in comparison. That is why VVT works so well even on single cam engines. Being able to control intake valve timing is far more important than exhaust valve timing.

Retiming the cam will have the most effect for good around 3400rpm or below and will have bad effects mostly isolated to above 3600rpm.

sometimes retiming the cam is trivial, I advanced my sohc metro cam 10 degrees with a hand drill (and some figuring).

BUT, another strategy that might help for hiway travel is to retard the intake cam, I'm not sure, but if you can "detune" the engine at hiway rpms so that it makes less power, so you can hold the throttle open more (and have less throttle lossses)

I don't think you will have any problem with 1:0.8 2.93, the thing to remember is the hp curve will probably take you to a better bsfc position with lower gearing. Hp is a function of torque and rpm so as you move left, you move up for a given power requirement, and the power requirement isn't changing because of a gearing change:

http://ecomodder.com/forum/attachmen...p;d=1216077759