T Nation

Vehicle Maintenance Help

I’ve got a nice snap on temp gun that I’ve been determined to find a use for to justify the price lol I’ve only tried the check the temp on the cat thing a couple times and it never worked out how I wanted.

I do have a cool little pressure gauge that goes into the upstream O2 port that is useful.

You could also monitor vacuum and get a rough idea on something like that as it would loose the ability to scavenge the cylinders properly.

I’ve done the carb cleaner test to find leaks in the past. Seems to work well enough for me. I found that I had an o-ring on my injector with tears in it. That gave me rough idle, but MPG was about the same and it didn’t ever stall out.

Yea this is a little different. It’s not a leak, it’s the inability to maintain correct vacuum due to an exhaust flow restriction.

Goes along with the theory of “back pressure”. Which is a giant myth lol. All about velocity!

Cylinder scavaging happens when for a brief moment both intake and exhaust valves are open and it uses the velocity of the exhaust leaving the chamber to cleanse the cylinder of all burnt fuel/air/exhaust. Throws off the whole engine (aka giant air pump) when the exhaust has a restriction.

I think it can make sense for a given tune (lowering back pressure on a stock tune might not work out well, and I think that is where the myth comes from). However, increase in back pressure means more load on the piston and loss of torque on the crank, and therefore is using horsepower to push out exhaust. So if can tune for a lot less back pressure (especially in a forced induction car), it should increase efficiency, which means more horsepower. In an NA car, the velocity thing seems to play out more. Getting a vacuum behind each exhaust burst, and sucking the next one with it.

You’re not making the car weak by lowering the back pressure. The reason your civic doesn’t like 3” exhaust had nothing to do with the lack of back pressure. It’s velocity. You create turbulence and then fuck up the air trying to leave and ruin it’s ability to scavenge a cylinder with a bunch of trash air in it.

You can’t burn things twice, so when you can’t get all that dirty used air/fuel out of the combustion chamber (scavenging) , you lose valuable space to stuff new FRESH burn able fuel and air.

Lost me here. But maybe tune means something different to you then me lol

I agree. It seems with forced induction going bigger on exhaust, and lowering back pressure and then tuning seems to usually bring more hp. I think the velocity thing plays out more in NA cars. I think after the exhaust goes through the hot side of the turbo, that it doesn’t do much pulling of the next exhaust burst. Having a bigger exhaust means less turbulence, and more efficiency (if the car is tuned for it).

Basically, what you want to do is make it the least amount of work for the upward travelling piston to push out the exhaust. If that means certain bends to get the most scavenging (pull with vacuum), then that is what should be done.


Also, just wanted to say I didn’t miss this, and I like you even more for this statement :joy: my truck had a stopped up cat, now it has two empty cats lol

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I’m thinking of going catless downpipe and catless exhaust on my Mazdaspeed 3. I guess with that and an intake and conservative tune they can get 50-60 more hp.

Heck yea! It’s amazing what you can get out of some of these cars by just letting them breathe better

Few other things needed like high pressure fuel pump intervals, and an accessport (can do all the mods for 2000ish). If you get really crazy, the stock block can handle 350 whp (many/most say 400whp, but then you are getting into crazy mods, like meth injection, but I guess that is the number that is thrown out all the time in the stock block). IMO, with fwd it would just not be all that useful. If I mod it, I’ll shoot for 300 whp. Not bad for a small hatchback.

I’m just glad people are now making upgrades pumps and stiff for DI engines! For a while you couldn’t get anything.

I had an integra that was turbo’d close to 5ish. It was stupid :joy: you could steer, or you could accelerate, but you had to choose ONE :joy: