260Z Dyno testing - exhaust system analysis with pics and videos

[LeonV]

Dyno Testing My 260Z

This is an analysis based on my dyno testing from 8-20-11.

Test Parameters

The reason for the testing was that I wanted to log a baseline for my 260Z before going further with more engine modifications and to make a comparison between different configurations.

The baseline settings are as follows:

• early 260Z with 1972 L24/E88 that was installed by the previous owner
  
• triple 40DCOE-18 Weber carbs with K&N-style filters, not jetted yet, just bolted on and sync'd
  
• Cannon long-runner manifold
  
• stock exhaust manifold to single 2.5" crush-bent pipe
  
• stock cam and internals
  
• refreshed cooling system with aluminum radiator, new water pump and hoses
  
• heater loop is blocked off at both ends
  
• Mobil1 synthetic 10W-30 in engine, Redline 75W-90 in transmission (GL-4) and differential (GL-5)
  
• MSD coil, NGK plug wires
  

The two pipes to be tested:

\

Test 1: Long 2.5" pipe with straight-through muffler in the rear

VIDEO

Initial Thoughts and Numbers:

I know actual dyno numbers don't mean a whole lot but I came in thinking that I'll be happy to break 100hp! The Z feels very torquey and quick but we all know that the almighty butt-dyno isn't very representative of reality, so for all I knew I could've been making 85hp...

I was also pretty convinced that I would not be over 120 hp as the carbs are not jetted and I have not even touched the spark timing. However, according to the dyno that day, I am making 121 hp @ 5200 rpm and 136 lb-ft @ 4000 rpm! At the very least, this indicated that my engine is fairly healthy and that my tune is not completely off. A happy side-note but we're here for comparison purposes not peak numbers. On to part two!

Test 2: Stock downpipe, cut after the merge

VIDEO

Results

SOLID lines are long-pipe runs

DOTTED lines are short-pipe runs

*Note: there are local peaks at about 5850 rpm, and they are there for both runs so it very well could be due to the tune.

The long-pipe made slightly more torque on the top end, but the short-pipe made more low-end torque.

Analysis

On the surface, the results look counter-intuitive. After all, shouldn't a short pipe be better for top end and a long pipe better for low end? How about the "mechanic" that's been "doing this for years" that says "well son, when there's less backpressure you lose all yer low end torque. You need some backpressure." Well in this case, running it with no muffler and a cut after the merge increased low end torque.

So please, get any thoughts of backpressure out of your head when speaking of exhaust tuning. In fact, forgot the word completely, it is not a very useful term in the exhaust theory glossary, besides wanting as little of it as possible.

The results come out as expected after you look at the differences in the two pipes. The critical difference between the two being the length of the secondaries before the merge into a single pipe. Take a look at the second picture from the beginning of the post, showing both collectors. Take note of the differences. The long pipe exhaust merges the secondary pipes quite a bit earlier than the short pipe, meaning that the short pipe secondaries are actually longer.

Thinking back to wave tuning, whenever there is a discontinuity in a pipe (e.g. a change in pipe diameter or merge), the compression wave coming from the open exhaust valve gets reflected back as an expansion wave. A tuned length of pipe allows the expansion wave to travel back to the exhaust valve just before it closes, further dropping pressure in the cylinder. This draws out more burned exhaust gasses as well as draws in more air-fuel mixture during the valve overlap period (scavenging). Keep in mind that a tuned length is only good for one engine speed. As pipe length before the discontinuity (merge) changes, so does the time it takes for the wave to travel out the exhaust valve, down the pipe, reflect and come back to the exhaust valve. A shorter pipe before the merge decreases the length of time it takes for the wave to reach the exhaust valve, therefore it is more efficient at higher engine speeds. The opposite is true for a longer length of pipe before the merge (longer wave travel time = lower rpm efficiency).

This test also shows that a single 2.5" pipe is ample for my L24 spinning to 6000 rpm, because it essentially mirrors the short pipe and actually outperforms it at high rpm. The large diameter pipe allows for the collector to do its work without hindering performance post-collector.

Conclusion and Discussion

The results nicely agree with the exhaust theory that we've covered in this thread.

If I took the long pipe with the muffler and cut it after the collector, it would perform better, but the difference would likely not be discernible. On the other hand, if the pipe coming off the collector were smaller than 2.5" then pumping losses would begin to climb and performance will decrease across the board, especially at high rpm since frictional losses in a pipe increase with the square of velocity.

In the near future, I am having twice pipes made and I'm curious as to the effect of keeping the secondaries separate. It may cause a small hit in performance, judging by how much of an effect moving the secondary merge point had, but carb and spark timing tuning will be a much more drastic change. I broke off a stud at the exhaust manifold-to-exhaust connection point, and although there was no detectable leak, I may just put on my Nissan Motorsports header and build the pipes off of that.

[surfer.tech]

I'm actually curious about those headers. I've been trying to decide between these headers and the Kameari ones. The Kameari one is 3X the price.

[LeonV]

Well, I just installed them, so I guess we'll see! They're in better shape then the rest of my manifolds (stock 240Z, Clifford Research, or Hooker Super Comp). Install was fairly painless, besides the light burn I gave myself when I was checking for leaks (they get hot real fast!). I did disconnect the steering rod at the steering coupler but that was just 2 bolts.

After a little maneuvering they dropped right in. Clearance is tight at the floor with only a few millimeters to spare, but as the engine spins they will tend to move away from the floor.

They sure sound mean as hell, though! The exhaust note is noticeably different then with the stock manifold, but I only ran it for 30 seconds or so. Should be off to the exhaust shop tomorrow!

