What's up with cobras?

[82ndCowboy]

So I noticed in another thread the comments about cobras being bad for a vmax?
Just thought I stir up shit and ask why?
One would think opening up the exhaust and letting exhaust out faster would allow air and fuel in faster.
Thoughts? Ideas? Opinions ?
Discuss.

 

[Pighuntingpuppy]

Robs HP. Lots of folks did the dyno. Has to do with the lack of cross over in the exhaust. They look great but hold back the bike. Stock exhaust allows the engine to breath better with the cross over in the exhaust.

 

[MaxMidnight]

One would think opening up the exhaust and letting exhaust out faster would allow air and fuel in faster.
Thoughts? Ideas? Opinions ?

If only it were that simple then all exhaust would be the size of drainpipes.

Have a read of this and this which explains what is happening in the exhaust.

In addition to the above the designer of the exhaust also has to consider how minimise noise without affecting performance and if the aesthetics will also alter its efficiency.
They will also have to factor in the time and cost of prototype fabrication and development and have sufficient knowledge about what alterations are needed to maximise the exhausts performance.

Ignore any of the above and you will get a product that will reduce rather than enhance the performance.

Discuss.

 

[Traumahawk]

So I noticed in another thread the comments about cobras being bad for a vmax?
Just thought I stir up shit and ask why?
One would think opening up the exhaust and letting exhaust out faster would allow air and fuel in faster.
Thoughts? Ideas? Opinions ?
Discuss.

It has to do with how they are made. They are basically a 4 into 4 slip on. With the stock exhaust or most slipons...2 cylinders are paired together...so that when when one exhaust pulse flows out....it creates a suction and helps to pull the next one out so you get better scavaging. Better cylinder scavaging....means better cylinder filling....which means more A/F mixture and more hp. Because the cobras are a 4 into 4 slip on....you lose this benefit. Hope this helps.

 

[02GF74]

ok, so what's 'going on with dragster engines; many seem to have short separate exhausts?

 

[Parminio]

It's about keeping the exhaust hot. That's why they make header tape.

The hotter the air is, the faster it moves. The colder it is, the heavier it gets and slower it moves. If you put too big a pipe on an engine not producing enough power, the exhaust cools down too fast and literally slows down the air flow out of the pipe and it causes all kinds of problems.

People talk about "back pressure" but few actually know what it is and how it works. Putting on bigger pipes without tuning the engine to make use of them actually hurts your performance because the engine isn't able to produce enough exhaust to keep it hot enough to move fast enough to make use of them.

It's not about "bigger pipes". It's about tuning the pipes to your specific engine.

At Cobra, all they really did was make a big ass slip on pipe that can move about twice the exhaust the engine is capable of producing, so what happens is the air gets too cold too fast and the engine has to literally work twice as hard to get rid of it.

It bogs it down.

Note that you see flames shooting out dragster pipes. They have no problem generating the heat to use that short, fat, bigassed pipe.

 

[Vinmax]

ok, so what's 'going on with dragster engines; many seem to have short separate exhausts?

View attachment 73232

Dragster engines are not designed to have a nice flat torque curve and great midrange drivablity. They're desiged for pretty much a very very narrow window of RPM's (usually right at redline). And the open headers (Zoomies exhaust) that they use are for that very small RPM range only.
There's a lot more science to it than I'm telling here.
Vinnie

 

[Parminio]

Dragster engines are not designed to have a nice flat torque curve and great midrange drivablity. They're desiged for pretty much a very very narrow window of RPM's (usually right at redline). And the open headers (Zoomies exhaust) that they use are for that very small RPM range only.
There's a lot more science to it than I'm telling here.
Vinnie

That's not entirely accurate.

You'll notice that all the pipes are the exact same length. That is essential to performance. Just because they're short and large doesn't mean that no thought goes into them. They are meticulously engineered.

They produce so much exhaust that they don't need long pipes to benefit from scavenging. The exhaust leaves with such heat and velocity they get the effect from the surrounding air at any speed.

