Timdevlin Posted February 7, 2016 Share Posted February 7, 2016 Hello! This is my first post. We have a 78 micro mini with a 20r engine. I just had some exhaust work done but the guy was trying to charge me a bunch because he thought j was crazy. I read that people were running 2.5" exhaust and it was working well for them. He wouldn't do what I want for less than 600 so I just had him run 2.5" from my headers that I put on to a 2.5" converter. Then reduce down to my original exhaust. I know it's not perfect but I will finish with a 2.5 muffler and tailpipe soon. While it was idling yesterday I pulled of the air filter. It was idling very high. When I removed the filter it went down to a comfortable idle. Could this be timing or fuel/air mix or would an after market air filter work for the new exhaust. Also any suggestions on air filters. Thanks guys! Quote Link to comment Share on other sites More sharing options...
zero Posted February 7, 2016 Share Posted February 7, 2016 The stock exhaust and stock air filter is more then adequate for a stock 20R or 22R engine. There is no gain in performance by installing larger pipes or a less restrictive air-filter. Not unless what is on there now is plugged in some way. When it comes to aftermarket air-filters - there is NO magic. Either they filter dirt as well as the original (and that means they can plug up easier). Or - they do NOT filter as well, don't protect the engine as well, but do NOT plug up as fast as an OEM set-up. There is no way around that unless the aftermarket setup has more surface area then the original - which is usually not the case. A stock 20R has a volumetric efficiency of approx. 80%. So for every revolution of the engine - it moves 53.6 cubic inches of gas (as in air, exhaust). So at 2500 RPM - a 20R is flowing 134,000 cubic inches. That is equal to 77.5 cubic feet per minute (CFM). A stock Toyota air filter on a 20R is rated to flow 192 CFM. The stock 1 3/4" exhaust flows 240 CFM. Now - if you need to run your engine at 6000 RPM - then maybe you see some gain. Not something I'd expect in a motorhome. And note - I left out calculations for thermal expansion of exhaust gasses here. At 6000 RPM - a stock 20R wants to suck in 186 CFM of air and the stock air-filter is rated at 192 CFM. So, the stock air-filter when clean is designed to work well at up to 6000 RPM. Looks like good engineering to me. Quote Link to comment Share on other sites More sharing options...
Timdevlin Posted February 19, 2016 Author Share Posted February 19, 2016 thanks Jdemarris I wish I would have found this Forum earlier. I read one Forum that said it would help. I know that is not enough to go on. However I had a leak in my exhaust manifold. Now I have a 4 into 1 header going to 2.5" pipes. Then that goes to a 2.5" converter. then nothing. How would you suggest to finish the system. I am going to replace the stock air filter first and see if that helps with the airflow issue. Again, that could be timing. It is shuttering when I shut it off after running. I have a lot to learn I appreciate your time and suggestions. Quote Link to comment Share on other sites More sharing options...
zero Posted February 19, 2016 Share Posted February 19, 2016 By "shuttering" I assume you mean it is dieseling when you turn the key off? That is usually a caused of the idle-speed too high. Engines with carburetors have electric solenoids that when energized - let the engine idle fairly high so it idles smooth. When the key is turned to "off" that solenoid is supposed to collapse and the carb drops to a very low idle position. A broken or unhooked vacuum line can also cause it to idle to high and cause "dieseling" AKA "pre-ignition." On the subject of your exhaust - if you really want 2 1/2" pipe running all the way out the back - it will be a lot of work. WAY overkill with no gain. My Ford diesel with a 445 cubic inch engine and turbo has a 2 1/2" exhaust. I'd put a reducer on it and get some non-magnetic stainless-steel flex pipe. It will last forever and it's easy to work with. Quote Link to comment Share on other sites More sharing options...
fred heath Posted February 19, 2016 Share Posted February 19, 2016 2" ID pipe is all you need. 2 1/2" is overkill on the 20R engine. Quote Link to comment Share on other sites More sharing options...
Timdevlin Posted February 22, 2016 Author Share Posted February 22, 2016 Thanks guys. I appreciate your help. I don't know where I read the 2.5" I will reduce and put the appropriately sized muffler. I am sure there is a vacuum line issue because of the covering of the exhaust manifold that I removed to put the 4 into 1 on. Quote Link to comment Share on other sites More sharing options...
WME Posted February 22, 2016 Share Posted February 22, 2016 I ran 2" from the cat back to the muffler then 2.5" out back behind the rear tire. The reason was that the tubing was not mandrel bent and had some tight bends. The muffler was from a Ford 3/4 ton pickup with a 460 and a single exhaust. Very quiet with only a 22re feeding it. Quote Link to comment Share on other sites More sharing options...
payaso del mar Posted June 10, 2016 Share Posted June 10, 2016 interesting link for those considering a K&N or similar: http://rubicon-trail.com/public/K&N filter test.html seems decent test methodology. kind of has me convinced to go back to paper filters, altho I've been running K&Ns (the flat panel in the stock airbox type) on all my vehicles for many years, even here in "dust storm hell" (we regularly can't see the 7000' mountains that are 5 miles from my house) and the Taco has decent compression at 225K and the Subie still had original cross hatching visible on cylinder walls when I pulled head at about 130K Quote Link to comment Share on other sites More sharing options...
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