Explain wobbly engine torque curve please

[hot66]

for reference, here is one of my plots

[mycar]

I see BobWatsonEngineering has registered recently, maybe James, or Bob can jump in here and offer an explanation,

Mike.

[oldtimer]

Went back to my plots..

there are 3 separate peaks in the torque curve , first at 3750 rpm , second at 4500 rpm , last and highest at 6000 rpm

The engine spec is standard MFI 2.4S with 2.4S cams but 2.2S pistons runing in 2.4S barrels which raises the compression ratio a bit compared to standard. Some gas flowing has been done , and an RSR lightweight flywheel fitted
So yes , some gain in power over standard to be expected. Had put BPs 97 octane fuel in on way to Bobs . Maybe can go to Tesco 99 for a bit more BHP

Oh yes , as Bobs dyno is common factor , I also noted he changed gear from first to second then to third and no higher giving max speed 154km/h on printout.

[butzsi]

None of these graphs show ignition timing and CO% which will have an effect,
also on MFI cars 3000rpm is the transition between idle and main racks. Choice of exhaust system will also effect torque and power curve at specific rpm ranges. Lots of other factors to consider ..

Chris

[chris68]

Really good thread! Very interesting that so many people's cars display the same torque profiles.

I agree with Butzi. Would like to see ignition advance and CO readings over the rev range.

Also, it would be nice to eliminate rolling road from the equation (although Bob's dynomometer is no doubt subject to regular calibration).

Chris

[Robster]

Hi,

my 914/6 runs 2.4E engine with 2.4S barrels and pistons / weber carbs/ 2.4E cams and was dyno'ed on Bob's Dyno (well actually he rebuilt the engine). My power/torque curves were close to textbook smooth with none of the dip exhibited on some of the plots so maybe it's the S spec cam?

regards
Rob

[Mike]

Mentioned the 'wobbly' torque curve to Neil Bainbridge as we worked on commissioning his new engine dyno last night, which should be ready in the next couple of weeks.

He says the wobble as seen on graphs from tests on a rolling road is due to the 'Torque-Time Reaction'. This is the effect the variables in the drive train have on the torque
curve during the power run, things such as the tyres changing shape at different speeds on the rollers, movement in the drive shafts and joints, gearbox wind up and the
action of a spring loaded centre plate clutch, all have the effect of disrupting the curve when trying to measure it in this way.

This is why the engine dyno as opposed to the rolling road is a far better way of analysing your engine, as those variables are taking away and the amount of information
you can get is almost limitless, for instance on a rolling road you are lucky if you are actually measuring 3 or 4 different parameters, on the engine dyno there are over a
hundred different sensor channels that can be set up to measure a myriad of information, all to show exactly whats going on and to help you set the engine up correctly.

Will post a picture shortly of an engine connected to the dyno, looks like it's on life support!

[Ferry Man]

This is why the engine dyno as opposed to the rolling road is a far better way of analysing your engine

Sorry Mike, but my engine is in my car. That's why a rolling road is far better....

[Mike]

but my engine is in my car.

but the beauty of the 911 means it can be out in 22 minutes!

[impmad2000]

but my engine is in my car.

but the beauty of the 911 means it can be out in 22 minutes!

Hmm, still wandering whether this is what I should just do with my EFI setup !! I'd need to knock up a second
I guess the rate of change of power the S Cam gives as it starts to come on-cam is going to pull out all these mechanical peculiarities.
Cool stuff
Tim

[hot66]

I would much rather have my engine on a dyno than a RR .... my last VW motor we sent upto John Maher Racing for a full dyno session to get the DTA set up

Playing with an engine dyno was something we did as part of my engineering degree ..... but due to the way degrees are designed we must have spend at least 3 hours out of a whole 4 year course covering that side of things ( can't even remember what we did actually ... think it was more a study of fuel flow / efficieny on a ford 4-pot ! )


Anyway, what with my obbsession with having a nice running motor, I'm sure at some point I'll be paying Neil a visit

[BobWatsonEngineering]

This thread looks very interesting...

Bob is very busy at the moment so he will be setting aside some time on Saturday to write a full explanation

many thanks

James

[BobWatsonEngineering]

Sorry for the delay in response

Here is an explanation as to why your car dyno traces are wobbly

You are right to say books on ‘engine theory’ show a smooth idealised torque curves and the reason for this is the results were completed on an engine dyno. An engine dyno power pull takes approx 10-15 seconds whereas a car dyno power pull will take aprox 30-45 seconds depending on the vehicle. The wobbly torque curve follows the pattern of the power curve and is something you would not notice in performance while driving on the street. If the power curve at 4000 rpm went into a dive and then came out at 5000 rpm you would then notice that situation as the engine would be holding back. The main reason for the wobbly torque curve is primarily due to the S type camshafts and also the drag implications involved with car dyno testing. Although the traces are wobbly using a car dyno the final end results are normally within 2% of an engine dyno.

The 2.4S camshaft is very similar the 2.8 and 3.0RSR having high end power however the 2.4E camshaft has more mid range performance with a reduction of BHP at the top end. I have experienced some 2.7 engines running MFI with E cams and SSI headers with a power and torque curve totally linier with BHP figures 212-217 which is the same as a 2.7 RS spec engine.

Just to put all of your minds at rest regarding intake air leaks, wobbly dizzys, injection pump timing etc etc. the way it works is

There is absolutely no point in just putting your cars on the rolling road and giving it a power run without knowing exactly what the fuel and ignition calibration is.

Before the rollers actually turn a complete analysis of the engine is done.
a. Dizzy condition and dwell timing
b. Injection pump timing
c. Correlation between throttle bodies and injection pump (85% of the time this needs adjusting)
d. Fuel pressure
e.Throttle balancing
f. A simple test is carried out using brake cleaner to check for air leaks on the intake system

It is interesting point out that with the CO machine connected quite a few cars run fairly high HC’s (Hydrocarbons) between 1700 – 1800 ppm. When the basic adjustments have been made and corrected the HC’s come down to between 250 – 400 ppm which makes the engine much more efficient saving you fuel costs.

Only after all of the above have been carried out the car is run in order to calibrate the fuel mixtures at various RPM’s normally 50% and 100% throttle angles. Adjustments are made at this time only to the main rack, when satisfied with the fuel mixtures a power run is carried out to check overall fuel mixture throughout the rev range and also a power figure. If not totally satisfied with the overall fuel mixture adjustments to the main rack are carried out in order to optimise the fuelling. On completion low speed checks are carried out 1500-3000 RPM for smooth engine running on light throttles, this is where there is a trade off between the idle rack and the main rack however once the main rack is set correctly adjustments to the idle rack clean up the low rev range.

One point to mention is a lot of people talk about curing the engine popping and back firing on overrun whether it be MFI or carbs. I have found on numerous occasions that it is not micro switches, shut off valves or in the case of carbs blocked idle jets; quite simply it has been a leaking exhaust header or silencer gasket.

To conclude it is the S camshaft design and car drag which causes the wobbly curve and as for the smooth torque curves in textbooks these were completed on engine dynos and not a rolling road.

If you would like to know any other information please do drop us a message on here or at bobwatsonengineering@live.co.uk

Many thanks

Bob

[Ferry Man]

Thanks Bob, very informative.

I thought the rest of you would like to see a poster that Bob has on his bedroom wall. Dyno guys dream of this...

Veyron. BHP topping at over 1,000 and look at that flat torque curve from practically tick over.

[left4dead]

I thought the rest of you would like to see a poster that Bob has on his bedroom wall.

Well, I, for one, would never have go to see that so thanks for sharing Paul

Never realised the two of you were so close