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my car idles at 6,000 rpm
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Discussion Starter · #1 ·
This is a mystery that I’ve been wondering for so long, if my question has been answered before, could someone be so kind to give me a link; I did some researches and couldn’t find any result. Anyways, earlier E46s like 99-00 323/328 have a factory recommended tire pressure (up to 4 people) of 29 PSI front and 33 PSI rear, but newer E46s (01+ 325/330) have a higher recommended pressure like 32 and 35 (I could be wrong on these figures) for front and rear respectively. Given that these cars are mechanically identical (some ZHP owners might think otherwise… j/k :D ), what's the reason that BMW changed the recommended tire pressure? I personally run 32 and 34 for my car; 29F 33R setting is too spongy to my liking.
 

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Were the tire and wheel dimensions the same from older and newer cars? I know the wheel designs changed, but I don't know if they also affect the tires also.
 

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my car idles at 6,000 rpm
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60 Posts
Discussion Starter · #3 ·
allaboutme said:
Were the tire and wheel dimensions the same from older and newer cars? I know the wheel designs changed, but I don't know if they also affect the tires also.
If I am not mistaken, wheel dimensions between early and newer E46s are identical, at least sports package wise. 323ci and 325ci both have 225 45 17" on all 4 corners; 328ci and 330ci have 225 45 17 front and 245 40 17 rear.
 

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Mapman said:
Inflation.

The psi isn't worth what it used to be worth.
:rofl: Good one...If anyone does know why this is, I'm curious too.
 

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my car idles at 6,000 rpm
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Discussion Starter · #6 ·
Mapman said:
Inflation.

The psi isn't worth what it used to be worth.
:rofl: 'kPa' must be EURO then
 

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Because suspensions are stiffer than on early models

My 2003 330i bounces on the rear tires if the tire pressure drops below 35 PSI. This was especially noticeable when new because of higher suspension friction.

I recently replaced the shocks and struts on my 96 Passat GLX VR6 with Bilstein HDs and the ride absolutely sucks when the tires are BELOW 34 PSI.

The stiffer the suspension springs, dampers and anti-roll bars, the stiffer the tires need to be. Tires act as springs too, but have almost no damping compared to the suspension. A stiff suspension forces more deflection of the tire and this can easily result in the car bouncing on the tire side-wall. The resultant bobbing seems to coincide with the resonant frequency of the human head, leading to a pain in the neck.

Raising tire pressure enough forces the suspension to move and the damper to work, thus storing less energy in tire deflection. Higher pressure also raises the tire's natural frequency to prevent the pain in the neck effect.

Much softer tires would lower the resonant frequency, but the car would wallow around on the un-damped tire sidewalls. The tire foot print would be unstable, steering response would be sluggish, cornering grip would be reduced and the risk of rolling the tire off the rim would make this even more dangerous.

BMW's recommend pressures are MINIMUM in my opinion, not optimum. My Continentals seem to like being at 32/36.5 Psi F/R to 33/ 37 measured at 70 degrees F. Operating pressures are a few Psi higher because of incresed temperature. Many people seem to like much higher pressures, so I suggest you experiment to find what works for you.

Bottom line: If you think your car's ride is too hard at a given tire pressure, try both reducing and INCREASING the tire pressure to find the happy medium. If you are already at 40 PSI then don't bother going up.

Front suspensions are softer than rear for correct phasing - this prevent pitching at highway speeds. Also, the rear sees more load from extra passengers and cargo. Thus, the front tires can be softer than the rears - this reduces front edge impact harshness. Many use the same pressure F/R to "reduce understeer". One should not confuse steering lag (slow response on center) with understeer. If the front tires are hard enough to move the contact patch with the rims, steering lag will not be a problem. "Reduced understeer" will only become noticeable at high cornering loads.

Front tires that are too hard can result in continuous wandering. The optimal pressure will vary by tire make and model, vehicle load and exact wheel alignment set. My car become "nervous" with the front tires above 34 Psi set cold.
 

