I had a revelation (re: Brain Fart) while thinking about those running long tube headers and/or hi-flow cats who see lower boost numbers...


Here's an example:

Stock internals Lightning, stock manifolds, stock cats.
6# Lower
CAI

Sees 14# of boost in 60deg weather.


Person changes to long tube headers and hi-flow cats.

Sees 12# of boost in 60deg weather.

WHY?

Most (if not all) motors have overlap on the intake/exhaust cycle. This overlap is called "scavenging" and is used to help "pull" in the intake mixture as the exhaust valve is still open to increase power and efficiency. There's a balance to this scavenging, and as the exhaust side of the engine is opened up more or is made less restrictive (long tube headers, hi flow cats), the exhaust gases tend to move quicker out the exhaust port. This faster moving exhaust flow will actually pull in more air as the intake port opens, which in turn will lessen the "backup" (or "backpressure" if there's such a thing) showing a lower "boost" number.

Those who run just hi-flow cats can often see a lower boost number as well, if the exhaust is truly flowing faster through the exhaust system. Not always true, due to the next issue:

The caveat to all of this is that heat also helps expedite exhaust gases through the exhaust system (re: turbo cars running header wrap for higher exhaust velocity). Long tubes help speed exhaust flow, not through heat, but through a vacuum effect, as the exhaust "pulses" aid in pulling each other through the system. On a system using both long tune header and hi flow cats, these pulses have less interference, and hence, make the exhaust flow more "efficient." Just running hi-flow cats may actually aide in expediting the exhaust out of your system quicker due to less restriction, but your exhaust may run cooler, creating a catch-22. The hi-flow cats alone can aide at higher RPMs, because there's less time for the exhaust pulse to cool, and the losses are minimal. This is why those running hi-flow cats only tend to see a loss in torque - lower RPM range there's less of a negative pressure in the system due to cooler exaust tubing, less scavenging.

Why are "shorties" (re: Bassani headers, etc) not as efficient as long tubes? Mainly short tubes dont allow as much for the "vacuum effect" to take place at lower RPMs, but are actually better when running in excess of 6,000+ RPMs. Long tube headers also move the torque curve lower, where ours already is, thus enhancing it. Plus, the Bassani(and other short tube hears) are actually shorter than what works for most engines. Headers are typically "tuned" to the RPM range that you want the most power/torque out of.


Remember - there's a balance to all of this, and a true engine tuner will know the limits and gains. (Plus, I'm sure I'll hear from a meeeelion people that I missed this or that )



Summary:

- Save your $ for long tubes, skip the shorties (unless you just want the "sound".
- Hi flow cats wont necessarily "lower your boost output" (mine didn't).



Hope you enjoyed my afternoon rant. (Needed a break from the Windows Admin duties...)

 

Also found this from TruckPulls.com

"The shorter the primary tubes are the higher in the rpm range they will help power. Each exhaust pulse causes a high pressure wave to travel toward the collector. When it reaches the collector it is inverted and travels back toward the cylinder as a low pressure wave. It is this low pressure area that helps scavenge exhaust out of the cylinder during the overlap period. This happens when the low pressure area reaches the exhaust valve during the overlap period. The low pressure area helps draw out the exhaust and draw in more fresh intake charge. All the pressure waves travel at the speed of sound (1200-1300 ft/sec in the hot exhaust). With all the pressure waves traveling at a constant speed, you can see that the header can be only be tuned to a narrow rpm range. On a street car that needs low-end, the tubes should be longer, in a high rpm drag car they will need to be much shorter. To get you close to the optimum primary tube length, use this formula:

Length (in inches) = (CID x 1900) ÷ (rpm x pri.OD2)
"


Could go on and on, but I think that's enough to hurt the brain a bit

 

'Power:

Blower cams (like the OEM in the Lightning) have little or no overlap. I'm sure somebody out there can fill us in on how little, but I'm guessing very little, if any.

Reasoning is thus - a blown motor doesn't rely on exhaust scavenging for cylinder filling - it fills the cylinder rather efficiently by simply ramming the charge in under pressure.

With any degree of overlap, you will just blow the charge through the cylinder and out of the exhaust valve during the overlap period, under pressure at that (read - a lot of charge). That makes for very poor mileage, big emissions, and very hot cats, for no real purpose.

I think that a big, hairy, huge-overlap non-blower cam would really foul things up in a Lightning.

So I'm not how sure how much exhaust scavenging affects cylinder filling in a Lightning, due to the low/no ovelap of our (blower) cam.

Somebody will likely chime in soon with overlap specs for the Lightning cam, and hopefully for various high-performance N/A cams for comparison.

 

Maybe it's that little bit of overlap that a more efficient exhaust system takes advantage of on our motors...


It would certainly explain the issues of lower boost ...


I'd definitely like to hear from our tuners on this one. I do know what you mean Silver-Y2k... makes sense in that respect.

 

Less boost does NOT necessarily mean less airflow. Airflow matters, boost does not.

 

Right. Not saying there's less airflow, what I'm saying is that the scavenging (however much), may contribute to the lower boost number that people tend to worry about. My "theory" was that the more flow through the exhaust may be contributing to the lower boost numbers.

 

my aftermarket blower cams have 114 degrees of lobe seperation, most n/a racing cams sport around 108 degrees of LSA (lobe seperation angle) which is more overlap. the more overlap the worse the idle, emissions, and mileage. most blower cams have a good amount of duration and lift to hold the valves open longer and bigger to cram boost in/out, but not a lot of overlap, there is some overlap though to help "blow" out the exhaust gasses. which is great because we get power and not horribly bad gas mileage.

http://www.modularperformance.com/mi...tegory_Code=BC

you can easily compare them here

 

Why don't more people run header wrap?

 

i bought a magna case from jdm

i had a six lber on

jdm said run a four lber untill you get some long tubes

 

It's not a theory at all -- it's a well-documented fact.

Boost is just backed-up air. More flow out means less backup.

 

It's more of a problem with a turbo (backup)

The ratio with a supercharger is more linear.

I am surprised that know one is touching on the intake filters etc.

Doesn't the intake create limitations at some point?

Always amazed me how the top fuel dragsters just suck all the track stuff in.

Of course they don't drive them home after the race like we get to (most of the time)

 

Boost is nothing moore than a measure of backpressure. Because you loose a pound or two of boost isnt necessarily bad. The more air and fuel that you can get into the cylinder the more power you will make. ie cams,porting, more effient blowers etc.

 

Agreed, but this is not a bad thing. Same airflow at lower boost just gives you more overhead to add boost.

 

I dynoed my truck back to back. One with nothing after the throttle body. No filter, not even MAF (since i dont use one), and once with complete set up (MAF, filter box etc). Made no difference in HP.

That doesnt mean that filters aren't benefitial on the road besides filtering air etc. They also bring in fresh clean and cooler air. Still I thought it was interesting on the dyno that the MAF and filter didnt seem to restrict airflow any more than with just the throttle body.

 

This is how I know when I suck when it comes to engines and stuff. I read this, hoping to understand it, instead I get a HUGE headack. Oh it hurts. Im going sleepy.