Diff ratios

[Timwell]

Hey guys,

What is a good diff ratio? And how long is a piece of string?

Some background:
L28 with tripple DCOE webbers.
Standard 260z gearbox.
I Don't go over 130km/h.

I dont want to rev the crap out of the car when cruising on the highway. But on the flip side want the best acceleration.

IS 4.1:1 too high?

Thanks Guys,

Tim

[.]

4.1 should be a good ratio given your requirements. However there is a couple of variables you forgot to mention.

1. What gearbox are you running? (Each box has a diffent ration fifth gear)
2. What Rim and tyre combo? (The rim and tyre combo will alter the final ration to the road)

I have been running a 4.6 in my 240z which is great on shorter tracks and for motorkhanas but not so great for highway driving. I have a 280zx gearbox and I have 16x50 tyres. At 110KMPH the car is reving at a shade under 4000RPM in fifth.

[Zedman240]

I had a 4.4 with almost the same setup and on the freeway, I'd be revving at around 3500ish rpm? Best all rounder would be between a 3.9/4.1 With the 260 box first gear, which is 2.93, a 4.1 would be better but on a freeway, might rev a tad. If you can, try them both...

[260Coupe]

I have a 3.9 in my Zed which sees a fraction over 3000rpm at 100 kph, the old 4.1 had the donk spinning at 3500 at 100 kph (not great on a drive to QLD) , standard 260 gear box with 205/60/15 tyres

[dat2kman]

4.4 diff 225/50x 15 rubber, 0.83 fifth 7300 rpm, approx 215 kmh Eastern Creek
4.1 diff, 225/50x15 rubber 0.83 fifth 7400 rpm approx 235 kmh Mt Panorama

If anyone is after GREAT ecomomy and, wants BETTER 1st to 4th gear performance, I have for sale a  close ratio 260?? box with a 3.0 first gear, 1.8 2nd,  and it has a 0.75 fifth overdrive !!!
Tha will drop your highway engine RPM right down, need to overtake? drop it back a couple, and let her sing.

[Timwell]

Gents,

all good feedback...less the 7400rpm suggestion . It sounds like I could live with 4.1:1 as I have 16s with 50 series tyres.

Since I am have this car for fun an not for day to day driving I think the "who cares its only money and fuel" factor comes into play. So if you see a lime green 240 cruising around at 30km at 6000rpm...its probably me!!

I knew it was a how long is a piece of string question however I am impressed with the quality of the responses and can now make a sound decision.

thanks

Tim

[dat2kman]

7400, nah thats safe, my max is 7800, it has had a little done to the bottom end, no more than what Nissan were doing with these engines in the USA to go SCCA racing, They used to spin the 2.4's and 2.6's to 9000, and the L20A 2 litre jobbies would see to 10,000 rpm. ( Fairlady Z or 432Z twincam)

Biggest issue with a stock bottom end is harmonics, ie at certain rpm the rotationary vibration will set in and thus cause things to dislodge violently. By altering what is rotating and going up and down and a particular way, you are able to cause the harmonic to be "moved" further up the rpm scale.
At approx 3500 rpm all the L6 engines have their first harmonic vibration, but it is insignificant, the next one is the worry, hence the "redline" setting on your tacho.

[.]

I am happy to hit 7400 for 30-40 seconds on the track but I would not want to sit on more that 4000 on the highway for prolonged periods. I think I have accidently hit 8000 - 8500 for an instant during motorkhanas. 

[stevo_gj]

At approx 3500 rpm all the L6 engines have their first harmonic vibration, but it is insignificant, the next one is the worry, hence the "redline" setting on your tacho.

When you say harmonic vibration, do you mean it achieves resonance? I thought that resonance was a completely different ball game.

My understanding is that the redline is linked to the fact that when you increase the RPM the minor differences in masses of rotating components will become more significant and introduce high oscillating loads. Therefore when you reduce the differences in mass by balancing your engine, you improve the engine's ability to cope with higher RPM. Do you mean that to shift the redline further up the scale you need to change the resonant frequency of the engine as well as balance its components?

[Timwell]

Steve-o,

you are right in both senses. The true word is 'harmonic resonance' and it refers to the natural frequency of something. Like when you hold a rope and whip it back and forward it creates a wave. The rope has reached harmonic resonance. Other material will only create a solid wave at certain frequencies...like running your finger around a glass and making it sing.

So removing material from a rotating mass has two aims:
1. To reduce the rotational inertia (i.e: make it easier to accelerate) like a lightened flywheel, and
2. Balance the rotating mass, this is what stops/reduces the vibration which consequently reduces the likelyhood of reaching harmonic resonance, which in some cases can be catastrophic.

Tim

[stevo_gj]

Hey Tim,

I've just done a course on dynamics and vibrations, which is why I perked up when I heard this topic mentioned.

Thanks for clearing that up for me. When balancing the rotating mass you are simply reducing the amplitude of the rotating force, not actually changing the resonant frequency of the engine, which is a much bigger job.

Cheers

[MaygZ]

Steve and Tim,

everyday is a school day.  thank you.