try doing exhaust comparisons from a different angle. Take a stock L motor (say 390) w/ no exhaust, add a stock exhaust now 380, add a bigger aftermarket system max you can get is 390. Take that same L motor modded to 500hp no exhaust, add stock system, 440hp, add a good aftermarket, 490.

There is always the basic principal in any combustion engine, and it doesn't matter if its supercharged, turbo charged or NA....

AIR/FUEL IN
AIR/FUEL THROUGH
AIR/FUEL OUT

No air exiting exhaust valves sucks a piston back up.



Sorry, i',m a newbee here and iwant in the fun on this post too

 

new cobra picked up over 50 rwhp with an exhaust in MM & FF. sounds like more air is moving through that motor......from 365 to 420

 

is t the same diameter exhaust as the L, is the CAT more restrictive, its not necessarily more air going through it, it could be a more restrictive system.

 

some of the other posts were saying how it doesn't matter how restrictive the exhaust is b/c the roots style blower on these motors only move a specific amount of air blabla, so im just saying that opening up the exhaust does make a difference. im sure the cobra exhaust is restrictive also.

 

No... the other messages are saying that assuming that the stock exhaust is already sufficient to keep up with the air flow of the supercharger, that "improving" the size or flow of the exhaust is not going to increase power significantly....

In a naturally aspirated engine, the exhaust restricts airflow more the the intake... given a certian efficiency in the design of the engine...

Therefore, opening the exhaust creates more efficiency in the e "air pump" engine.

Also... headers ARE designed witht the intention to minimize the "piling-up" of gases in the exhaust... adjusting the length of the individual tubes SHOULD create a vacuum in the other tubes... that vacuum WOULD then help pull the exhaust gases out of the other tubes, which of course creates a vacuum in the cylinder... If that IS true, that vacuum WOULD help to rotate the crankshaft, therefore pulling the other pistons UP in th eintake stroke...

<headspin>

 

Wow, wasn't expecting to see this thread in the list!!!

I don't read MM&FF so I don't know what the article had in it, but I don't think there is anyway in hell that a car can pick up that much HP from an exhaust change. I could see a small amount, but there is absolutly no way to pick up 55 hp from an exhaust change!!

Call it Dyno error, operator error, math error, or whatever you want, but that's the biggest crock I've ever heard. If multiple people start to see those changes, then I'll believe it. But right now, hell no.

All right, your full of #$&* The engine is using all of the air the blower puts in. Where else would it go?? The blower puts in some air. The engine is a restriction (since its not just a staight through pipe). The engine is moving a volume of air per time. Take the mass flow from the blower and shove it into the engine and ideal gas law tells you the pressure is going up. Pressure doens't matter, though, mass flow does. Mass flow matters because the more air goes in, the more fuel goes in. Fuel contains the energy to do the work.

I don't it doesn't sound reasonable, but if everyone would just understand the equation for mass flow from the blower, it would make since. Intuition says if I decrease the restriction at the engine, there will be more flow. When in reality, the flow stays roughly the same. Not becuase the change you made doesn't make a difference (because it does), but the difference doesn't affect mass flow (which is what matters).

 

Th Vacuum puling up the pistons and gasses piled up in the cylinder hampering the piston from coming up are too different things. Gasses exiting the cylinder have almost immeasurable pulling ability when you are talking about a piston connected to a rod connected to a crank. If you put the total exhaust gasses coming out of all the cylinders in a vaccum on 1 cylinder you couldn't measure the pulling ability from vaccum. What i am saying is if you hooked up the biggest shop vac in the world to the cylinder, you couldn't move the piston 1/32nd of an inch, the same way if you pump compressed air into the cylinder you cannot push the piston downwards. The only thing that moves a piston is a big explosion on the opposite side of te crank rotation.

 

GN-

The "pulling" action is slight... I'll give you that.. and yes, that slight amount cannot pull up the from a static position. However... The crank is a rotating mass... once that mass is rotating, inertia makes it EASY to continue that rotation. Also, since oil is now allowing the parts to "float", the effort required to move that piston up is even smaller... so yes... a slight vacuum can increase the efficiency to a notable level, otherwise long tube headers would be of NO benefit on naturally aspirated engines.

99 White Light-

Concerning mass flow... I don't think it is "proper" to count the engine as a restriction since it is the device "pulling" the air through.... If i put a fan in my window to move air into the house, is that fan considered a restriction to the air moving thru the window? (Assuming the fan is on of course...!)

Since the blower is POSITIVE displacement.... meaning it moves the same amount of air with each cycle no matter what... Then yes, a restriction in the exhaust WOULD cause a higher pressure, as you mention.

But, even when you reduce the restriction from the exhaust, the blower still moves the SAME amount of air... in theory (only for a roots type postive displacement blower). The PRESSURE is reduced... therefore only an insignificant amount of recognized increased hp gained from long tube headers.... But I agree 55hp is silly!

 

i didnt make up the 55 rwhp number, straight from mm & ff, they maybe be inflating the number a bit but even 35 hp gain is substantial....dont you think?

 

The pulling is alot less than slight its really not there at all. If some way you could make it produce no exhaust gasses at all, the crank would turn better w/ no vacuum pulling. Its all about getting the burnt gasses out to get fresh fuel and fresh oxygen in to get a better bang. (isn't that just everone's quest in life, be it women or motors ) Burnt gasses not getting out of the cylinder in a timely fashion is the problem.Its like when you are building a house, if the plumber isn't done by the time the sheet rockers get there, there is a backup.

The vaccum doesn't pull on the cylinder at all, it just releives pressure in the cylinder so it can turn more efficiently. Air cannot pull a piston upwards in the slightest.

 

GN1270-
I think you're mistaken on the vacuum issue. When we put the vacuum pump on our race engine we picked up almost 40 hp on an engine dyno. That's with 16 in/hg @ 7800rpm.

 

forgot to comment on the headers. Like i posted earlier, doesn't matter what blower/turbocharger/NA motor you have, a motor flows air in, flows air through, flows air out. The way to Mod is first you deceide how fast you want to go, then you modify your fuel system (if needed) to handle that HP.It will gain you "0" HP but enables you t run at higer levels safely.You then start with the most restrictive part of the "three flows" and i beleive onthe L its the flow in, so you get a cold air kit (flow in is all set) now you do flow through (on a L i don't know what it is yet, maybe heads, maybe blower, deffinately not a 90mm tb though.... then your flow out, exhaust and headers. Now your gains from an exhaust system will depend on how much air you flow out, and how efficient the system is. You can pick up 300hp if you go from a stock L exhaust on a1000hp L to an aftermarket system. It all depends on how much of a restriction the exhaust is.

To recap, the only way to make a combustion engine faster, is to get more flow. The more you can flow, the faster you will go....Damn that rymed Cool...i found a catch phrase

 

Looks like a sig line to me ... Welcome aboard!!!

 

GN1270-
I think you're mistaken on the vacuum issue. When we put the vacuum pump on our race engine we picked up almost 40 hp on an engine dyno. That's with 16 in/hg @ 7800rpm

Not really. The vaccum you put on didn't pull the piston up at all,it freed up a blockage that enabled the engine to crank more efficiently. You helped the flow by sucking out exhaust. If that motor wasn't running, you couldn't suck that piston up a nano inch with a vaccum. vacuum...damn,,how do you spell that dam word??

 

And now we are back to square one...

If you make NO change to a positive displacement blower, you make essentially NO change to the air flowing in, Since the blower continues to ONLY use the same amount of air... and thus changing the exhaust nets no additional flow out....