Author Topic: 6.9 performance - when to declare victory?  (Read 369 times)

raueda1

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6.9 performance - when to declare victory?
« on: 09 July 2020, 05:30 PM »
Other than the mysterious always-on suspension light, my 6.9 project feels about done.  Engine is running strong and smooth, very driveable.  Shifts great, rock-solid temp under pretty harsh conditions, excellent oil pressure.  Magic carpet ride.  Can't ask for much more.  But yet, despite that I always seem to be chasing a little better performance with the nagging idea that's there's a little bit more if only I could get it dialed in better.  My benchmark is 0-60 mph acceleration.  This doesn't tell the story but it's better than nothing.  So, I'd appreciate anybody's views on where I'm at.

Before my "rebuild" I was pulling 0-60 in about 10 seconds - substantially below the claimed 7 sec.  This is using a GPS app on my phone.  Not the ideal measurement, but the results were highly reproducible.  Now it's down to 9 seconds, notably better.  The stretch of road isn't perfectly level but I clock it in both directions and take the average.  This is 9.5 sec going up and 8.5 sec going down.  So, seems that all the work did seem to make a measurable improvement, though still 2 seconds short of 7 sec.

However, there's a major qualification for all that.  I'm at 5000 ft/1500 m.  I've been reading a lot about loss of performance at altitude.  I keep seeing the claim that you lose about 3% of power per 1000 ft / 30 m in a normally aspirated car (i.e., no turbo or blower I guess).  So, assuming that the engine is running to spec (286 hp), that translates to an altitude loss of 43 hp.  That's significant!  Would it running at sea level get me to the magic 7 sec?  I don't know, but another 43 hp would certainly be a major kick in the butt. 
[sidebar:  Rumb is at about same altitude as me and gives his cars 5 deg advance for altitude.  On my recent Colorado trip we did that to mine and it made a very noticeable seat-of-the-pants improvement.] 

This all has me thinking that I'm actually pretty damn close, at the point of diminishing returns.  Time to declare victory?  Quit the endless tweaking, leave it alone and just drive!  What say ye all?  Thanks and cheers,
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

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Re: 6.9 performance - when to declare victory?
« Reply #1 on: 10 July 2020, 01:45 AM »
Upon introduction DB claimed 7.4 sec..2 persons..for a UK  spec machine. In other words, a Euro spec with no hang-on emission gear running on 97 leaded. The quoted variation was +- 7%.
Fact specs are historically optimistic.   So at best 7 secs at worst  8 secs.

We now come forward 40 years.
God knows what the fuel is we are being served.
The relevance of the altitude is significant.
I'm not sure what spec your car is  ..Euro/US/upgraded whatever.... BUT on a cool day ave 2 way run: if you can get 8sec with 2 persons you're doing OK.
I've been in quite a few 6.9's..Aus/Euro...even an AMG [must have just had the floor mats.......]    they don't feel like 7 sec cars to me.

Be interesting to compare the test using just 1 st gear and a whisker of redline versus an auto D upshift  versus a manual 1-2 upshift.

raueda1

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Re: 6.9 performance - when to declare victory?
« Reply #2 on: 10 July 2020, 08:13 AM »
Upon introduction DB claimed 7.4 sec..2 persons..for a UK  spec machine. In other words, a Euro spec with no hang-on emission gear running on 97 leaded. The quoted variation was +- 7%.
Fact specs are historically optimistic.   So at best 7 secs at worst  8 secs.

We now come forward 40 years.
God knows what the fuel is we are being served.
The relevance of the altitude is significant.
I'm not sure what spec your car is  ..Euro/US/upgraded whatever.... BUT on a cool day ave 2 way run: if you can get 8sec with 2 persons you're doing OK.
I've been in quite a few 6.9's..Aus/Euro...even an AMG [must have just had the floor mats.......]    they don't feel like 7 sec cars to me.

Be interesting to compare the test using just 1 st gear and a whisker of redline versus an auto D upshift  versus a manual 1-2 upshift.
Thanks, interesting idea with the sifting.  It's a Euro spec car running 91 octane, alcohol-free gas.  that's the highest you can get here.  The gas companies knock the octane down a couple points in Rocky Mtn states cause of altitude.  As you say, altitude matters. 

I forgot to mention temperature.  No more cool days here for a couple months.  All this was at about 95F/35C.  Not exactly a cool day. I just looked at some of those aviation altitude/temp charts which indicate that the air density difference going from 20C to 35C equates to another 2000 ft!  So there's a double handicap, really kind of worse case.  So guess what?  I'm inclined to declare victory.  ;D
« Last Edit: 10 July 2020, 08:31 AM by raueda1 »
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

daantjie

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Re: 6.9 performance - when to declare victory?
« Reply #3 on: 10 July 2020, 08:26 AM »
Yup I think that is wise Dave, to chase that last bit of performance will likely drive you nuts for marginal gain.

