bpu699

Lets say you are in 4th, going down a long straight (930, so 4th is top gear). The best gear to be in coming out of that corner is second...

Assuming you drive a manual...

Do you downshift 4-3-2? Or just 4-2?

The latter makes more sense, but the risk of a mis-shift, over rev, or poor rpm matching seems way higher unless you get it just right...?

How much of an over-rev can a motor take, before bad things happen? 500 over redline? 1000 over? 1500 over?

Just curious more than anything.

In another thread there was discussion of using the gearbox to slow down in case of brake failure...

But same question comes up. If you are at redline in 4th, and brakes fail, and you downshift to third, aren't you going to significantly over rev the motor? Better than hitting a wall I suppose...

Last track day I was going from 3-4 in the 930, and it went from 3-2. I wasn't anywhere near redline when shifting thank god as we were just cruising, and nothing happened. I have never missed a shift before. When setting the car up I actually have the rev limiter set at 1000rpm below redline (5700rpm not 6700). Figured it was easier on the car, that's where peak torque is anyway, and could keep a larger safety margin on the track. Kind of glad I did that. Going 3-2 the car definitely slowed down, and RPM definitely shot up...

At redline driving in anger... would have likely been a different result.

So, how do you guys downshift at the end of a long straight? And for those of you with PDK and not burdened with these issues, I know you are smiling...

TXE36

I generally row down the gears. Less chance of a mistake plus at any moment I can go to power. While 4-2 in a 4 speed tranny is not risky as far as getting the wrong gear is concerned, it is a risk for locking up the rear tires or becoming a money shift if one doesn't wait long enough to be safe.

If you can heel-toe at max braking, there really isn't much to be gained by not rowing through the gears.

Just my opinion.

-Mike

JustinL

Uh oh! This could get heated. Let's hope for some good Friday afternoon reading.

johneecatt

Agreed. Once you get in the habit, it will be second nature and safer.

bpu699

So, when going 4-3-2:

Do you at some point let off the clutch mid braking so that the motor/trans rpm's line up?

When I do it here is the process:

1) In 4th, coming to end of straight... apply brakes so rpm fall from 5500 or so to 3000
2) NOW, apply clutch, feather the gas to rev match, and throw it into 3rd...
3) NOW, apply clutch, feather gas, and throw it into 2nd...

Or, do you simply mash the brakes, clutch, and row 4-3-2?

In the 930, shifting is SLOW. You can't just do lightning shifts... the tranny won't allow it unless you manhandle it into gear (synchros are new).

I initially tried to learn double clutching, but that really seemed to take an eternity, and you and do that at every corner...

mark kibort

i dont know why there is such a debate on this, but there is one way to do this fastest and one way to do this not as fast.. both incorporate the one way to do it that doesnt wear the transmission and still provides you consistent rear wheel engine braking a long with the braking system.

you row the gears and brake as hard as your grip allows. and if that means heal toe. do it. if not, you can cheat a little and brake, then clutch , rev shift, and repeat until you get into the proper gear. double clutch is NOT a needed skill set or function, with modern transmissions . the entire idea of "bliping the throttle" is to spin up the layshaft as you drag the clutch (not full depressed ) across neutral and into the next gear.

skipping a gear is not good as you just make the snycnros do more work and that accelerates wear and get them hot too. (both bad)

also, for safety, its better to row, because of what might happen unexpectedly on the track. always best to be in the best gear at any point in the track. some close ratio gear boxe'd 911s have been raced and a 3rd gear is skipped from 4th to 2nd, but its not as common, and most gear boxes have spacing that doesnt bode well for this. brake , wait a fraction of a second, then start the shifting process.. this way, you never run the risk of over rev.

the most important thing for rowing each gear besides wear reduction is control. having the same rear wheel forces caused by each down shift in a progressive fashion is the best habit to get into. its predictable and gives more control on rapid decel. it becomes a kind of "abs" system for the rear wheels.

as far as setting rev limiter to 1000rpm lower than peak, thats ONLY if you are worried that your motor will last longer if you spend time at 1000rpm less than redline. in actuality, it also means you droop the car down to a higher torque zone of the engine, which puts more forces on the driveline, AND YOU LOSE available HP to the wheels (if that matters to you) what i mean by that, if you are well below max HP, it doesnt matter if the engine has a lot of torque. torque doesnt equal power (contrary to popular opion) in a nutshell, HP determines rear wheel torque, at any vehicle speed, so if you want to have the greatest acceleration rate with your car, it should be at the highest HP possible.
for example.... if you are at max torque with your 911, and you could have been at a higher HP level.... the higher HP level would produce MORE rear wheel forces, even at a lower "torque" level, due to gearing. think of HP as the capacity of the car to accelerate at any vehicle speed.

Hope that helps.

Mark

CosmosMpower

Up and down through all gears, no skipping.

TXE36

Hmm. My foot-cam paid off, as I didn't really know, but I blip for each shift but I don't release the clutch in between. Here is coming into T11 and T12 at COTA last year:

Rowing down the gears at COTA

Since I'm the financier, driver, crew chief, head mechanic, gofer, and bottlewasher for my race team, I have a lot of mechanical empathy I don't speed shift or mash anything.

-Mike

Thundermoose

I don't see the need to blip between shifts if clutch is in. You only need to blip before releasing clutch to match engine speed and transmission speed.

I think to save synchros though you want to do downshifts as the speed allows. I find that rowing just naturally kinda gets you there.

To OP - on upshifts, do you have your thumb pointed down or up when you grab shifter? I started doing 3-4 upshifts with thumb pointed down and it seems to help protect against 3-2 shift.

I'll take a slight nibble at your comment of shortshifting at max torque vs max hp. Please remember that HP = mass x velocity x acceleration. Max acceleration therefore occurs at the point that maximizes hp under the curve given that velocity is the same for both gears at the point of the upshift. If your car is making increasing hp to redline, the optimal shift point is at redline, not necessarily at max torque.

Veloce Raptor

I think it's a matter of personal preference, as there are other pros who skip gears. But I go up and down through each gear. Going down I blip and release the clutch each time. I downshift at the latter half of the brake zone. And...in a slower shifting car like a 930, what's the rush? Take your time.

TXE36

The weird thing is I thought I was releasing the clutch in between. With the clutch in, the only blip that mattered was the last.

You going to TWS tomorrow?

-Mike

tkerrmd

this

bpu699

I'm probably overthinking it...

I get 4 bar of oil pressure when warm, max. That's 60psi or so...

I had heard the old rule of thumb of 10 psi per 1000, so figured 6000rpm was a safe place to be...

As an aside, I have been in other folks older 911's... that seems to be about what they all show at under hot use. I almost never see 5 bar in my car or anyone elses when hot...

Overthinking it?

hunterpeaks

Not in a Porsche, but this guy has some of the fastest lap times at VIR in his BMW. He never shifts down through the gears. Always shifts two at a time. It has made me change the way I shift at races. #1- it takes thought right now to do this since I have been racing 9 years with the normal 4-3-2 downshift-blip, etc. #2- There is ALOT less going on in the car when you just go down from 4-2. You are focused on braking and not worrying with braking, blipping, pushing in the clutch, etc. #3 Getting it done like this is faster in my case at the two tracks I have tried it at, according to my data logger and previous sessions. Watch the video and see how smooth he is, especially in turn 1. Hunter

Thundermoose

I signed up for Saturday only.