I've always been curious because I don't really understand what it is. I've heard a few people talk about it, and from what I've gathered it seemingly has to do with downshifting... but what is it, and why does it happen?

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'88 WS6 Formula - LB9, Auto

1 5/8" SLP Headers, Power Flo Catback, CatCo Cat

 

Engine braking is when you use the engine to slow the car down. You are right about downshifting, thats exactly what you are doing. Since you have an auto, have you noticed that you slow down fairly quickly when you take your foot off the gas? Well, accel to any given speed and then take your foot off, note how long it takes to slow down. Now do the same thing, but pop it into neutral first, it takes a lot longer to slow down. Now finally accel. to the same speed as before and then drop a gear, you decel pretty quick. All of this happens because of the parasitic energy loss of the engine. Basically you are using the cars wheels to turn the engine instead of the engine turning the wheels.

 

yep, the throttle is shut and the engine rpm is up--the engine wants to slow down. A must in the mountains.



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350 with stealth intake, holley carb, 470 lift cam. 700r4 with .5 boost valve, vette servo, tci lock-up kit, B&M megashifter. Richmond 3.73 gears, powertrax locker, timkin bearings, synthetic lube. Custom 3 inch single into 2 2.5 pipes. 1 1/2 drop springs, 1 5/16 solid front sway bar, 1 inch rear bar, custom subframe connectors, custom LCA relocation brackets. Kobel ground FX, current red metallic paint. Lots of other stuff...
82camaro

 

Just to add a little something. During an decel the throttle blade(s) is(are) closed. This makes a lot of vaccuum in the engine and actually tries to pull the piston up during the intake stroke. That is why diesel engines (no throttle blades) are sometimes installed with exhaust brakes, which applies positive pressure to the piston during the exhaust stroke.

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'87 Bird 350 .060 over 700R4 + Lots more

 

To add to this, with an automatic such as a 700, in O/D when you let off the gas there isn't much decelleration affect. If you put it into D there will be more, primarily because the engine speed must pick up to meet the gear it is now in, and the more compression you have the more you will feel. Now if you put it into Man 2 it will really start to slow due to increased gear ratio pulling on the engine.

 

Large diesel engines use a compression brake, not an exhaust brake. Same principle, different method. Exhaust brakes restrict the exhaust to a maximum back pressure situation. Causing the engine not to expell the burnt gasses during the exhaust stroke, pressure is still pushing down on the piston after the exhaust stroke and during the intake stroke. Compression brakes use a valebody type system above the head to open the exhaust valve at the top of the compression stroke creating a vacuum in the cylinder. Therefore causing a reaction exactly opposite of combustion and sucking on the piston during the expansion stroke.

 

Oh some use both. Most have a compression brake (jake brake, c-brake, or whatever manufacturer of compression brake is on the engine). The ones that have an aditional exhaust brake could put you through the windshield when at full decelleration.

Back to automotive.

I have to assume a 700R4 has a similar setup to the TH350 and TH400. These trannys have an intermediate band around the intermediate hub (second gear). When you slow down and the tranny drops to second gear, fluid pressure applies this band to let the slower turning engine slow the car down. The only purpose of this band is engine braking.

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87 IROC-Z SuperPro ET Bracket Race Car
461 naturally aspirated Big Block

Best ET on a time slip: 11.242 altitude corrected to 10.89
Best MPH on a time slip: 121.52 altitude corrected to 125.89
Altitude corrected rear wheel HP: 497.9
Best 60 foot: 1.546

Racing at 3500 feet elevation but most race days it's over 5000 feet density altitude!
Member of the Calgary Drag Racing Association

87 IROC bracket car, 91 454SS daily driver, 95 Homebuilt Harley