I know you attach a pressure guage to the O2 sensor fitting. But what size is the fitting? I can find SAE fittings with nipples for the vacuum hose. But it doesn't quite fit right. I'm thinking the O2 sensor is metric and I don't want to force the wrong size fitting in and mess up the threads.
I want to check the back pressure to do a before and after comparison for my new Flowmaster.
Take an old O2 sensor out of a car in the junkyard, have the center machined out and install a fitting of your choice in place.
I personally know a machineshop that would be able to releive you of some of your $$'s to do this project All donations of $$'s go towards my own personal Fiero projects so your money would be well spent LOL
What do you plan on using to read the back pressure? A PSI gage would be way to high a scale for this. You will need a water column manometer to read the low pressure you should see. You can make a simple homemade water manometer for this purpose.
------------------ Advanced Dealer in the Great Material Continuum . . . Non Fiero Website: http://tractorsport.com You can't race a dyno . . .
I've got a pressure guage that will work. I just need to figure out how best to tap into the exhaust system. I can always weld a nut to the pipe, drill the center and screw in a nipple fitting. But I was hoping for an easy adapter for the O2 mount.
88 Formula...are you saying that you can measure manifold vacuum to determine exhaust back pressure? I have a pressure/vacuum guage that I'm going to use. But I don't understand using the vacuum scale to determine backpressure.
Nice engine install:-). Where did you get the 3.4 from?
If you can read backpressure off a vacuum gage on the manifold side you have some serious blockage somewhere in your exhaust or way to much overlap in your camshaft. That much back pressure would not allow a new charge to come into the cylinder. You should have no more that a few inchs of water column (if that), not a few inchs of mercury. 1 PSI = 27.71 in/H2O or 2.036 in/HG. Just my $.02 worth
------------------ Advanced Dealer in the Great Material Continuum . . . Non Fiero Website: http://tractorsport.com You can't race a dyno . . .
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10:26 AM
88formula Member
Posts: 2361 From: Worcester, MA Registered: Oct 1999
What goes in must come out! An engine is nothing more than a fancy air pump. The more air it pumps the more fuel that can be added and the more power it makes. If you are trying to determine if you have a clogged or restrictive exhaust you can use a vacuum gauge hooked up to engine vacuum to do this.
On a normal engine if you snap the throttle open and hold it there around 4+rpm the vacuum will drop to almost nothing and then quickly rise to a reading above what it was at idle. If the vacuum drops and then slowly rises or does not rise to a level that does not at least equal the idle measurements than you have a clogged or restrictive exhaust system; usually a clogged catalytic converter. If you hold the throttle at about 4500 rpm or more the vacuum should be higher than 20 inches of vacuum. If you see the vacuum increase and then start to decrease while holding the engine at high rpm than that is a sure sign of a clogged exhaust. The instructions that come with most vacuum gauges usually explain this test in detail. Also some vacuum gauges come with a cone shaped adapter that you can just hold against the O2 sensor port, but don’t hold it there to long because it will melt. Try it both ways!
The vacuum gauge I have reads both vacuum and pressure.
[This message has been edited by 88formula (edited 07-09-2001).]
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04:49 PM
PFF
System Bot
88formula Member
Posts: 2361 From: Worcester, MA Registered: Oct 1999
BTW the 3.4 I got was a used junk engine I got from a 95 Camaro. It was not in running condition because it sat outside and had rust in the cylinder bores. I rebuilt it too much better than stock (blueprinted) with ARP rod, main and head studs and a GM performance Camshaft. It has ported heads, SI valves, and a performance valve job with Crane Roller fulcrum rocker arms. It also has stock type (dished) hypereutectic pistons and a full balance including the damper, aluminum flywheel, and pressure plate.
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04:56 PM
Tom88gt Member
Posts: 395 From: New Orleans, LA Registered: Dec 1999
GOT IT! Auto parts stores sale spark plug anti-foulers. Got an 18mm one attached a 3/8 brass fitting and sealed the threads with JB weld. Then a 1/8 nipple fitting with threads into the 3/8 fitting. Perfect fit into the O2 port and my pressure guage hose slips right onto the nipple. Cost...about $5.
Does the car have to be driven while checking the back pressure? At idle I get a reading of 0 psi. Reving the engine makes no difference. Still 0 psi. I have a couple of generic books on building performance engines and they seem to imply that the car must be driven during the test.
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02:49 PM
Alex4mula Member
Posts: 7410 From: Canton, MI US Registered: Dec 1999
0 PSI?? Wow! That's a true free flowing exhaust you have! May be wrong but I don't see why the back preasure would be different under load. But higher reading should occur at higer RPM (i.e. more air flowing).
As I stated before and I'll state again if you are seeing back pressure in the PSI range you have some serious blockage and not just a restriction.
This needs to be tested in the water column range not PSI but then again thats just my $.02 worth Bring your mufflers on over and I 'll show you on my flowbench how much restriction there is
------------------ Advanced Dealer in the Great Material Continuum . . . Non Fiero Website: http://tractorsport.com You can't race a dyno . . .
Most comercial back pressure gauges measure 0-10PSI. As I remember, More than 1-2 PSI indicates a major blockage. (I have notes someplace can't find them now.) Gauges that read higher than 10PSI don't have enough resolution to see a 1-2PSI change. (Brucepts The water column would be more accurate but most people don't understand in/h2o. That's why the comercial gauges read PSI.)
That homemade gauge plug idea was along the right idea... DON'T drive the car with it installed. JB weld won't take the heat for that long before it blows out. You would have been better to get someone to braze it together.
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07:26 PM
lowCG Member
Posts: 1510 From: seattle,WA U.S.A. Registered: Jun 99
Red,hi-temp silicone would work better than JB. I need to do this as well;the muffler guy back in MI decided to attach my 2-1/2" stainless system with a piece of 2" tubing,not sure if it's a restriction or not since it's only a 7" piece.
I didn't see the JB Weld part, no, it won't survive. But while the book says to rev the engine in park, that will tell you if the cat is clogged but not give an accurate reading of the loaded backpressure.
Also, you have to pick an RPM. I use 5000-5500. That way you always have a reference.
My 3800 SC with the stock exhaust would hit 10 PSI under load, but never go over 2 in the driveway. With the 2.5" and Spintech, it won't hit 3 PSI underload.
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11:10 PM
PFF
System Bot
Jul 17th, 2001
Tom88gt Member
Posts: 395 From: New Orleans, LA Registered: Dec 1999
Took my Jeep out for a test spin (the Fiero's exhaust is still out waiting for my new flowmaster). Anyway at idle it reads around 0 psi.
Driving normally it only gets around 1 psi or so. Even driving at 70 mph/2000 rpm and flooring the gas going up hill gets maybe close to 2 psi. down shifting to 4th gets the rpms up to around 3000/75 mph and backpressure to about 3 psi. If I drop down to 3rd and push it to 4000 rpm at 80 mph I can get almost 6 psi.
My exhaust is a generic turbo muffler from the local auto part store and the cat was hollowed out when I bought it. I wonder if I replace the hollowed cat with a straight pipe for smoother flow would make much of a difference in this application. A Jeep is driven at lower speeds and rpms anyway.
The J&B weld? I'll inspect it closely after the exhaust cools.
All donations of $$'s go towards my own personal Fiero projects so your money would be well spent LOL
