I still dont get it I guess. If you are talking cruisng, then what difference does it make how much power you are making? You aren't calling on any power. I have already had every type of exhaust known to man behind an ls motor. Under no circumstances would I put less than a single 4 or dual 3" exhaust behind it. I actually much prefer the single 4" with flowmaster merge.

 

who goes wot for 2-3 seconds? Whats the point? If someone mods their car/truck for performance I can assure its gonna see wot blast for way longer than any drag strip

 

It's fine that we both disagree. Whether your calling none or little power to cruise you still need scavenging and velocity in the exhaust to produce more power. you still need hp/tq to push the car to overcome all the forces to keep it moving. Point being at 2500rpms cruise let's say your making 350ftlbs and 250hp with a single 3" exhaust. The same situation but a 4" exhaust might be making 30hp/30ftlbs less due to the massive size and even less exhaust going through it. Now say you want to give alittle gas to speed up you already have 30hp more sitting there compared to the 4" which you might have to stomp on it to get the revs up to make the same power. This is an exaggeration and pulling numbers out of my *** but just trying to convey a point. Putting too big of an exhaust on a motor actually hurts overall performance vs too small which just restricts it at the point where it becomes a restriction on the top end. So why do I not care about 20hp at 6000rpms with a 4" exhaust? Bc I'm getting 20-30 more between idle and 4000 where I spend most of my time with a smaller exhaust.

I have literally felt the difference with just a muffler swap on a stock truck exhaust. All was the exactly the same (inlet and outlet DIA and all pipes were reused going in and out of the muffler. I lost a noticeable amount of tq below 3k where I found I had to cruise at a higher RPM to maintain a given speed 2200rpms vs 1800rpms. The truck felt doggy below 3k. Removed the shorter high flow muffler and put a more larger stock like muffler back in and gained the Low end back. In exhaust a restriction is a good thing as long as it's a designed system for the specific application.

I personally have run every type of exhaust on my 3rd gen which had 4 different motors at at least 7 different exhausts. Stock 350tpi with stock exhaust, modded 350 TPI with dual 3" exhaust, 383 HSR with different dual 3" with xpipe, 521 BBF with dual 3.5" collectors into 3" exhaust, xpipe and turn downs at the axle, same 521 with same dual 3" with custom over axle dual 3". Now my supercharged 5.3 with 4" flowmaster merge to a 4" catback dual muffler which I just sold and now I have a 3" summit catback and CME tips. Multiple changes in each system (different mufflers to get the sound I wanted). Honestly my favorite to date is my 3" summit catback. My 4" was too much for the street and down low out of boost it felt alittle sluggish. Now with the 3" car feels more responsive down low and doesn't seem to be any slower up top. I also get to floor the car more and not draw as much attention.

For what it's worth I had a 14k motor before this 5.3 and I can count on one hand how many times I had it above 4500 on the street each time I took it out. It was just too fast and dangerous to be reckless on the road. Not to mention the attention it drew wasn't conducive to my driving record. Street cars don't see redline as often as a dedicated race car. And I cruise at 1600-2000 70% of the time. I only get the floor it on back roads, and highway on ramps. I'd rather the car feel stronger where I spend the most time.

There's a saying that it's more fun to drive a slow car fast than a fast car slow.

I will agree it's stupid to put a single 3" exhaust on a 650hp motor as that would warrant a 4" exhaust but for a 500-550hp motor the 3" just isn't enough of a restriction to upgrade honestly. I'm sure I've gotten off track but the main point is design the exhaust system where you plan to spend the most time. Always go smaller than too large and keep in mind that the flow changes power in different areas of rpm range.

 

If your at WOT (I mean 100% TPS) for more than a few seconds on the street then your either dangerous or your car is too slow. I log my driving and I have yet to be at 100% TPS (gotten close) for more than 3-4s on the street before I need to back out bc I'm going way to fast. 30mph highway on ramp, gas it 90% TPS and 3-5s later I'm at 100MPH+ with cars whizzing around me. Same goes for side streets. Can be in high boost for more than a few sec before I'm doubling the speed limit. People mod their cars performance so they have more power everywhere! no one here or I the world would build high performance motors if they only made more power at WOT. We build motors bc i make Stock WOT 275hp at 3000 instead of 5500rpms. Which means I only need to go 3000rpms to have just as much fun as I would at 5500 with the stock motor. Plus everything above that 3000rpm is just gravy as it's faster and more fun than I could ever have with the stock motor that didn't have the power.

Unless you drive the autobahn I don't see how you could be at WOT very long on the street. remember dead stop drag racing 12 sec cars are pushing 110+ mph which means in 12s your at 110mph. Just about a stock LS is in the 12s zone so most of us are much quicker than that. Which means 10s cars are pushing 130mph+. So in 6-7s your already going to fast for the street. Even if I stomp it at 80% TPS I will kick out the rear tires on the road which still isn't WOT.

If you want I'll look at my 30min datalogs and tell you how long I'm at 90+% TPS I will and I will assure you 30s would be a long time which is like 5% of the time. And I go out to beat on it and tune WOT.

