Questions about RPM - PeachParts Mercedes-Benz Forum

skyridr02 · #1 10-18-2010, 06:37 PM

Hello I am kinda new at this but here goes...I have a 79 240D 4spd with the 3.0 5 Cyl diesel from a 300D. I finally got a 4spd (240d) transmission installed and have been driving it... With out a tach I kinda of have to guess at the engine speed. It seems to me that it's turing some high rpms to be doing hwy speeds 55 and above...normal? I was on the hwy last weekend and the spd limit is 65, the car has no problem running that fast but sounds like it's really wound up...I hasitate to exceed 55 or 60 for fear it will overwind. Do these cars IP have governers to prevent engine overspeed? Any way to attach a tach? ....

Thanks in advance for any help.

Daman858 · #2 10-18-2010, 06:45 PM

Seems to me that the differential ratio is the culprit here. Don't imagine a tach hookup would be easy.

winmutt · #3 10-18-2010, 06:53 PM

Tachs pretty easy, takes 2 boxes I think. http://www.superturbodiesel.com/std/adding-a-tachometer-t-97.html

You need to swap your rear diff out. I recommend a 2.88, maybe even a 2.47 if you are up to snuff (and then some).

aaa · #4 10-18-2010, 07:14 PM

You can't "overwind". The injection pump won't let you. I've taken my 240d to 75 (lol). It gets pretty darn loud though, and can't be good for fuel mileage.

destroy · #5 10-18-2010, 07:38 PM

I've gone 110 mph in my 300SD absolutely no problems whatsoever. The revs were fine, and it was even up a slight incline. I've got 16" wheels on it, but still.

As for the tach, if you still have the stock and haven't already - try undoing the cap for the sensor. It's on the front of the passenger side wheel well under the hood. Unscrew the cap, and then pull out the pins until the plastic assembly separates. Put some cardboard or something on the back side of the glue/silicone and then put it back in the cap. The cardboard acts as a buffer to push the pins down into the contacts. For whatever reason over time they don't seem to make contact and this is a really common fix. Worked intermittently on my old junker '83, and seems to be working absolutely perfect on my '81.

vstech · #6 10-18-2010, 08:05 PM

if all you changed with the 240 was the engine, you have a 3.69 rear gear, so, at 75mph your engine is turning 4K
I would hunt down a pre 82 300D, and pull the diff, and speedometer from it.
then you would have a slightly milder 3.45 ish rear gear, and get the tack down a tick or two.
if you go with a 300D newer than 81, it will have the 3.07 gear, some have reported no issues with the clutch slipping in 1st with this gear, but it's a tall one! the 2.88 and 2.47 or taller are GREAT for low RPM cruising, but it's a big pait to get moving.
your choice.

skyridr02 · #7 10-19-2010, 12:08 PM

Thanks for the advice. I am slightly confused now however....how did the original 2.4 4 cyl preform in this car geared the way it is and how is it that much different than the 'transplant' engine 3.0 5 cyl as far as rpm and gearing is concerned?
It's my understanding from reading from the forum that that's a popular swap from the slightly less powered 2.4 to the 3.0. So am I to assume that the 2.4 4 cyl was running at farily high rpm to acheive say 70 mph?

I remember driving my grandparents early 80's something 240D with a 4spd, but I don't remember high rpms at hwy speeds? Guess I am more familliar to my 02 powerstroke! 4000 rpm and it would be in pieces...LOL!

vstech · #8 10-19-2010, 12:22 PM

yup. RPM same between the cars. rear gear same, then rpm same, unless your transmission has been changed to an overdrive, or the clutch is slipping.

skyridr02 · #9 10-19-2010, 12:58 PM

Guess I won't be crusing the autobahn at 105 in this old beast LOL!! Well's it's my weekend fun car...55 or 60 seems fine...went on a cruise this past weekend to our local mountains and got 32 MPG! can't complain to much!