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Old 04-30-2008, 04:28 AM   #1
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Speed at Redline in Fifth Gear?

Has anyone under safe conditions approached redline in fifth gear in a base Boxster with 5-speed manual transmission? A recent dyno run got me thinking about actual top speed of our cars. Are the 2.7L cars limited by gearing and redline limiter or power available in order to be able to reach maximum speed in fifth gear.

At 7K redline I am going about 103 mph in third gear. I have never driven to the redline in fourth gear on the road, but on the dyno, the limiter cuts in just over 7K at 138 mph.

Any thoughts about gearing and power of the 2.7L engine pushing toward redline in fifth gear. I would sure like to find a track, salt flat or someplace to see where the cross-sectional profile and wind resistance of our cars runs into the power output and rpm limiter at our top speed.

Ed



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Old 04-30-2008, 10:03 AM   #2
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986 3.2 BoxsterS does 250 indicated redline in fifth. And then you still got six ! :-)

Seriously I remember a rpm vs speed graph for every gear in the original manual. For 2.7 as well as 3.2S.

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Old 04-30-2008, 10:39 AM   #3
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I don't know about the 2.7L and 5 speed, but my 03 S is drag limited in 6th gear not gear limited. If 4th is good for 138mph and the top speed is about 155ish then I would say a stock 2.7L is drag limited as well.
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Old 04-30-2008, 11:02 AM   #4
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"I don't know about the 2.7L and 5 speed, but my 03 S is drag limited in 6th gear not gear limited."

Interesting, is this from first-hand experinece?


"If 4th is good for 138mph and the top speed is about 155ish then I would say a stock 2.7L is drag limited as well."

I also think the 2.7L cars are drag limited. The graphs of rpm vs mph in the owners manuals show what speed the cars would reach at redline, say on a dyno. But, of course, that does not imply they could ever reach the top speed on the road. If I read the graph correctly, speed at redline would be over 160 mph. I also know the specs for our cars is in the 155 range. I was wondering if anyone has had a venue to see when drag limiting kicks in in fifth gear with the 2.7L cars.

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Old 04-30-2008, 11:26 AM   #5
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Well i dont know if any boxster engine is capable of hitting redline in 5th gear cause the Max MPH that the manual has for each boxster is usually pegged at 6k rpm. In my 1997 2.5 liter boxster 5 speed, the manual states that its max MPH is 155 however with my intake filter and MAYBE desnorkling I've gotten it to 157 mph and if i recall the RPM's were in 5500 - 6000 RPM range... Just fighting the air the body needs to displace and when my foot pegged to the floor for atleast 10 more seconds, the MPH's wouldnt go any higher. So i dont think you can hit redline UNLESS you have some form of forced induction like a Supercharger or Turbocharger.
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Old 04-30-2008, 02:48 PM   #6
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Ed,
There is an airfield in Texas (near Austin?) where they regularly do top speed runs on asphalt. Much safer and more predictable than the salt flats I think. Maybe a nice waypoint on your trip this summer .

Many many cars hit an invisible wall about 150mph where horsepower cannot overcome aerodynamic drag. If you need 250hp to go 150mph, you may need 500hp to go 200mph, and 1000hp to go 250mph. Imagine how much hp is required to push a SR 71 Blackbird to 2500mph .
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Old 04-30-2008, 03:29 PM   #7
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drag limited on mine at 147 before mods. now i have evo intake and headers. not bad for a 99 only losing 2 mph after 9 years.
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Old 04-30-2008, 03:40 PM   #8
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Quote:
Originally Posted by SilentThunder
drag limited on mine at 147 before mods. now i have evo intake and headers. not bad for a 99 only losing 2 mph after 9 years.
You can get to higher MPH than that. I have only an intake filter mod and desnorkel mod and i was able to get to 157 MPH in my 1997 Boxster with 125,000 Miles on the engine.
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Old 04-30-2008, 03:59 PM   #9
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GPS speed or your 100% accurate speedometer?

ive gotten 160 mph on the "icy driveway" dyno :P
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Old 04-30-2008, 04:02 PM   #10
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Quote:
Originally Posted by CJ_Boxster
You can get to higher MPH than that. I have only an intake filter mod and desnorkel mod and i was able to get to 157 MPH in my 1997 Boxster with 125,000 Miles on the engine.

You must've had a good tail wind pushing you
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Old 04-30-2008, 04:06 PM   #11
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Quote:
Originally Posted by Adam
You must've had a good tail wind pushing you
My car is lowered too with somewhat light stock wheels so that probably helped a bit as far as momentum and wind resistance.

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