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cj79

power to weight ratio

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I'm probably being very stupid here but here goes...

 

OK so a BMW Z4 has a 2.0l, 4 cylinder engine producing 181bhp. It goes from 0-62 in 7.2 seconds and weighs 1750KG.

The TR6 has a 2.5l engine, 6 cylinders producing (well with the modifications i've had done) mine apparently should be about the same, 180 ish bhp. The original (150 bhp) spec had it rated at 0-60 in 8.2 seconds, and it weighs far less at 1085kg... With a bigger engine and far lighter, why is the performance not way higher?? I know its a modern engine etc but still!
Chris

 

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Its the torque that matters.

Stuart.

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This is where I wish we had the 'bloke eating popcorn' emoticon :ph34r:

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This is where I wish we had the 'bloke eating popcorn' emoticon :ph34r:

Or one selling tickets :lol::lol:

Stuart.

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" Its the torque that matters."

 

And how it's measured. The measurement techniques of 1973 and 2013 are two very different things.

 

Rear wheel horsepower and bhp at the flywheel are also two very different things, it's the former that matters.

 

For sure no '150bhp' CP standard production engine ever put out anything like the mythical 150 mark in real terms, and by the same token the average '180bhp' TR6 engine is a long way shy of a three darts top score.

 

But gullible folks like big numbers, and a rolling road can cheerfully produce whatever figures the operator chooses to dial into it - which is why chassis dyno figures are effectively worthless in absolute terms.

 

Cheers

 

Alec

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I've often read that 'rolling road figures are worthless'

 

Never really understood why.

 

Surely it's not that hard to measure torque on a rolling road, and isn't BHP just the rate of delivery of torque?

 

Now I can see why the extrapolation of RWBHP to engine BHP might be inaccurate, but why is the measurement of RWbHP a problem?

 

Steve

Edited by SDerbyshire

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Have a look around the internet, it's not an instant answer topic !

 

David Baker of Puma Race Engines has penned several good explanations over the years.

 

Here is one example

 

http://www.max-boost.co.uk/max-boost/internet_articles/Puma%20Race%20Engines%20Technical%20Guide%20-%20Measuring%20Engine%20power%20-%20engine%20dynos%20and%20rolling%20road%20dynos.htm

 

 

Cheers

 

Alec

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Decent article with reasonable assumptions and opinions, as always there are other opinions out there some with a different viewpoint but all considered as close to a majority in agreement as makes no difference.

 

One particular point is well made, as declared by David Baker it's what the engine delivers on the road or track which is important and not the figures given to the owner, for example I've never built a 4 cylinder engine which has 200 hp at the back wheels but the engines I built were faster than those who claimed to have 200hp. The manner of delivery of power is at least as important as the highest figures achieved by the engine.

 

Mick Richards

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Interesting article which explains the differences found on various rolling roads.

 

When the first rolling road appeared at the national weekend at Malvern a few years ago, I asked the operator what he would expect to see from a good 150bhp TR.

He reckoned a good one might make 125bhp. I spent a large part of the day there and the best figure I saw for a 150 was 122, that was for a completely standard TR5, original exhaust and manifold .

 

There were a lot of long faces that day, especially the foreign guy who had his 125 spec TR6 supposedly rebuilt to 150 spec at great expense. His car only made 85 which was similar to what most of the 4 cylinder cars were making.

 

A lot of the injection cars that were down on power were blowing black smoke, obviously too rich.

 

I was a happy bunny as my Stag powered example made 165bhp with a Holley carb and tubular manifolds, maybe not a definative figure but since all the cars are a similar size and weight, at least it was a reasonable comparison on the day which is probably the best we could hope for.

 

A few years later I saw figures of 175bhp at Malvern, and two weeks later 155bhp at Enginuity down in London. I know which one sounds better, but were either of them right?

 

Neil

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I can add that my TR6 has been on the Cambridge motorsport RR with Chris Connelly tuning it, then later on, in similar conditions it was at Enginuity's RR and the results were almost identical. I have no connection to either place.

 

 

However, as said else where, best ignore the actual numbers, a RR tuning session can give great return for your £ in finding, fixing problems and knowing the car is nicely setup. Recommend it.

 

Mark.

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Back in the mid 1970's I entered my absoloutly, bog-standard '69 TR6 Pi CP car in a Drag Racing Series at Blackbushe (as my Racing Car had a problem and I didn't want to scratch my entry) I clocked a genuine 15.37 Standing Start Quarter mile at 84mph at the finish line. (I still have the BARC timing ticket framed with a picture of the car leaving the line trailing tyre smoke off one rear wheel)

 

before the event we clocked my TR on a Standing Start 0-60mph at 7.5 seconds.

 

I'm not sure what a modern BMW Z4 can do the SS Qtr mile in...perhaps somebody here can enlighten us?

