94 honda civic vx issues

[Ecky]

You might see a MPG loss by swapping to a larger pipe, and probably no power gains either with the stock intake manifold being the limiting factor. A new muffler would be entirely "cosmetic".

[RatherDashing]

Originally Posted by Ecky

You might see a MPG loss by swapping to a larger pipe, and probably no power gains either with the stock intake manifold being the limiting factor. A new muffler would be entirely "cosmetic".

Understandable.i dont care bout more hp.how big of a mpg loss we talkn bout?

[Ecky]

I don't have a VX and I don't have a before and after of my car either - when I got it, it had a 3inch pipe and an obnoxious muffler and I've been too cheap to replace it. Couldn't give you any numbers.

Whatever the losses are though, it's not from backpressure as is commonly thought but actually quite the opposite. The stock system is tuned to maximize "scavenging" which to my understanding is when the exhaust pulse of one cylinder travels down the pipe and causes negative pressure exactly when the valves open up on the next cylinder to fire, and actually pulls the exhaust out.

It seems the ideal pipe size is "just big enough" because it maximizes scavenging without restricting.

[RatherDashing]

Understandable.im not even if throwing the muffler im looking at on it would even give it much more sound.on my previous 89 crx with a stock d15b2 engine i put a cheap ebay 4in muffler on it and supprisingly it sounded really nice.then i had the cat converter taken off and a high flow cat converter straight pipe put on that was really cheap too and it sounded even better.idk driving a quiet stick shift really gets to me haha.and if throwing on an apexi n1 4in muffler on it gives it descent sound and i can still have high mpgs then id enjoy the hell outa this car

[Ryland]

I'm on my 2nd VX and my brother owns a VX and something I've noticed is that they do not like after market over sized exhaust, that for whatever reason it leads to an early 02 sensor death and on a VX that is a $200 to $350 part that keeps getting more expensive! keep a good stock exhaust on the car and your o2 sensor should last 200,000+ miles, over sized exhaust like I had for a while and the o2 sensor lasted around 10,000 miles! fixed the exhaust and replaced the sensor and it's been going strong for around 80,000 miles in one car and 40,000 in my other, when someone says they were shocked that an o2 sensor was going to cost them $350 from the parts store they are shocked when I ask them if they have a loud exhaust, I've also seen a 2-3mpg increase in mileage switching to stock exhaust.

If your shift light is not working it might be from being heavy on the gas pedal, pushing on the gas tends to push it out of the peramitors that tell the shift light to turn on.

[RatherDashing]

Alright.well my idea of putting a larger muffler on can be completely thrown out the window at this pt and im ok with that. Wut about the air intake on it now? Wud it hurt at all putting an aftermarket one on?

[Ecky]

I would think it would be a bad idea on vehicles that use a MAF sensor, but your and my Honda have MAP sensors. A cold air intake will give you a bit more power and a bit less economy, while a warm air intake should do exactly the opposite.

You should get more opinions on the other effects, I'm not terribly knowledgeable on the subject beyond what I've said.

[Ryland]

If you want to make a new intake then get some books on intake design, but if you do the math in them for the perfect intake size you will most likely end up with something about the same size as what you have now, going larger will hurt your mileage, getting rid of the chamber that is under the air filter in the bumper will also hurt your mileage and make your car louder, warmer air will help your mileage colder air will hurt your mileage.

[RatherDashing]

dang. well iv made up my decision to leave this thing stock. my next question is out of curiosity but what about GFchips.com. would this really positively effect the vx much more?

[Ecky]

From GFchips.com:

The ideal air fuel ratio (mixture) is termed stoichiometric. This means that there are 14.7 parts air to 1 part gasoline. By achieving this air fuel ratio an engine is able to run more efficiently, meaning better gas mileage, better emissions and more horsepower. Through rigorous research we were able develop a system that adjusts the factory ECU's settings in order to produce this ideal fuel mixture.

When you are full throttle your engine load is increased dramatically over normal freeway cruising, so the chip tells the ECU to compensate for the larger volume of incoming air by injecting more fuel into your engine and advancing the timing, giving you increased overall horsepower
When you are cruising on the freeway or in the city the load on the engine and incoming air is little so the chip tells the ECU to use less fuel in order to achieve maximum fuel economy

The factory ECU already does this in all cars, and the VX does it better than most cars in part because it uses a very sensitive O2 sensor - hence the $300 replacement costs. The VX's lean burn intentionally leans out to 18-22:1 under low load conditions (lean burn) to save you even more fuel.

Honda has tuned the system such that it doesn't burn any valves when running these super-lean AFRs.

Ideal ignition (spark) timing can only be utilized once ideal fuel ratio has been achieved. By advancing the spark timing the piston is allowed to have a longer power stroke, and achieve a more complete burn of the gasoline vapors inside the combustion chamber. These factors are essential in creating more efficient power for a smaller amount of gasoline.

Due to government regulations on emissions and power, many manufacturers program your car's ECU with very modest fuel and spark settings that negatively influence your car's power and gas mileage.

Advancing ignition timing can also be achieved in your car by loosening the bolts on the distributor and advancing (turning it toward the firewall) ~2 degrees. This should probably be done with a timing light (and not done lightly), as advancing too far can have some pretty serious consequences (such as a blown motor), hence Honda shipping them from the factory with somewhat conservative settings.

That said, your car is an extremely fine-tuned machine from the factory and almost any aftermarket modification to it will just make it worse.

EDIT: If you want a mod that will improve gas mileage and improve your car cosmetically at the same time, I'd suggest lowering it ~2".