 

probaly hundered of thousands of videos on youtube you can watch of cars at wot for a long long time...not me, but thousands of others

 

at 2500 rpms at cruise you might be 2-5% throttle and at 2-5% throttle you would be lucky if you were making 50hp. This discussion is starting to feel childish because I feel like Im explaining academics.

 

I want you to go video tape or log your car driving for a normal trip you take the car out for. Please... I bet you don't send 1/4 of the time at WOT. Thousands of cars?! How many cars are out driving daily? Hundreds of millions or billions. Your argument would be "they aren't performance cars" even then your number is insignificant and your still wrong as this also applies to low HP OEM style cars. Smaller DIA exhaust promotes low end HP/Tq via more efficient scavenging and exhaust velocity. Its the same damn reason a turbos hot system is better UNDERSIZED than oversized.

 

Why would you assume that? I actually can see 1-2psi of boost at 2500rpms cruising with a locked converter fairly easy (locked converters Increase engine load). and still be under 30% TPS... still think I'm making 50hp? For arguments sake your still going to think that your 50hp worth of exhaust is optimally flowing through 4" of exhaust pipe vs 3" of exhaust pipe? Exhaust only cares about volume and velocity, You can't have both optimally so you have to compromise. Increase the volume and you loose velocity/scavenging at certain RPMS which makes the car less responsive. Decrease the volume and increase the velocity, responsiveness goes up but you limit Peak power. Lets also not forget that Exhaust velocity is high and the gas is HOT coming out of the motor into the headers, as it reaches the tail pipes the exhaust has cooled significantly and thus has slowed is velocity even more. This is compounded as the exhaust gets larger.

I think you have no data or just believe that bigger is always better. Prove me wrong and this would be all over. I will happily provide injector flow duty cycle, RPM, MAP pressures, TPS Voltage and TPS % at cruising at 2500rpms and you can eat your own words. 2500 rpms coasting down hill is not the same on a flat or a incline grade, technically all cruising still if RPMS don't increase. For base I idle with 4% TPS and a .53 TPS voltage.

Pulled from an online source for performance exhaust sizing.

It is exhaust velocity that will suffer and the velocity is more governed by engine displacment and RPM range in relation to pipe size. Maintaining exhaust velocity will create a low presure area at the exhaust valve, which will help cylinder evacuation. Too large of pipe on an engine will allow exhaust expansion (slowing the flow) which will cool the exhaust stream (again slowing the flow), and cause additional turbulance within the pipe (again, slowing the exhaust flow). I copied (and cited) this from Axle Performance.

"Too large of diameter exhaust will cause a power loss and loss of low end torque because a larger pipe has less exhaust stream velocity than a smaller pipe. If the exhaust pipe is too large, then the exhaust flow will be slowed with less velocity. There are exceptions, forced induction or high-volume race engines require a larger diameter exhaust due to the flow characteristics of the engine. Supercharged and turbocharged vehicles can have an exhaust gas volume of 1 to 2 times more than an equivalent displacement naturally aspirated engine. In this case, one is able to use a larger diameter of exhaust for greater performance.

Heres an interesting read on a track exhaust for an M3 Vs stock exhaust. Car lost 64ftlbs at the rear wheels at 3400rpms by increasing the exhaust size for top end power...this is just due to the loss of exhaust velocity. That was compared to STOCK exhaust (manifolds etc).http://www.rogueengineering.com/Rogu...aust_p_88.html




Header primary sizing in relation to exhaust velocity tq loss below 3500rpms, pulled from http://garage.grumpysperformance.com...-headers.1303/


Smaller diameter pipes flow less volume than larger ones, but the exhaust in the smaller pipe flows faster. Until you reach the RPM where the sheer volume of exhaust gases require bigger primary tube diameters, smaller tubes will scavenge the cylinders more efficiently. If you are using the engine in the 1,500 to 3,500 RPM range, which is typical for a street-driven vehicle, you definitely want 1-1/2" to 1-5/8" primary tubes for any small block and 1-3/4" to 1-7/8" for a big block engine. Any bigger and you will lose a considerable amount of low end torque. beyond 3,500 RPM it is a question of where you want the power peaks. As you can see from the charts on this page, small tube headers do not lose their edge in horsepower and torque until you exceed 5,500 RPM.

Even if you're running a radical camshaft and blower, you're better off sizing your headers smaller rather than larger, unless you plan to spend most of your driving at full throttle. We size our headers correctly for even the most heavily modified street motors.


With this all being said, I dont feel like this is a "childish" discussion, I think people who are set in their ways are harder to convince when something new and different (information or way of thinking) comes along. I'm at fault here as well... I grew up being taught that Pluto was a planet, now they say its not one... guess what, its still a planet in my eyes. I dont dissagree with you about larger exhaust making more peak HP, that is 100% correct if the exhaust was a restriction in the top end. I'm only trying to argue that while gaining a small percentage up top you will be loosing power under the curve as well (its physics!) this could typically be more than the gain up top. So if your car is street driven would you rather more power under the curve or slightly more up top?