BHP question and g-tech pro..

Post Sun Jun 19, 2005 7:53 pm

325i standard and healthy is 170bhp, at the flywheel, How much is lost through the transmission etc, is it approximately 30bhp???

reason I ask, is I've been suspecting my 325i to be underpowered, and I measured today with Fozzymonsters g-tech pro (Cheers Fozzy) and it shew 117hp, meaning will be approx 150hp at the flywheel...

Does this sound right to you....

Post Sun Jun 19, 2005 8:23 pm

You serioulsy cant rely on a gtec to measure BHP can you? Is it that good?

If your worried I think a visit to a rolling Rd aint a bad idea.

Be aware of one thing though, its hot and it will show less power than normal.

I just been out in the C2 and it really felt sluggish compared to yesterday as temperature is much higher today.

Post Sun Jun 19, 2005 8:35 pm

M5pilot wrote:Be aware of one thing though, its hot and it will show less power than normal.

I did think that, but its felt slower than other 325's since I got it 6 weeks ago...., I dont know how accurate they are, but I think they're a good guideline.

might give it a power run on a rr soon

Post Sun Jun 19, 2005 10:33 pm

I think the recognised loss for BMWs is around 17% drivetrain loss, although it depends when you changed your gearbox and diff oil last:)

Post Sun Jun 19, 2005 10:38 pm

losses cannot be calculated as a percentage.

Example:

325i makes 141bhp at the wheels = 170 bhp at fly based on 17% loss

Bloke with same 325i goes and throws in a 2.7 engine pumping out 210bhp

The gearbox and diff, hence the losses are exactly the same = 29bhp in this example.

17% of 210bhp = 175bhp. Losses canot jump from 29bhp to 35bhp just y changing an engine.

Post Sun Jun 19, 2005 11:24 pm

That's very true actually, now I think about it percentage is a stupid measurement!

Post Sun Jun 19, 2005 11:34 pm

M5pilot wrote:losses cannot be calculated as a percentage.

Example:

325i makes 141bhp at the wheels = 170 bhp at fly based on 17% loss

Bloke with same 325i goes and throws in a 2.7 engine pumping out 210bhp

The gearbox and diff, hence the losses are exactly the same = 29bhp in this example.

17% of 210bhp = 175bhp. Losses canot jump from 29bhp to 35bhp just y changing an engine.

nicely said sal!!!! the transmission losses are a fixed value in terms of bhp lost, as the components can only have a certain level of friction.

i.e the more power to begin with, then the more that will be at the back wheels

Lots of folk get the wool pulled over their eyes by rolling roads that give @the fly BHP figures by taking the at the wheels figures and adding on up to 40%

i'd say to anyone using a rolling road to only trust the @the wheels power

Post Mon Jun 20, 2005 12:01 am

RR should perform a coast down test to measure transmision loss's otherwise it's guess work

Post Mon Jun 20, 2005 12:04 am

Andy335Touring wrote:RR should perform a coast down test to measure transmision loss's otherwise it's guess work

even thtn the coast down isnt entirely accurate as the tranny isnt under load, also tyre pressures can affect the resistance of the rollers

Post Mon Jun 20, 2005 12:21 am

Power loss in transmission can be very APPROXIMATELY treated as a percentage, since some of the frictional losses depend on the force being transmitted at a certain rpm (ie: power).
A gearbox will lose more power if it's having 220BHP fed into it compared with 150BHP.
The Pumaracing website has some useful rules of thumb for this.

THe GTech works by going:
force on sensor -> acceleration ->integrate for speed -> integrate for distance
Power is force x speed, the accelerative force is estimated by multiplying the acceleration by the approximate mass of the vehicle. All very rough and ready. Best test for power output is a top speed run, though this isn't always easy to do without risk of being caught.

Post Mon Jun 20, 2005 12:28 am

that pumaracing website is really good for making you challenge the conventional wisdoms of tuning

I'd advise everyone to have a read!!!

http://www.pumaracing.co.uk/mainmenu.htm

Post Mon Jun 20, 2005 12:31 am

Only up to a point though!

Post Mon Jun 20, 2005 12:43 am

tomstickland wrote:Only up to a point though!

totally!challenge only means to test

Post Mon Jun 20, 2005 4:29 pm

Mine came up as 143bhp at the wheels; the RR guy (who seemed to know what he was talking about...) reckoned they lose 28, giving 171 at the crank.

Post Mon Jun 20, 2005 7:01 pm

just to add mine makes 182 at the wheels and 213 at the fly.....

thats 31/213= roughly 15%

at 25degC ambient....

why is mine losing less?

Post Mon Jun 20, 2005 7:02 pm

hogweed wrote:Mine came up as 143bhp at the wheels; the RR guy (who seemed to know what he was talking about...) reckoned they lose 28, giving 171 at the crank.

thats makes sense as minen is losing 31bhp

Post Mon Jun 20, 2005 7:19 pm

What aboot dynamatting?

Post Mon Jun 20, 2005 7:52 pm

Jimbob wrote:What aboot dynamatting?

