Dyno run on my old +2

Hi guys,
Treated myself to a run on a dyno to tune the +2 for Xmas…
Figures I got back were:
118 ftlbs of torque at 4500 rpm Although its above 100 from 2500-5600
118 BHP calculated at the fly wheel at 5500 rpm, 80 BHP at the wheels…
The engine is a big valve, with the only documented rebuild being the head 5 years ago…
Everything is standard.
BHP seems a tiny bit low, what does everyone think?
The tuning itself did me proud, the carbs we’re perfectly in balance and only one of the two carbs was off…
Managed to get the CO to between 4.8 and 5.3 for the whole rev range with the hydrocarbons below 400 ppm (which the dyno guy was very impressed with for an “old engine”!)
Does this sound any good
Only thing he didn’t mange was a flat spot I have when I floor the throttle at anything below 1500 rpm…
Anyone any ideas ?
Cheers
Tim

Tim, 80 bhp at the wheels seems low, but considering a 20% drivetrain loss, how did you come up with 118 at the flywheel?

  • believe they measure the wheel-power, then measure the drag when the transmission breaks the drum, and then calculate the engine power…Why 20% ??

Dag

Dag, are you referring to measurements taken on an engine dyno? I know there are various ‘rolling road’ dynos out there but aren’t all numbers taken a the wheels, since that’s all it knows, and other numbers extrapolated from there? 20-30% drivetrain loss seems to be the rule of thumb so if 80 is at the rear wheels, 118 seems a bit high ‘at the flywheel’.

But I guess to answer Tim’s question, I don’t know why it’s low. Compression? Timing?

The loss of power could be down to poor gas flow. When I rebuilt the head on my S4 the inlet valves had a large coke build up on the back. This restriction would certainly claim several bhp. Another area is cam timing, as most heads have been skimmed by now and the original sprokets were not the most accurate the cam timing is likely to be a few degrees out. Adjustable cam sprokets and a dial gauge are a good investment.

Rolling roads are pretty useless for getting absolute power readings. Very good for backing up any tuning you do by taking before and after readings for “at the wheel” figures. There is a new type of chasis dyno that bolts to the wheel hubs in place of the wheels and thus gets rid of a large set of variance from tyre slip, pressures etc.

I am sure we all be very interested to see the plots. Any chance you could scan them and post them up?

If the air corrector jets are not too BIG the engine will respond by pulling briskly from an idle with a wide open throttle which is applied in about 1/10 of a second. The stock specified jets are a certain recipe for a flatspot. They didn’t understand the flow dynamics. Neither did Weber!

Please don’t call bullshit on me because I can demonstrate this by removing any flatspot in seconds by rejetting so the air corrector does not flood the well with air and prevent the fuel to start flowing at low vacuum signals. At low rpms the vacuum signal grows in strength as the air velocity increases up to a whopping 3mm of water (1/8"). At 6000 rpms it can only suck water up a soda straw 3mm. Since gasoline is slightly less dense than water it is lifted by about 4mm. Little wonder putting a HUGE air corrector into the side of the soda straw kills the fuel flow at low rpms and IS the cause of the dreaded flatspot. The air corrector size is the crucial factor.

You can find out the size of the flatspot is by doing my WOT test. Slow the engine down while in 4th gear to a target rpm say 1300. Go to WOT and hold it there. After the accelerator pump shot is burned up if the engine continues to run the main circuit is flowing okay. If it dies like the igntion was switched off, it’s not. The main circuit should start to contribute fuel by 1300 rpms if you want the engine to run flawlessly.

The light bulb has come on for me finally and I’m understanding this stuff well enough to realize the emulsion tube is not being exploited to it’s fullest possibly. :laughing:

Tim, what size are your air correctors?

Merry Christmas everybody and may your Lotus dreams come true and your Webers sing without a hiccup!

The claim can be made but not believed by some like myself. That’s dazzle the customer bullshit. It sounds like a smog machine which is only accurate if the rpms are held steady for several seconds. The time response of those instruments is normally very slow and therefore they dampen a lot and you only get the mean value. I would not go to any tuner that is not invested in the lastest wideband ultra-fast O2 sensor technology stuff. This stuff is cheap enough so you can buy the instrument and DIY for the cost of several hours on the dyno.

