Power curve goes flat then peaks again

[Pentoman]

This is a wheel dyno graph for a bog standard cossie Merc 2.5 16v, can you tell me perhaps why the power curve rises fairly linear to 6000rpm, then goes flat, but has a last spurt at 6900 to 7200 where peak power is made; right at the limiter. Doesn't really seem right, surely it should peak a bit sooner and then drop off.

Factory claims are max power at 6200rpm which is where the graph looks like it should stop at but it sort of stretches out. Perhaps it's not a bad thing and gives you a wider rev range to play with, but I wonder if there is power missing just there that I should look for. The peaks are 10hp and 10 lbft short of factory claims which I've no qualms about.

Don't suppose the timing chain would have anything to do with it...?

[skid-mark]

doubt the timing chain has anything to do with it, it's prob down to the ecu controling the fuel etc if its running standard chip prob what the manufactures have programmed in as a pre caution, does the tourque drop off once you get past the flat spot.

[Pentoman]

Oh what an idiot, forgot the graph.
http://www.vr6oc.com/e107_files/public/1225642170_...
There it is!
Torque peaks at 4900 which is where it's claimed to. Just that weird top end.

[phumy]

Pentoman

Why is Psteam (Steam pressure) shown on the ambient conditions on your graph?

[Pentoman]

Erm, god knows. What does it mean is it bad? All the runs on that day have that on there, here's another for example http://www.vr6oc.com/e107_files/public/1225642170_...

The chip etc is standard. How would this characteristic be a precaution? What could I do to investigate it further?

Edited by Pentoman on Tuesday 3rd March 23:55

[phumy]

I have no idea why steam pressure is indicated on both of the graphs all i can imagine is that the dyno is from the Victorian era and is steam driven or that your car is steam driven

Actually 9.6hPa equates to 0.0096 bar pressure, in enginnering terms about the same as a fart building up when your stuck in a lift with a young lady and dare not let it peeeeep

Any idea how they calculate the drag power? Or is it a guessing game?

[Pentoman]

Drag power was done by letting the car 'run down' on the rollers to measure the drag, that's when the bottom curve was created.

[Jack_and_MLE]

On my car, totaly different one C21 with VVC, I had a little flat spot which was due to a restrictive exhaust

Could be the same for you?

Jack

[ringram]

Variable valve timing?
Also intake design can cause peaks at more than one point.
As well as exhaust design etc.

You would need to understand each component and its design goals to see what was responsible for the final peak it may be a combination of things.

[Pentoman]

No variable timing. The engine is a pretty traditional design - four big valves, central spark plug, basic chain driven cams, distributor but electronic ignition, and fuelling by a moving flap in the intake connected to the injectors. The intake runners are long (300mm?) and curve round to a large single throttle body. Tubular exhaust, 4-2-1 I think, with no catalyst.

Since it lacks fancy modern gizmos and it's just old-school breathing and mixing that gets the power, I figured the curve would/should be fairly straightforward.

I haven't seen another example on a same manufacterer of rolling road, but the other runs I have seen have been more naturally curved at the top.

The reason I mentioned the timing chain is one guy (from Sweden who builds turbo version of these engines) once said chain stretch can put the valve timing out and shift the power up the powerband. Likely?

Additionally my dyno operator implied that for example it could be a weak fuel pump leaning out the mixture at the top end (causing a slight power increase). I want to state he was just blindly speculating though and trying to move quickly through 30 cars in one day.

I was going to get a fuel pressure test done when I find someone who wants to do it and who I trust.

[phumy]

Pentoman said:

No variable timing. The engine is a pretty traditional design - four big valves, central spark plug, basic chain driven cams, distributor but electronic ignition, and fuelling by a moving flap in the intake connected to the injectors. The intake runners are long (300mm?) and curve round to a large single throttle body. Tubular exhaust, 4-2-1 I think, with no catalyst.

Since it lacks fancy modern gizmos and it's just old-school breathing and mixing that gets the power, I figured the curve would/should be fairly straightforward.

I haven't seen another example on a same manufacterer of rolling road, but the other runs I have seen have been more naturally curved at the top.

The reason I mentioned the timing chain is one guy (from Sweden who builds turbo version of these engines) once said chain stretch can put the valve timing out and shift the power up the powerband. Likely?

