`68S torque curve.

[Mycar]

Preamble.

When I first got my car I found it to have the wrong distributor fitted (0231 159 006). With help from from forum members here, I realised I needed a 0231 159 002 and with a lot of hunting around, I finally found one. (it has a black label, not yellow one but let`s not go there.)

So then, I slotted the new distributor in place and ran it for a while. The difference was noticable especially at the top end but I had terrible trouble timing it so onto the rolling road the car went.



Here`s the first trace,



It was after this session the distributor was removed and was found to have incorrect springs. It was repaired and the curve checked.... all good now.

Second run, some weeks later....



Now to my question..... not before time, I hear you say.

The second graph shows a great hole in the torque delivery, (top, brown trace) What would cause this ? What would have such an effect on the torque between 3000 and 5000 revs ? Maybe this is how it should look... I know these 2.0S carburetor engines have all their power at the top end but even so, the published torque curves look a lot smoother than this.

Not to muddy the water too much, one concern I have is that my fuel pump isn`t up to the job but the same pump was used on both runs.

I would appreciate your thoughts, regards, Mike.

[sithot]

That's a helluva flat spot.

Tom

[Mycar]

Like a pancake.

Looking on the bright side though, I`m getting 172bhp.

Mike.

[edmayo]

Do you have the distributor advance curve graphs as they are printed in the shop manual? The 002 is the correct distributor for your 68S. It has a very quick advance curve, you need to see if yours matches the graph in the manual.

[Mycar]

Here`s the curve for anybody that`s interested,



And the timing details,



I`ve had two highly recommeded experts work on this distributor. The second found fault with the firsts work, and supposedly all is now good. I don`t know a third expert and haven`t got access to a distributor machine myself.

Regards, Mike.

[sithot]

I had a dyno sheet like the one in question once but it was on a Moto Guzzi 1100 Sport with big 40mm Dellortos.

We tried everthing and in the end it was the airbox causing the problem.
At the time Guzzi was attempting "ram air" on these bikes and it really didn't work with the Dellortos (or at least nobody could re-create what Guzzi "imagined" was happening). No wind tunnel.

The fix was found by accident during a dyno run. While making a "pull" the owner of the shop lifted the airbox lid. The curve immediately changed.
The airbox was removed and all subsequent charts showed no drop off.
That was a sweet running motorcycle after "the fix".

I ended up with a some K&N filters on the bike. There was a great "howl" between your knees when you wicked it up sort of like taking your father's Buick and flipping the air cleaner lid.

What are you running on this car; stock airbox or something for the "go fast" appearance?

Tom

[Mycar]

"go fast" appearance?
Tom

appearances are not always deceptive.

Like you say, I`m using one of those fancy K&N filters, it`s about 2000 miles old.

And I`ve had the Webers de silted, cleaned and coated my fuel tank and got new leads and plugs.

Mike.

[tfiv]

I dug out the engine dyno results I got from Rothsport when my 67S engine was rebuilt (its a little confusing because of the metric conversion and that yours are chassis dyno results). When I get home tonight I'll see if I can figure out how to scan and post the sheets.

There is definitely a belly in my torque curve. It starts falling off from 114 ft lbs at about 4400 and bottoms at 5000 and 97 ft lbs. By 5500 its back to 126 ft lbs and then stays pretty flat, peaking at 131.8 ft lbs and 6600 rpms. Peak hp was 168.9 at 6900.

This illistrates exacty why airport gear ratios make such a huge difference in these cars.

[tfiv]

appearances are not always deceptive.

Like you say, I`m using one of those fancy K&N filters, it`s about 2000 miles old

It's been well documented that the stock airbox makes torque.

[BURN-BROS]

The dip could be a number of things, but my guess is that the dizzy curve is too conservative @ that specific rpm.

A custom curve in which the secondary spring comes in sooner in the rpm should correct the problem.

The dip in the dyno results correspond directly in the lapse in advance from 3000 rpm to 5000 rpm.

It is a tricky area as you can easily get a detonation issue.

Lastly, it is difficult to curve the earlier dizzys and they suffer from premature wear. It is the main reason I modify a different dizzy that uses a big cap from the 911 SC. I can curve them the way I like and they can easily outlive the engine.