FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: cjshaker on August 02, 2016, 10:20:08 PM
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I've always used the engine to help slow the car down, for several reasons; one being that it helps to 'settle' the car and make it much more stable while slowing down rapidly. But I've also considered the fact that it's hard on the engine, or at least I assume so. I tried recently to just put it in neutral and brake to slow down and didn't like the feel of the car as it seemed to bobble a bit due to the soft shocks. It also caused me to brake much harder (stock disc brake system), to the point that I thought lock-up may be an issue. My question is how hard do you guys think it is on the engine to slam the throttle blades closed and go full vacuum at 5000-6000+ rpm? Actually, I don't slam them closed, but ease off the throttle quickly. Still, it seems that it has to be hard on the engine, nearly equating to another 1/4 or 1/8 mile run at the end of the first. What do you guys think? I know a quality modern brake system would be better, but that's not something I want to do on this car.
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I often thought about that. I figure it can't be as bad as doing the quarter mile, it's not like we are downshifting, wonder what the big guys think
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Sounds like your doing it right. Ease off the throttle a bit and do not slam the throttle shut 100% at the end of a run. A major drag engine shop I ran out of long ago told me to do the same as instantly snapping the throttle shut puts enormous loads on the rods and pistons, let alone other engine components. The why makes sense too: snapping the throttle shut removes the cushion of air above the pistons as the engine rapidly decelerates, forcing maximum rod stretch...not a good thing.
This shop's owner (ProMotor Engineering, Des Plaines, IL ) now builds NASCAR engines by the score and Jerry Baker is still a renowned builder. At the time, they building crazy rpm SBC's for Comp Eliminator, most doing way over 10,000 rpm. A string of rod failures had Baker in fits, until he saw a few runners doing the instant lift at the stripe. All was well after he instructed the drivers to not do that!
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JMO, but I ask my guys to let the brakes slow the car down. If you go from a full-throttle situation, to a full engine-brake situation, think about what the crankshaft looks like......noodle time. :)
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Reverse pattern - lift and brake. Forward pattern - kick to neutral and brake. Haven't figured out what I'll do with the glide in the dragster but I should be able to kick it to neutral and brake. I've got thousands of passes, never noted any difference in engine wear or life. Been running a reverse pattern now for about 4 years, I just lift and get on the brakes.
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Just too add to this. As Brent said "think of the crankshaft", put the trans in neutral and use the brakes. I was told this years ago by an engine builder who admonished me not to " motor the rods"! That was on our 7600 rpm 289 with stock rods .
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I always push the clutch in & put er in neut. but then again I never had any horsepower to speak of. Just observing though, not many racers do this. most just back off the throttle. One thing about coasting though, you better have a balanced drive line.
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lots of things to think of, does your automatic transmission have clean neutral. what happens when the drive shaft is running at 7000 RPM then the input shaft speed drops to 2000 RPM. some automatics do not like this. how many guys have shifted from drive missed neutral and hit reverse? i know a few. think about road race and circle track cars. as far as my own car goes I just lift. I have never had any issues, but I do glue my intake gaskets to the head with weather strip glue. this has ben an issue with circle track motors. just my O2
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The higher the compression, the bigger the problem. Also, the heavier the pistons, the bigger the problem. It is best to disengage clutch and brake allowing the engine to come down to idle normally. Also, if you use the engine to decelerate, some tires will start the car to porpoise if the tire pressure is really low and make handling difficult. I always slapped the shifter to neutral, hit the brakes hard, released them, and reapplied the brakes to slow down with complete control. Joe-JDC
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I always lift fairly quickly, shift to neutral and then brake - but not getting on the brakes real hard initially because of the tires.
I wanted to read the plugs so I did not the engine working hard after the run.
Bob
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In ECTA racing we run a mile. And although the shut-off is another mile, since we run one at a time, we are encouraged to make the first turn-off if we can do so safely.
I don't believe my 5R55N trans 'motors' the drivetrain but I confess I'm not sure. In any case, I simply flip the fuel switches to 'off' and get hard on the brakes. Engine's still goin' strong with about a zillion miles on it.
KS
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My race engine has aluminum rods, and I was advised that the rod small end can fail under engine braking loads. My new C4 has a clean neutral solenoid, so i flip the toggle switch to put the trans in neutral, then use my brakes
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>what happens when the drive shaft is running at 7000 RPM then the input shaft speed drops to 2000 RPM. some automatics do not like this.
This may be the case with the Powerglide, will have to ask around - first time running one. I think as long as the motor is running and pumping fluid it's not a problem. A real problem would be killing the motor and coasting down. That could fry some clutches and/or thrust bearings.
>how many guys have shifted from drive missed neutral and hit reverse? i know a few.
Not if they have the NHRA/IHRA required reverse lockout on the shifter. If you don't, well - read the rule book next time LOL. It's mandated for a reason.
>some tires will start the car to porpoise if the tire pressure is really low and make handling difficult.
My dragster is a hard tail, so that is a precaution. Smooth breaking, ease off the power, know where the chute handle is mounted
>My new C4 has a clean neutral solenoid, so i flip the toggle switch to put the trans in neutral
Interesting, never head of that. With a reverse pattern valve body, you'd have to pass through 2nd to get to 1 which free wheels in most C4 reverse VBs. 2nd on coast down isn't really a good idea LOL.
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Interesting Topic and it would be good to know the "facts" however I put the clutch in and don't use engine braking. Some racers do and claim no negative side effects but I can't see any reason to stress the motor on deceleration unless you need the extra stopping. I also keep the RPM up and a little and coast to the turnoff road.
One thing I do know is you'll never see a high winding Comp Eliminator engine braking. They click it off completely at the finish line.
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Thanks for the replies. First off, no automatic here, only stick, so the tranny wasn't a concern for me, although I imagine it isn't the best on them.
I've watched some of the big dogs in Stock and SS and most seem to shut the engine off or at least go to neutral, then restart for the drive back to the pits. Again, talking about stick cars mainly. My concern, as David mentioned, was for the rods. Mainly because I imagined the extremely high vacuum under deceleration had to stress them out, even if they're steel. I could also see it causing some ring flutter, although i haven't heard of that before, as well as trying to suck the valves open if seat pressure wasn't sufficient. I don't notice my car porpoising, although I have seen that on rigid rails and big tire cars as mentioned.
I think I'll start using brakes only because I just can't see it being any good on the engine. I need to get my rotors turned because under hard braking from 100+ mph I can feel they're slightly warped. Thanks again for the replies.
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>One thing I do know is you'll never see a high winding Comp Eliminator engine braking. They click it off completely at the finish line.
The high end Comp people I know run a transmission with an onboard circulation pump. Like $4500 Powerglides.
For automatic racers - a good discussion here
http://www.yellowbullet.com/forum/showthread.php?t=172108
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I push the pedal in and use the brakes.
My clutch costs a lot.
Brake pads are cheap.
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I normally my Jerico 4 speed in neutral, and let the engine drop down to idle somewhat slowly. Both my Fairmont and Mustang have rear sump oilpans, so I try to avoid jamming on the brakes, causing the oil to slosh foreward, and possibly uncovering the oil pump pickup screen. Besides Mission and Pacific (Seattle) both have long shut down sections, so no need to brake really hard. Bremertons much shorter, I have shut the engine off after kicking the trans in neutral if heavy braking was needed. I would never leave the car in gear and let the engine slow down the car, I doubt the connecting rods would like that very much.