Diff mph question.

Mon Apr 20, 2009 11:06 am

I have had the 3:64 diff on for the m20 motor I was running and never saw more than 137mph on gps readings. Now on an m30 motor with the same diff, I am getting the same readings. Although i'm getting there "slightly" quicker. So as I see it, the diff is the deciding factor in the over all speed of the car.

Is this correct ??? And if so then I need an m3 diff 3:25 lsd ???

Mon Apr 20, 2009 11:53 am

Its a combination a gear ratio and diff ratio but why are you driving so fast?

Mon Apr 20, 2009 12:08 pm

A 3.46 diff would be slightly better i think, Rav335uk has one fitted on his M30 and i do believe that he has done more than 140mph

Mon Apr 20, 2009 12:18 pm

Are you hitting the rev limiter in 5th or not?

Why anyone needs to do more than around 120mph is beyond me. 120mph will be fast enough on most UK tracks and anything more on UK roads is pretty crazy due to stopping distances. If it is a fun car, i'd fit the shortest diff I could and enjoy being a hooligan to legal'ish speeds.

Mon Apr 20, 2009 12:20 pm

yes it is correct to say the differential gearing decides the cars top speed, but its also dictated by the amount of HP a car engine can generate. Torque normally dictates acceleration and HP normally plays a big factor on top speed as essentially HP is the ability to produce torque as speeds, so an engine with a higher HP will be able to produce a given amount of torque at a higher RPM and thus turn faster, if you mate that to a correct gear ratio then an engine of higher HP will be faster then a lower HP engine (assumeing the same torque)

your M30 probabaly does not produce any more power then the M20, but it produces more torque, so while it runs out of puff at 135 with the diff you have, running a lower ratio (you can find a 3.46) will allow the car to reach higher speeds, upto the point where the resistance (air drag/engine drag/etc etc) can no longer be overcome by the engine and your car will reach its top speed, but if you run too low a ratio then you will loose acceleration and possibly even top speed as the engine cannot produce enough torque to overcome the higher turning resistance found in a loe ratio gearbox (remeber you old bicycle - remeber trying to start off in top gear?? hard work yes..)

so the ratio does dictate the top speed, but only when mated to the correct engine. a 3.25 may be too much for the engine, but you can only find out by trying

mrlee

Mon Apr 20, 2009 4:08 pm

Guy's this was on an air strip (brunting thorpe) long enough for me to go flat out with out a care in the world. NOT on the road. In fact it feels slow on a runway, as it's so wide..

Any hoo, No only getting to around 5800rpm in 5th. Can bounce of the valves in every other gear. Had exactly the same with the m20 in there too.

Mon Apr 20, 2009 4:11 pm

i wish i had an air strip near by... i dont speed on public highways(does the norwich ring road count??) would love to have a go on an air strip... just to see what it will do..

Mon Apr 20, 2009 4:13 pm

It was a 2 hr drive there

Mon Apr 20, 2009 4:17 pm

zaust wrote:It was a 2 hr drive there

worth it thought!!! i want to see 150 out of the b28is... no more ... no less.... inlet swap is needed.. i have sourced the parts... i wonna feel what 240bhp feels like in a dirty six! not as quick as the e30!!

Mon Apr 20, 2009 4:21 pm

A 3.46 diff would suit nicely

Mon Apr 20, 2009 4:32 pm

So if you didn't hit the rev limiter in 5th, the engine hasn't got the power to overcome the aerodynamic effect or you simply run out of road. If the latter is the case, putting a 3,46 would make the problem even worse

Summary: Worn out m30b35.

And, driving fast in a straight line is lame!

Going fast around corners is where it is at.

Mon Apr 20, 2009 4:38 pm

Jon_Bmw wrote: Going fast around corners is where it is at.

lol... i wrote the book..

Mon Apr 20, 2009 5:25 pm

e30bmlover wrote:
Jon_Bmw wrote: Going fast around corners is where it is at.
lol... i wrote the book..

Bumlover, you're my hero. I'll pm you my address so that you can send me your autograph.

On a broader point, an e30 is shaped like a brick, and I imagine the coefficient of drag is pretty high. In zaust's situation, the only way to improve top speed would be to change to a higher, (shorter) ratio, ie 3.73/3.91:1. When you are reaching terminal velocity at peak power revs, then the limitation is drag, and you need to exponentially increase power in order to boost top speed, as drag is proportional to the square of speed (and you also need to continually adjust gearing to suit).

But it's all pretty pointless anyway, it's all about the twisties.

