Measured Backpressure on the Vette
 

Long saga, but finally thanks to ThrottleCrazy and 94cobra69ss296 got to measure the back pressure on my Vette. It has some "resistance to flow". It's about 1 psi on idle and 3psi at full throttle. The cats are the main issue, but unfortunately with recent CA law those have to be EO numbered. I could put on a set of LT headers and hi-flow cats and keep the stock stuff for "contingencies" and probably pick up a fair bit of power. I'm at 391WHP now. The motor with headers and hi flow cats (but an LS1 and not LS6 intake) and a few other differences, made about 430 WHP. It's a guess, but I'm think LT headers, hi flow cats, (maybe a new set of injectors as at least one of my is dodgy sometimes) should put conservatively at 410+ I would think :huh:

Will the LS6 intake not fit? You only have 2 1/4" pipes too right? Need to step it up to the 3" pipes and some long tubes.

I have the LS6 intake. The Vette the motor was on had the LS1. I mean to say in this case I breate even better than it did. I have 3" pipes (I think, definitely bigger than 2 1/4") after the cats. I have stock manifolds and cats and that's where the bottleneck is.

After the cats? :toetap:

The stock manifold are not pipes. I thought you were referring to after the cats. Are we gonna have another semantics smack down.

Good do all that then drop a big cam in it!

If I remember correctly the tubing up near the cat was pretty small. I would guess that it was only 2.25. I would also recommend larger exhaust but I don't think you would need larger than 2.5. You would definately make more power with a free flowing exhaust system.

Hey Ben do you have a dyno chart you can share here? How did you measure "back Pressure?" I may want to try it on my exhaust.

There is a pressure gauage that inserts into the O2 bung ahead of the cat.

Here's my last dyno sheet, with the setup as it is. 391/369.

Would love to, but then I wouldn't pass the sniffer anymore. I'm have a 114LSA 224/224. My emissions are still safe, but pushing the boundary.

Awesome, now I just have to wait for you to uprade your exhaust and dyno again :)

I'd like to see that too. I've had some discussions about the negetive effects of backpressure and I would love to remeasure the backpressure on your Vette after the upgrades and then see what the difference is on the dyno.

I want to do it for sure, but new headers, cats, replumbed exhaust and new tune is going to be well over a grand and probably closer to two. I'll be putting that into the project car first. But I will get to this. Not quite sure how best to do it to isolate the effects of back pressure. Just inserting some hi-flow cats in place of the stock cats, measuring back pressure, and if essentially gone, dynoing might be the best way. Changing to LT headers, new injectors and tune, etc., there's just too many contributing factors to increased power.

I agree. Just because there is no longer back pressure doesn't mean that the flow is optimum. Are your cats bolted in between the exhaust manifold and the aftercat or is it welded? If it is just bolted we could probably make a test pipe to replace the cat and then measure the back pressure again.

They're bolted. Pretty easy to remove actually. Good idea. :pot_stir:

Is it a straight shot? If so, it would be really easy to make them. Just go to Autozone and pick up some tube.

If I may hypothesize here, the torque curve will simply shift to the right on the dyno graph with only an exhaust size upgrade with no tune or cam replacement in mind.

edit: wait wait....if I have this right in my head, increasing the exhaust size too much would cause the curve to move to the right (RPM range). Why? Well I figure too large of an exhaust would take a higher amount of exhaust pressure to create velocity, in other words higher RPM's needed. Too small an exhaust we have a restriction of the velocity trying to release.

I don't think so. If anything he'll see an increase across the entire range that the dyno sheet shows but mostly in the upper rpms. Remember that he already has a restriction that even at idle is creating 1 psi of pressure in the system and 3 psi with just a rev. I had a plugged cat on my Suburban and it didn't even have back pressure at an idle. It did however have 3 psi on a rev but only on one side.

Hmmmm I have my head rapped around this now. Need to find something else to occupy my time haha. We'll leave it up to the dyno :)

edit: YA! Without using any formulas and just winging it, anywhere between 2.5" and 2.75" should yield a better power curve. ^^^^This guys is right!! As I said though I think too large and the gains will only show on the top end. I don't know why I'm bringing all this up, it just snapped into my head this whole exhaust theory ordeal I heard about.

You do realize that your car is already really fast don't you?

Btw the torque curve can never just shift. If it changes, it will still cross over at 5250rpm.

I understand, the whole unit conversion causing the curve to cross that exact point every time. You get what I meant though no? lol Just more torque higher up in the range.

Yes. But Sean needs to be put back in his place.
http://www.motorgen.com/garage/times...43&SlipID2=136

That is incorrect. A change to a larger cam will shift the torque curve up. Horsepower and torque will still cross at 5250 but the peak torque will be at a higher RPM.

My guess considering what your current set up and readings are is about 7.5% increase at the rear wheels across the board with headers and straight pipe. 420rwhp/396rwtq. Ron how much did you gain with just the x pipe instead of the high flow cats? I know the 1/4 mile time went from 12.1 down to 11.6.

I'm not sure because I never dyno'd it but I picked up 4 MPH in the 1/4.

That is correct. Both numbers will just cross at a higher point in the 5250 rpm range. The curve has still shifted.

What?

He's agreeing with you.

I was questioning his "Both numbers will just cross at a higher point in the 5250 rpm range". I'm just not sure how they can cross at a high point in the 5250 range. Maybe he meant at like 5251 or 5252.:rolling:

By the way Throttle Crazy is my brother so I have to give him a hard time.

The intersection of hp and torque can still meet at 5250 rpm but be higher numbers.

I knew what you meant. I just wanted to give you a hard time.:D

Why, I would neeeever do that to you.:rolleyes:

Are you talking about the torque curve moving and hp numbers staying the same? Are you saying that by changing cam profile you can move the torque curve without effecting hp?

No to both.

LOL at these guys:lmfao:

you were trying to make a joke, but the fact is that torque and horsepower intersect at 5252 rpm, horse power was invented by James Watt's after observing that a horse could lift at a rate of about 550 foot-pounds per second for about an 8-hour shift. horsepower = torque * rpm / 5252

That's not a shift, that's an expansion or a change. A shift denotes moving the exact curve either up or down the rpm axis. :judge:

Another example of a change, being an expansion, not a shift. :)

And before you say it it shifting up the axis, stop. Because since you can't make hp at zero rpm, the angle of the torque line will be different to originate at 0 and 0.