15 replies to this topic

[mad4mins]

hi

ok just found my dyno test print out from a month ago (which was done at Slarks)....and thought i'd compare my results to this months MiniWorld dyno session (as you do) ... and a standard 998 has more bhp at the wheels than i do....!!!

so my car is a 998, with
- 12g940, 1275 head,
- 266 Kent cam,
- inlet manifold with single SU and cone K&N,
- LCB exhaust manifold and big bore zorst...

standard - diff, gearbox, clutch, flywheel and driveshafts

Slarks tuned it for me and got 65.3bhp at the flywheel, but only 35.9bhp at the wheels... so are the standard items above making the engine loose 29.4bhp, the engine is runnng fine..

anybody got any ideas?? is this normal?? should i change the diff ???

cheers for any help

Aaron (M4M)

Attached Files

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Edited by mad4mins, 28 October 2006 - 02:59 PM.

[Total Richie]

Just looked at Mini World and there power outputs where all rated at the flywheel

Also you can expect up to 40% power loss through transmission systems depending on setup.

[mad4mins]

yeah the first one has 48.8bhp at the fly and 39bhp the wheels, thats only a loss of 9.8bhp....

so would it therefore be worth upgradeing the diff to try and reduce my 29.4bhp loss..???

[steveobennet]

if your transmission side is used then it may be taking more power. is the clutch slipping? if you plan on doing any more to your engine i would suggest a re-con gearbox with a competition diff pin, and also a uprated clutch, just to be on the safe side

[mad4mins]

ok sounds like an idea... the clutch was replace with a new standard one less than 500 miles ago (due to student budget) and it works fine.. but might upgrade the gearbox an diff like you say

[Total Richie]

Because the gearbox is also them engine’s oil sump then fresh oil would make a difference anyway. If your oil has become thick and lumpy then this would increase resistance around the gears. Also you can use an oil additive just before you dump the oil to brake down any sludge in the sump.

[mad4mins]

funny you should say that, i changed the oil last weekend... and it felt much nicer when i drove back to uni sunday night... and the oil did have traces of water in the system when they did the dyno test (guilty !!) lol

[Bungle]

the fly wheel power is only a guess when worked out on a rolling road

[Total Richie]

the fly wheel power is only a guess when worked out on a rolling road

I completely forgot about that!

[Jammy]

The average loss through the transmission is about 30%.

[fikus01]

35.9bhp at the wheels!!!! i would be happy with that!! face facts you drive the car by the wheels not the crank!! you have at the wheels what a std lump is supposed to have at the crank!! looks like you've got 13" wheels!! big wide tyres are a good place to look for power loss!! and if you're really into power can i suggest the 3.4 or 3.7 diffs!! they aint great for cruising (especially the 3.7) but power and acceleration are good

[mad4mins]

yeah but i'm losing nearly 50%!! ahh!!

true forgot about the big wheels

[fikus01]

the computer could have been having a bad day when it calculated the bhp!! u will lose power in friction so maybe drop gears of you have a siexed thrust washer or 1 of the gearbox bearings possibly!! but seriosuly if it feels good to drive and it doesnt have any flat spots i would say its ok!!

if you're that worries take it to another rolling road and ask for a power run!! they dont cost much!!

[Sprocket]

I have discussed this many times before on forums and will do again.

Power losses are a result of friction within the drive train. The drive train consists of the clutch, the entire gear system (transfer gears, gear set and final drive/differential) the CV joints, wheel bearings and the tyres.

Each part except the clutch (when it is sized correctly) absorbs energy as a result of friction, the amount of energy absorbed depends on the components condition, worn parts absorb more energy. This energy is converted to heat and is dissipated to the atmosphere by radiation.

The rundown indicated power loss is a cumulation of the total system.

How to improve these losses.

Make sure the wheel bearings are well greased and in good condition. Make sure the brakes are not binding IE seized caliper. Make sure the CV joints are well greased and in good condition. Rebuild the gearbox and tranfser gears carefully, replacing all of the bearings and thrusts keeping the tolerancies within limits. Fit sctraight cut gears if you can put up with the noise and afford it. Use a top quality modern oil (incidently 20w50 is thick and generates a loss in itself, Im not going into this argument again) And lastly if its a twin roller system put 50% extra air in your tyres when on the rollers. If its a single drum leave the tyre pressures as standard. CAUTION!!!! that last statement is for you to use only if your tyres are up to the job. Not every tyre is the same and the max tyre pressures and tyre condition should be taken into consideration before increasing the pressure. BE WARNED check the numbers on the side wall first and asses the tyre condition.


The rundown figures are normaly quite acurate, but there are some inacuracies withing the system. I havent been to a rolling road where they test the rundown of the rollers themselves and i have heard of 8bhp losses on the rollers alone. Not only this but the system needs time to gt up to running temp. Its no good putting the car on the rollers first thing in the morning just after the shop has opened. to be honest the operators dont give a f$&k. So, if you want accurate figures, be sly about it, book it in after some one else and dont mention the losses of the rollers because they will look at you with a rather blank expression as there is no way to calculate the drag of the rollers except on very expensive dynos. I have heard of people using a motorbike to get the rollers up to a speed of 70mph and then roll off. How many people are willing to do that!!!!

Visard recons on a well built gearbox and drivetrain giving a loss of 12bhp, but i suspect that was with a straight cut gear set. 18bhp seems to be average and 24 is about the most i personaly would tollerate.

Percentage of power for a transmission loss is nonsensicle. An engine with power of say 150bhp will have almost exactly the same looses if the same drive train was used with an engine of 45bhp

[mad4mins]

wow mini sprocket

will change the diff and the gearbox.. as per your experb explantation!!

cheers

M4M..