Mon Apr 20, 2009 5:31 pm

djk wrote:
e30bmlover wrote:
Jon_Bmw wrote: Going fast around corners is where it is at.
lol... i wrote the book..
Bumlover, you're my hero. I'll pm you my address so that you can send me your autograph.

lol... i was only pulling ya leg!! my driving abilities are ok.. i can handle a rwd car as its all iv driven... round a track i would lose.. badly... but if you are intrested i have a monthly newsletter and you can join my club for a small fee of £500 per year... for that you will get the monthly newsletter... it will contain my preachings of Aachen and why i have turned to the dark side... no refunds!!

Mon Apr 20, 2009 5:53 pm

e30bmlover wrote:
djk wrote:
e30bmlover wrote: lol... i wrote the book..
Bumlover, you're my hero. I'll pm you my address so that you can send me your autograph.
lol... i was only pulling ya leg!! my driving abilities are ok.. i can handle a rwd car as its all iv driven... round a track i would lose.. badly... but if you are intrested i have a monthly newsletter and you can join my club for a small fee of £500 per year... for that you will get the monthly newsletter... it will contain my preachings of Aachen and why i have turned to the dark side... no refunds!!

What a bargain price for porn such as that...

Mon Apr 20, 2009 6:00 pm

zaust wrote:Guy's this was on an air strip (brunting thorpe) long enough for me to go flat out with out a care in the world. NOT on the road. In fact it feels slow on a runway, as it's so wide..

Any hoo, No only getting to around 5800rpm in 5th. Can bounce of the valves in every other gear. Had exactly the same with the m20 in there too.

Do you know where "peak power" is being produced. I think you are going to have to work out the ratios to put "what you require" into this area if that makes sense.

As a comparison my engine should be about the same power output as yours and with the "sport" box & 3.64 diff goes over the 140mph mark according to GPS on the straight at Bedford, unfortunatley had to then brake for the chicane

Mon Apr 20, 2009 6:19 pm

daimlerman wrote:
e30bmlover wrote:
djk wrote: Bumlover, you're my hero. I'll pm you my address so that you can send me your autograph.
lol... i was only pulling ya leg!! my driving abilities are ok.. i can handle a rwd car as its all iv driven... round a track i would lose.. badly... but if you are intrested i have a monthly newsletter and you can join my club for a small fee of £500 per year... for that you will get the monthly newsletter... it will contain my preachings of Aachen and why i have turned to the dark side... no refunds!!
What a bargain price for porn such as that...

lmao... worth every penny!
any takers?? ( i wont hold my breath )

Mon Apr 20, 2009 6:47 pm

12345kevin wrote:
zaust wrote:Guy's this was on an air strip (brunting thorpe) long enough for me to go flat out with out a care in the world. NOT on the road. In fact it feels slow on a runway, as it's so wide..

Any hoo, No only getting to around 5800rpm in 5th. Can bounce of the valves in every other gear. Had exactly the same with the m20 in there too.

Do you know where "peak power" is being produced. I think you are going to have to work out the ratios to put "what you require" into this area if that makes sense.

As a comparison my engine should be about the same power output as yours and with the "sport" box & 3.64 diff goes over the 140mph mark according to GPS on the straight at Bedford, unfortunatley had to then brake for the chicane

Don't know the peak power yet, It is a very strong engine now. I have rebuilt most of it. As far as I know it's a box for a 3.0. So the ratio's should be extremly close to the tech1 1 set up I had in there.

I have in the meantime discovered the afm is not working correctly, and not letting me adjust below 4.09 C0

so that has got to be done as it may be hindering me big time at high rev's. And also explains bogging down in traffic.

Mon Apr 20, 2009 7:01 pm

would there be much difference between facelift and prefacelift cars punching through the air? My faithfull Alpina was said to do 141 - 144 mph , would a facelift car with the same engine be able to beat that? I know I have a long diff at the back.

Mon Apr 20, 2009 7:49 pm

Don't know the peak power yet

assuming its the M30 3.5 then your peak torque is at 4000rpm and the peak HP is at 5400rpm, thus if you are revving more then 5.4k irregardless of the limiter setting then you will not go any faster

as for pre and post facelift the basic shape is the same so why would the drag be any different, your faithful apline probably had more HP then a standard 2.5 - or was a very healthy engine.

Mon Apr 20, 2009 8:00 pm

It's about 190-195bhp from the various articals I've seen.

