Engine Braking, Does It Make Sense?
01-15-2007, 11:13 PM
#76
Rennlist Member
When Jim suggested why the necessity of synchros I was about to ask the requirement for autos...........then I reminded myself where I live now.
4 brakes good one tranny bad...............Otwell or somebody?
4 brakes good one tranny bad...............Otwell or somebody?
01-16-2007, 04:58 AM
#77
Rennlist Member
Hmmm, i think you may have never paid anyone TO REMOVE AND REPLACE A
$100 THROWOUT BEARING!! (oh, and by the way, probalby the $300 pressure plate too!)
anyway, you must not be aware how i downshift. if you match rpms, you get no wear. Ill challenge anyone to downshift by double clutching coming down from 6000rpm down to 4000rpm vs my blip and matching the rpm to the gear speeds. It must be working, as i have over 70race days doing this on the holbert car, and the synchros are in great shape.
there is a way you can do both. care for your synchros and care for your clutch and related components. Jim mentions the load on many of the clutch related parts by holding in the clutch at a stop sign or in traffic. some folks will put more wear on a throwout bearing on one day in LA traffic, than i do in the entire 5 seasons of racing and commuting to the track in the holbert 928!
by double clutching, you can run the risk of not knowing the exact engine speed to use for gear speed matching. there is a fluidity with shifting as we all know. audibly, we learn the speeds and rpms. An example is if miss a shift at the track and i try to find the right rpms for the gear and speed, i rarely make it smooth. its double clutching by accident! if you dont match the speeds exactly, you run the risk of putting more wear and heat on the synchros than if you did a skill full match of rpms on a downshift without double clutching.
Good down shifting is an artform, especially in a 928. ive driven one for 20 years and im finally getting good at it!
Anyway, Read Jim's post. as usual, he is dead on.
Mk
$100 THROWOUT BEARING!! (oh, and by the way, probalby the $300 pressure plate too!)
anyway, you must not be aware how i downshift. if you match rpms, you get no wear. Ill challenge anyone to downshift by double clutching coming down from 6000rpm down to 4000rpm vs my blip and matching the rpm to the gear speeds. It must be working, as i have over 70race days doing this on the holbert car, and the synchros are in great shape.
there is a way you can do both. care for your synchros and care for your clutch and related components. Jim mentions the load on many of the clutch related parts by holding in the clutch at a stop sign or in traffic. some folks will put more wear on a throwout bearing on one day in LA traffic, than i do in the entire 5 seasons of racing and commuting to the track in the holbert 928!
by double clutching, you can run the risk of not knowing the exact engine speed to use for gear speed matching. there is a fluidity with shifting as we all know. audibly, we learn the speeds and rpms. An example is if miss a shift at the track and i try to find the right rpms for the gear and speed, i rarely make it smooth. its double clutching by accident! if you dont match the speeds exactly, you run the risk of putting more wear and heat on the synchros than if you did a skill full match of rpms on a downshift without double clutching.
Good down shifting is an artform, especially in a 928. ive driven one for 20 years and im finally getting good at it!
Anyway, Read Jim's post. as usual, he is dead on.
Mk
Originally Posted by SwayBar
'Silly, no reason'? I think the following is much sillier:
So let's get this straight. You are advocating preserving a $100 throwout bearing, yet on the other hand, you state that it's 'silly' to preserve one's syncros in their very expensive 5-speed transmission?
Sure, one does not need to double-clutch just like one does not need to put their tranny in neutral with the clutch out at a stop light. Nonetheless, both are good practices for the sake of reliability, but one of them is much more costly to fix.
One would think that since you're concerned about a $100 bearing wearing out that you would be much more concerned about a part which cost much much more to fix. Each time you downshift without double-clutching, you're spooling up those syncros, heating them up and subsequently wearing them out over time. Fixing those syncros will prove to be infinitely more expensive to fix versus replacing a cheapo, easy to get to throwout bearing.
Additionally, since you're not a factory driver and must replace worn-out parts yourself, one would think that you would take much better care of such an important and expensive part of your car like the transmission, especially since those good 928 5-speeds keep getting harder to find.
On the bright-side, once those syncros expire, and they will without double-clutching on downshifts, at least you won't need to replace your throwout bearing too.
So let's get this straight. You are advocating preserving a $100 throwout bearing, yet on the other hand, you state that it's 'silly' to preserve one's syncros in their very expensive 5-speed transmission?
Sure, one does not need to double-clutch just like one does not need to put their tranny in neutral with the clutch out at a stop light. Nonetheless, both are good practices for the sake of reliability, but one of them is much more costly to fix.
One would think that since you're concerned about a $100 bearing wearing out that you would be much more concerned about a part which cost much much more to fix. Each time you downshift without double-clutching, you're spooling up those syncros, heating them up and subsequently wearing them out over time. Fixing those syncros will prove to be infinitely more expensive to fix versus replacing a cheapo, easy to get to throwout bearing.
Additionally, since you're not a factory driver and must replace worn-out parts yourself, one would think that you would take much better care of such an important and expensive part of your car like the transmission, especially since those good 928 5-speeds keep getting harder to find.
On the bright-side, once those syncros expire, and they will without double-clutching on downshifts, at least you won't need to replace your throwout bearing too.
01-16-2007, 05:00 AM
#78
Rennlist Member
all the ratios of a GT are the same as any other 928. (except for 4-5th)
mk
mk
Originally Posted by RyanPerrella
I wanted to post this.
