Low end increase performance

[henry]

What is the best way to increase performance in the low end (first gear?) in a 2.0L FI.?

[engineerted]

Swap out your rear end ratio, 3.90 std on 2L, 4.30 std on 1.8L, most of the 1978's came with the newer style rear end, this would be a direct swap, other than the speedo gear ratio.

Ted

[henry]

Can you please explain me a little bit more about this?

Thank you.

[launieg]

Be aware that there are reports of various ratios in these 1978 models with the modern diff housing. For example, I have a May 1978 parts car that has the new diff housing, but seems to have a 4.1 ratio. For me, this would be a nice compromise, giving me more pickup for my 1980 FI 2 litre without running the revs too high at highway speeds. Our highest speed limit around here is 90mph, and most of my driving is at 50mph or less, so it would be a nice compromise, I think.

I believe that others have reported the same diffs (from 1978 models) have had either 3.9 ratios and 4.3 ratios, but when I look at the old threads, that is not entirely clear. (Look at this thread http://www.fiatspider.com/f08/viewtopic ... tio#p42616

I guess the answer is to always check the rear end before installing by rotating the axle or drive, counting the turns in the counterpart. For example, I got exactly 41 turns of the diff drive-out to exactly 20 turns of one road wheel, with the other firmly on the ground (not turning). I'm supposed to have a 4.3 according to the car's manual, so maybe I should recheck it.

[baltobernie]

Can you please explain me a little bit more about this?

The "lower" the gearing, the quicker the engine can turn the gears. That's why your car accelerates faster in first gear than in fifth. When you lower the final drive ratio, acceleration in fourth gear feels more like third, etc. So first gear will really spin quickly.

Now to confuse things a bit: "Lower" gearing another way of saying "higher" numerical axle ratio The 4.3 Ted is speaking of is "lower" than a 4.1 The 4.3 car's wheels revolve 1 time for every 4.3 turns of the driveshaft. The 4.1 car's wheels would revolve a few inches further with the same number of driveshaft revolutions.

The bad news: Better low-speed performance comes at a price, and that price is poorer high-speed performance. I swapped my original 4.3 rear for a 3.9, and my highway revs went down 500 RPM for the same MPH. Yes, I did lose a little bit of acceleration, but now the fillings stay in my teeth at the 70+ MPH everybody drives around here.

To further confuse the issue: Are you sure you're not asking how to get more performance at lower RPMs? As opposed to more performance in first gear? The 2-litre FI motor has the most "grunt" of all the Fiat engines to begin with. Depending on your budget and regulatory emissions laws where you live, you may be able to achieve quicker acceleration from a stop using other methods, and still have a comfortable highway car.

[engineerted]

Is this the same Henry that posted on Guy Croft's site? If so, you need to provide more information on the issue. If I may, I think the issue you have is that it seems that the car does not accelerate very fast in first gear. The Fiat 5 speed gear box has a very low first gear, Hell; I can take off with engine at idle. Thus you find yourself shifting in to second very quickly. The only way to change that is to change the 1st gear ratio, and that’s not going to happen. So drop in the 4.10 or 4.30 rear end and take off in 2nd gear, or swap out that 2l motor for an 1.8 or 1.6 and rev the snot out of it in 1st gear.


Ted

[baltobernie]

Ted,

That's what my buddy John did with his IDFd 2-litre. We put in the auto rear end, so now he has a usable 1st gear and comfortable highway cruise.

If I had the time and money, I'd investigate the possibility of manufacturing a fifth gear pair that would result in a larger gap to 4th than presently exists. No speedo gear swap, no acceleration change, just better highway cruising. I know Alvon would buy one!

[Europa]

If I had the time and money, I'd investigate the possibility of manufacturing a fifth gear pair that would result in a larger gap to 4th than presently exists. No speedo gear swap, no acceleration change, just better highway cruising. I know Alvon would buy one!

I would too! I've been thinking about the 5th gear ratio for a while now. 4th gear is 1:1 and 5th is just a small drop to 0.881:1.

[racydave]

Short of changing out your rear end, you can re-size the tires to a smaller diameter for quickness. You might also try tweeking the ignition curve. Wild guess, but maybe a small change in the throttle position switch?