I am puzzled as to why the engine is making top HP and Torque at RPMs lower than what Nissan spec is. Any guidance would be appreciated on what to check for possible causes/explanations. Does anyone have dyno graphs for similar NA stock cars? Per factory spec the engine is suppose to deliver top BHP at approx 5,500rpms and max torque at approx 4,000rpms. This is a 280zx 2+2 NA 5sp, 1981 model with p79/f54 FI head/block combination. When Dyno'd on a MustangDyno the vehicles showed to HP (124, or with WCF 129) at 5,00rpms, and torque (144, or 150 with WCF) at a low 2,500rpms. Fuel ratio showed slightly rich (14.2), though I would not consider that excessive (my novice opinion). The HP and torque numbers appears within reasonable expectation for a well tuned NA vehicle (again my novice opinion), especially with the Mustang equipment which I was told was known to be a bit on the 'conservative' side compared to Dynojets so that it reflects more real world scenario. I am just puzzled why power is achieved at such lower rpms than what spec is suppose to be...Appreciate to hear possible theories why this could be. I am planning performance upgrades but want to make sure the stock engine is best tuned as possible, get a baseline, before I start.

BACKGROUND:
The car is ALL stock, except the only mods on the entire vehicle ever done is a lightened flywheel (about 18lbs) and a new aftermarket CAT (supposedly more free flow), with a crush bent exhaust system from the CAT backwards (used 2 inch tubing - very few and slight indentations), with (IMCO) new muffler. Kept the original exhaust manifold and front pipe. Car has true 15,000 miles. Recently tuned and always maintained per FSM and by certified Nissan mech with over 20years experience working on Datsun/Nissan vehicles. Recent services incl cyl pressure checks (wet/dry), oil pressure checks, checked for vacuum leaks, timing adj, valve clearance, cleaned the fuel system, check and clean all elect connectors, distr cap and rotor, etc etc. The only 'issue' I have and never so far able to troubleshoot is that the engine is running slightly uneven at idle...various constantly with about 50rpms +/- all the time. Contributed it to perhaps the slight richness in A/F ratio. We did not use a fan in front of the vehicle to simulate (aid) air flow at high speed. I also tested the various components and the only oddity I found was on the TPS...no registration of WOT connection. The idle switch makes correct contact and disconnects at around 900rpms. I read somewhere that the manual transmission cars may not have come with a TPS with WOT switch.

 

Isn't "factory spec" according to flywheel hp on a brand new motor? You can't expect your 30 year old car to dyno EXACTLY like the factory flywheel numbers show back in 1981. There are too many different variables for a factory hp rating, and a strap down dyno.

If you just can't live with that, and really feel like something is wrong with your engine, you should start with the cylinder head. Check the valve train, cam, spark, and injectors. Those are the main things that will affect the power range through the rpms.

Or... just throw in a mild to wild cam... that will raise your powerband.

 

my dyno, with a low compression N47/N47 head/block combo:

 

I had most of my power in the lower. Until i poped on a reman dizzy, what a difference a working timing advance can do for you.