After almost 30 years of driving, modifying and racing Zcars I decided I wanted to know the answer to the big question: how much power do you really get out of a SU-carbed L28. I'm not talking about a stroker, turbo, or nitrous engine but an Old School L28 motor with a cam and header, running SUs, electronic ignition and maybe a little compression boost thrown in.  

Keep in mind that ZCAR engines were designed for road racing, autocrossing, and rallying. Road racing requires an engine that can hover above 5000rpm or so as you jockey for position outmaneuving other cars around corners. Acceleration off the line isn't as important since you spend very little time in the low rev ranges. Drag racing uses an engine that pulls hard off the line while still accelerating at full power past the finish line. It's all about max torque and horsepower from start to finish. This is easy for a naturally high-torque V8, harder for a smallish 6-cylinder with a redline of 7,000. Keep in mind that the F54 L28 was designed to put out about 145 stock HP. It's unrealistic to expect that with just a few tweaks it will produce 300 horsepower...it's only a straight-6 engine after all.

So I spent over a year on my rebuild taking my time and making sure the machinework and parts integration were correct. The trick is finding the combination of parts that maximize power over a broad range while avoiding power spikes and dips. While some people claim to see 220+ horsepower at the wheels on their street L28s, I'd like to see those dyno sheets. Detailed reading on forums show that carbureted L28s on 93 octane seem to max out around 160-170 at the rear wheels. Stroker motors can break this ceiling as can expensive fuel-injection and three 2-barrel Mikuni/Weber carbs, but I have none of those. So I took my L28 240Z to Bristol Dyno in East Hartford, CT. They use a Dynojet Research model 248 intertia dyno and are very familiar with testing sport compacts. I did three runs to create a baseline to work from. The results were good but less HP than I expected...see my excuses below:

My Dyno excuses:
In a perfect case of Murphy's Law my alternator actually failed on the way to the Dyno. I didn't know it until my cooling fans refused to come on because the battery was only at 9 volts @#$%^. So my first run was actually with an undercharged battery, the other two on a battery charger. That says something about an MSD ignition's ability to run at low voltages.

Also I had just put in a new ZX distributor the day before and after the dyno discovered the rebuilder had put the reluctor on upside down. The reluctor is unidirectional part and would have run the ignition pulse in reverse doing some odds things to the spark. My carb needles were not flush with the piston bottoms as well. With carb tweaking, correct spark, a working alternator, and some air horns I'm certain I can pick up more HP next time. Coming soon.........

FUEL RATIO RESULTS
Sadly, Datsun never created a SU needle for the L28, so for 30 years people have had to use L24 needles for performance. While there are lots of SU needle profiles on the market you' ll notice those are for the British SUs (Jag, MG), not Japanese Hitachi SUs. This is why the SU books on the market never mention our N27 or N54 needles. Luckily back in the 1970s someone determined that the British "SM" needle was good for Z performance which has made it pretty much the only choice for performance L28s since then. Some people claim the SM is perfect, some people claim it's too rich and damages the engine. Who's right?

• 14.7 is considered the best stoichiometric fuel ratio for emissions
• 12.8-13.2 is considered the best range for performance with high quench chambers and hot ignitions.
  

When I had my engine above dynoed I had them sample the 02 levels in the exhaust. Guess what they found? It turns out that SM needles on my engine produced a mixture of 12.1 to 13.5 from 2300 all the way to 7,000. While this is considered on the rich side it's still within an acceptable range. The one deviation was a rich spot between 3k-4.5k. I don't believe this is due to the carb pistons rising too slowly or quickly, I suspect the SM needle is simply too rich in that rev range. My plan is to get the mixture around 13.5 all the way across, so I have some different taper SU needles on order that should lean out the bottom end while maintaining the top end. Stay tuned.

VERDICT: SM carb needles are worthwhile for performance on a free-breathing L28 if you can tame the low end rich spot. NOTE: the spikes at 7k are from letting off the throttle.

HORSEPOWER RESULTS
My dyno runs were to gauge rear wheel horsepower. Rear Wheel takes into account drivetrain losses and tire squirm. Flywheel horsepower is based off the motor only and is generally about 20-25% higher depending on who you ask. So I can say truthfully that my engine produces 200 horsepower but in reality only 165 hits the street. The results below show a fat and usable curve with horsepower above 150 from 4600rpm all the way to 7000. Since the fuel mixture is OK at 7k I believe the drop-off starting at 6.5k is due to the cam grind and/or carb flow. Interesting that the 3-4k rich spot in the fuel mixture above corresponds to a slight horsepower dip. NOTE: this scale is in "STD" mode. Subtract about 4hp for SAE.

The 1st run was with the intital timing set to 12btdc. For the next two runs I bumped it to 14btdc. It looks like that move picked up a couple of HP at the top end. I also leaned the mixture nuts out a 1/4 turn after the 1st run, but I don't think it affected anything.

TORQUE RESULTS
Torque curves always look like the engine can't breathe. In reality, smaller engines actually develop high torque just off idle which diminishes at higher speeds. What is excellent here is that I have over 150ft/lbs from 2400 all the way to 5800. That's a decent curve with a healthy bump at 4800. Notice again that the rich spot from 3-4k corresponds to a slight torque dip there too. That looks like that area of the rev range is my project to concentrate on.

So is it worth tweaking a 25 year old 6-cylinder to see if I can pickup more horsepower when it's common today for a 4-cylinder to pull 200HP? You bet!! There aren't too may cars anywhere that have the excitement per hosepower ratio of the Datsun Zcar. This saga is just beginning for 2008.

2007 DYNO RESULTS F54 L28 with SU CARBS

- REAR WHEEL-
165.6 horsepower@5800rpm (200 flywheel)
169.6ft/lbs torque@4800
O2 mixture 12.1-13.5, 2300rpm to 7000