Contour Enthusiasts Group Archives Forums Modifications Duratec Performance ? about back pressure how little is to little?

After my problems with my off road y-pipe my mechanic said that my engine needs back pressure. this sort of makes sense

gutted main cat- increase felt
gutted pre cats- feels like a decrease

Did I do more harm then good? should I get a high flow main cat? some of you are running 3 inch exhaust and y-pipes without a problem why am I experiencing this. or will I be ok with an offroad y-pipe? should I get one with a cat when I send this back?

thanks for the input.

:(could someone please help me out?

I'm really baffled here don't know what it is but my car feels slower ever mod I've done has done nothing except my intake.

what would ya'll do in my situation.

I'm going to go out on a limb here .. and expect the flames to follow. IMHO you will probable not be able to get to the point of 'not enough back pressure'. My recolection is that the need for some minimum level of backpressure was to prevent burning the valves.... and I personnaly recall many a Sunday at the drags with open headers and no burned valves as a result .. so obviously I hadn't reached the 'minimum' level yet.

If what your mechanic was saying about back pressure was true, then a set of headers (i.e. no pre-cats), no main cat, and a low restriction cat-back would not provide enough backpressure to allow efficient operation. I don't buy that.

I think it is much more likely that something went wrong in the process of gutting your pre-cats. It could be you didn't get everything cleared out .. it could be your computer hasn't optimized for the greater airflow ... it could be a lot of things. But I don't think adding backpressure will make things better .. of course you could try the old potatoe up the tailpipe trick from Beverly Hills Cop to prove me wrong

But that's just my opinion ... I could be wrong.

Here's an idea .. I noticed in your sig that you have 'ground and polished' your MAF sensor. If I was looking for a place to 'debug' that would be my first stop.

I think back pressure is a myth. I think when you open the exhaust, you change the RPM point of maximum scavaging. That is, where exhaust gas leaving the pipe effectively creates a vacuum in the cylinder, helping pull in more air fuel mixture.

Larger diameter pipes mean lower exhaust gas velocity. This means less scavaging.

Of course, at higher RPM's there is more exhaust gas trying to get out, so it must go faster to get out.

So larger pipes can improve peak HP, but usually at the price of a bit lower low end torque.

So if you've opened up the exhaust, you probably have a bit more on the top end.

The question is, where do you spend most of your time? Top end, middle, or bottom end.

I reserve the right to be wrong, I'm an EE by training, not an ME.

Cheers,

TB

THANKYOU. thats what I've always believed but I had 4 mechanics around me nodding there heads and going engines like this need backpressure to make power. dyno says no. common knowledge says no. as soon as I get my car back tommorrow for the custom cat back. I'll never go to %&*#ing tuffys again.

Backpressure creates torque, but decreases power, so you could possibly free up your exhaust enough to make your car slower if not enough horsepower is created to compensate for loss in torque.

maybe with the whole gutted cat thing you have a turbulence problem. anyone ever complained about losing power with a straight pipe instead of a cat?

just a thought

-jason

Well, do you have your res still??? I am running stock headers and precats, off road y, and a res. You can see what it did for my dyno in my sig.


As far as backpressure. I could not really tell you. I have heard reasons for both sides. I see less back pressure more hp, more TQ. I am going to eliminate the pre cats in the near future. Are you running MIL's?

This is just the myth we are trying to dispell!

Loss of exhaust veloctiy causes torque loss. This happens when you pick an exhaust pipe diameter that is too large... Velocity and scavaging drop; therefore torque drops (Horsepower is a direct derivative of torque BTW)

Removing obstructions in your exhaust will not cause you to lose torque as long as the exhaust pipe diameter does not get too large. It only increases horsepower achievable (sometimes TQ increases as well depending on how bottled up the system was to begin with)

There is all sorts of phyics going on that are being ignored.

You have to look at Static Pressue, Velocity, Volume, a butterfly beating its wings in china... etc.

A tuned system (ala HO Quad4) really can't be improved that much.

I remember readint in Grassroots Motorsports about their project Porsche 944, and how they put a Borla system on it and it was *SMALLER IN DIAMETER* than stock. And yet they had power increases from it.. Can you say velocity?

I would bet that gutting the cat and pre-cats cause some sort of flow problems that reduce velocity and thus reduce power.