96LT1TX

I just installed my cam, and w/ my lt1 it seems that backpressure in my car w/ the cutout capped stalls the car up when I shift gears? is this an illusion? If this is so, at the track I might actually run better shifting into 2nd @ 2800 or so rpms instead of 2400 or so w/ it un-capped...

I just want to get the best ET out of the car w/ the cam... w/ headers and full exhaust I only ran a 13.7@103 (2.2 60'...)

btw: I just bought nitto 555r 275/35/17s too
any advice appreciated!
Eddy

Projectz28

its probably just in your head. You will make more power with it open. I picked up 13RWHP on a dyno with it open.

96LT1TX

yes, you gained that @ peak rpm... what was your tq losses if any? on lt1s (at least mine) w/ stock cam max tq is about 2500rpm... so if you switch above 2500rpm wont it act like a stall and get all the tq when you switch? Im not saying I switch into 2nd or 3rd at exactly 2500 rpm just making an example... next time I can rev past 3000rpms i might have to tell yah
W/ just headers and full exhaust I gained like 5rwhp w/ the cutout... im just not sure how much under the curve I have lost or what not is lost w/ the loss of backpressure.

Capn Pete

I agree that you should un-cap your cut-out at the track.........that's pretty much what their original intent was, so you could drive around with quiet exhaust on the street, but open it up to get the most power at the track.

Am I reading you correctly though? Do you actually shift gears at ~2500 rpms???! That just doesn't make any sense at all??? With an LT1, you should be shifting at no less than ~5500 rpm, and with your cam, you should probably be shifting at ~6000 rpm or higher. What are you talking about with 2400 - 2800 rpm?!??!?! (or are you referring to the speed that you bring the engine upto before you launch? ).

Projectz28

even on a stock cam peak torque is still higher than 2500. Everything you hear about backpressure causing you to lose low end tq is plain silly. I gained 13 RWHP and 25 RWTQ. When I switched to longtubes I picked up 39RWHP and 55RWTQ. Backpressure is your enemy. The more air you can get out the more you can get in. The more you get in the more HP you make. Remember... and engine is nothing more than a glorified air pump...

Uncap the sucker and let her rip. The only thing you lose with it uncapped is your hearing.

96LT1TX

lol, I meant that after you shift out of 1st gear at redline (now 6200) the revs match in 2nd at about 2500rpms...

Steve Y

No it's not. Too little backpressure will definately cause a low end torque loss. Try running headers straight into the air and look at a dyno compared to a full exhaust.

Steve Y

If you have the stock M6 trans., then your 1st and 2nd gear ratios are 2.66 and 1.78. That means if you shifted out of 1st at 6200 rpm, you would be at 4150 rpm in 2nd.

Capn Pete

Sorry, I still don't get what you mean? Stevy Y is right, the revs after your shift would be up around ~4000 somewhere, not 2500. Are you sure you're not shifting 1st to 4th?!

96LT1TX

well w/ stock 3.42s switching out of 3rd into 4th I thought my rpms were kinda low... sorry I'm in withdrawal from driving my car past 3krpm (new cam) so maybe im remembering it wrong

ill have to repost once I can drive it fast again

Got-LT1

Read this:

"No, you absolutely cannot have too little backpressure. That is one of the biggest and dumbest fallacies that exist in relation to 4 stroke IC engines. Why? I'll tell you.

Pumping Losses --- The concept of backpressure means that there will be a high pressure area at the exhaust port. When the valve opens, the escaping exhaust gases have to push against that high pressure area. How can a parasitic loss be good for your engine? The only engines that NEED backpressure are various small engine designs, mostly 2 stroke. The problem with them is they have the tendency to be too effective at driving the exhaust out, loosing compression. I assure you that on a big 4 stroke engine such as an SBC you want the exhaust to have the free-est path possible. In fact, it would be ideal to have a low pressure zone at teh exhaust port. A low pressure area would help to pull the exhaust gases out. Freeing up more power that your engine would have wasted pushing the gases out...which brings up the next topic beautifully.

Scavenging --- We've heard this before, but what does it mean in relation to our discussion of exhaust. Easier for me to use an example. So your engine fires and now it's on the way up on the exhaust stroke. Exhaust doesn't come out at a consistent rate, it comes out in pulses. Each pulse is a high pressure area, and as it moves, it leaves alow pressure area behind it. Aha, there's our scavenging. You want that low pressure area to be at it's peak when the exhaust port opens on the next exhaust stroke. Thats another reason why headers make better power than manifolds. besides just flowing better, instead of all the pulses being dumped in a log fighting with each other, the tubular runners allow the exhaust pulses to stay seperate and create a nice low pressure area behind it. This is also where tuned and equal length headers come into play. Tuned headers are sized such that the length of the tube corresponds the speed of the exhaust pulses so that the low pressure area is maximized at certain rpms. No surprise that short headers are better for high rpms than longtubes.
Problems can surface if you use too large of a primary diameter, loss of torque. The morons are quick to spout 'you lost backpressure and thus torque.' Next time you hear that you will smile and know that that person failed physics in high school. The problem with using too large of a primary is this. The exhaust pulse only has so much gas and energy in it. If the tube is too large, the pulse expands to much, losing energy and thus velocity. When it loses velocity, it can potentially stall and stop moving in the tube, or at least slow down. aha! Too large of a header actually CAUSES backpressure, and thus lost power. We feel this power loss as a loss of torque because usually this effect is much more pronounced at low rpms as much less gas is moving.

The same principles apply to the entire exhaust system, from primaries to collectors to pipes to mufflers. I am too tired to explain it all, books have been written on these topics. I have just scratched the surface, but hopefully you all understand a little better why their is no such thing as good backpressure. I know some of this has been a little oversimplified, but it think it gets the message across.
thank you please drive through~~"

Steve Y

This stuff you posted is theory. Dyno testing is much better. Long tubes have been proven to make more high rpm power than short headers over and over. Yes larger tube diameter loses low end torque. Larger tubes offer less backpressure. Try sucking through a small straw or a big one, which is easier? Again, too little backpressure from an exhaust has been dyno proven over and over to cause a low end torque loss. Same with big short ports on an intake manifold. Theory is never as good as scientific tests (dyno testing in this case).

Dragon Racing

I have yet to see anybody dyno their car with the cut out capped and then un-capped and not see an INCREASE in torque from having the cut out un-capped.

Steve Y

I am talking LOW END TORQUE (like from idle to 2000 rpm). That's where too little backpressure usually results in a loss.

Projectz28

2 things.... first is if we are talking racing here, not even a bone stock LT1/LS1 is at less than 2000 rpm when racing. On a stock convertor the flash rpm is usually just over 2000. After that at no point during a 1/4 mile run is the motor at 2000 or less.

second, "if" you can prove it its not on a chassis dyno. I have yet to see a dyno sheet with a starting rpm of 2000 or less.

I'll give you the benifit of doubt here. Lets say you do loose some low end torque (idle to 2000), the gains you can and will see above that FAR outweigh the losses at such a low rpm.


on a dyno, I picked up 25 RWTQ "peak" with an open cutout. But I can also tell you the overall gain was easily over 15 across the board. Dyno pulls started at 3500 rpm right thru 6600.