There are several plenums available for your Z31. The stock plenum is generally regarded as not being a very good design in terms of flow capacity and hence limits the power output of the engine. To date there has been NO conclusive data posted by a non-biased third party as to which plenums flow the best, and hence make the largest horespower gain over the stock plenum. The purpose of my project here is to set the record straight once and for all as to which plenums flow the best AND what the expected horsepower gains would be on a ~300 wheel horsepower modified car with stock heads (which, I believe, is about the point where upgrading the plenum would be worthwhile).
Part 1: Flow Bench Testing (2/20/06)
Part 2: Dyno Test Results (expected sometime in 4/06)
Part 3: Summary and Conclusion (after dyno results are completed)
The flow bench used is an industry-standard SuperFlow 600 unit (SF-600s) at 28 inches of water. I have to extend very special thanks to Bob Makley of Finish Line Motorsports in Hastings, MI for his time and very reasonable pricing for me to work on this project. The head used in testing is a stock early casting (non-w series) with stock valves and seat cut. The decision to use a completely unmodified head was based on the fact that very few people install ported heads along with the hunch that both the flow and dyno comparisons would greatly favor the modified plenums if modified heads were used. The runners in the lower intake (shown) were smoothed to remove casting marks and flaws. As the 240SX throttle body seems to be a popular 'upgrade' for the stock plenum, this stock plenum's elbow was ported and the 240SX unit fitted in order to measure gains (if any at all) from this modification.
"Engloid" Plenum
Modified Z31 plenum.
Typical sale price $450-$600 used.
Install notes:
stock fitment
| Valve lift (in inches) | Flow rating (in CFM) |
|---|---|
| .100 | 63.82 |
| .200 | 117.28 |
| .300 | 140.43 |
| .400 | 152.39 |
| .500 | 158.36 |
"L&P Performance" Plenum
Modified Z31 plenum.
Typical sale price $450 + SH new.
Install notes:
stock fitment
| Valve lift (in inches) | Flow rating (in CFM) |
|---|---|
| .100 | 64.08 |
| .200 | 119.68 |
| .300 | 144.99 |
| .400 | 156.87 |
| .500 | 162.24 |
Stock Z31 Plenum
Typical sale price $15-30 used (but you should already have one anyway) .
Install notes:
stock fitment
| Valve lift (in inches) | Flow rating (in CFM) STOCK TB |
Flow rating (in CFM) 60MM TB |
|---|---|---|
| .100 | 57.24 | 61.0 |
| .200 | 113.99 | 113.39 |
| .300 | 134.46 | 134.46 |
| .400 | 144.92 | 144.92 |
| .500 | 147.90 | 147.90 |
Pathfinder Plenum
Typical sale price $15-50 used.
Install notes:
- Requires 88 Maxima distributor cap or pathfinder distributor.
- Does not clear stock hood.
- Unsure of emissions/idle control fitment
| Valve lift (in inches) | Flow rating (in CFM) |
|---|---|
| .100 | 62.7 |
| .200 | 115.79 |
| .300 | 138.94 |
| .400 | 150.89 |
| .500 | 155.37 |
Early Maxima Plenum
Typical sale price $15-30 used.
Note: Almost exactly the same as Z31 plenum except lacking "elbow".
Install notes:
- Needs modification to clear distributor.
- Does not clear stock hood.
- FPR needs to be re-located
| Valve lift (in inches) | Flow rating (in CFM) |
|---|---|
| .100 | 64.22 |
| .200 | 117.28 |
| .300 | 140.43 |
| .400 | 149.4 |
| .500 | 153.88 |
In flow testing it appears that the L&P Performance plenum is the best overall, showing substantial gains at all valve lifts. The Engloid plenum is in second place, followed closely by the Nissan Pathfinder and Maxima plenums. The poor performance of the stock Z31 plenum becomes apparent at higher valve lift. The result on the Maxima plenum is an eye-opener, as it points to the "elbow" on the stock Z31 plenum as being a major point of restriction. My speculation would be that (given no need for an emissions-legal part) a modified Maxima intake similar to the L&P method would yield even better results. I am now even more thoroughly convinced that on a car with modified cylinder heads any of these other plenums would provide a very substantial horsepower gain over the stock unit.
Part 2: Dyno Results: Not yet completed
Dyno testing will be done on a Dynapack 3000 chassis dynamometer capable of handling over 800 wheel horsepower.
"The Dynapack is a unique dynamometer with many advantages over other types.
First, unlike many dynos, it has active load control so you can tell it EXACTLY what you want it to do. It can hold engine speed at 3300rpm regardless of throttle position, it can do a power pull from 2000rpm to 7500rpm in exactly 8 seconds, it can do a power pull with steps instead of one constant ramp, the possibilities are endless. This ability is a must if you're doing any serious engine mapping. This dyno is top of the line."
In addition, the Dynapack is one of the most consistent dynos on the market, showing some of the smallest dyno-related power variations from pull to pull. Wheel horsepower and torque from about 3000 to 6500RPM will be measured three or four times and then the average will be taken for a baseline.
Only three of the shown plenums above will be run on the car due to the cost of dyno time and the labor and fabrication involved in swapping the parts. Dyno time is stupid-expensive and both the maxima and pathfinder plenums won't fit without modifications to the plenum, distributor cap, or both. In addition, I would have to make new intercooler piping and change it for the different throttle body location on each plenum. Horsepower gains with the Pathfinder and Maxima plenums will be roughly estimated based on the results of the plenums being tested correlated to the flow data.
For those reasons I will only dyno the Stock Z31, Engloid and L&P Performance plenums with 3 or 4 pulls on each averaged together. The A-B-A 'scientific method' for measuring performance gains could be used, but that would mean a lot more work and a lot more money. You, as a Z31 owner, should understand that.