Tim, please don’t be insulted or offended by what I’m telling you, it’s the truth.

p.s. I’ve come to the conclusion that Lotus must have felt having a bit of a flatspot on the Webers was the norm. There isn’t any other plausible explanation for their choice of air correctors. :open_mouth:

Hi There

I think that 118bhp at the flywheel sounds quite good. I am sure that what Keith says about the air correctors is true (must check mine), but I found that 30mm chokes in the carbs also improves the driveability at low rpm.

Best of luck and happy holidays!

Berni

Tim

I would have expected 90 to 95 hp at the dyno for a Weber head Big Valve in good standard condition on a rolling road power generation (non flywheel) style dyno that I have tested my cars on. Why less than that could be due to a huge range of issues.

  1. Internal engine problems - low compression, carbon fouling, valve timing, ignition timing, eg.

  2. Abnormal high losses on the dyno - low tyre pressures.

  3. Bad dyno calibration.

  4. Atmospheric conditions in the dyno room - pressure, temperature humidity

I would not put to much emphasis on the estimated engine HP at 118 hp as a 38hp calculated loss through the drive train and dyno to 80hp at the dyno sounds high to me.

Have a look at this web site for more info:

pumaracing.co.uk/simulate.htm

Rohan

  • I was referring to a rolling road. I have done them a couple of times, and been told/ seen that when they have measured the maximum power at the wheels , they slip the car into neutral, and then measure the drag in the transmission as it breaks the rolling drum down. If you compare that to a free spinning drum, I guess you can "easily " estimate the drag/ loss in your transmission. It will still be a “calculation/ guess” , but gives you a good clue according to what they claim…

Dag

Rolling road measurements are only really useful as a relative measurement, IMHO. I’ve had measurements of 166 and 147 BHP from the same car in the same state of tune from two different roads (an Elise). So they can vary by that much.

Some more rolling road knowledge here (and a pic of a Plus 2).
emeraldm3d.com/em_r_road.html

Niall

Hi Guys,
Thanks for the large number of comments…
Few added bits of info…
engine is a big valve with twin dells
Ignition is an Aldon ignitor
testing done on rolling road, not engine dyno.
I agree with what you all say about accuracy of absolute figures on the dyno, which is why I am not unduly worried.
Certainly during the day the car gained 10 bhp, and the torque levels stayed higher at lower rpm. The car also started on the dot this morning for the first time ever so worth it to me.
We tried a range of idle jets to try and remove the flat spot with no success.
I have previously also tried a different idle aircorrector with no improvement.
Keith tell do you think your fix would work with Dells.
The operator worked out the transmission losses using the dyno.
One worry I had was that the power seems to start dropping over 5500 rpm. Is this usual?
Cheers
Tim

Hi Keith

My air corrector jets are 155’s. What size do you think I should have? Also would changing the choke size, from 33mm to 30mm throw all of the main jet/air settings out?

Many thanks

Berni

Yes, Tim with almost 100% certainty. You guys were fiddling with the wrong jets to remove the flatspot.

I would love the opportunity to interact with a tuner before they screw with your carbies. With my insights they should be able to correct the normal problems in a record short period of time.

I’ll describe a little more complete picture of the flow behavior of the well. The mainjet has two main forces applied to it and the others I’ll ignore, gravity and vacuum. The gravity is cancelled by the small gap between the OD of the emulsion tube and the well ID. The surface tension of the gasoline holds the fuel up to the brim of the emulsion tube which is about 2mm below the passageway leading down to the auxiliary venturi. Once the fuel is sucked up to the brim it does not fall back down and does not get affected by the changing fuel level in the float bowl. This is good because it also makes less prone to be affected by high cornering forces or vibrations.