 

Considering that the TR6 was just a cheap car (1300 quid) on an old fashioned ladder chassis of a former model, designed as a mere export dollar earner, I think it is an unfair comparison to a modern German engineered BMW...but really...it's not that far off is it!

Edited by Denis

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I'm probably being very stupid here but here goes...

 

OK so a BMW Z4 has a 2.0l, 4 cylinder engine producing 181bhp. It goes from 0-62 in 7.2 seconds and weighs 1750KG.[/size]

 

The TR6 has a 2.5l engine, 6 cylinders producing (well with the modifications i've had done) mine apparently should be about the same, 180 ish bhp. The original (150 bhp) spec had it rated at 0-60 in 8.2 seconds, and it weighs far less at 1085kg... With a bigger engine and far lighter, why is the performance not way higher?? I know its a modern engine etc but still!

Chris

I've just goggled the 2.01 BMW Z4 ...2005-2008 model...it has a kerb weight of 1295kg and a 0-62 in 8.2.

 

Also, I guess it had an OHC motor, which must be stronger than a vintage long stroke OHV...and I guess the BMW has better gearing and more a sophisticated drive train!

 

My 'TR Companion Book' shows the Gross Vehicle weight of a pre '71 TR6 as 1308kg and a 1971 onwards as 1360kg.

Edited by Denis

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Denis,

 

if comparing weights, the kerb weight of the Z4 at 1295kg needs comparing with the same thing for the TR6, which is 1092 kg (depending on the year).

 

the gross weight is another figure.

 

Bob

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For what its worth, after my tr6 rebuild I took it to be weighed at the local council weigh bridge, came in at 1108kg, full fuel tank, including spare wheel, tools etc. ( but no one in the car) 69 UK car..

 

However, it is possible, that like myself, it may have put on a few pounds since then...... :)

 

Mark.

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AH I was basing my figures on an online (autotraders I think) ad for a Z4, the only reason the comparison came to mind is that I was also reading the new classic cars mag at the time with all TR's tested article in it where they spec the TR6 weight as 1085kg. I've never had mine tested on a rolling road, the 180bhp figure came from the guys who did the original work for it in the early 80's when my uncle bought it, i've since had the engine rebuilt last year and replaced the components like for like with the non standard ones, as many of you say it's probably way off the 180 mark, not that it matters really, I just (finally) got the ignition timing right and the run I had in it yesterday was fantastic :)

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Handling and brakes are far more important

 

Indeed they are, shall we start a 'how big are your brakes' thread ;-)

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I've just goggled the 2.01 BMW Z4 ...2005-2008 model...it has a kerb weight of 1295kg and a 0-62 in 8.2.

 

Also, I guess it had an OHC motor, which must be stronger than a vintage long stroke OHV...and I guess the BMW has better gearing and more a sophisticated drive train!

 

My 'TR Companion Book' shows the Gross Vehicle weight of a pre '71 TR6 as 1308kg and a 1971 onwards as 1360kg.

 

Well, yes...... But the main problem with a Z4 is that it's a BMW, and by definition, looks like some variant of an upside down suet pudding dish. I mean, you only have to look at a BMW Mini.., once, twice.., and you suddenly realise that in fact , every BM looks like that . ..... fat and dumpy.

 

Quite unlike a TR6 or especially a TR5..... drop dead gorgeous. :)

Edited by amclpreston

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Well, yes...... But the main problem with a Z4 is that it's a BMW, and by definition, looks like some variant of an upside down suet pudding dish. I mean, you only have to look at a BMW Mini.., once, twice.., and you suddenly realise that in fact , every BM looks like that . ..... fat and dumpy.

 

Quite unlike a TR6 or especially a TR5..... drop dead gorgeous. :)

Should have gone to spec savers

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I also think (as sort of mentioned earlier) that delivery of power is important.

Had a lotus élan and was great if you wanted your power delivered in a frenzy at >5000 rpm. Fun for the first 30 mins but tiring.

Then there's turbo charged engines, that torque whoomp when the turbo comes in is addictive but knock it off turbo and where did all the power go?

Since I had the TR I have come to love flat torque curves. OD 4th tootling through town at 30 mph and car still pulls when you put your foot down. Dropping from OD 4th to 4th at 60 and overtaking on an A road is sorted. Cruising at high speed on the motorway is low stress to me and the engine.

Wonderful.

All this whether it has 120 or 160 bhp.

Tim

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Hit the nail on the head Tim,

 

it's all about torque delivery, not the numbers, but how it drives.

 

Some folks understand that instinctively, others never will. Some stick with TRs, others don't.

 

Cheers

 

Alec

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Indeed

 

Yesterday i drove my '6 up to the Heritage Motor Museum for a meeting, 185 mile round trip in the sunshine.

 

When I neared home i realised that i had just driven 90+ miles in a 39 year old car having used only 4th gear !

I did come in and out of Overdrive a few times but with light motorway traffic and some A roads there was no need for any more gears.

 

the smooth pull of the Pi 6 when its working right is fantastic :-)

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