I in no way meant to suggest that engine braking will slow the car anymore effectively then the front brakes. Our brakes are great and if i was racing my 928 i would slam the brakes every chance i get cause i know they could probably take it. But I do see the occasional use for engine braking, it dosent reduce your speed much, the 928 GT is a pretty close gearbox, as in ratios are fairly close, not a giant change in RPM going from one gear to the next, if anything you engine brake and go down 2 or 3 gears and slowly let the clutch out. I think its kinda fun, its something you cant do with an auto and i still dont think it accelerates any clutch wear to the point that its wasteful to do the occasional engine braking.
Obviously to stop fast use the brake pedal.
I in no way meant to suggest that engine braking will slow the car anymore effectively then the front brakes. Our brakes are great and if i was racing my 928 i would slam the brakes every chance i get cause i know they could probably take it. But I do see the occasional use for engine braking, it dosent reduce your speed much, the 928 GT is a pretty close gearbox, as in ratios are fairly close, not a giant change in RPM going from one gear to the next, if anything you engine brake and go down 2 or 3 gears and slowly let the clutch out. I think its kinda fun, its something you cant do with an auto and i still dont think it accelerates any clutch wear to the point that its wasteful to do the occasional engine braking.
Obviously to stop fast use the brake pedal.
01-16-2007, 05:05 AM
#79
Rennlist Member
either with or without clutch input, the move to neutral on approach is a smart one. its easy on the clutch, as it doesnt even have to be fully depressed if you match the rpms here too, or wait until they are realy low and there is very little load on the gears, as the shifter will just very easily slip into neutral.
the key wear is the fact that at a light, you are sitting with all that massive spring force deflected, and the throwout bearing is now spinning.
MK
the key wear is the fact that at a light, you are sitting with all that massive spring force deflected, and the throwout bearing is now spinning.
MK
Originally Posted by fabric
I had never heard this, you're ok to just pop it out of gear? This just seems like it wouldn't be good.
01-16-2007, 12:27 PM
#80
Addict
Rennlist Member
Rennlist
Site Sponsor
Rennlist Member
Rennlist
Site Sponsor
Simply stated 1/2 of the gears in the transmission are always turning with the wheels the other gears always turn at the speed of the drive shaft, So double clutching letting out the clutch with the trans in neutral allows you to speed up or slow down the drive shaft and those gears NOT spinning with the wheels so they will mesh. Blipping the throttle to downshift with the clutch pedal on the floor does NOTHING to match the speed of the gears BUT BIG BUT when you let out the clutch after blipping matching engine speed there is no chirp of the tires less load on the clutch and less possibility of spinning out of control if you just happen to be turning. For the record I only double clutch when driving a car with bad weak syncronizers but always blip the throttle on downshifts...When I do down shift. The great movie chase scene in Bullit the "Mustang" sound effects are actually from the camera car an Ex CAN-AM big block Chevy sports racer with a Hewland gear box and Hewlands do not have any stinking syncronizers 
01-16-2007, 01:27 PM
#81
Three Wheelin'
Originally Posted by mark kibort
Ill challenge anyone to downshift by double clutching coming down from 6000rpm down to 4000rpm vs my blip and matching the rpm to the gear speeds. It must be working, as i have over 70race days doing this on the holbert car, and the synchros are in great shape.
You still need to match revs when double clutching. Once you get good at it, double clutching doesn't take really much extra time.
I would usually double clutch when going into 2nd or 1st gears, it definitely provided for a smoother transition.
01-16-2007, 03:03 PM
#82
Rennlist Member
I understand. I just dont see a huge benefit. Dennis is really good a this trick, but it does cost him quite a bit of time on the track. probabably .5 second and another .5 second for the added distraction. 
at laguna, i do 10-15 shifts per lap, all at near redline and all downshifts are in the high rpm range(rpms always over 4000, min) this goes on for 30min on every session. 255-400 shifts per race., 2-3 30min sessions per race day and the hobert car has 77 race days under its belt. something near 60,000shifts over my 5 full race seasons, 1/2 of which are downshifts, and the synchros are just fine. PLUS all the rest of the 20,000 miles ive driven to and from the tracks!
got to love porsche durability!
mk
at laguna, i do 10-15 shifts per lap, all at near redline and all downshifts are in the high rpm range(rpms always over 4000, min) this goes on for 30min on every session. 255-400 shifts per race., 2-3 30min sessions per race day and the hobert car has 77 race days under its belt. something near 60,000shifts over my 5 full race seasons, 1/2 of which are downshifts, and the synchros are just fine. PLUS all the rest of the 20,000 miles ive driven to and from the tracks!
got to love porsche durability!
mk
Originally Posted by fabric
You still need to match revs when double clutching. Once you get good at it, double clutching doesn't take really much extra time.
I would usually double clutch when going into 2nd or 1st gears, it definitely provided for a smoother transition.
I would usually double clutch when going into 2nd or 1st gears, it definitely provided for a smoother transition.
01-16-2007, 04:38 PM
#83
Rennlist Member
There you go Dennis. That's a full second per lap you can pick up. Let us know how that works out. I'll bet it is a habit you can't change at this point. You might lose a second trying. I suspect Mark is right though. Had you never started double clutching your lap times might be better. Probably not a second better, but something.
01-16-2007, 06:40 PM
#84
Addict
Rennlist Member
Rennlist Member
Originally Posted by mark kibort
I understand. I just dont see a huge benefit. Dennis is really good a this trick, but it does cost him quite a bit of time on the track. probabably .5 second and another .5 second for the added distraction.
mk
mk
The synchros in the gearbox on my car looked pretty good too after about 100 track days. The only problem was that the pinion shaft had twisted into two pieces.