They benefit from it as much sitting still as they do at 300 mph. The RPM range of a typical top fuel dragster is 2500 to 9500 RPM, not a great deal different as that of a typical Vmax.

 

[Vinmax]

That's not entirely accurate.

You'll notice that all the pipes are the exact same length. That is essential to performance. Just because they're short and large doesn't mean that no thought goes into them. They are meticulously engineered.

They produce so much exhaust that they don't need long pipes to benefit from scavenging. The exhaust leaves with such heat and velocity they get the effect from the surrounding air at any speed.

They benefit from it as much sitting still as they do at 300 mph. The RPM range of a typical top fuel dragster is 2500 to 9500 RPM, not a great deal different as that of a typical Vmax.

I agree that thought goes into zoomies, but a street naturally aspirated motor with lots lower compression is very different from a blown motor like in that picture above, that's used for a more narrow RPM range.
I don't think a zoomie system on our Vmax would be working out too well for hp and torque over a wide RPM range. Plus of course, you also would not be able to use much of a muffler system on them.
It's been proven that the scavenging effect design built into any good exhaust system will help hp and torque over removing that type of system and going straight pipes.

In fact, no high performance vehicles that need both low and high RPM power/torque (road race stuff) run zoomies. Formula cars and bikes don't use stuff like that. You really only see that in drag racing for running off the line and keeping the RPM's at close to max the whole way down the track. Top fuel uses the zoomies for downforce as well. At max rpm/hp/torque, they push down with something like 1000 lbs of force. The car already makes more hp than they can use (can't use all the horsepower if they can't get it to stick to the track), so they use those pipes for the downforce as well.

 

[scout477]

ok, so what's 'going on with dragster engines; many seem to have short separate exhausts?

View attachment 73232

Positive Manifold Pressure, Supercharger, vacuum is irrelevant

 

[Fire-medic]

Here's the best example I know-of for the benefits of a tuned exhaust: the 1965 Ford Indy engine, 255 c.i., 12.5:1, 'way-oversquare bore-vs. stroke, Hillborn mechanical fuel injection, they are reported to have installed 30 of these in Mustangs. Mario Andretti set new FIA records at Bonneville with one, named the "Autolite Mustang." The old records: standing 5 km, 125 MPH; standing 5 mile, 139 MPH.

The new records: 5 km, 171.47 MPH, 5 mile, 175.87 MPH. 495 HP @ 8300 rpm.

 

[Woody]

Cobras are what I would
Run on a harley. Lmao

 

[Traumahawk]

Here is a good video discussing exhaust gas speed, and exhaust savaging.

 

[Screwloose]

Here is a good video discussing exhaust gas speed, and exhaust savaging.

Liked the video, very interesting.

 

[82ndCowboy]

If only it were that simple then all exhaust would be the size of drainpipes.

Have a read of this and this which explains what is happening in the exhaust.

In addition to the above the designer of the exhaust also has to consider how minimise noise without affecting performance and if the aesthetics will also alter its efficiency.
They will also have to factor in the time and cost of prototype fabrication and development and have sufficient knowledge about what alterations are needed to maximise the exhausts performance.

Ignore any of the above and you will get a product that will reduce rather than enhance the performance.

Discuss.

Awesome info there Max.
This is why I like stirring up shit. You can actually learn shit.

 

[82ndCowboy]

ok, so what's 'going on with dragster engines; many seem to have short separate exhausts?

View attachment 73232

Drag Pipes on a VMax.
Sign me the fuck up!

 

[82ndCowboy]

Cobras are what I would
Run on a harley. Lmao

Why in the world would you run a Harley?
har har har

 

[82ndCowboy]

This was a great class.
Now all we need is a hot teacher in a mini skirt.

 

[C2protect]

If you expect power the way a Vmax was designed. DO NOT expect it from Cobra.

 

[Woody]

This was a great class.
Now all we need is a hot teacher in a mini skirt.

I brought my pencil !