The Big Problem With Modern Manual Transmissions - Rev Hang

Hello everyone and welcome! Today is my YouTube Birthday, and Engineering Explained is officially 9 years old! Thanks for joining in, I really appreciate it! As for the video, I have wanted to make a video on rev hang for a long time, but wanted to make sure I had it right. I've discussed this with numerous powertrain engineers, read through academic literature that relates to the subject, and boiled it all down in this video. Quite excited about this one, hope you enjoy!!

It doesn't matter how hard or soft I shift, there's still a jerk in the driver's seat.

I had a throttle body freeze up while going down a hill in winter, now THAT was some rev hang...

I surprisingly understood everything. And I'm a dentist. As they say.. if you can't explain it to a ten year old, you don't probably understand it yourself.

Idk why I'm watching this, my car is carbed.😂

good thing that my throttle is cable operated then again my scooter doesnt have any gears...

The answer to any modern car problem: “it’s to control emissions”

I would love to see a modern engine with today's materials and tolerances ran with all mechanical/carb then converted to all electronic and see the actual mpg differences and emissions.

You failed to mention that cable driven throttles usually have some mechanical non linearity built into them. If you look at the part the cable goes around, you will notice it's oval. Otherwise good video. I have just converted my car to drive by wire with an aftermarket ecu, and can confirm no rev hang, it's all in the tuning, as you say, it's a deliberate thing built into Oem cars, not a limitation of the technology in general. Electronic throttle, when you can set it up yourself is awesome.

I’ve seen mechanical throttle bodies with a “cammed” lever so that the airflow increase is more proportional to input at the accelerator pedal.

I don't know why I'm watching this, I can't even afford to fix the suspension in my 1993 Audi 80. Edit: As of last month, sadly she is deceased, had a crash at a car meet, front ended up totalled, wrecked intercooler. Spent some good years with her, Rest in Peace.

Literally couldn't be a more intuitive and thorough explanation. That was an amazing video.

Every video I see like this just gives me another reason to hang on to my 90's cars.

I would love to see you take a shot at tuning a car. With all of your knowledge, that may be interesting to watch.

You totally nailed it. I used to do emissions testing at Ford back in the 80s when cars were transitioning to 3-way catalyst necessitating, prior to EFI, feedback carburetors to attempt to control to stoich AFR. But with carbs there were all sorts of AFR excursions during transient tip-in tip-out modes, and on a tip-out, all the wet fuel clinging inside the intake manifold suddenly vaporized and sent this huge HC spike which blew through the oxidation catalyst. Remember dashpots on the carburetor? This slowed the closing of the throttle to it's idle setting to attenuate the fuel vaporization rate, the first form of rev-hang. But even on my 93 Mustang with EFI and cable actuated throttle, it has the annoying rev-hang, but this is the idle speed control valve acting, like drive-by-wire throttle, to bypass air to achieve the same emission control goal. Just some retro perspective to ponder. The guy below said it succinctly, 'it's to control emissions'. Today, there are regulations to be met, vastly more stringent than the 80s. Give credit to the engineers that achieve them and build engines like the Coyote, LT6 and Hellcat.

I really enjoy your videos. As a car enthusiast, I feel it’s important for me to understand how cars and all their components work. Your videos make that easier for me!

4:07 This engine is super clean.

Me: presses pedal to 25% hoping for 50% air Gets 25% air. :O

Excellent description! The other day, I drove a 2018 Jeep Wrangler with a manual and I noticed the rev-hang and the rpm being too high when I hit the next gear. I drive an older Alfa Spider that has no rev-hang and I can feel the rpm match as I up shift. With the Jeep, I was always at too high rpm. I owned a 2017 Toyota Corolla iM and I also noticed the rev-hang. I thought it was due to how freely the motor revs. Now that I've watched this video, I understand the process. Thanks again for another excellent video!

This was a great video! It made me do some thinking about a 2002 Mustang 4.6L I had years ago. It had terrible rev hang, but mechanical TB. The fix was to restrict the airflow of one of the lines connected to the TB by inserting a metal plug in the line with a small hole drilled in it to allow a small amount of airflow. I can only assume that this line was how the ECU added air when lifting off the throttle similar to what you explained here. By restricting that airflow, the rev hang was reduced.