The vacuum is the dominant force to control the flowrate through the mainjet. It’s achieving the highest pressure differential that’s the key and getting only bubbly two-phase flow. Install too big an air corrector and along with delaying the flow from the main circuit it also reduces the highest level of vacuum which can be generated. This reduces the ability of the mainjet to respond to the changing vacuum and deliver a linear slope of fuel versus rpms through it’s orifice. There’s more to it than this but I will not go there now. :wink:

You can go too small with the air corrector also. When it stalls out while cornering hard on left-hand turns, it’s too small. You can only do this test by actually driving the car and not on the dyno.

Berni,
You’ll have to answer your own questions by doing the tuning. Do the WOT test first. It takes just a few seconds to do it and it can be done on your neighborhood streets without upsetting or alarming anyone. The test requires you to slow it down to about the 15-20 mph range. Tell me where the main circuit starts to work in rpms and I’ll advice you on what to do next.

A 155 air corrector might be okay for your engine. The main choke size controls the amount of vacuum signal the auxiliary venturi can generate. Bigger means less vacuum. If you don’t have a flatspot and the mixture is ideal over the whole rpm range for WOT and cruise then the choke is sized okay. On the otherhand if no matter what size the air corrector is you can’t remove the flatspot then the choke is too big. It really is that simple. :smiley:

Keep doing the WOT test to find the right choke and air corrector combination. I can tell you right now that a 30mm choke is as big as you should go for a stock engine. I borrowed some 33mm chokes from Mike Ostrov and tried them. I got a huge flatspot as a result. I’ve got 30mm chokes with about 120 air correctors installed in my 40 DCOEs right now.

BTW, when I say it’s a flatspot what that manifests itself is a surging feeling while at a steady state partial throttle cruise. Damn annoying is what it is! :angry: When it’s really bad the car will shudder violently to the point is can’t be driven in that gear, at that speed.

Tim

The losses on the rolling road dyno go up roughly linearly with transmission and wheel speed. As you reach the maximum point in the power curve the losses are actually increasing faster than the power output of the engine is increasing. Thus the rear wheel measured power output peaks slightly earlier than the actual engine flywheel power.

After multiple dyno runs on multiple engines I have estimated the loss for my Elan and +2 at around 4 kw per 1000 rpm. This will vary somewhat depending on the types of tyres you run, oil used in the transmission, etc.

I usually see about a 500 rpm difference between the actaul engine peak power and the rolling road rear wheel peak power. The peak power in a big valve sprint twin cam is between 6000 and 6500 rpm so for the rolling road dyno to show it at 5500 rpm is not unexpected.

regards
Rohan

When I say flat spot I mean that it if the car is running between 1.5K and 2.5K RPM and I push hard on the gas, the car hesitates and then takes off…
Keith, is this what you mean?
Cheers
Tim

That’s a bog. Look at the accelerator pumps in that case. A bog that lasts about a second can be from going too fat or lean and/or the duration is not matched for your engine. You’ll have to figure out which. With a fast wideband O2 sensor the shot AFR can be easily seen and the fault condition corrected with the least amount of fuss and bother. Your tuner guy should have done this before your car was put on the dyno.

Just checked my pumpjets on the Dels and they are 45, which is correct according to elan books…
However the books on dell ortos that I have suggest 35-40, so I wonder whether it is bogging due to running rich.
May be time to buy some new jets.
Cheers
Tim
(I agree with you about the tuner, I did mention that I thought that perhaps the pump jets were at fault, but I bet he didn’t have any…! Sad really, he was recomended by some single seater hill climbers that hold records at some of the hills 'round here)

Tim,
Without the right diagnostic tool you’re just guessing what to do. It’s a hopeless situation really which will soon be frustrating and expensive. If this stuff were easy any tuner would be able to rectify all these faults pronto. Most don’t have a clue.

I predict in the next couple of years the folks that posted technical articles on how to do this or that will be horrfied to find they didn’t know diddily squat afterall. There is huge surge of understanding of how a carbie really works with the recent introduction of the ultra-fast wideband O2 sensor technology. Even the carburetor manufacturers didn’t understand it completely. They just trial and errored it until they got it to work. The real research to understand it is done later in academia.