Back pressure is always bad for a 4 stroke!! What happens when you open up the exhaust too much is you lose VELOCITY, which leads to reversion and an INCREASE in backpressure.

 

Sorry, but you're wrong. Some amount of back pressure is needed in a good system (excluding drag racing) for proper scavenging depending on the desired optimal rpm operating range. Reversion is NEVER a good thing and is different from back pressure. Velocity is in the domain of high rpm, high horsepower engines. Too much velocity leads to more of the intake charge being sucked out of the cylinder during cam overlap. Proper back pressure prevents this.

 

so smaller pipe from the collectors back to the y-pipe (original muffler location) or smaller pipes all the way to the back? i want to know for sure before i mess with it. theres nothing like spending a bunch of money on something and having it turn out to not work like you had hoped.

 

And how is backpressure beneficial to scavenging?

 

thats weird im able to smoke mine with the 3.55 and have same stuff as you. whens the last time you done a tune up ?? check you cat and plenum agian, and if you want more low get the harland sharps on there

 

During cam overlap when the intake valve is opening to allow the fuel/air charge into the cylinder and the exhaust valve is not quite closed. If the exhaust velocity is too high it will suck some of that intake charge out the exhaust. By controlling back pressure you reduce the amount of intake charge that ends up going out the tailpipe during that overlap. That's what is referred to as a tuned exhaust. Reversion is when the exhaust pulse goes back towards the exhaust valve and inhibits the incoming intake charge from entering the cylinder. Ideally, the exhaust back pressure is enough to help draw the intake charge into the cylinder, but not so unrestricted that it pulls it past the exhaust valve. This is considered proper scavenging.

This is why freeing up the exhaust too much can hurt low rpm performance. Less of the fuel/air mix remains in the cylinder due to overscavenging, so less power is made. In a closed loop EFI system the 02 sensor will signal a rich condition to the ECM due to more unburned fuel in the exhaust and cut back fuel delivery even more, resulting in even less performance.

 

I've always been taught that valve overlap is to help draw in the fuel/air charge, the escaping exhaust is helping draw the fuel/air charge in, creating creating a higher VE. I see your point about letting the fuel air charge into the exhaust could be a bad thing but I'm sure it greatly depends on the engine and how much overlap is in the cam.

In any event, you can't have your cake and eat it too, I'm happy with my 3" exhaust.

 

That's exactly right and matches what I said. You want the exhaust flow to be at a level that helps with scavenging, but not too much so that it hurts it. The problem is that we need these engines to operate on a wide rpm range and ideal tuning is usually restricted to about a 1200-1500 rpm range. It has to be compromised somewhere. You can't have great high rpm performance without sacrificing some of your low rpm performance. And vice versa. The stock system is designed for an all-around performance level, but it suffers from design considerations such as cost and ease of manufacture/assembly, so performance often takes a back seat. That's why the aftermarket exists. Unfortunately, you can't tune out the compromises necessary to make a vehicle perform well in all circumstances. What you have to do to be happy is be honest in knowing where you need your gains and accepting of what losses you will have by achieving them. If you want a high rpm screamer, then accept that you will lose some low end performance. If you want your truck to pull like a tractor then don't expect it to run like a drag racer.

 

Also test your MAP sensor.

Some mornings I had no low end, others the truck was quite peppy. I replaced the sensor, and now every morning its peppy.

I recommend testing it before you replace it though, unless you have $75-$100 just lying around.

 

Yes I'm using the 91 tune. Yes my plenum is good, and I'm pretty sure my cat is good considering it's a magnaflow that's only 6 months old. I probably need new spark plugs since I haven't changed them since I replaced the plenum gasket. Would that help with low end though?

I would like to get the Harland rockers, but again, money is an issue, and I'd like to get 4.10s first. Do they help that much with low end?

Do you think my exhaust could be too free flowing? I have the stock manifolds going to a magnaflow kit made specifically for my truck (includes y pipe), then 2.5" tubing into a SI/SO flowmaster 50 series muffler, then the stock tailpipe.

I would really like my power curve to be a lot lower, preferably in the 1500 to 2500rpm range. What can I do to achieve this?