Jagged dyno graph...why?
05-18-2004, 02:25 PM
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Jagged dyno graph...why?
The car in question is a 1992 Nissan Sentra SE-R.
The first run it laid down 103hp/99tq. That was fine and dandy. It was lower then I thought, but I didnt buy this car for its brutual straight line speed...heh. Anyways, the graph itself looked very scary. The HP and TQ lines are extremely jagged and sporadic. Every 100rpm, the HP and TQ fluctuates around 10hp/ftlbs up and down. To be inside the car you would never feel this, so I never thought anything was wrong…and kind of still don’t. But I also know that this dyno doesn’t lie and its been very accurate so far. Take a look see and keep on reading…
My first thought was IGNITION problems. I pulled the plugs and they were quite clean, nothing damaged (until I dropped one…doh) and all burning clean and evenly. I am running NGK BK-R6E’s. I have them gapped to about .039/.040.
While I was changing the plugs I went ahead and threw a new set of wires on it. It really didn’t need it, but the jerky-ness of the graph was unsettling and I was really hoping for one of those easy fixes. No dice. If you are wondering, the local part store did not have a cap or rotor in stock or I would of bought those as well. I did take the time to pop the cap off and look at everything and it was in great shape, so I can practically count that out as a possibly culprit.
I put the plugs back in, put the wires on and ran it again. It did pick up some power…from 103/99 to 110/101, but the graph’s layout was even MORE jerky…WTF!?! Please shoot out some ideas on why this is happening and what I can do to smoothen this out.
Here are some more raw facts to help anyone get a hypothesis working:
- '92 SE-R, 5spd
- Timing set at stock 15*
- 87 octane
- stock clutch w/ 139k on it. Did not appear to be slipping and wouldn’t think it would show slippage in that manner on a dyno graph anyhow.
- New plugs, new wires, cap and rotor w/ 15k miles, looked OK.
- No missing during driving, no misfires.
- Have NOT done a compression check.
- Bone stock JDM engine with estimated 50k on it.
- As you can see the Air/Fuel ratio is in the “ok” range for me. Lean at low rpms, and gets a little richer as the cams cut on (5k) and it starts making what little power it makes. Yea, it’s a tad bit TOO rich for a n/a car…but its something I can live with.
Any info would be appreciated. This is not a dyno problem, nor a software problem. I tried to convince myself it was, but I did 3 runs and they were all this way. Then ran other people's car's and they all were smooth. So, I need to find out what's going on. The dyno I was using does not have load control if anyone was wondering, so I cannot diagnose through that either.
Thanks in advance...
The first run it laid down 103hp/99tq. That was fine and dandy. It was lower then I thought, but I didnt buy this car for its brutual straight line speed...heh. Anyways, the graph itself looked very scary. The HP and TQ lines are extremely jagged and sporadic. Every 100rpm, the HP and TQ fluctuates around 10hp/ftlbs up and down. To be inside the car you would never feel this, so I never thought anything was wrong…and kind of still don’t. But I also know that this dyno doesn’t lie and its been very accurate so far. Take a look see and keep on reading…
My first thought was IGNITION problems. I pulled the plugs and they were quite clean, nothing damaged (until I dropped one…doh) and all burning clean and evenly. I am running NGK BK-R6E’s. I have them gapped to about .039/.040.
While I was changing the plugs I went ahead and threw a new set of wires on it. It really didn’t need it, but the jerky-ness of the graph was unsettling and I was really hoping for one of those easy fixes. No dice. If you are wondering, the local part store did not have a cap or rotor in stock or I would of bought those as well. I did take the time to pop the cap off and look at everything and it was in great shape, so I can practically count that out as a possibly culprit.
I put the plugs back in, put the wires on and ran it again. It did pick up some power…from 103/99 to 110/101, but the graph’s layout was even MORE jerky…WTF!?! Please shoot out some ideas on why this is happening and what I can do to smoothen this out.
Here are some more raw facts to help anyone get a hypothesis working:
- '92 SE-R, 5spd
- Timing set at stock 15*
- 87 octane
- stock clutch w/ 139k on it. Did not appear to be slipping and wouldn’t think it would show slippage in that manner on a dyno graph anyhow.
- New plugs, new wires, cap and rotor w/ 15k miles, looked OK.
- No missing during driving, no misfires.
- Have NOT done a compression check.
- Bone stock JDM engine with estimated 50k on it.
- As you can see the Air/Fuel ratio is in the “ok” range for me. Lean at low rpms, and gets a little richer as the cams cut on (5k) and it starts making what little power it makes. Yea, it’s a tad bit TOO rich for a n/a car…but its something I can live with.
Any info would be appreciated. This is not a dyno problem, nor a software problem. I tried to convince myself it was, but I did 3 runs and they were all this way. Then ran other people's car's and they all were smooth. So, I need to find out what's going on. The dyno I was using does not have load control if anyone was wondering, so I cannot diagnose through that either.
Thanks in advance...
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