Mon Apr 20, 2009 8:36 pm

early ones from E28 M535 185hp at 5800 and 290lb/ft at 4000

later E34 lumps show 211hp at 5700 and 305lb/ft at 4000

Think difference mainly down to fuel injection system, so it depends on what he has and how tired it is.

Mon Apr 20, 2009 8:39 pm

Felix79 wrote:It's about 190-195bhp from the various articals I've seen.

or are you talking about your C2???!!! In that case no idea about C2 2.5 but the 2.7 were 210 so 190 sounds about right - must be quite a good laugh to drive!!

Mon Apr 20, 2009 8:43 pm

Yeah my Alpina for the 190-195bhp. It uses the eta lump from the 525 with the reworked Alpina top.

Mon Apr 20, 2009 8:47 pm

Jon_Bmw wrote:So if you didn't hit the rev limiter in 5th, the engine hasn't got the power to overcome the aerodynamic effect

That's pretty much on the money.
Back in the good old days, gearing used to be worked out by the car just maxing-out in 4th (or 5th if it was direct, not overdriven). You may have been lucky enough to get a few more mph in o/d top or 5th, but it wouldn't have been a lot more.
An old fashioned overdrive, or a gearbox with an overdriven 5th, was used to keep the revs down and economy up when cruising.
Having said that, thinking may have changed with the times. Facelift E30 performance figures equate to pretty-much maxing-out in 5th.

Mon Apr 20, 2009 8:48 pm

nice!! love the Alpina ''cow catcher'' front spoiler, proper 80's car styling!! really miss my tech 1, the older rear lights make them so much more dated and that is the appeal for me, both mine now are facelifts so the rear lights just look too modern!!

mrlee

Tue Apr 21, 2009 7:53 am

So then. My alpina front looks good but wont let me go over the 140 mark ??

Tue Apr 21, 2009 1:30 pm

What size is your engine?

Even though my Alpina is a 2.5 ( slightly bigger than a 325's ) it's got a fair bit more bhp than the 325.

Also as previously mentioned top speed is also effected by gearing of 5th ( or top gear if you prefer ;) ) the ratio of the diff and then finally areo.

Tue Apr 21, 2009 1:52 pm

one more thing to consider is the wind, you may not think it plays much role but it does as the air resistance increases exponentially at high speeds, so a 10mph head wind could require many additional horsepower to overcome as essentially the car is travelling through the wind at 10mph more (this is why speed runs are reveresed and averaged within a set time limit - it negates the effects of wind)

mrlee

Tue Apr 21, 2009 2:56 pm

never mind all that. can anyone use this airtstrip? or is there a hole in the fence

Tue Apr 21, 2009 3:01 pm

See if millbroke will let us have a track day!

Tue Apr 21, 2009 6:01 pm

Have a play around on this gear ratio/speed calculator http://www.unixnerd.demon.co.uk/revs.html. 2.93 diff peak rpm 5400 theoretical speed 161mph.. even at yout torque peak of 4000rpm it's 119mph (I am aware this would never happen in a brick of an E30)

Next question who's got a diff for sale :lol:

Tue Apr 21, 2009 8:47 pm

charlE30 wrote:Have a play around on this gear ratio/speed calculator http://www.unixnerd.demon.co.uk/revs.html. 2.93 diff peak rpm 5400 theoretical speed 161mph.. even at yout torque peak of 4000rpm it's 119mph (I am aware this would never happen in a brick of an E30)

Next question who's got a diff for sale

That help's.

and yes I now need a 3:46 lsd or a 3:25 lsd.

Tue Apr 21, 2009 11:17 pm

zaust wrote:I have had the 3:64 diff on for the m20 motor I was running and never saw more than 137mph on gps readings. Now on an m30 motor with the same diff, I am getting the same readings. Although i'm getting there "slightly" quicker. So as I see it, the diff is the deciding factor in the over all speed of the car.

Is this correct ??? And if so then I need an m3 diff 3:25 lsd ???

an m3 3.15 or 3.25 will increase the top speed of your car compared to the 3.64 check this site all about diff calculations... http://www.unixnerd.demon.co.uk/revs.html

Wed Apr 22, 2009 12:52 pm

charlE30 wrote:Have a play around on this gear ratio/speed calculator http://www.unixnerd.demon.co.uk/revs.html. 2.93 diff peak rpm 5400 theoretical speed 161mph.. even at yout torque peak of 4000rpm it's 119mph (I am aware this would never happen in a brick of an E30)

Next question who's got a diff for sale

I had assumed that peak power was at higher than 5800 - as it seems it is at 5400, a taller diff will be the thing.