speed and rpm question

hyperollah · Nov 18, 2003

i own a manual 1987 325is and i believe i have a high ratio limited slip diff. on the free way my engine revs about 5000 rpm when traveling 80 mph. I want to know if this is hapening to anybody's car. Is this normal high revs. and on a full tank i get about 250-270 mpg. please reply

bahnstormer · Nov 18, 2003

what gear are u in? 5k is a bit steep
my old 89 325i did 3500rpm at 75 if i remeber correctly.
if the previous owner replaced the diff with a hear geared
one then yeah, u could see 5k at 80, but i bet u have great
zip off the line?

mikev · Nov 18, 2003

hyperollah said:

i run 4grand at 90mph in 5th (roughly as i can't check for a while [:(]

) and get around 300miles from one tank of fuel (20Mpg) but if i drove like most people on the road it'd be 27Mpg and 350+ miles!!

OreoGaborio · Nov 18, 2003

The pic below says that if you have a 3.73, you're at about 3.5k at 80 in 5th.... a 4.10 would give you about a 10% increase in rpm at a givien speed (4.1 / 3.73 = about 1.1), which would put you just under 3850...

from rumors that BMW tachs often read a little high, i woudn't be too suprised if a 4.10 is what you had... but i also wouldn't be suprised if it was even higher... but those would cost a good amount of money, since they're rare & often custom

hyperollah · Nov 18, 2003

when i drive in 5th gear my engine is at 5000 rpm. this car is somewhat quick off the line, real fun to drive but the highway revs are ricdiculous. thanks for the help

Ben · Nov 18, 2003

when im going 80mph in 5th the rpms are at 2600, and 90 at 3000..........

OreoGaborio · Nov 18, 2003

well yeah... but that's a supereta

the super have a 2.93 rearend, too? woudnt suprise me

Ben · Nov 19, 2003

i like having the nice low rpms at high speed. Btw, my car is currently limited to 122mph (i think) now when i put a 325i engine in what will happen? will i be able to go faster than a normal 325i?

OreoGaborio · Nov 20, 2003

you won't want a 2.93 w/ a 325i.... it'd be pretty sluggish w/ that tall of a gearing, since they came w/ a 3.73 stock

Ben · Nov 20, 2003

will it be more sulggish than the supereta???

OreoGaborio · Nov 20, 2003

it'd be close.... if you're getting at what would happen if you swapped your supereta engine for a 325i engine & kept the rear diff, then you probably wouldn't get any gains in performance, just becasue of the long diff

instead of swapping in the 325i engine, you should blend the two to make a 2.7i... you're already 1/2 way there w/ a supereta, the only thing you really need is a 325i camshaft & double valve springs which you can probably pick up at a junkyard for like 100 bucks!!!

it woudn't be a true 2.7i though, because your intake manifold is slightly smaller than the 325i, but bigger than the regular eta

check it out www.strictlyeta.net ... there's more info in the 2.8i section (he did a 1mm overbore & custom pistons... big bucks) as well as in the differences sections

Ben · Nov 20, 2003

the problem is i already bought the 325i engine.................

OreoGaborio · Nov 20, 2003

that's fine... you want that... now blend the two engines together!!!

2.7 bottom end (crankshaft, rods, SuperEta pistons or even better, Euro Eta pistons)
2.5 top end (head w/ 325i camshaft, double valve springs, etc)
2.5 intake manifold, TB, exhaust manifold etc

mikev · Nov 21, 2003

OreoGaborio said:

could you just pick up the gearbox diff etc from a 325i??

Ben · Nov 21, 2003

OreoGaborio said:

the problem with that is that i have to give the supereta to the guy who sold me the 325i engine. thats why i got it so cheap............

OreoGaborio · Nov 21, 2003

aw... bummer.... oh well, it's still an improvment

and yes mike... the diff from a 325i is swappable to any other e30

ODDferkOUT · Nov 22, 2003

whoa.. my car is well under 3000 rpm even at 95 mph...

speed and rpm question

hyperollah

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