what dose it do exactly?
i know u need it for low end torqe,
but what dose back pressure do to create low end power?
dose it slow the flow of intake and exhaust for a split second to let the engine and fuel charge to catch up with the incoming air

any help would be greatly appreciated
thanks

 

Its the suction effect of one cyclinder pulling the exhaust charge of another cyclinder to promote better cyclinder filling for each revolution.

Realisticly, you don't need to worry about it.

Unless you get nascar/circle track headers it isn't really an issue.

I've never seen dyno proven torque loss on a street car with any headers you can actually drive. On a street car, your headers will NOT be the restriction.

And on a race car, you aren't really in any of the RPMS that torque loss could even theorhetically occur.

So basicly, don't worry about it.

Some more info on your setup would probably provide some better answer.

 

i was just wondering what it realy dose,
i have custom exhaust,but have the stock headers
it is a 3" mandrle bent pipe all the way back to a
2 chamber flowmaster ( flowmaster is all i have right now
until i can get a gmmg or somthing eles )
no cats.
and befor, i could spin my tires swithing through the gears
and now that i took the cat off and put the pipe on it only chirps the tires, low end droped, but the top end pulls realy hard.
just thought i would figure out exactly why
thanks

 

oh! for got to say it also has the T/B and intake ported ram air, and cooling mods to run cooler, all of this was before the exhaust though

 

any body,

 

There is no such thing as an engine "needing back presure," that is a myth. What it needs is exhaust velocity; a moving column of exhaust gasses that is still moving the next time the valve opens helps pull the exhaust out of the cylinder. If you put some masive 2.125" headers on your engine, the exhaust pulse might come out and just stop, causing a loss of exhaust gas velocity. I don't think this is your problem, though. You might want to Atap your engine and see if everything is in line.

 

One of the local tuners who ARE not going to be mentioned talked me into headers and X-pipe on my roots blown C5. Should expect at least 15HP, opps we lost 15HP/T. Neeldless to say I paid the bill and never got one phone call returned again.

The result was that I lost 1# of boost, The engine ran rich 10.5 and there ARE codes from the O2 sensors P1153.

With less boost I was able to advance the timing and lean out the fuel with no knock and get back close to were I started.

The theory as explain to me is that the Heads need more flow to maintain manifold pressure since more air is flowing through the valve overlap. The MAF still sees the same air flow as before but the cylinder charge is now richer. ie. same flow and same fuel but more air being wasted. The headers ARE now affecting the Volumetic Efficiency, the Injector Offset, and the LTFT amoung other things. These ARE usually not tuned for new Headers.

I suspect that a NA car will not experience these conditions to the same extent.

The roots blower is fixed compression it cant make up for the increased flow unless we undersize the pulley or restrick the exhaust flow.

The benefit of the header is that it will remove more heat from the chamber, this alone was worth the expense.

But then again I could be wrong.

 

In a scenario I experienced yesterday at dyno:

Difference between cutout open and cutout closed Torque Dyno readings: 18 lbs/ft rwtq.

Cutout closed max torque @ 3750 RPM
Cutout open max torque @ 5000 RPM

I'd rather see 18 more rwtq @ 5000 RPM.
HP peaks stayed at 6250 RPM in both cases, with 23 HP gain on an open cutout.

 

not to hijack the thread but since you dont need backpressure if i were to run dual cutouts, one right after each 1 3/4" header, and a 150 shot would the cutouts hurt or help? i understand what you are saying about the charge pulling the air through but would all my exhaust be gone before the next time the valve opens? should i put them farther back in the pipe to make sure the gas is still in there by the time the valve opens again?

 

Running dual cutouts would lose the scavenging quality of the x or y-juction, wouldn't it?

 

I recently bugged a bunch of folks about this issue.

There does seem to be a relationship between HP/TQ and exhaust.

Too restrictive of an exhaust and a car does not make as much HP. Too free flowing the torque might, and the car might not necessarily go faster.

BUT most of our stuff works well with 3" exhaust.

 

FLP's with dual cutouts here. I picked up 18hp the entire range, and 26ft lbs torque the entire range NA. On the gas I have no idea, it was first time dynoing on the gas and I didn't have much time to fool around with opened and closed cut-out's on the gas.