Lotus Elan

Rear wheel BHP

PostPost by: rocket » Mon Jun 22, 2009 11:50 am

I had my plus 2 130/5 on a dyno last weekend,out or curiosity having recently come across the rebuild details of the p/o.He had recorded a reading of 117BHP.My dyno run last week produced 114BHP,engine has done 18000 since rebuild so im pretty pleased with the figures.Anyway my reason for this post is to ask if anyone has dyno readings from there twinks for comparison?
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PostPost by: elansprint » Mon Jun 22, 2009 12:51 pm

that difference could well be down to the difference in dyno's that seems like good rear wheel bhp presumably you have a tweaked engine?
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PostPost by: rgh0 » Mon Jun 22, 2009 1:16 pm

Is the measurement you are quoting rear wheel horsepower or some "adjusted" number that attempts to back calculate engine output from rear wheel measurement? A standard 1558 sprint spec twincam typically gets between 90 and 100 hp at the rear wheels. Most dynos would be lucky to be 5% accurate at the rear wheels and large variations also occur just with tyre type and pressure plus temperature and atmospheric pressure in the test cell on the day.

A tuned 150hp engine will show around 125hp at the rear wheels. A full race 170hp engine will show around 140 hp at the rear wheels.

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PostPost by: rocket » Mon Jun 22, 2009 1:30 pm

Engine did have some hot cams fitted at rebuild,work was carried out by QED but all it mentions on invoice is fitted customer supplied cams..


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PostPost by: neilsjuke » Mon Jun 22, 2009 2:56 pm

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PostPost by: jimj » Mon Jun 22, 2009 3:10 pm

Dennis Welch, now Jeremy Welch, rebuilt and upgraded a pal`s Healey engine last year. He uses a dyno to optimise the tuning but refuses to divulge dyno figures. He maintains that one day on one dyno is not likely to be accurately replicated elsewhere at a different time. He feels that unscrupulous tuners may exaggerate figures, (Colin Chapman for one?) and all that matters is that your particular engine is performing at it`s best.
Incidentally, I was priviledged to be at the Great Jim Jackson`s 60th birthday do last Saturday. Rocktastic. What a guy. I understand that Pete elansprint71, President of CLOG, was invited but couldn`t make it and Gareth theelanman didn`t even reply !
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PostPost by: reb53 » Wed Jun 24, 2009 10:10 am

I've always read dyno figures here, and other places, and wondered how come I got the Friday afternoon engine as I was sure mine had nothing like the figures quoted when I had it done years ago.

I just found the old test sheet, dated 1976. So the car was 5 years old and had 32000 miles on it.
Have to bear in mind that this was a small town garage and, being 33 years ago, the dyno probably wasn't state of the art.
At 73 mph in third, or with my diff, 5822 rpm, it had 87 hp at the wheels.
What the significance of 73 mph is I don't know but it is actually printed permanently on the test sheet.
It was the first, and only, time the guy had done a Sprint and he was both surprised, and impressed, as there was more to come but he didn't want to push it as he wasn't looking for an ultimate figure, just a good improvement after a tune-up.
He used to do the local rally boys TC Escorts and didn't see a lot over a 100 for those.

So, after Rohan's reassurance, maybe all my horses haven't bolted after all.

Ralph.
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PostPost by: Coupe » Wed Jun 24, 2009 10:59 am

135.6 bhp at the wheels.
Not sure when tested as information supplied with car when purchased late 2007.
1825 not 1558 cc
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PostPost by: rgh0 » Thu Jun 25, 2009 9:00 am

I just found the old test sheet, dated 1976. So the car was 5 years old and had 32000 miles on it.
Have to bear in mind that this was a small town garage and, being 33 years ago, the dyno probably wasn't state of the art.
At 73 mph in third, or with my diff, 5822 rpm, it had 87 hp at the wheels.
Ralph.[/quote]

My data says the mid-range of expected rear wheel measured power of a sprint spec 1558 is 88hp at 5800 rpm. So your 87 is spot on.


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PostPost by: pamitchell » Thu Jun 25, 2009 5:03 pm

What would be a good power loss factor for a standard Elan drivetrain in calculating flywheel BHP vs. the chassis dyno number?
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PostPost by: neilsjuke » Thu Jun 25, 2009 5:27 pm

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PostPost by: msd1107 » Thu Jun 25, 2009 6:37 pm

Phil,

15% is a figure that is used often.

However, there are many factors that influence this, so you could go several percentage points higher or lower.

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PostPost by: rgh0 » Fri Jun 26, 2009 8:26 am

I use 4.0kw to 4.4kw per 1000 rpm in third gear as typical . Most dyno rolling road tester will do it in third gear to avoid excessive roller speeds in top gear. In third gear gear box losses are substantially higher but wheel tyre loss are lower. The end result of doing it in either gear is similar in rear wheel output terms.

I think this models the actual loss better as most losses in the system are frictional in nature and vary with drive train speed to a large degree. Torque being transmitted is the other major determinant and this is relatively constant over the range of interest in the dyno test enabling a fixed number per 1000 rpm. There are also the losses in the dyno rollers themselves that contribute to this number so it is not true power at rear wheels but power at the dyno torque or power sensor you are measuring

In a rolling road test you see the power peak at the rear wheels at lower rpm than it does at the engine. This due to the drive train losses at the top end of the power curve rising faster than the engine power itself so as revs rise rear wheel power falls even as engine power continues to rise. Raises interesting questions about what is the best shift point on the track


An alternate model is a fixed percentage of hp and i have seen numbers from 25% to 15% quoted or a fixed amount of say 10hp plus 10% I have also seen used for 4 cylinder engines in the 120 hp range

They are all approximations and when combined with the variability in dyno calibration in the first place lead to a huge range of derived engine power numbers from rolling road data.

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Rohan
Last edited by rgh0 on Sat Jun 27, 2009 2:07 am, edited 1 time in total.
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PostPost by: denicholls2 » Fri Jun 26, 2009 4:20 pm

As always, Rohan, an excellent summary. I learned a lot from a few words about interpreting my own results, an S2 Europa dynoed at 88 RWHP, so between 101 and 110 FWHP (also third gear), which is about right for a healthy Hermes. (Some argue that with one less driveshaft, the Europa loses less in the translation and might instead spec me as low as 96.5 FWHP).

I find it intriguing that people always look to map RWHP to FWHP, probably because the manufacturer specs cars to FWHP, FWHP is always a bigger number, and so it's become the standard talking reference. But if you stop to think about it, you're measuring how much power your engine delivers to the task of accelerating the car to which it is attached. For this purpose, RWHP is the best measurement and a more logical standard, so why bother converting it?

And to stir the worms a bit more, torque (and the RPM at which it is available) is arguably a better indicator of the car's ability to move than either horsepower number...
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PostPost by: jimj » Fri Jun 26, 2009 4:54 pm

Yet when Autocar magazine does back to back tests, often a lighter car with more bhps and more lb/fts, even when produced at lower revs with a flatter torque curve, can be slower accelerating throughout the speed range. It can`t be gearing so it must just be magic.
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