Torque figures

[SteveRST]

My 2.1 Pinto is quite lightly tuned. Was rolling roaded about 2 wks ago.

160 lb.ft @ ~3800 rpm.

136 bhp @ ~5800 rpm.

Was told this was very good torque. Great driveability. But it doesn't like to rev.

Blatmans calculation is correct, but this just shows the torque at peak power / high revs. i.e. 7200 rpm in Blatmans example.

IMO, Peak torque is more likely to be made lower down the rev range, around 3.5 - 5k rpm.

              bhp x 5252

Torque =  ----------

               revs pm

So 180bhp @ 7200rpm = 131 lb.ft

But if you have 100 bhp @ 3500 rpm, your making 150 lb.ft

I was told that its best to think of BHP being the byproduct of Torque & rpm. In order to make more BHP, you have to make good Torque and keep making that torque as revs rise. Problem is, that as revs rise there is less and less time to 'suck-squeeze-bang-blow' efficiently, so torque does tend to drop off.

BHP = Torque x revs  /  5252

So 150 lb.ft torque at 3500 rpm is only 100 bhp.

But if you can continue to make 150 lb.ft torque at 7000 rpm, you are making 200 bhp.

steve

[Matt Seabrook]

No torque peak was 5500rpm and yes on the rollers it was making 200bhp ish at 5500rpm I think you are getting a little mixed up Blatters

[ChrisG]

Lets try a backwards calc.

(Torque x Engine speed) / 5,252 = Horsepower

(190 x 5500) / 5252 = 198bhp.

Either Matts/SBD's torque figure is wrong, or you need to calculate it at max rpm (8000rpm in Matts case according to the graph), which gives 289bhp. I don't think so... So, somethings wrong somewhere with Matts/SBD's quoted figures...

Have a look here.

 

Its producing 198bhp AT 5500rpm when producing 190lbft, however, its not producing the same 190 odd lb/ft torque at peak rpm hence why your calcs go AWOL  

That calculation doesnt give you a peak torque or HP figure for the engine, merely the torque/bhp produced at those particular revs. As an aside, at 5252rpm torque output and bhp output should always be the same numerical number, thats why the graphs always cross there.

[predawson]

I *think* you can calculate it, but I'm not 100% sure... Try

Horsepower x 5252 / max rpm

So for a Vx 180bhp x 5252 / 7200rpm = 131.3ft/lb's

But I'll stand to be corrected...

Forumla correct except it is usually used to calculate power from torque and not the other way around. Power can't be measured and therefore needs calculating from measured torque

Worth noting that the point at which max power occurs is not the point where peak torque occurs so calculating max torque with the max power rpm will not give you what you max torque is.

[Blatman]

Lets try a backwards calc.

(Torque x Engine speed) / 5,252 = Horsepower

(190 x 5500) / 5252 = 198bhp.

Either Matts/SBD's torque figure is wrong, or you need to calculate it at max rpm (8000rpm in Matts case according to the graph), which gives 289bhp. I don't think so... So, somethings wrong somewhere with Matts/SBD's quoted figures...

Have a look here.

 

Its producing 198bhp AT 5500rpm when producing 190lbft, however, its not producing the same 190 odd lb/ft torque at peak rpm hence why your calcs go AWOL  

That calculation doesnt give you a peak torque or HP figure for the engine, merely the torque/bhp produced at those particular revs. As an aside, at 5252rpm torque output and bhp output should always be the same numerical number, thats why the graphs always cross there.

Sorry but I can't accept that a 4 cylinder engine can produce 190ft/lbs of torque at 5500 with 198bhp showing without it having forced induction or diesel for fuel...

[ChrisG]

Im not saying the bhp/torque figures Scott quoted are right, just that your calculations are wrong  

[Blatman]

I never claimed them to be correct, BUT they seem to be universally in use for these sorts of things.

I found this too:

Rolling roads that inaccurately set the ratio between roller speed and engine speed give misleading torque outputs, but realistic power outputs. When you get curves from a rolling road the power curve is generally trustworthy (apart from its scaling against the rpm axis) and the torque curve needs to be taken with a pinch of salt.

From page 6 here,

I was looking for a posting over there that dealy with high torque figures for engines with apparently low (by comparison) power figures. I haven't found it... yet...

[Matt Seabrook]

Its producing 198bhp AT 5500rpm when producing 190lbft, however, its not producing the same 190 odd lb/ft torque at peak rpm hence why your calcs go AWOL  

That calculation doesnt give you a peak torque or HP figure for the engine, merely the torque/bhp produced at those particular revs. As an aside, at 5252rpm torque output and bhp output should always be the same numerical number, thats why the graphs always cross there.

Sorry but I can't accept that a 4 cylinder engine can produce 190ft/lbs of torque at 5500 with 198bhp showing without it having forced induction or diesel for fuel...

So you are saying if I put diesel in my car you could believe it then?  

[ian crocker]

BMEP is the true measure of an engine's efficiency and it is directly related to torque per litre.  F1 engines are the pinnacle of engine development and deliver around 97ftlb per litre.  Nuff said

My SEiGHT makes 280ftlbs from 4.2 litres but it's not very highly tuned.  Most I have heard of is 360ftlbs from a 5.2, again not very efficient.  But then they do go a long way between rebuilds

[Matt Seabrook]

 Found it Blatman claims 193-200lbft from his sprint car See here

[ian crocker]

 Found it Blatman claims 193-200lbft from his sprint car See here

Rumour has it that BMW are going to buy it from him and add an extra 2 cylinders for their next F1 engine      

[Matt Seabrook]

[Blatman]

 Found it Blatman claims 193-200lbft from his sprint car See here

Er, I make no claims for the torque in that thread at all. In fact, read the last posting

248 x 5252 / 6500 makes 200 but it ain't right, and I wouldn't ever claim it to be. I have also gone on record as stating that I believe that particular set of rollers to be over reading.

I claim 235bhp at 7000rpm for the Cossie, which according to my flawed calcs gives me 235 x 5252 / 7000 = 176 which is still high, bit it's a lot closer to the truth than anything posted so far...

I do have a torque curve from Northampton Motorsport at home. I'll post up the numbers when I get home...

[ChrisG]

Blatters, the equation is correct, its the numbers you're putting into it thats wrong  

Forgetting Scott's power/torque outputs for a mo, in your calcs you seemed to be using a peak power figure in an attempt to calculate peak torque, which you can't do. The only time this would work is if peak power was at the same revs as peak torque. If you have a power graph, to find peak torque you need to read off the power figures over the entire rev range in steps of say 100rpm, apply the equation to it for each power/rpm figure and then see where the peak is, eg

80bhp  @ 3000rpm = 140lbft

120bhp @ 4000rpm = 158lbft

140bhp @ 4500rpm = 163lbft

160bhp @ 5500rpm = 152lbft

175bhp @ 6500rpm = 141lbft

180bhp @ 7200rpm = 131lbft

179bhp @ 7300rpm =  129lbft

ie peak torque = roughly 4500rpm but that number in no way correlates with the 180bhp peak power figure at 7200rpm.

Sorry if its me getting the wrong end of the stick tho

[Blatman]

Hey don't be sorry, we'll not get to the bottom of this without discussion. As i said in my opening post, I stand to be corrected, and educated. However, whilst the numbers may be imprecise, I believe that using the peak RPM figure at least allows us to get in the right ball park for max torque...