Car feels faster than it is and interesting loss of power in higher RPM, Went and did a dyno run to see what my car produces. Options

[Michael N]

Today I had there afternoon off and went to a local shop to get a dyno run on my car to see what it actually produces rather than me guessing on what it produced. The local shop was running a special of 3 pulls for $85.00. The shop actually did 4 pulls and we awesome with me and my car. They are used to seeing high HP muscle cars etc but didn't pick on my car too much for being so low in HP. They did the pulls in 4th gear. The shop uses a dynapack dyno that bolts to the hubs which I like as there is no need for tied down points. Overall I am happy with the results as the engine is now 40 years old without a rebuild yet. It is a 2.4 liter 911S engine with MFI and produced 141.2 at the wheels.

What would cause the power to drop consistently between 6000 and 6500 Rpm? It drops and come back on again.

[brant]

did they run an Air fuel ratio while on the dyno?

I'm guessing your AF ratio might be contributing to the power curve your asking about

[Cap'n Krusty]

While not real common with the S pumps, there's always a chance the 3D cam is worn. Also, you could have an issue with the pump/throttle plate correlation. That's the first thing to go soft in an MFI engine, and the easiest thing for an inexperienced tech to mess up, especially without the proper tool set. While the redline is 7300 RPM, the max power output on a 2.4 S is at 6500 RPM. You can expect a bit of a drop off between those numbers.

The Cap'n

[gothspeed]

what kind of manifolds and what diameter are the intake runners, top to bottom and length? What kind of exhaust, headers runner diameter/length/ collector type, muffler?

What size are your valves, intake/ exhaust and what are your cam specs? Is your fuel pump getting enough juice? what kind of air cleaner set-up?

[Michael N]

what kind of manifolds and what diameter are the intake runners, top to bottom and length? What kind of exhaust, headers runner diameter/length/ collector type, muffler?

What size are your valves, intake/ exhaust and what are your cam specs? Is your fuel pump getting enough juice? what kind of air cleaner set-up?

Everything was pulled out of a wrecked 1973 911S back in 1978. The car came into EASY auto dismantlers and the PO had a connection there to get it complete. I believe that it is all original for that car including the air cleaner. The muffler is a modified (changed the angle of he tip to match the rear valance) 911 unit and running original 914-6 heat exchangers.

[gothspeed]

Here are some dyno graphs from various 911 Carreras . That upper RPM dip on your early engine is not there on the later engines ...... so it may seem to be timing or caburation issue?

http://www.911chips.com/dyno.html

[messix]

if you are using carbs it could be a harmonic standing wave [stand off] in the intake. you could try a taller velocity stack on the carb, or smaller venturies in the carb.

never mind just reread and see you are mfi..... mabe too restrictive fro the s cam.

[brant]

jetting... (using old fashioned language)

worn 3d cams would still change the air fuel ratio
you need to make the runs with A/F measurements

[charliew]

Put a af sniffer in the tail pipe and don't ask for best guesses. 99% it's af problems probably caused by metering controls that have gotten old. It could be only vibration of some connectors that just need replugging to get recleaned on their contact surfaces. When power falls off and comes back it's usually a worn throttle position or cam sensor on later stuff or fuel supply. If it gets comlicated also check the 12 v power supply that it stays up at higher rpms. My shadetree experience says manifold size, valve size etc doesn't cause dips in power. That is caused by sensor feedback or metering glitches. Fuel pressure, timing and so on.

I would do one pull in 3rd to see if a shorter interval doesn't show the dip. If it doesn't then suspect fuel supply or still maybe vibration.

On the brighter side at least it's a consistent problem and will make it much easier to fix.

[gothspeed]

Put a af sniffer in the tail pipe and don't ask for best guesses. 99% it's af problems probably caused by metering controls that have gotten old. It could be only vibration of some connectors that just need replugging to get recleaned on their contact surfaces. When power falls off and comes back it's usually a worn throttle position or cam sensor on later stuff or fuel supply. If it gets comlicated also check the 12 v power supply that it stays up at higher rpms. My shadetree experience says manifold size, valve size etc doesn't cause dips in power. That is caused by sensor feedback or metering glitches. Fuel pressure, timing and so on.

I would do one pull in 3rd to see if a shorter interval doesn't show the dip. If it doesn't then suspect fuel supply or still maybe vibration.

On the brighter side at least it's a consistent problem and will make it much easier to fix.

I asked all those questions to see what if anything was done to the motor, NOT as a diagnosis in and of itself. Many mods can throw the COMBO out of balance ........ intake system changes can affect intake resonance which CAN manifest in power dips or spikes. Obviously there are other things that can cause this but one should start by getting the most information of the base platform first.