JoeMX82

Like most of you I have seen the never-ending discussion of what a clean DME report should like and what would make an over-rev range 1 vs. 2 vs. 3 etc. The consensus seems to be that hitting the rev-limiter on the way up will result in only a range 1, where as a missed shift/ money shift will get you into the range 2+. I decided to test this theory on a brief 1st gear rev-limiter bump and the impact on my DME report and was surprised to find these results..... Car is completely stock in case you were wondering and thought I would share as I have not seen anyone else post this kind of data.

Before the data here is how you read an over-rev in summary. I could be slightly off but based on our 7,300 red line and 3 ignition cycle you will essentially see 365 over-revs for each second spent in a zone of the over-rev report (7,300/60*3).

I scanned my DME as a baseline and then immediately after the rev-limited test - below is the net impact.. As far as the test, I had the car fully warmed up and let the car hit of the rev-limiter in first gear and immediately pushed the clutch so essentially as quick a reaction as possible. Curious to get others insights here but I think the computer might be exaggerating the impact on the DME here which is a theory I have seen others float but without proof:

The following are net impacts between the two readings:
Range 1 = 450
Range 2= 196
Range 3= 9
Range 4+= 0

My thoughts: There is no way I was on the rev limited for anything more than a few tens of a second, certainly not a second and a half. Also, not sure how I could enter range 3 (7,700+) for two one thousands of a second. I was also watching the tach and certainly did not appear to go past the 7300 mark.
I'm sure many of you will want to respond with congratulating me on devaluing my car or some other nonsense - save it as really don't care about that. I'm sharing this as I think it is very telling and also points to people worry way too much about a DME report in general.

andino

Thanks for "devaluing" your car for us. I personally didn't take much stock in range 1-4s when I was searching for my 997. Could be my naivete with the platform but I firmly think that a car that's driven and well maintained is better than one that's babied and never driven. I could see 5-6 being a bigger problem as that's money-shift territory but 1-3 and some 4s means the car was just driven and occasionally driven spiritedly or hard on track.

Skwerl

I saw a post once, I think on PistonHeads, where a shop had a car up on a lift and revved the engine precisely to redline via computer control and still managed to add some range 3s or 4s to the report. It's been a few years but the gist was that the DME is not an infallible record (something you also see when you have DMEs with more range 4s than 3s and other assorted impossible weirdness).

The fact that there are scores of 997s with "bad" DMEs but yet none of them are rattling apart at the seams is further evidence that they aren't the kind of be-all-end-all red flag the BaT peanut gallery would make you think.

Tj40

Interesting, thanks for posting. given that the DME is counting pulses that (at 7500 rpm) are 0.00266 seconds apart it wouldn't take much of an error in the clock here to skew the results.

For example a 2% error in the clock rate - measuring 0.00261 of a second instead of 0.00266 gives 7653 rpm instead of 7500 - is this the cause - I dunno it would have to be a bigger offset than that but the 9 range 3's shouldn't be there so something funky happened.

cwheeler

Interesting! Thanks for the data point!

RacerWannabe

Very cool - thank you - interesting experiment here and worthwhile for aiding interpretation of DME reports.

Vince Vincent

Thank god for more of these reports and information. Thank you forum poster! Do we want a 997 thats been streetraced all day/every day? No, but the purist crowd that want a 0' across the board DME report drive me crazy. It is a sports car, money shifts are never good but with a competent driver seem few/far between, so can we stop bashing normal DME reports or bounces off the rev limiter every now and then?

roadie13

Interesting experiment, thanks! This agrees with my seat of the pants experience (certainly in the ranges 1-3).

Unframed

I don’t sweat range 1-3 overrevs unless there are a lot of them.

JoeMX82

so I agree but my car has around 4K range one and 1k range 2, like 200 in range 3 along with 45 range 4. Regarding range 1 I always wondered how it has like 13 seconds of banging the rev limiter as that seems a bit excessive but still didn’t bother me when buying. After this experiment of barely touching the limiter and registering as 1.5 seconds it makes a lot more sense to me. Essentially car has probably touched rev limiter around 8-9 times vs. what I first was thinking based on just the just the DME figures.

8KaboveMSL

Awesome data. I had DME pulled on car after I bought it (I know, learned from that) and Revs were very normal, so got lucky. I've since hit rev limiter 3 or 4 times, mostly in 2nd while passing someone on 2-lane road and didn't;t want to shift while passing. Learned lesson and make sure I'm in 3rd before trying to pass in those scenarios (BTW, the 997 just seems to want to keep going and then you hit the Reve limiter, that high end torque is amazing).

I am planning on getting some suspension work done on it in the spring (once we are out of stormy weather as my indy keeps cars outdoors) and will. ask him to pull the DME report again and will compare w/ previous and post results here.

I've been wondering what the impact of hitting rev limiter on DME report so happy to see that I'm not the only one! Thanks for posting!

qikqbn

I have said this exact scenario in many dme threads in the past. Stopped, in neutral, and bouncing off the rev limiter will register range 3 all day long and not hurt the engine. Thanks for doing it "scientifically" and reporting what you found. As many of us who have tracked these cars or run them through the canyons like a bat out of hell... A little dme rev range 3 love will not hurt these engines. A missed shift range 4 caught just in time, not so much either. A whisper or kiss at the bottom in range 5 will raise my eyebrow, but no big deal if the hours passed are 200+. It is the deep 5-6 that would make me walk. Range 1-3, Party is just getting started.

Petza914

Can you do one more test where instead of pushing in the clutch (which might actually allow the momentum of the rotating mass to continue to increase speed as the engaged load is removed), and instead just let off the gas. I'd like to see if there's a difference. Since we're talking fractions of a second here, timing as to when the gas is relweaed and the clutch deprreseed might account for the range 3s. I believe pinging the Rev limiter when accelerating should log ranges 1s and 2s, but not 3s.