Some ramblings on gear ratios and rev matching |  |  |
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| Some ramblings on gear ratios and rev matching grant - Sunday, 1 December, 2013, at 11:56:49 am |
I thought i'd post some thoughts and things that i find useful.
Rev matching is often a case of anticipating the % that the motor speed must fall or rise to get a smooth engagement. In general, you can do this by "fee", and in many cases its easiest to - say - over-rev a bit, catch it as it falls, and call it "good enough".
But if you want an idea of how much revs do need to fall (or rise) to mesh perfectly, its good to know the gear ratios. For example, they tend to be more closely spaced as they go up - the drop from 1-2 may be 50% (in fact, it is) while 4-5 may be 20%.
Some years ago, I decided to find out in my car. It turns out either lady luck smiles on us 5-speed 986 owners or someone is Stuttgart wants to make our rev-matching lives easy. My method was as follows: measure the car's speed at either 1000 or 2000 rpms to get a "mph / 1000 rpms" figure.
For the 985 5-speed (Audi 01 tranny) the answer is as follows.
1rst 5 mph / 1k rpms
2nd 10 mph
3rd 15 mph
4th 20 mph
5th 25 mph
really. Its that easy.
that means
1-2 = -50% engine speed for same road speed
2-3 = -33%
3-4 = -25%
4-5 = -20%
On my 986S 6-speed (Porsche crunch-box), 1,2,3 are the same; 6th = old 5th and 4,5 are spaced between 3 and 6 (duh). I dont have their exact mph/1k rpm number yet; something to do when i have the car uncovered (its sleeping, like a Norwegian Parrot).
I think this disparity throws a lot of learners off. They get a ratio that works between, say, gears 3-4 and then suddenly between 1-2 or 5-6 its not as smooth. That's because the ratio is in fact different. But its a smooth a predictable decrease from lower to higher.
Ever notice how hard it is to get into 1st gear? That's because the synchros have to work quite a lot -typically idle to X rpms, before the gear can be selected. Try blipping the throttle and then applying gentle pressure as the revs fall. Suddenly it will slip in like, a they say, butter.
/ rambling
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
Rev matching is often a case of anticipating the % that the motor speed must fall or rise to get a smooth engagement. In general, you can do this by "fee", and in many cases its easiest to - say - over-rev a bit, catch it as it falls, and call it "good enough".
But if you want an idea of how much revs do need to fall (or rise) to mesh perfectly, its good to know the gear ratios. For example, they tend to be more closely spaced as they go up - the drop from 1-2 may be 50% (in fact, it is) while 4-5 may be 20%.
Some years ago, I decided to find out in my car. It turns out either lady luck smiles on us 5-speed 986 owners or someone is Stuttgart wants to make our rev-matching lives easy. My method was as follows: measure the car's speed at either 1000 or 2000 rpms to get a "mph / 1000 rpms" figure.
For the 985 5-speed (Audi 01 tranny) the answer is as follows.
1rst 5 mph / 1k rpms
2nd 10 mph
3rd 15 mph
4th 20 mph
5th 25 mph
really. Its that easy.
that means
1-2 = -50% engine speed for same road speed
2-3 = -33%
3-4 = -25%
4-5 = -20%
On my 986S 6-speed (Porsche crunch-box), 1,2,3 are the same; 6th = old 5th and 4,5 are spaced between 3 and 6 (duh). I dont have their exact mph/1k rpm number yet; something to do when i have the car uncovered (its sleeping, like a Norwegian Parrot).
I think this disparity throws a lot of learners off. They get a ratio that works between, say, gears 3-4 and then suddenly between 1-2 or 5-6 its not as smooth. That's because the ratio is in fact different. But its a smooth a predictable decrease from lower to higher.
Ever notice how hard it is to get into 1st gear? That's because the synchros have to work quite a lot -typically idle to X rpms, before the gear can be selected. Try blipping the throttle and then applying gentle pressure as the revs fall. Suddenly it will slip in like, a they say, butter.
/ rambling
Grant
Grant
gee-lenahan-at-gee-mail-dot-com
| Good post, Grant. Bob G - Sunday, 1 December, 2013, at 12:52:18 pm |
Laz - Sunday, 1 December, 2013, at 9:36:01 pm |
| Just in case Bob's point is not clear.... grant - Monday, 2 December, 2013, at 8:54:03 pm |
(aside from the A&C reference...)
Shifting to 1st requries that you synchronize to whatever speed the tranny shaft is turning. Might require, say, 2800 rpms.
But shifting to 2nd gear lowers that (read my other post) to 1/2 that - 1400 rpms - only about 500 rpm above idle. Voila! Now shifting to 1st is easy - its DOWN shift.
But if you blipped (note- spell check does not think "blipped" is a word) the throttle to match in the first place, all this could be avoided.
QED.
G
Grant
gee-lenahan-at-gee-mail-dot-com
Shifting to 1st requries that you synchronize to whatever speed the tranny shaft is turning. Might require, say, 2800 rpms.
But shifting to 2nd gear lowers that (read my other post) to 1/2 that - 1400 rpms - only about 500 rpm above idle. Voila! Now shifting to 1st is easy - its DOWN shift.
But if you blipped (note- spell check does not think "blipped" is a word) the throttle to match in the first place, all this could be avoided.
QED.
G
Grant
gee-lenahan-at-gee-mail-dot-com
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