rolling road figures

Talk about the E-Type Series 1

Tom W
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#61 Re: rolling road figures

Post by Tom W » Sat May 30, 2020 12:08 pm

I’ve just bought myself one of those road dyno boxes. I’ve recorded a few runs now. It seems more appropriate to document my findings here, than in the road dyno thread.

Apart from one, where I recorded 1080 lb/ft! (Guess that’s wheelspin), I’m getting between 275 lb/ft and 290 lb/ft, and 220 and 225hp. The torque graph is pretty flat between 2250rpm to 4000rpm, where it drops off quite steeply.

I think the absolute numbers for the road dyno should be taken with some caution, though it is similar to the figures I got at the rolling road. Changing parameters like weight, drivetrain loss, frontal area etc make more of a difference to the figures than you’d expect to make from basic tuning. I can’t find a definitive answer for the values on the web, though I haven’t searched very hard yet.

However, it looks like it’s going to be very useful for seeing the effects of modifications. It should mean the car’s well set up when it goes to the rolling road. The rolling road is more useful though, as you can plot other parameters, such as emissions, against the power curve. This gives a much better idea of what change might be required to to fix holes in the power curve.

For those interested, my car has 45DCOEs, a Bell large bore stainless exhaust and tubular manifolds, and 123 Jag distributor.

When I’ve fixed the leaky pump jets on the Webers, I’ll try another run.

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Tom
1970 S2 FHC

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Heuer
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#62 Re: rolling road figures

Post by Heuer » Sat May 30, 2020 1:13 pm

Tom

I adjusted the drag, frontal area and loss figures (I ended up with 0.55, 17.5 and 0.18) until I got the Road-Dyno figures to closely match the full dyno. Once you have done that you can run as many tests as you want and compare to the base figures. Stood me in good stead and works to compare changes I make.
David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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Tom W
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#63 Re: rolling road figures

Post by Tom W » Sat May 30, 2020 1:29 pm

Thanks. Are those for the FHC? What did you have for weight? I could only find dry weight in the manual. What’s the weight of a FHC with 1/4 tank of fuel? I’ll re-run the data with your figures and see what differences it makes.
Tom
1970 S2 FHC

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#64 Re: rolling road figures

Post by Heuer » Sat May 30, 2020 3:32 pm

No those figures are for my OTS. The FHC has a better drag coefficient. I used 3180lbs as the weight:

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As I said, tweak the variables until you get a good match with your rolling road figures and then you can use it as your base line. From now on you will only be looking for differences from base. Not 100% empirical but pretty damn close.
David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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Tom W
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#65 Re: rolling road figures

Post by Tom W » Sat May 30, 2020 7:17 pm

Using David’s figures for losses, I get some very different results. Drag coefficient should be a bit lower for the FHC, but it’s still an interesting comparison.


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This suggests the rolling road that I had my car tested on reads lower than the one David used to calibrate his road dyno. I think it’s to be expected that rolling roads don’t all read the same.

I can play around with the figures to try and get something to match the rolling road runs I had done, but I’m not sure how much value there is in that. I’ve tuned the car between the rolling road run and buying the road dyno, so it’s slightly chasing a moving target. Once I’ve settled on the figures for the road dyno, as David says, it is most useful for checking how much improvement tuning or a particular mod makes.

I will continue to do some more experiments, and properly check the weather forecast too. The temperature and pressure corrections make a difference to the values, so guessing here introduces some errors too.
Tom
1970 S2 FHC

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#66 Re: rolling road figures

Post by Heuer » Sun May 31, 2020 10:25 am

Don't try and compare your dyno results with mine as my engine was built for high torque - so 340lb/ft and 280bhp. It has different cams, 9.5:1 compression and Edis/Megajolt ignition.
David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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Tom W
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#67 Re: rolling road figures

Post by Tom W » Sun May 31, 2020 11:46 am

Surely that’s half the point? As well as doing back to back tests on the same car, we can get a comparison between different people’s cars and specifications, using essentially the same dyno.

I’m not trying to claim I get the same power and torque figures as yours, David, but if I use the same correction figures as you, I get much higher numbers than my rolling road session showed my car made. I doubt my car is any lighter, or the transmission any less parasitic, so apart from the difference in drag between the FHC and the DHC, the accuracy of the results should be reasonably comparable, assuming both road dynos record the same. And it does show that your spec produces more power ant torque.

I haven’t had my engine rebuilt to a spec with an expected power and torque output, so I’ve no idea what it’s supposed to make. As far as I know it’s standard internally.