Here in Vancouver of course we are at sea level and the only Ethanol (what a scam :o) free gas we get is 94 Octane (M+R/2), and you pay a hefty premium of at least 20c/litre for it at the pump.
« Last Edit: 11 July 2020, 08:23 AM by daantjie »
Daniel
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raueda1

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Re: 6.9 performance - when to declare victory?
« Reply #4 on: 10 July 2020, 12:48 PM »
Yup I think that is wise Dave, to chase that last bit of performance will likely drive you nuts for marginal gain.

Here in Vancouver of course we are at sea level and the only Ethanol (what a scam :o) free gas we get is 94 Octane (M+N/2), and you pay a hefty premium of at least 20c/litre for it at the pump.
Yup.  Moreover, I have no good way to know if I have it or not except that it seems damn close.  But for all I know I may have it already, but the heat and altitude mask it.  I'm now pretty convinced that that's the case. 

Victory!   ;D ;D ;D
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

Randys01

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Re: 6.9 performance - when to declare victory?
« Reply #5 on: 10 July 2020, 05:35 PM »
We get 98 ULP or even 100 E10....not bad for a Colony.! I think on a cool day at sea level on decent fuel using manual upshift you would romp in 8 secs!
The euro motor has the edge but not really able to realise its full power on the kerosene as I expect you would have to retard your timing a few degrees.
I'm surprised with all the things working against you that you squeezed 9 !

ptashek

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Re: 6.9 performance - when to declare victory?
« Reply #6 on: 10 July 2020, 07:02 PM »
My standard euro 450SE does 0-60 in 9.6s, on RON95 fuel (5% ethanol) - so about the same as US 91. Sea level, about 13*C/55*F, 80% humidity (i.e. typical Irish day)
I'd say 9s for a euro 6.9 at high altitude sounds good enough.
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TJ 450

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Re: 6.9 performance - when to declare victory?
« Reply #7 on: 10 July 2020, 09:23 PM »
I wouldn’t dare try it with mine as it pings like crazy when you floor it... I found out recently it has a 450SL distributor and has had it since I bought the car.

Tim
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raueda1

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Re: 6.9 performance - when to declare victory?
« Reply #8 on: 11 July 2020, 08:15 AM »
Interesting stuff about the fuel.  The rule of thumb here (Rocky Mountains) is that the effective octane of the gas is 2 units more than what it would be at sea level, i.e., our "regular" gas (85 octane (R+M)/2) performs same as 87 at sea level.  Or, in my case, 89 octane performs like 91 would at sea level.  To be clear, our octane is (R+M)/2, not RON as in rest of world.  So, what I'm burning is about the same as you guys' 98, 99 or 100 RON.  My base timing is advanced to 5°ATDC to compensate for altitude.  Rumb suggested this and he was totally right.  Seems like a lot but car performs significantly better and I've never had any knocking or pinging issues.  Many problems, but not that.  ::)
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0

floyd111

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Re: 6.9 performance - when to declare victory?
« Reply #9 on: 11 July 2020, 08:28 AM »
 ..or maybe save up for a tesla Roadster (1.9 sec/60mph) ;D
And give the old W116 a wax and a cruise instead..

Randys01

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Re: 6.9 performance - when to declare victory?
« Reply #10 on: 20 July 2020, 06:14 PM »
Tks Dave for pointing out the difference in the Octane rating system.......

raueda1

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Re: 6.9 performance - when to declare victory?
« Reply #11 on: 15 August 2020, 07:21 PM »
More data!  I find myself in Arizona after a 500 mile drive where I averaged 58 mph over 9 hours including all pit stops, going through small towns, etc.  That's pretty damned fast.  Anyway, now I'm at 3200 ft instead of my usual 5000-7000 ft.  Wow, another significant kick-in-the-pants boost!  This further supports the substantial altitude performance degradation thing.  Maybe everybody "knows" it, but feeling it is another matter altogether.  Wow. 

Next step will be rechecking cam timing when I get home - it was a little bit off after rebuild and now I've got enough miles for a little stretch. I'm looking forward to a Cali trip and see what happens at sea level.  Just keeps getting better and better.
-Dave
Now:  '76 6.9 Euro
Before that:  1966 230S, 1964 220SE coupe, 1977 Carrera 3.0