how much boost?

[nemz]

im not exactly sure how much boost im running, but tonight i done some air bx modding to allow alot more air flor through and thought id take it for a spin and give it a bit of a wholoping, now when giving the car my mmg boost guage would go upto +7 and sit there easially, now as far as ive heard that dosnt mean 7psi and 7psi is only about +4? correct? so how much boost is that? p.s last night on the high way doing 100 i was able to shift down accelerate hard and spin the wheels and take off hard can stockies do this? im not quiet sure how fast my car is and i wanna know now

[Steve]

7x100mmHg = approx 13.5PSI

http://www.onlineconversion.com/pressure.htm

[Jimbo2000]

hi steve, do you know what the conversion rate for rwkw-kw is?

[Steve]

kw to kw???? I dont understand a kw is a kw whether its at your rear wheels, front wheels or engine???

If you have a look at www.onlineconversion.com it has a few different conversion calculators, but I really dont understand what it is you are asking?

[Jimbo2000]

heh

i basically want to know what hp my car is, it it 205 rwkw

[Jimbo2000]

yeah i know its a stupid question

[nemz]

ummmmmm welllllllll id say...

about 230kw

[b2barker]

try 270kw

[Steve]

Jimbo2000, I believe driveline loss for the R33GTSt is pretty close to 60-65kw, so if you have a rwkw figure just add 60 and it will be bloody close to at the engine.

So if you have 205rwkw, around the 265 - 270 mark at the engine.

[Jimbo2000]

thanks lads.

i know its silly, just wanted to compare to all the new cars you see in papers and vids quoting hp and kw figures at the engine

thanks again

James

[Cubes]

When you make more power you also lose more power through the means of heat and more friction so the only real way to work out the rwkw to fwkw is to use a percentage value.

I.e a Stock R32 GTST makes around 160kw at the flywheel.

On a dyno it will make ~115.2rwkw. So that is a percentage loss of ~28%.

A 300kw (400hp) engine * .28 = around 216kw at the rear wheels.

Hence why a 225kw VX Commodore makes around 164kw at the rears. 225 * .28 = 63kw loss which equals 162rwkw.

Its an estimate for manuals not auto's.. Its not exact but close enough.

Auto's lose a lot more power.

[b0neR]

I head that as a general rule of thumb between 20-30% Drive line loss is usually apparent in most vehicles. So if you wanna convert rwkw to engine kw then your rwkw figure would be about 75-80% of your engine figure i.e. 75-80rwkw = 100kw etc.

[Cubes]

Don't fall in to the trap of thinking you can simply minus 28% from 100% to give you 72% which you would think will give you the rwkw to fwkw conversion as it doesn't work.

You can apply the accounting future value formula to work out the rwkw to fwkw. OR if you have the rwkw to fwkw % you can use the present value accounting formula to get the desired result.

I think... It should work.. Theoretically.. I'll have to check.

[Steve]

There are two trains of thought on driveline loss. One that it is a percentage of the engine figure, another that it is pretty close to a fixed amount.

the fixed amount theory sits alot better with me, for example if a car is making 1000rwkw (which has been done), the percentage theory means a driveline loss of 200 - 280kw - that is monserous amounts of heat - probably enough heat to melt any driveline.

Also, I cant get around frictional losses varying directly with the power being applied - it just doesnt add up - it would also follow that if a car was making 100kw at the engine, that the driveline loss would only be 20-28kw?? Sure friction accounts for an increase in loss, but that much?

[Cubes]

My old NA VL 3ltr commodore when it was dead stock apparently makes 114kw at the flywheel.

I had it on a dyno and it made 80rwkw.

That is a loss of around 34kw. Applying the .28 percentage gives me a loss of 32kw which I guess the 2 kw would be the auto trans that was in it at the time. Its a rough guestimate but it seems to be pretty spot on 99% of the time.

I've also had my first car a TE Cortina 4.1ltr 4 speed when it was stock with only a 2.5" exhaust, on the dyno and it made 73rwkw. They apparently make just over 90kw at the flywheel @ 3700rpm with stock exhaust etc so that should equal around 65rwkw, maybe the exhaust picked me up a few kw here or there.

It guess it would be a little harder predicting the drive train loss when it comes to some of the older aussy high performance auto gearboxes as some of those really sucked the juice out of the motor.

I guess it isn't just friction losses its also efficiency through out the motor its self. Not sure really.. That 28% for the Manuals seems to work.

[Sydneykid]

So Joel, if you double the power output from 114 kw to 228 kw (pretty easy with a turbo) it will now magically loose twice as much at 64 kw (previously 32 kw). Sit in front a 1 kw heater for a while and then tell me where the extra 32 kw went. You would have a heap of melted aluminium that used the be your gearbox if that was true.

As you have guessed by now, I agree with Steve, if it looses 32 kw with 114 kw I see no reason why it would loose much more.

[Cubes]

When the car was absolutely dead stock making 115rwkw and 160kw at the flywheel, that’s a 45kw loss through out the drive train, engine internals etc...

Now when upping the boost, exhaust, fmic, ebc I made 164rwkw on the same dyno with the same ambient temperature.

So does that mean that I am really making 209kw at the flywheel?

Why is it that a 225kw Commodore makes 164rwkw?

And say if I were to make 250rwkw then I am really making 295kw at the flywheel.

In reality/practise it just doesn't seem to work out the the way you are stating Gary/Steve.

I do agree with what you are saying however the facts in front of my don't suggest so.

I have never done physics so Gary.. Whats the scientific theory behind it?!?!?!

Have you had an engine on a dyno say a RB25DET that apparently makes 187kw at the flywheel, then worked it up to make say 300kw at the flywheel, slapped it back in the car and seen if the drivetrain loss has increased or stayed the same as the stock car?

[Sydneykid]

Hi Joel, it's pretty simple high school physics really. Newton's theory says that energy can neither be gained nor lost, it can however transformed into other types of energy, ie torque into heat via friction etc.

Each car transforms a different amount of torque, a rear wheel drive car will transform more than a front wheel drive car because it has no tailshaft universals and right angle drive to the differential. Obviously a four wheel drive transforms even more, it has 3 diffs, 2 sets of drive shafts etc.

Then there is the small gearbox versus the large gearbox, ditto diff. So a Commondoor with a huge gearbox and diff, big tailshaft and universals, large wheel bearings etc etc wil obviously transform more torque into heat.

Some results we have seen;

An R32 GTST standard 160 kw makes 110 rwkw

An R33 GTST standard 180 kw makes 125 rwkw

An R32 GTR standard 225 kw (the accepted 206 +) makes 165 rwkw and 150 4wkw. Losses of 60 kw and 75 kw respectively. Calculating the percentage that would be 29% and 33% respectively.

I have previously installed, into an R32 GTR, an RB30/25 that made 360 kw on the engine dyno. On the rollers it made 295 rwkw and 278 4 wkw. Losses of 65 kw and 82 kw, pretty close to the previous RB26 losses.

If I had experienced the percentage loss of 29% it would have only made 255 rwkw and 241 4wkw at 33%.

I could give you at least 10 other examples of engines we have run on the engine dyno and then on the rollers, and they would show the same result. Sure there is a little more losses but nowhere near what you would expect using the percentage theory.

Think about it in reverse..........

What if I put in a straight cut gearbox (substantially less friction), the car will show more power at the rear wheels, but it will have the same at the engine. How does the fixed percentage method allow for that?

Hope that helps

[Penfold]

In reality its probably a combination of the two - something like the Duckworth-lewis formula in one-day cricket. The loss through the drive-train obviously varies between different car makes - where you compare a commodor to a skyline & indeed a gtr compared to a GTS - with different drivetrains. Also, any modifications will assist as well in raising the ratio - a nice well layout machine with big exhaust will loose less through the drive train than another. And on that topic, a newer car will probably be more efficient than an older car (as bits gradually decline & increased resistance - especially if not well oiled & maintained etc.

So you can probably have some sort of guess but for any given car it will be a fixed loss through the drive-train + an adjustment factor ie say 30kw for the drivetrain but also say multiply it by 1.12 after that. Obviously if you've got a part in your drive-train that is a little inefficient then the more it turns, pulls, etc then more it will more power away.

Anyhow it sounds reasonable to me - but feel free to tell me im wrong

Cheers,

Penfold

[Sydneykid]

Hi Penfold, exhaust system losses (heat tranformations) are inlcuded in the power the engine makes. The losses we speak of are driveline losses.

When Nissan quote 180 kw for an RB25DET the standard says it should include running all the stuff necessary to make the engine work including stuff like the water pump, oil pump, cambelt, alternator etc. Plus all the standard inlet and exhaust stuff has to be connected eg; they don't test it without an airfilter and no exhaust pipe.

So the only losses you should see are gearbox, diff, tail shaft drive shafts, wheel bearings, brake drag, tyres on rollers etc.

Hope that helps explain