Well, guess what, the stock system sucks. I know big surprise.

I’ve read the posts about reverse-flow problems, the many filter and airbox mods/replacements, etc. So a few weeks ago I decided to try and measure how much restriction to airflow was created by the stock air inlet passages on my ’95 PEP. Other flow-affecting mods are SS downpipe and catback.

My test setup was to run a 3/16” clear vinyl tube from inside the airbox back in to the passenger compartment where the other end of the tubing was attached to 4-inch diameter mechanic’s vacuum/pressure test gauge. Then I went for a test drive while monitoring the test gauge’s responses.

Worst case I found 7-8 inches of vacuum IN FRONT OF THE FILTER ELEMENT! So regardless of the type of filter, the OE air passage between the air box and the front of the car cause a significant amount of flow restriction.

During this test I was running a Purolator brand filter element with ~9K miles of daily use. Had this been a higher flowing K&N, or even just a new element, I’m sure I would have seen higher vacuum readings.

Since I wanted to keep a stock appearance and also maintain as much air to the intercooler, I decided to perform the Robinette Cold Air mod http://www.rx7.org/Robinette/coldair.htm using two each 1-3/4” diameter aluminum flex hoses. Following this I retested and measured zero vacuum under all conditions. Butt-o-meter said there was noticeable improvement in power, particularly at higher RPM’s. Secondary boost recovered faster and held higher at redline. Transition spiking also increased by 2-3 PSI forcing me to add a manual controller for the wastegate; already had one for pre-spool.

 

Good work.

 

Good information, I always wanted to do this test but I was too lazy. Now I don't have to!

 

damn, good write up...

 

Good info.

I would be curious to hear if you get vacuum with a K&N. Any plans to test with a K&N?

 

adam c, my future plans do include switching to a K&N, but right now I've already installed another OEM style paper element; WIX this time.

 

i can test with a k&n... i have a stock box at the moment and freshly oiled K&n and my boost gauge is just lying around on my desk waiting for a project

 

wouldnt it show even more vaccum since the filter element is less of a restirction all the restriction plus the increased air induction due to the performance filter would occuer at the duct???

 

RotorMotor, it would be very helpful if you performed these tests with a stock filter for baseline. It wouldn't make much sense to compare your results to FDjunkie's, since you'll each be using slightly different test setups, and testing slightly different motors in terms of mods & health.

-s-

 

true... also the other thing im currently lacking is the top cover duct thing that goes over the IC and air box... (i installed a koyo, and everyhting seems to sit to high despite every effort to lower it).

also wouldnt you need two tests for every setup?? youd need a pre filter vacuum test, and a post filter vacuum test to compare anything. id think one would also want a test with NO filter at all to see what the upper bounds would be... in other words, the value that we are striving towards (no restriction).

another thing that would need to be considered is RPM, and what the turbos are doing. depending on those 2 factors, the vacuum value you would obtain would be altered. im down to experement with various filters (i should have the apexi filters for my project car hopefully soon) but we need to isolate the variables so we can see what exactly is affecting our readings... sorry that sounded really dorky, but anything we find will be useless unless the tests are controlled... so lets talk -heath

 

i wish i had a gauge setup where i could plot pressure/vacuum vs RPM.... then i could just go WOT and we could get some nice plots of how particular filter setups compared.

 

FDjunkie, great test! I've always wondered about that. I'm glad the hoses-in-the-bottom provided such a measurable improvement.

-Max

 

Your to smart for you own good, you make my head hurt max