Post Mon Jun 20, 2005 7:57 pm

lol shit just realised that implies sound-deadening

I've heard of something along those lines anyway that is like a RR but a lot more involved...which therefore should give a more accurate reading?

Post Mon Jun 20, 2005 8:00 pm

hahaha, dynamometer is what i was thinking of (not sure how i came to think of this whilst eating a chinese tho...)

Post Mon Jun 20, 2005 8:34 pm

u can out those thingies on the hubs which is more accurate.... but u still get gbox and diff losses...... but no wheel spinning losses as it is direct to hub

Post Mon Jun 20, 2005 8:36 pm

Karan,

yours was done on a different dyno, losses will be different.

Post Mon Jun 20, 2005 10:00 pm

How accurate was the measurement of the cars weight when using the G tech?

Post Mon Jun 20, 2005 10:12 pm

Russ wrote:How accurate was the measurement of the cars weight when using the G tech?

You input the weight yourself in lbs.. 2 door 325i 1640kgs x 2.2046 = 3615lbs, that is the weight fully loaded, - approx 170lbs per person missing

thats what i did anyhow, am I correct???

Post Mon Jun 20, 2005 10:21 pm

Wilson wrote:
Russ wrote:How accurate was the measurement of the cars weight when using the G tech?
You input the weight yourself in lbs.. 2 door 325i 1640kgs x 2.2046 = 3615lbs, that is the weight fully loaded, - approx 170lbs per person missing

thats what i did anyhow, am I correct???

a 325i does not weigh 1640kgs!!! no wonder u got a low bhp

Post Tue Jun 21, 2005 6:36 am

whats it weigh???

I was going of the plaque under the bonnet... (you know I wondered if that was right!!! is it more like 1200 - 1300kgs?????)

Post Tue Jun 21, 2005 7:53 am

I gathered all the vehicle weights in the following thread :-

Clicky click

I took all the info from the BMW handbooks, but BMW seem to publish slightly different figures from time to time. The wights are all in Kg, so multiply the figure by 2.2 to roughly convert it to Lbs.

The plaque under bonnet shows the MAX permissible vehicle weight, fully loaded with passengers, luggage, fuel etc

assume 1250Kg for your 325i or 2750 lbs.

Post Tue Jun 21, 2005 8:06 am

I'm an idiot!!!!!!!!!!!!

Kgs to exact lbs is x 2.2046.. surely if I input weight heavier than it actually is, it would show more hp, because it would need more hp to move all that weight that fast!

Post Tue Jun 21, 2005 8:57 am

yeh u r right.... what 0-60 times r u getting?

Post Tue Jun 21, 2005 9:39 am

Karan wrote:yeh u r right.... what 0-60 times r u getting?

Havent tried that yet, will post up here when done

Post Tue Jun 21, 2005 10:01 am

Wilson wrote: I'm an idiot!!!!!!!!!!!!

Kgs to exact lbs is x 2.2046.. surely if I input weight heavier than it actually is, it would show more hp, because it would need more hp to move all that weight that fast!

You nugget!! I said 1200-1250 kgs when I gave it to you....we even discussed the pounds to kilos ratio.....muppet!! :lol:

And I've never tried the BHP part of the G-Tech, so no way to compare....
Be interesting to see what it puts out with the right weight in tho.....
Mark.

Post Tue Jun 21, 2005 12:04 pm

fozzymonster wrote:You nugget!! I said 1200-1250 kgs when I gave it to you....we even discussed the pounds to kilos ratio.....muppet!!

I Know I know... forgot what weight we said!!!!!

Post Tue Jun 21, 2005 12:14 pm

Wilson wrote:
fozzymonster wrote:You nugget!! I said 1200-1250 kgs when I gave it to you....we even discussed the pounds to kilos ratio.....muppet!!

I Know I know... forgot what weight we said!!!!!

Must have been the heat we were in at the time.....was about 30 degress on Sunday when we were chatting....man did I burn my legs!!

Post Tue Jun 21, 2005 12:18 pm

fozzymonster wrote:Must have been the heat we were in at the time.....was about 30 degress on Sunday when we were chatting....man did I burn my legs!!

Yeah... My shoulders sting like hell!!! Even Celvin burnt!! :lol:

Did you get your electrical thingy done?? or did you call it a day?

Post Tue Jun 21, 2005 12:26 pm

Wilson wrote:
fozzymonster wrote:Must have been the heat we were in at the time.....was about 30 degress on Sunday when we were chatting....man did I burn my legs!!
Yeah... My shoulders sting like hell!!! Even Celvin burnt!!

Did you get your electrical thingy done?? or did you call it a day?

TBH after you left I vegged out......was too hot to think.....did have another read through the how to's and instructions...but no way I had either the energy or patient to try and wire that up.... will do it this week sometime....

Have you got a bleed kit for the brakes? I'm wanting to swap my calipers over this week, but aint got a bleed kit....can buy one but I've spunked all my cash on the new wheels!! :lol: