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[Doctor Gollum]

Be around 200lbft, less gearbox tax

[Webby the Bear]

So that's 200lbft at the flywheel. Minus the "drivetrain loss"

[Doctor Gollum]

Officially, 168bhp and 167lbft, so I suspect 140-150 on both at the wheels

[LC0112G]

HP = (RPM * Torque) / 5252

So if you get a pretty graph, check that HP line crosses the Torque line between 5200-5300 RPM. If it doesn't then something is wrong (the operator is fudging the results!).

For a N/A petrol car, torque is usually flat-ish from around 2000 rpm to about 5000 rpm. Therefore, as the RPM rises, the power output rises fairly linearly between 2K and 5K RPM. Power usually reaches a peak somewhere around 5500 RPM because the torque curve starts to decline above 5K RPM quicker than the RPM is increasing.

Torque is basically determined by the amount of air you can get into the cylinder (hence the saying that there is no replacement for displacement) - For your car is 2.5L/6 times the VE (Volumetric Efficiency) of the engine. VE is a measure of how good the inlet and exhaust manifold design are at allowing air into and out of the cylinder.  At low RPM there is plenty of time for the cylinder to fill with air. As the RPM increases there becomes les and less time for the air to enter (and exhaust to exit) the cylinder, so VE starts to tail off above a certain RPM, and this is what causes torque to decrease. You can tune the inlet/exhaust to try and improve VE (which is effectively what the multi-ram bagpipes is), but if tuning is taken to extremes it tends to result in a very peaky torque curve, and a big spikes in BHP at certain revs.

A nice wide flat torque curve makes for excellent drivability and acceleration. However, maximum BHP is achieved by tuning everything to produce a spike in the torque curve. You may win a Top Trumps bragging war with higher BHP by doing this, but in the real world the car with the flat torque curve will pi$$ all over you.

[Doctor Gollum]

Which is why the smaller 2.2 dti produces as torque as the 3.2... VE being 0.55 plus turbo vs 0.533 

[minifreek]

Had my 2.5 Elite RR'd and the resukts was quite impressive....

Operator got 235BHP FWP and 195BHP WHP was happy with that result....

This was after changing a few components etc... not a standard factory engine or heads....

[Webby the Bear]

thanks for input guys.

just to clarify in simple terms (so i dont get boggled with science) any loss between BHP and WHP will have the exact same loss (percentage) in torque (between flywheel torque and torque at the wheels)?

[LC0112G]

thanks for input guys.

just to clarify in simple terms (so i dont get boggled with science) any loss between BHP and WHP will have the exact same loss (percentage) in torque (between flywheel torque and torque at the wheels)?

Yes. The Dyno actually measures torque - the turning force that the tyres exert on the rollers. It can't measure power directly, and can't know where in the drivetrain the losses are occurring. The headline BHP figure is calculated by multiplying the torque on the rollers by the RPM of the rollers.

The usual way to measure losses in transmission is to rev the car up to red line on an unloaded roller and then lift off the throttle completely. If there were no losses, then the wheels would continue spinning at the same RPM for ever (that bloke Newton). However, in the real world the losses will cause the wheels to slow down, and the quicker they slow down the more the losses must be. The rate of slowdown can be converted to a power figure. Unfortunatley this isn't a perfect science, because there are losses in the engine which should be included in flywheel BHP which are cancelled out by the coastdown technique.

I've never really understood the obsession with flywheel BHP though. What really matters is available torque at the wheels over as wide RPM range as possible.

However, one tip that seems to work in getting good BHP figures is to pump the rear tyres up hard - 40psi. And make sure all your electric fans are working.

[anV6]

Had my 2.5 Elite RR'd and the resukts was quite impressive....

Operator got 235BHP FWP and 195BHP WHP was happy with that result....

This was after changing a few components etc... not a standard factory engine or heads....

235bhp from an originally 170bhp Omega engine?  And you are not running any forced induction? If accurate, it's impressive indeed. Please do tell which mods you have made to your engine to get these figures. 

[Kevin Wood]

Had my 2.5 Elite RR'd and the resukts was quite impressive....

Operator got 235BHP FWP and 195BHP WHP was happy with that result....