Additionally my dyno operator implied that for example it could be a weak fuel pump leaning out the mixture at the top end (causing a slight power increase). I want to state he was just blindly speculating though and trying to move quickly through 30 cars in one day.

I was going to get a fuel pressure test done when I find someone who wants to do it and who I trust.

If you have a 190 it will have a chain tensioner on it so i doubt a slack chain would put the valve gear out of time, unless of course, the slack is taken up only on on side of the "drive". Then it souls have an effect, however if it is that slack you would have heard it. I used to own a 190E many years ago and it had a round 100k miles on the clock and the chain was beginning to rattle, then had a new one fitted, so from that if its not rattling its probably ok

[skid-mark]

Pentoman said:

Erm, god knows. What does it mean is it bad? All the runs on that day have that on there, here's another for example http://www.vr6oc.com/e107_files/public/1225642170_...

The chip etc is standard. How would this characteristic be a precaution? What could I do to investigate it further?

Edited by Pentoman on Tuesday 3rd March 23:55

well manufactures tend to flatten the power curve out towards the end of the rev range like what you've got so that the engine does not have any more to give at 6500 rpm and would then req a gear change this is like a built in rev limiter/engine protector, you also have a flat spot between 2000 rpm and 3000rpm this is optimum crusing speed for getting the best economy, if you was able to get your car chipped or remapped they would make that power curve a full curve by altering those flat spots.
your car is probably leaning out at the end of the rev range where it peaks as the injection/air flow is giving all it can give but you would probably find if your car could rev more that power curve would drop right off as your engine is just not built for high end power.

unfortunatly i don't know what you can do to to get that power at the flat spots with that old school mechanical injection system you would have to see specialist might be down to just old engine or layout like exhaust design/cams/fueling etc/gas flow,
as for the others that were on the day depends on what management etc they're using later cars tend to have a more smoother power curve as they try to make the most out of the engine with all the emissions related equipment hampering on the power like egr valves and catalic converters etc and have much better fueling tecnology/valve and head design.

[Pentoman]

phumy said:

Pentoman said:

No variable timing. The engine is a pretty traditional design - four big valves, central spark plug, basic chain driven cams, distributor but electronic ignition, and fuelling by a moving flap in the intake connected to the injectors. The intake runners are long (300mm?) and curve round to a large single throttle body. Tubular exhaust, 4-2-1 I think, with no catalyst.

Since it lacks fancy modern gizmos and it's just old-school breathing and mixing that gets the power, I figured the curve would/should be fairly straightforward.

I haven't seen another example on a same manufacterer of rolling road, but the other runs I have seen have been more naturally curved at the top.

The reason I mentioned the timing chain is one guy (from Sweden who builds turbo version of these engines) once said chain stretch can put the valve timing out and shift the power up the powerband. Likely?

Additionally my dyno operator implied that for example it could be a weak fuel pump leaning out the mixture at the top end (causing a slight power increase). I want to state he was just blindly speculating though and trying to move quickly through 30 cars in one day.

I was going to get a fuel pressure test done when I find someone who wants to do it and who I trust.

If you have a 190 it will have a chain tensioner on it so i doubt a slack chain would put the valve gear out of time, unless of course, the slack is taken up only on on side of the "drive". Then it souls have an effect, however if it is that slack you would have heard it. I used to own a 190E many years ago and it had a round 100k miles on the clock and the chain was beginning to rattle, then had a new one fitted, so from that if its not rattling its probably ok

The slack [is] taken up on the exhaust side only; I remember from I removed to rebuild the top end. Chain was apparently fine looking so wasn't replaced. Doesn't rattle. Was yours a single row chain (was it a mid-80s 190?)?

Phumy that's interesting about flat spots coinciding about cruising rpms I hadn't thought of that. Seems a bit stupid to make a sports car non-sporty but I suppose that's what Mercedes do best!

The throttle response absolutely sucks on these but I read complaints of that from brand new so am treating it as normal behaviour.

[phumy]

Mine was an '86 190E, a wonderful car.

[skid-mark]

the only thing i can think of too get you some more power is a supercharger but i don't know if this can be done on a merc 190