Engine power, speed, etc

[simdel1]

This looks promising: http://www.pumaracing.co.uk/TOPSPEED.htm I've not had chance to read it properly, but it seems like it could do the trick.

 

[TTM]

Yes thanks, I've been looking at this one and got interesting results.

 

[TTM]

Good evening all,

There are some interesting web pages on this, but I was wondering if someone on here knew of a relatively easy but accurate way to calculate how much power was needed to reach a certain speed.

Any clue?

Thanks.

 

[barks944]

So whats this in aid of then?! 944 Landspeed record?

 

[TTM]

Well, I used the formulas from the Puma website and with the datas from a roughly standard (shape & weight) late 944T, this is what I get. Does this look realistic?

 

[simdel1]

Unfortunately there's some funny units for the road speed that I don't recognise. [] Seriously though, it looks to be sensible. 530bhp to get to 210mph (340km/h) sounds reasonable for something that has a small frontal area. I'd be interested to know how close a standard turbo is to taking off at that sort of speed though! As a comparison. The Ferrari F40 needed 471bhp to get 200mph. That car is considerably more 'draggy' than the 944 too.

 

[TTM]

Whoops sorry, I just updated the graph as I initially forgot to take account of rolling resistance.

 

[robwright]

Just going by your graph then a 220 turbo fitted with a Tial and assumed to be in reasonable condition could feasibly achieve around 171 mph. I wonder how long it would take to get there?

 

[tscaptain]

At high speed aerodynamics will be the determining element and not bhp surely? To make a stupid analogy if I could fit my garage with wheels and the engine from my car it would struggle to make 330 kph...[] More appropriate would be a table showing how much more bhp is needed for a given object (a car in this case) to achieve a certain increase in speed. Different again for a boat, for example. If calculators are your thing you can spend hours here........[] http://www.wallaceracing.com/Calculators.htm

 

[TTM]

Captain, interesting page thank you! The graph already takes account of the Cx (0.33) and frontal area (20.3 ft²) of a standard 944T. Weight 2980lbs/1350kg (that would be a "heavy" late car) Estimated transmission losses 15% Here is the graph expressed in mph.

 

[944 man]

Surely gearing (gear ratio, final drive ratio and wheel diameter) and torque characteristics need to be factored in too? Id always thought that this makes any calculator rather simplistic and inherently inaccurate?

 

[Hilux]

I would have thought that the line should be less linear and would tail off much like a rolling road graph As speed increases the drag, friction and resistances become greater so a given amount of bhp incrementally would give less benefit ie: a proportionately larger bhp would be needed for an ever higher top speed. The Veyron for example needs 1002 bhp to do 252mph - a doubling of bhp (from the graph suggestion of 500ish bhp for 230ish mph) for a 25% speed increase on your figures - if you see what I mean. I could be talking dogs danglies but its just my two penny worth []

 

[TTM]

Well yes Simon, and this point is also highlighted on the Puma page in the Gearing section. For example let's consider the factory 968 turbo S. Factory figures are 305 bhp at 5400 rpm and a top speed of 282 km/h, and its gearing diagram below shows that 282 km/h equates to ~5800rpm in 6th gear. The Puma page mentions that top speed should be reached within a +-500rpm range around peak power, which makes sense with the gearing diagram. As you suggest Simon, the engine would still need, I think, to have a rather wide torque curve in order to help push the car against the air until it can stay in this hypothetical 1000 rpm range around peak power. So perhaps gearing should be chosen so that the engine "switches" from the peak torque rpm range to peak power rpm range without effort. I think the example of the factory 968 turbo S is an interesting one as peak torque is reached at a remarkably low 3000rpm, which correlates well with the fact that peak power is reached at an also remarkably low 5400 rpm for this type of "fast sports car". This just got me thinking that there might be another type of ratio to look at [8|]

 

[TTM]

Paul, good point. The graph for the Veyron is as follow, with its particular Cx, frontal area and weight. It's still quite in line with official figures, isn't it?! []

 

[George Elliott]

I agree with the shape of that last graph, it shows the laws of diminishing returns. Bugatti know all about it. Just one point, Porsche's turbo horses seem a lot stronger than Maranello's fragile beasts. George 944T

 

[333pg333]

How fast are you trying to go Thom? [8D]

 

[TTM]

I would like to break the symbolic 300 km/h barrier (the standard gearing demands running past 7000rpm, which is an engine speed I don't want to risk on my engine). Just trying to evaluate if it sounds doable with my car which I hope will make just south of 400bhp at 18psi with the new exhaust, as I don't want to bleed my car budget for a custom 6 speed gearbox if it won't allow me to at least reach this speed. At 300 km/h my hypothetical custom 6 speed gearbox will be running the engine 400rpm slower than the factory's 968TS unit, with a supposed 90bhp difference in engine power, so I think it's not a too unrealistic goal. Fuel consumption in top gear on the motorway should be nicely reduced too.

 

[DivineE]

Apparently v.easy to remove cover from back of gearbox and replace turbo 5th with S2 5th leaving the final drive. That should bring the revs needed to something a little more sensible p.s. Although I have already suffered my fair share of abuse for saying this when it first happened.. I did genuinely hit the rev limiter in 5th in a totally standard 220turbo with a sticking boost enhancer (by pressing the throttle in squirts of less than 2secs to avoid the overboost trip on the standard chips). So you dont need that much power to get to 176mph (speedo was of the clock). Hey I was 18 years old, cut me some slack

 

[TTM]

Well... I would say building a gearbox to reach such a "politically incorrect" speed on a 944 turbo probably doesn't stay within the financial realms of the usual owner. And I didn't say I was going to try this on the public road anyway (too dangerous for other users), thankfully we have some decent airfields here too. (I modified the title of the thread, I hope it's doesn't read as "politically incorrect" as the first one) Back on topic now, the S2 5th remains too short I'm afraid, I'm not sure it is worth it. Did you monitor speed with Satnav? The speedo is highly erratic...

 

[Hilux]

ORIGINAL: TTM Paul, good point. The graph for the Veyron is as follow, with its particular Cx, frontal area and weight. It's still quite in line with official figures, isn't it?! []

Thats the sort of curve I would expect (very well described by George as diminishing return) []