This was after changing a few components etc... not a standard factory engine or heads....

235bhp from an originally 170bhp Omega engine?  And you are not running any forced induction? If accurate, it's impressive indeed. Please do tell which mods you have made to your engine to get these figures. 

See, this is the sort of claim that rings alarm bells.

A 4 valve per cylinder NA engine will do very well to produce 90lb.ft per litre of capacity. Physics sets this limit, and we're talking race tuned engine here. For a road car engine, 80lb.ft/m is more like it. Even then, it's a trick engine on individual throttle bodies, not a plenum.

On that basis, then, a 2.5 will make a maximum of 200 lb.ft.

Factory specs say 167 ft.lb, so it's a bit off the theoretical maximum as we would expect for a production car. Anyway, let's say we tune it to get 200 lb.ft torque.

Maximum power always occurs well past the torque peak, especially on the V6 where the torque peaks quite low in the rev range. Let's say it's at 90% torque at maximum power. I suspect we're being generous here, but stay with me.

As LC0112 says, HP = (RPM * Torque) / 5252

Rearrange this and you get (HP * 5252) / Torque = RPM

If we plug the numbers in we see that our 2.5 will have to rev to 6856 RPM to produce 235 BHP. The factory rev limiter is around 6700.

If we try it with the manufacturer torque of 167, we get maximum power of 170 bhp at 5952 RPM, which is about right, so the maths looks plausible.

The final test would be to look at the dyno plot to see if the torque output defies the laws of physics and to see if power actually peaked at over 6800 RPM. If not, it's likely BS.

Look at any tuned NA engine where the output is genuinely substantially above standard and you'll see forged pistons, steel rods, lairy cams and it'll rev to kingdom come. This is because NA engines are basically torque limited and to make more power you have to make them rev higher.

It's so easy to fiddle the corrections on a rolling road for a "pub figure" but "Ye cannae change the laws of physics".

[YZ250]

I'm not in to the figures malarkey myself. If, hypothetically, I floored it off the lights and the car next to me outruns me, and I can't catch him, then he has a faster car. No maths involved there, quite ingenious really isn't it.   

But ....Webby is an inquisitive chap so I look forward to his findings.   

[Doctor Gollum]

Kevin, to be fair to Minifreak, and perhaps in context he wasn't explicit enough... he did say not a factory standard engine, with modification...

If it's the engine I think it is, it's from a Vectra B racing series, so the rev limiter might be set a little higher , and the tune rather more aggressive

[anV6]

For the record, I didn't mean to doubt him. I'm genuinely interesting in learning about the mods of his engine. 

[Kevin Wood]

Kevin, to be fair to Minifreak, and perhaps in context he wasn't explicit enough... he did say not a factory standard engine, with modification...

If it's the engine I think it is, it's from a Vectra B racing series, so the rev limiter might be set a little higher , and the tune rather more aggressive

I'm also not aware of the spec of Minifreak's engine so I didn't mean to cast doubt. I can see how my post could be read that way. I'm merely stating what the physics of a 2.5l engine producing that power are. If it's had cams, porting, an increased rev limit and so on, it's quite possible that that's a genuine figure.

For every genuine claim of the type, though, there are umpteen who have fitted a cone filter, blanked off the EGR, bored out the throttle body, fitted a 4 BAR fuel pressure regulator and de-catted it!

[anV6]

Kevin, to be fair to Minifreak, and perhaps in context he wasn't explicit enough... he did say not a factory standard engine, with modification...

If it's the engine I think it is, it's from a Vectra B racing series, so the rev limiter might be set a little higher , and the tune rather more aggressive

I'm also not aware of the spec of Minifreak's engine so I didn't mean to cast doubt. I can see how my post could be read that way. I'm merely stating what the physics of a 2.5l engine producing that power are. If it's had cams, porting, an increased rev limit and so on, it's quite possible that that's a genuine figure.

For every genuine claim of the type, though, there are umpteen who have fitted a cone filter, blanked off the EGR, bored out the throttle body, fitted a 4 BAR fuel pressure regulator and de-catted it!

Wait, wait, wait! How much extra power would all that get you? At least 5bhp?

I know what you mean. And dyno houses also exaggerate numbers to make the customer feel good.