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914World.com _ 914World Garage _ got a question

Posted by: bd1308 Aug 3 2005, 10:39 AM

you said in a previous post that one should not go below 3500RPM in 5th gear, as the airflow is not sufficient enough for proper cooling.

i have some problems with this....

1) in my car, at 70mph, I am around 3200RPM in 5th...sometimes i am even running 3000RPM and occasionally run even 2800RPM for short periods of time. Are you suggesting that I should not shift into 5th gear at all?

2)I have also heard that the type 4 cooling fan efficiency goes down after around 4000RPM....it was mentioned by someone on this board, but the user name escapes me. Is this true?

I tried running 75 in 4th gear and I was running more than 4000RPM.....

I think i am missing something....

Posted by: bondo Aug 3 2005, 10:56 AM

What you need is a tire with variable diameter!

Posted by: grantsfo Aug 3 2005, 11:11 AM

Is your speedo correct? If it is you have some damn tall tires on that car. I think the biggest thing you have to watch is engine load below 3500 rpm. If you were doing 60 on a slight grade in 5th I would shift up. If you were cruising on flat or downhill 5th at 3000 rpm + should be fine. If it was a real hot day with a lot of heat radiating off the pavement it might be a good idea to shift into 4th.

I have close to 400,000 miles on these engines and have never had to tear into one yet for overheating using the principles above.

Posted by: Aaron Cox Aug 3 2005, 01:46 PM

with the rubber band 205/50 tires, 80 mph is like 3800

Posted by: bd1308 Aug 3 2005, 02:06 PM

i keep up with traffic, and i'm doing 4000rpm in 4th gear. explain that?

did i get the ultra rare ring gear upgrade or what?

Posted by: MattR Aug 3 2005, 02:35 PM

Do you have any vinyl graphics? That will make you go faster.

Posted by: bd1308 Aug 3 2005, 02:39 PM

no...but I have the VTEC sticker....added at least 10hp and improved high RPM performance.

Posted by: Jake Raby Aug 3 2005, 02:48 PM

How tall is your rear tire?? Tire size impacts final drive incredibly....

Most of us are not running stock height tires, almost always shorter..

4,000 RPM in 4th is way better than 3000 in 5th for the head temps..

Posted by: Joe Ricard Aug 3 2005, 03:00 PM

I'm a personal fan of 4000 in ZD 5th. 195/60-15 Ya get there so much sooner. But then again my car is yellow so it IS faster.

Posted by: bd1308 Aug 3 2005, 03:11 PM

QUOTE (Jake Raby @ Aug 3 2005, 02:48 PM)
How tall is your rear tire?? Tire size impacts final drive incredibly....

Most of us are not running stock height tires, almost always shorter..

4,000 RPM in 4th is way better than 3000 in 5th for the head temps..

okay. done.

it sounded cool too running 4k rpm. i'm running 195s Falken ZE-502

Posted by: JmuRiz Aug 3 2005, 03:18 PM

Interesting, I did notice my car is much happier running over 3200 on the highway than when it's below 3000...maybe this is why? Came home from picking it up years ago going ~75mph the whole way and got over 30mpg.

Posted by: MattR Aug 3 2005, 03:18 PM

QUOTE (bd1308 @ Aug 3 2005, 01:11 PM)
i'm running 195s Falken ZE-502

The aspect ratio (second number) determines rolling diameter.

Posted by: Jake Raby Aug 3 2005, 03:20 PM

How tall in inches are those tires??

Remember that a shorter tire will throw off the calibration of the speedo, so you are turning less engine RPM to gain more speedo MPH.... But because of the shorter tire you have to have more engine RPm to run the same true speed.... "Every action has an equal and opposite reaction"

Posted by: bd1308 Aug 3 2005, 03:21 PM

195/60R15

Posted by: lapuwali Aug 3 2005, 04:21 PM

The stock tires were 165/85-15, which is roughly 661mm tall (26" for the metric challenged). A 195/60 would be 615mm tall (24.2"), so your tire is quite a bit shorter than stock, thus it has a smaller circumference than stock, so the tire spins faster at a given road speed, so your speedo reads high by about 7%. 70mph indicated is 65mph actual, assuming the speedo was accurate to begin with (not bloody likely).

It's actually pretty difficult to find new tires that are as tall as the stockers for 15" wheels these days.

Posted by: 914forme Aug 3 2005, 05:04 PM

QUOTE (lapuwali @ Aug 3 2005, 06:21 PM)
so your speedo reads high by about 7%. 70mph indicated is 65mph actual, assuming the speedo was accurate to begin with (not bloody likely).

This is why I run a know run with a GPS, I find my true speed or pretty darn close, and avoid finding it via a radar gun.

My 914 runs way shorter tires than stock, My Jetta went plus one in wheel size and keep the same aspect ratio so I went way up. That was drop the revs on the jetta and get it closer to 2000 RPM and 70 MPH, its a TDI, it likes low revs

Grab a GPS and find a measured mile and see what you get.

This is the exact reason Porsche used a lower 5th in the -6s was to keep the revs up and help the fan do its job. Works for our -4s also.

Due to lovely mandates and our sue them attitude society your speeedo will read low even with everything including the planets being aliened. Just the way it is, the odo's are usually a little more accurate. But I would not use them to measure a true mile or kilometer with.