Does bouncing off rev limiter hurt the engine cranck ?

[cosworth]

Not necessarily on 996 TT boxer engine ,in your knowledge does bouncing off rev limiter consistently hurt or damage engine especially at the bottom end ? I've read some theory saying that bouncing the limiter shuold cause rod and main bearings wear and rod bolts to stretch as well as generic engine stress more than revving the engine to higher rpm BUT without hitting rev limiter. Is the jolt due to the suddenly fluctuation from load to unload condition that occur in milliseconds range when rev limiter is bounced off and ignition is cut by the ecu. This rapid sequence of fire and misses and fire and misses repeated in any cycle should load/unload the cranck too suddenly causing stress and overheat on the bearings and stretch rod bolts dangerously. Onestly until this time i bounce off my rev limiter with quite confidence when shifting, thinking it is there for a reason and it was doing its job , but after having read this theory i'm a little concerned. Also some times running at top speed in 6th gear on the German Autobahn i have bounced off my ecu tuned 7150 rpm rev limiter sitting on it for 5-10 seconds .... i hope this has NOT damage my engine.

[cosworth]

Also Watching this videos it seems that bouncing off rev limiter do NOT damage anything LOL

You Tube

You Tube

You Tube

You Tube

[unterbergamotte]

[cosworth]

Originally Posted by unterbergamotte

i hope this is not for my english...i know it's terrific

[johnspeed]

Originally Posted by cosworth

Not necessarily on 996 TT boxer engine ,in your knowledge does bouncing off rev limiter consistently hurt or damage engine especially at the bottom end ?
I've read some theory saying that bouncing the limiter shuold cause rod and main bearings wear and rod bolts to stretch as well as generic engine stress more than revving the engine to higher rpm BUT without hitting rev limiter.
Is the jolt due to the suddenly fluctuation from load to unload condition that occur in milliseconds range when rev limiter is bounced off and ignition is cut by the ecu.
This rapid sequence of fire and misses and fire and misses repeated in any cycle should load/unload the cranck too suddenly causing stress and overheat on the bearings and stretch rod bolts dangerously.
Onestly until this time i bounce off my rev limiter with quite confidence when shifting, thinking it is there for a reason and it was doing its job , but after having read this theory i'm a little concerned.
Also some times running at top speed in 6th gear on the German Autobahn i have bounced off my ecu tuned 7150 rpm rev limiter sitting on it for 5-10 seconds .... i hope this has NOT damage my engine.

There are two streets of thoughts...But I dont think its 'that' bad because basically its just causing a miss in your ignition with still a constant load on the engine,IMO...Rev limiters have been around for ever and before all this electronic magic,some racers shifted when they hit it....Thats what a rev limiter is for...

[cosworth]

Originally Posted by johnspeed

There are two streets of thoughts...But I dont think its 'that' bad because basically its just causing a miss in your ignition with still a constant load on the engine,IMO...Rev limiters have been around for ever and before all this electronic magic,some racers shifted when they hit it....Thats what a rev limiter is for...

I'm also on this street of thought and i hope it's the right "street"..i don't think it's a big problem although actually in the cycle where there is NOT ignition/spark (when the limiter kick in and cut it) the engine is Unloaded (like when you lift the foot from the gas) because sure the cranck still run by inertial force BUT in the cycle of ignition cut, the piston is NOT pushed down since there is NO combustion so it is pulled down by the inertia of the rod...
So we have the normal fire cycle where there is a compression force act on the rod and rod bolts , and the cut missfire cycle where there is a tensional "pull" force act on the rod / rod bolts.
This sequence repeated so suddenly when the limiter kick in ,could add more stress on this parts (especially rod bolts) than in normal condition although it should be able to sustain this additional load / stretch force.

[johnspeed]

Originally Posted by cosworth

I'm also on this street of thought and i hope it's the right "street"..i don't think it's a big problem although actually in the cycle where there is NOT ignition/spark (when the limiter kick in and cut it) the engine is Unloaded (like when you lift the foot from the gas) because sure the cranck still run by inertial force BUT in the cycle of ignition cut, the piston is NOT pushed down since there is NO combustion so it is pulled down by the inertia of the rod...
So we have the normal fire cycle where there is a compression force act on the rod and rod bolts , and the cut missfire cycle where there is a tensional "pull" force act on the rod / rod bolts.
This sequence repeated so suddenly when the limiter kick in ,could add more stress on this parts (especially rod bolts) than in normal condition although it should be able to sustain this additional load / stretch force.

I always thought and was told by a few companies that the ignition miss posed no harm on the engine other then the sound of the engine??..I would think that if it was that bad on the motor,that there would be warnings on systems with them and others would bring this point up??..I am going to have to look into this closer now..

[Engine Guy]

The rev limiter is there to prevent an engine from hitting RPM levels that exceed designed limits. The electronic cut out limiters do create some issues but that is a very complex subject to say the least.

I would say --> remember the harder you run something the quicker something wears out.

Now it is possible to create harmful over revs by mechanically forcing an engine to rev past its safe limits by doing things such as down shifting into to low a gear.

[cosworth]

Originally Posted by johnspeed

I always thought and was told by a few companies that the ignition miss posed no harm on the engine other then the sound of the engine??..I would think that if it was that bad on the motor,that there would be warnings on systems with them and others would bring this point up??..I am going to have to look into this closer now..

i also would think there would be warnings on car's manual...
i've found this thread :
Rev Limiter worries

K24madness says the things for wich i'm a little concerned.
The point is : Are rod bolts able to handle this additional stress/stetch ? i would think yes.

[Engine Guy]

Originally Posted by cosworth

K24madness says the things for wich i'm a little concerned.
The point is : Are rod bolts able to handle this additional stress/stetch ? i would think yes.

What tom mentions in his first post in that link is very well put.

A rev limiter incident is really not a big deal unless you actually surpass that number by a good amount. The only way to really do that is to mechanically force it to go past.

Maybe really hard acceleration in first would do it if you pushed in the clutch just as you are coming into the rev limiter and as the engine un loaded the inertia allowed the engine to continue past the rev limiter, but even in that instance the over rev would be very mild.

Like Tom says, at the track a racer will be touching the rev limiter numerous times lap after lap.

I always tell my customers the story of the light bulb. You buy one of those tri-light bulbs for your desk lamp. Run them on low and they are good for 1500 hours, run them on med and they are good for 1000 hours, run them on high all the time and they are good for 500 hours. Now plug that baby into 220 and she goes KaBoom real fast!!!!!

I am going to tell you a saying that one of the Japanese engineers I worked with always used to tell our drivers when they were driving slow.

Imagine a thick japanese accent.
"Why we give you race car when you drive like station wagon"

[AlexN]

I think the only damage it will do is financial, when you come to sell the car and get the overrev report from the PPI.
The prospect buyer will try to lower your price because of that

[Guybrush]

That 996TT driver must have burned his front differential - I dont see the fronts pulling at all.... any thoughts?

Anyone done that with a working diff? Can it be done?

[johnspeed]

HEY,Wouldnt launch control kinda fit into this subject also??

[Duane996tt]

Originally Posted by Engine Guy

The rev limiter is there to prevent an engine from hitting RPM levels that exceed designed limits. The electronic cut out limiters do create some issues but that is a very complex subject to say the least.

I would say --> remember the harder you run something the quicker something wears out.

Now it is possible to create harmful over revs by mechanically forcing an engine to rev past its safe limits by doing things such as down shifting into to low a gear.

Upshift rev limited bounces no issues...that's what it's there for...Downshift rev limited bounces big problem because the ECU can't stop it.