Power Loss/ Labored Performance @ 5000 RPMS

[XTCoX]

Well this problem has successfully confounded me. I have an E31/N42 (dished pistons) with a .290 dur. cam, and the 280z valves, Crane XR3000 ignition, and im having a hell of a power loss in the upper end of my power band.

 

Ive tried richening up the mixture on my SU's, which seems to have had little to no effect... Tried increasing the fuel pressure to the carbs, no effect... I tried advancing the timing... and this is why im so confused: The timing light tells me that I am only able to get about 5 degrees (if that) of idle advance without knocking.

 

Now, this is after spending at least an hour driving around and playing with the timing to find out where it is safe to run without knock. I find a safe place for the distributor cap to sit, and then go back and check it with the timing light, and its less than 5 degrees. In looking for an answer on here, and in my tuning books, a cam with that kind of duration should be able to run 15 degrees of advance no problem.

 

Im in CA so I get 91 octane, and even with the Lucas Cylinder Lubricant in the tank its still knocks. So, any ideas?

[Leon]

I had a similar issue on my L24. It turns out that the new distributor cap didn't fit well, and did not seat correctly. Just to rule out the possibility, make sure the cap is seated well around its whole perimeter, all 360 degrees. I only noticed this when I pulled my distributor. That's my off the wall advice!

[XTCoX]

Hmmm. Will check, but are you referring to the power loss or the knock?

[Leon]

Hmmm. Will check, but are you referring to the power loss or the knock?

 

Both as they are intertwined. My engine would knock around 3000rpm and up, even with retarded timing, high octane, and a rich mixture. Stupid distributor cap...

[XTCoX]

I checked the distributor this morning, and its fine. Maybe I should have been more specific; It knocks under pedal-to-the-metal conditions, toward the upper end. I thought it was the vacuum advance at first because of this, but its already shimmed up to make it VERY stiff, so, as it is, Im suspicious of it even working at all.

 

Maybe its just me... What kind of 0-60 should I be looking at with an E31/N42, 280Z valves, MSA Stage 4 cam, R180, 4 speed tranny, SU's, and a 6-2-1 header?

 

Maybe I just think that there is a power loss because I feel like im not going fast enough... It might just be my butt dyno needs a tune up

[XTCoX]

Update:

 

I have been running my SU's at 5 turns out to try and compensate for the supposed problems that they have with keeping up with the larger cams that are available for the L28. However, I recently backed the A/F mixture nuts off to 4 1/2 turns out, and the upper RPM range improved... a lot. Now its not as good as it could be, because of the retarded timing, but leaning out the mixture helped drastically; not sure why.

 

Anyway, any idea on the timing issue?

[BluDestiny]

Your getting Knock when your compression is at 8.5:1? That's pretty close to stock IIRC

[XTCoX]

How do you know its 8.5:1? A stock 240 has a 9:1 CR, and my engine has the .290 duration cam in it, and the 280z valves... Wouldn't that be closer to like 9:1 or 10:1?

[BluDestiny]

http://www.atlanticz.ca/zclub/techtips/calcs/engine%20builder/index.html

 

I was referencing 8.5 to the stock 280z(8.3ish) since you're using a 280z/early 280zx block

 

Also from my understanding of a longer duration cam( not an expert, just read it on how stuff works), you're letting in more air, especially with the "bigger" valves (not sure if a mmm or two can make that much difference in a street application) which is probably leaning out the mixture and really straining your SU's, unless you have bigger jets or something. You said it got better when you did lean it out though, so I don't know what to say to that.

 

I could totally be wrong though, I'm not an expert engine builder or anything, this is all just info. Please correct me if I'm wrong

[XTCoX]

Well my understanding is pretty much the same as yours, which is why Im so stumped... My only possible explanation for this is that im still running the old 240z stock air cleaner, which, (with the mixture richer) could be restricting air flow. However this is a weak explanation and I doubt its right, but its all Ive got. Im probably going to try an and lean it out more to see what happens, but that stupid knock is going to seriously get in the way of my tuning...