i've heard the term often and assume it's for the exhaust. can someone clarify this for me? thanks!

 

test pipe/ race pipe etc. these are strait pipes that elminate the cat, freeing up more hp, less restriction. They're also not for street use. lol

 

They've also been known to allow for a really stinky exhaust and at times, additional black smoke from the tail pipe.

 

dont forget flames!

 

A testpipe is just a open pipe that replaces the cat.

Your cat looks like this to the exhaust flow

 

and stinky,but no smoke,if your engine is well tuned,and healty.
Never saw any flames though.

 

proven dyno hp gain of how much?

 

Might be because I'm tuned...its easy to do it if I free rev and do ONE bounce of the rev limiter...just like a quick one. I think it is because with the EMS there are 3 different limiters. The first is a soft-cut limiter, a lot like those in new cars (thinking of my gf's TC), its set to 9050. Then there is an ignition cut set to 9100, then there is a fuel cut at 9200 if it ever got that high. I think what happens is that in a free rev from 4000rpm or so the soft-cut limiter cant catch it at 9050 and it hits the ignition cut sending unburnt fuel into the pipe which quickly ignites when the ignition is resumed and a backfire commences.

However, there were a few times after I installed the cat-back before I installed the EMS that I managed to shoot flames, normally they were between shifts, I think.

 

You can't ignite unburnt fuel in the pipe.
The only way you would throw flames,is running quite a bit of overlap.!!!

 

You don't need spark source to ignite the fuel in the pipe. In extreme case, the unburnt fuel plus exhaust gas can get hot enough from the engine. All it takes is to find enough oxygen closer to the tail pipe and bam! Very easily done with test pipe and aftermarket computer, even in my miata.