Ducman82

So i had my first Dyno pull today. It seems i have some kind of stutter or hiccup that caused a noticeable drop in power. Fuel pressure was good, and it seems to idle ok. what should i check? If you check the graph it appears around 3200-4300 RPM's.

Thoughts?

Lizard928

be really helpful to see an AFR chart

Ducman82

Ya, don't have that..... :-(

pcar928fan

Is that indicative of a poorly functioning flappy???

Iwanna928

It stated the AFR minimun was 10.0 or so which would be way rich. I would suggest a dyno tune. Probably about 150 bucks. You can get your timing right and play with the AFR until you are happy. I think you have 30rwhp that is missing and about 20 tq. Should be around 200 or a few more at the wheels!

jorj7

No floppy on 16 valves...

Ducman82

I do have the Barn door flappy. Disregard the bottom section of the chart, no sensors were hooked up to the car. This shop that i went does all other engines for porsches but ours. he messed with a 87 928 and did not like to mess with it. so he sticks with that he knows, but i did get a cheap run on the dyno! im going to check my plugs again tonight and see how they look. they checked out fine last week, thats why i am kinda stumped....

IcemanG17

okay a couple thoughts

1: 81 L jet 5 speed....so you have the MILDEST cams ever made for a 928....with somewhat low compression
2: Your numbers are a tad low...but not horribly....I would say a STRONG 81 5 speed with your mods would be 200whp....how many miles on your engine?

what is your timing? on early cars the timing is very important....when I had mine set wrong it only made 140whp.....set right it was nearly 200whp...

Ducman82

not sure on the timing. i need to get a new timing light (busted old one). i just turned 120k on the baby.

Iwanna928

120k Just warming up!

Ducman82

i just changed out my plugs. this set MAYBE had 2k on them. (left side of picture is pass side, right driver, front to back). i see some kind of scaling i have not seen on a plug before. plugs look to be burning just fine. I did have a leaky FPD that would flood the car when hot and sitting, and crappy oil breather set up. but i have fixed all that. so im hoping its just old stuff.

Ducman82

the plugs may be the key. she seems to run a bit smoother. i took her for a test drive out by the flight line here on base and punched it. im sure it lost that "stall" or hesitation feeling. another thing i noticed is my AC/Heater control head even when in the off position, would still pump and and AC.... so i need a new one, and its on the way. maybe that was an extra load?

AO

You might be losing spark on the top end. A tired green-wire (my guess), cap, rotor or coil could cause this.

But what's more concerning is that your dyno operator wasn't set up to measure AFR. I would not go back there until he does. This is very irresponsible on the operator's part. Bad things happen to motors that go lean. How can he tell if it's going lean without AFR? Danger Will Robinson! DANGER!

hacker-pschorr

What was done to the exhaust?

Ignition module in the fender is just as important. JDS (John Speake) can rebuild these now.

I found the ground wires going to my ignition module heavily corroded, I doubt that was helping anything.

I would also put a small port in the exhaust manifold and measure backpressure on the dyno. Anything over 1-2psi you have issues.

jcorenman

Are you sure that the throttle is opening all the way? Max power is down, but so is the max-hp RPM-- 4500 rpm should be more like 5500 for that engine. The torque curve is falling off a cliff at higher RPM's, sort of like maybe the throttle is only half-open...