It would be an interesting comparison to try my car on the same rolling road you used, or vice-versa.
Tom
1970 S2 FHC

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#68 Re: rolling road figures

Post by Heuer » Sun May 31, 2020 6:12 pm

I used Hi-Tech Motorport near Coventry.
David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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Fred
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#69 Re: rolling road figures

Post by Fred » Mon Jun 01, 2020 10:14 pm

Gents,
I tested my Series 1968 1.5 with the 2 Strombergs, when it was relatively new, on a chassis dynamometer at Texaco Research Center. We got 160bhp at the wheels at 5000 (or so, can't remember exactly) engine rpm, in 2nd gear (speed limit obviously so not in 3rd or 4th).
Fred

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#70 Re: rolling road figures

Post by Heuer » Tue Jun 02, 2020 10:41 am

Tom

This is the Hi-Tech graph of my car:
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David Jones
S1 OTS OSB; S1 FHC ODB
1997 Porsche 911 Guards Red

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Tom W
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#71 Re: rolling road figures

Post by Tom W » Tue Jun 02, 2020 11:49 am

Thank you David,

If I’m reading it correctly on my phone screen, that’s 250bhp, and what looks like 260 to 270lbft between 4000-4500 rpm. Your modifications have definitely enabled the engine to rev more. I don’t rev mine beyond 5000rpm anyway, but by then with my car the power is falling off rapidly, so there’s no benefit. Your peak torque is made higher in the rev range too.

This is the final graph I had from my dyno session at Project Heaven. The shape of the curve is similar to the road dyno graph, which is very encouraging, but I would trust the absolute numbers shown here as more accurate than what I’ve got from the road dyno so far. I’ve also fiddled with the car between here and taking the road dyno runs, so it might not be comparing like for like either.

They record torque in Nm, but doing the conversion, it works out as about 295lbft. Maximum power was a little over 216bhp. Apparently, these are figures at the crank.


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Fred, it’s interesting to see what the Stromberg equipped cars make, I don’t think many people have posted dyno runs for those.

Regards,

Tom
Tom
1970 S2 FHC

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Tom W
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#72 Re: rolling road figures

Post by Tom W » Fri Jun 05, 2020 3:56 pm

More interesting than playing top trumps with headline figures, and certainly more useful is to do some analysis on the dyno output.

You need raw dyno data that can be entered into excel or similar, so you can draw your own dyno graph. This is mine from the data on my rolling road print out.
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From there you can turn the torque curve into a series of graphs that show maximum tractive force available at the tyres in each gear.


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It can be seen from this, that on the E-type, that with with a large space between each gear, and a low rev limit, the maximum potential acceleration is always achieved in the lowest gear you can use for that speed. The dotted line shows the theoretical curve that joins the point on each curve where maximum power is made.

By viewing the data this way, we can see in where improvements in the torque curve will have most benefit to the maximum acceleration the car can achieve. In the case of my car, I’d benefit from smoothing the torque curve beyond the point of maximum torque, and improving the rev limit, to try and reduce the drop in tractive force experienced between gears. Anything that improves torque as a whole, and drags the whole curve up will be a benefit too. It’s apparent though, that any effort to only improve the torque curve before the point of peak torque will not benefit maximum potential acceleration, except for 1st gear, which is probably traction limited anyway.

This isn’t the whole picture though, as we don’t spend all our time driving around at wide open throttle, maximum revs and maximum acceleration. The shape of the curve at lower revs is very important to how the car feels on the road, more torque here will make the car accelerate more quickly for any given gear ratio. There’s a bit of work I could do here to smooth the curve slightly, but generally it’s very good. Decent torque from around 1750rpm all the way to 4000rpm, equates to the car pulling very well from 30 to over 70mph in 3rd. What can’t be viewed on the curves, but also makes a big difference to the feel of the car, is transient throttle response. This is essentially a measure of how quickly, on snapping the throttle open, the engine can get itself up to the torque curve.

The other useful thing that can be done by analysing the data this way is seeing what effect different gear ratios have.

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Here, I’ve overlayed the curves I would have were I still running the 3.54 diff in place of the 2.88 (dotted lines represent the 3.54). Interestingly, although the holes between adjacent gears have moved, the general trends follow the same curve. Where the 3.54 scores, is only in first, where the extra tractive effort is least useful. I suspect, though I’ve not tried it, that the 0-60 time will actually be better with the 2.88. The car will now do 60 In 2nd with the 2.88, so there’s now only one dip in the tractive effort curve to negotiate between 0-60, as well as the time saved from only having one gear change itself.

One final interesting point to note, is that my car will only make maximum power at 2 points within the legal speed limit, and maximum torque at 3 points (just).
Tom
1970 S2 FHC

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Craigwin1963
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#73 Re: rolling road figures

Post by Craigwin1963 » Thu Oct 22, 2020 12:28 pm

Took my 3.8 on Monday and very happy with the results after a fully rebuilt engine on +40 Mahle pistons, triple Webber’s, tubular manifolds,123 distributor, compression is at 205 on all cylinder so possible 9.0 to 1.


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