[steve91tt]

Nice work. Your setup is close to my car so I love seeing data to help me calibrate my butt dyno. Did you get any air to fuel ratio data (AFR) with the dyno run? As you start making changes to intake and exhaust you may change how much air the engine is sucking in (volumetric efficiency?). I'm no expert, but from what I've read, the engine may need a jetting tweak to take advantage of the extra air that you will introduce with changes to intake and exhaust. If you don't increase the gas with the air then you may see the dyno numbers drop off as the car starts to go lean (unless the car was running rich to begin with then the extra air will have a double positive effect). In other words, if you don't keep the AFR relatively constant from run to run then it will be difficult to tell what whether your changes are helping the engine breathe or messing with the carburetion.

[surfer.tech]

I'm just curious how much power that header makes. I've been reading about this header and supposedly this is one of the best power making headers out there. Kameari claims on a stock healthy engine, power gain is 12-15%. I don't doubt them because they have proven they can build badass L series engines. But price wise, is a few extra ponies worth it?

[LeonV]

Nice work. Your setup is close to my car so I love seeing data to help me calibrate my butt dyno. Did you get any air to fuel ratio data (AFR) with the dyno run? As you start making changes to intake and exhaust you may change how much air the engine is sucking in (volumetric efficiency?). I'm no expert, but from what I've read, the engine may need a jetting tweak to take advantage of the extra air that you will introduce with changes to intake and exhaust. If you don't increase the gas with the air then you may see the dyno numbers drop off as the car starts to go lean (unless the car was running rich to begin with then the extra air will have a double positive effect). In other words, if you don't keep the AFR relatively constant from run to run then it will be difficult to tell what whether your changes are helping the engine breathe or messing with the carburetion.

Steve,

I decided not to do AFR logging, since I didn't feel the need to do so (no tuning runs, just baselines). I plan to do AFR logging in my next baseline, as well as tweaking the mix and spark. Basically, I'll do another baseline with nothing changed except the exhaust and then go on to jetting, etc. I could've logged AFR and made jetting changes, but my budget is not unlimited and I don't have any spare jets to play with. I'll do my best to keep the comparisons valid.

As far as volumetric efficiency goes (VE), it is directly proportional to torque output. Therefore, the VE changes will be seen in the torque curve. I don't think that the header will make a big enough difference to wildly swing AFR, based on other's test results. Like I said in my conclusion, spark and mixture tuning should yield more gains then just switching out exhausts, although the header coupled with twice pipes should have a noticeably different torque curve from a stock manifold and single pipe.

[LeonV]

I'm just curious how much power that header makes. I've been reading about this header and supposedly this is one of the best power making headers out there. Kameari claims on a stock healthy engine, power gain is 12-15%. I don't doubt them because they have proven they can build badass L series engines. But price wise, is a few extra ponies worth it?

I'm curious as well, especially with the stock cam! I do highly doubt that I will gain anywhere near 12% just by virtue of the header. On my L24, that would take it from 121whp to 135whp! It's just not gonna happen, maybe on a hopped up L28 or stroker but even then I'm not convinced you can gain 12%, let alone 15%.

Is it worth it? It depends on cost and your goals. To someone building a race engine, yes it is worth it. To others, maybe not, it depends on how much one is willing to pay. I got the header for a good price and it's in good shape, so I'm going to use it. If it alone doesn't improve performance at all then I'm at least happy with the sound it makes!

[steve91tt]

Good point on the AFR's. I suspect you are pretty close to being jetted correctly given the excellent output of your L24.

Have you thought about installing a wideband O2 sensor? It's a great tool for tuning carbs and they don't cost nearly as much as they used to (ca. $160). Depending on how much dyno time you have planned the wideband may pay for itself by allowing you to get the jetting right before you hit the rollers. I installed one in my L24 and it made tuning my triple Mikuni's a fairly painless exercise. It also allows you to keep your eye on what the carbs are producing and diagnose any future issues that may come up with your triples.

As I said, my track car setup is quite similar to what you are running so I'm very interested to seeing more results from your testing.

Edited August 22, 2011 by steve91tt

[LeonV]

Good point on the AFR's. I suspect you are pretty close to being jetted correctly given the excellent output of your L24.

Have you thought about installing a wideband O2 sensor? It's a great tool for tuning carbs and they don't cost nearly as much as they used to (ca. $160). Depending on how much dyno time you have planned the wideband may pay for itself by allowing you to get the jetting right before you hit the rollers. I installed one in my L24 and it made tuning my triple Mikuni's a fairly painless exercise. It also allows you to keep your eye on what the carbs are producing and diagnose any future issues that may come up with your triples.

As I said, my track car setup is quite similar to what you are running so I'm very interested to seeing more results from your testing.

Steve, looks like we think alike!

I have a WB02 system (14point7) sitting on my desk all ready to go. I am having my exhaust guy put in two bungs, one per pipe when I head over there today. I really don't want the bungs on the header though, hopefully we can fit them on the pipes. If not, my NLA header will have some new holes in it. It's going to be one loud ride with these open headers, gotta make sure to grab some ear plugs!

For now, I may just fine-tune my idle circuit with the wideband, since I don't want to change jets until after I establish a new baseline.

Thanks for your input!

[steve91tt]

Very cool. I look forward to comparing your AFR numbers to mine. For reference here is a link to my build...

http://www.classiczcars.com/forums/showthread.php?42156-New-240Z-build&highlight=steve91tt

[240260280z]

[LeonV]

Z is at the exhaust shop as of yesterday! Hopefully, I will get it back in the next day or two. Time to schedule another dyno run.