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Could it be that BMW changed their "control" tire that the recommended PSI was based on? Was there a differnet conti used on the different generations? Otherwise I guess dynosor implies the answer...the E46 is sensitive to tire pressure and BMW changed their minds on the optimal setup.

dynosor said:
My 2003 330i bounces on the rear tires if the tire pressure drops below 35 PSI. This was especially noticeable when new because of higher suspension friction.

I recently replaced the shocks and struts on my 96 Passat GLX VR6 with Bilstein HDs and the ride absolutely sucks when the tires are BELOW 34 PSI.

The stiffer the suspension spring, damper and anti-roll bars, the stiffer the tires need to be. Tires act as springs too, but have almost no damping compared to the suspension. A stiff suspension forces more deflection of the tire and this can easily result in the car bouncing on the tire side-wall. The resultant bobbing seems to coincide with the resonant frequency of the human head, leading to a pain in the neck.

Raising tire pressure enough forces the suspension to move and the damper to work, thus storing less energy in tire deflection. Higher pressure also raises the tire's natural frequency to prevent the pain in the neck effect.

Much softer tires would lower the resonant frequency, but the car would wallow around on the un-damped tire sidewalls. The tire foot print would be unstable, steering response would be sluggish, cornering grip would be reduced and the risk of rolling the tire off the rim would make this even more dangerous.

BMW's recommend pressures are MINIMUM in my opinion, not optimum. My continentals seem to like being at 32/36.5 Psi F/R to 33/ 37 measured at 70 degrees F. Operating pressures are a few Psi higher because of incresed temperature. Many people seem to like much higher pressures, so I suggest you experiment to find what works for you.

Bottom line: If you think your car's ride is too hard at a given tire pressure, try both reducing and INCREASING the tire pressure to find the happy medium. If you are already at 40 PSI then don't bother going up.

Front suspensions are softer than rear for correct phasing - this prevent pitching at highway speeds. Also, the rear sees more load from extra passengers and cargo. Thus, the front tires can be softer than the rears. Many use the same pressure to "reduce understeer". One should not confuse steering lag (slow response on center) with understeer. If the front tires are hard enough to move with the rims, steering lag will not be a problem. "Reduced understeer" will only become noticeable at high cornering loads.

Front tires that are too hard can result in continuous wandering. The optimal pressure will vary by tire make and model, vehicle load and exact wheel alignment set. My car become "nervous" with the front tires above 34 Psi set cold.
 

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blazingbeat said:
This is a mystery that I've been wondering for so long, if my question has been answered before, could someone be so kind to give me a link; I did some researches and couldn't find any result. Anyways, earlier E46s like 99-00 323/328 have a factory recommended tire pressure (up to 4 people) of 29 PSI front and 33 PSI rear, but newer E46s (01+ 325/330) have a higher recommended pressure like 32 and 35 (I could be wrong on these figures) for front and rear respectively. Given that these cars are mechanically identical (some ZHP owners might think otherwise… j/k :D ), what's the reason that BMW changed the recommended tire pressure? I personally run 32 and 34 for my car; 29F 33R setting is too spongy to my liking.
For slightly better fuel mileage, maybe.
 

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my car idles at 6,000 rpm
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Discussion Starter · #10 ·
Thanks for all of your informative responses! :thumbup: Like you guys mentioned, many people here run different tire pressure settings, and most of those settings are higher than BMW recommended. I was just wondering if those ‘factory recommended specs.’ for newer E46s (tire pressure, torque spec for wheel lugnuts and so on…) would apply on early E46s.
 

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blazingbeat said:
Thanks for all of your informative responses! :thumbup: Like you guys mentioned, many people here run different tire pressure settings, and most of those settings are higher than BMW recommended. I was just wondering if those 'factory recommended specs.' for newer E46s (tire pressure, torque spec for wheel lugnuts and so on…) would apply on early E46s.
My '99 323 feels like ass with less than 40 all around.
 
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