Just a question about the v-10

[Nefarian]

I keep reading all these posts about people cutitng out their cats and mufflers or some combination of the two, does the viper rely on a certain amount of backpressure like so many other cars do? I know the jeep 4.0 doesn;t like not having full backpressure on it, whats the setup on on viper that is so differnt?

 

[HP]

There have been reports of some power loss from opening up the
exhaust(ie cat removal or larger pipes) - but this was on otherwise stock motors. It would be hard to generalize this, since there have been changes
over the years to the stock production setup, and exhaust modifications aren't
usually the first aftermarket change. Most make these changes for other reasons(ie side sill heat,sound). There is a point in upgrades, where the old exhaust will be your limiting factor.

 

[joe117]

I always wondered why removing backpressure would hurt power. I've heard people say it for years. The only thing I can figure is some kind of reflected pressure waves forcing scavanged mixture back into the chamber and if you upset the stock exhaust it might not work right.
Anybody know the real reason?

 

[HP]

Velocity(speed) of gases. Smaller diameter=faster speed.
Another contributor could be related to diameter of pipe vs heat retention.
Better heat retention - faster gas speed
Its the suction effect or lack of it, from the expanding gases in the exhaust.
In the situations where this is a factor, you would still just trade off a little power at low and midrange, then overcome it at WOT for a little gain.

 

[joe117]

HP, I think you are about as close to right as anyone but someone has to know the real reason.

Heat retention, seems like cooler exhaust gas would shrink and thus flow easier through any system.

Like I said, I don't know.

 

[HP]

I somewhat see your confusion, but if you equate hot/expanding gases with
speed, then you can see that the opposite would be stagnation, which you don't
want. It's better visualized if you take into account the movement of the
piston and the valves. Someone like Gregg Good could do a better job
explaining this phenomenon.

 

[joe117]

Seem's to me that the gasses are as hot as they will ever get when they pass the exhaust valve. Why would they be expanding in the manifold or pipes?

 

[HP]

Seem's to me that the gasses are as hot as they will ever get when they pass the exhaust valve. Why would they be expanding in the manifold or pipes?

From the back-firing and popping that goes on in exhaust, that part is an easy
sell. The explosion that occurs in the cylinders, creates a volume of gases
that are many times greater than the volume of the cylinders, and are only
allowed to expand outside the confines of the motor - that's why on 2 cycle
motors - the exhaust manifold is called the expansion chamber.

As far as heat goes, you're right - the exhaust valve temperature is about the
peak temperature, so the key is to limit the temperature drop.
If you just focus on one gas - steam(byproduct), my headers(at the heads) run at around 275deg -once the temp in the exhaust pipe drops below the boiling point of water, the volume of steam collapses and you get water.

 

[McGuireV10]

HP, I think you are about as close to right as anyone but someone has to know the real reason.

Actually HP is exactly right. There was a long, very detailed article about it in the October 2000 issue of Hot Rod Magazine. If you're extremely masochistic, they even have the relevant calculations. You can figure the exhaust gas velocity range of your exhaust system, as well as predict the range of optimal scavenging vacuum for a given cylinder size and shape. However, using those calcs is pretty tedious -- you have to figure the diameters of the bends in your headers, the volume of the header tubes and the collector, lengths between the bends, and other similarly complicated details.

 

[joe117]

Well, I agree that the gasses want to expand since they are under pressure. But, as they leave the hottest part and move down the pipe to the coolest part, they are cooling and so will be under less pressure than they started with. Of course this is assuming a constant exhaust system cross section area.

Anyway, we started talking about why back pressure is a good thing for some engines. From what McGuireV10 says about the piece in HR magazine, it look's like it isn't the back pressure that's needed. What is needed is an exhaust system that is designed to optimize flow and just sticking on a bigger pipe or cutting off the cats isn't always going to be better than stock.

 

[HP]

Backpressure may be the wrong word to use. I think gas velocity is better.
When a motor is at WOT - then usually the biggest, less restricted exhaust
pipe will make more power. But at low and midrange operation - scavaging
comes into effect. Scavaging is just the sucking(vacuum) effect caused by
the motor dynamics and moving gases. This helps pull out the burnt gases and
pull in the intake charge. So bigger exhaust pipe - best WOT flow, but smaller pipe gives higher gas velocity - and better low-mid range torque.
If you can relate this to hydraulics(like brake systems or fuel flow) it's
sometimes easier to visualize - you trade flow for pressure(vacuum)
Master cylinders use small diameter pistons(with a long stroke)to move
larger slave cylinders a small distance with great pressure. Fuel regulators
constrict flow to increase pressure. I don't know if these are the best examples, but there are some similarities to trade off when changing pipe diameter.

 

[McGuireV10]

Actually the HRM article makes the case that adequate scavenging is always an issue. The article was primarily focused on whether the trend in dedicated drag cars towards huge exhaust was really better (like 4.5 inch and bigger, reminds me of that Flowmaster poster with the tailstanding Nova), and of course a dedicated drag car is ONLY concerned about WOT operation.

But you're right, the term "backpressure" suggests adding something is desirable, whereas the real issue is to avoid losing something -- gas velocity. However, the equation is further complicated by the question of how much exhaust you're pushing in the first place, and an 8L engine is pumping out a hell of a lot of exhaust compared to most other engines out there.

So, to go WAY back to the original question, with all of this in mind... imagine the amount of exhaust which has to be constantly expelled by a 4L Jeep engine versus the 8L Viper engine. This is why you can open up a Viper exhaust system so much compared to a much smaller, less powerful engine like a 6-cyliner or a small V8. The stock Viper exhaust system is simply a lot farther away from being too open to maintain exhaust gas velocity for its engine size.

 

[HP]

McGuire's right, this thread is more meaningful when applied to some other motor.
By the time you add smooth tubes and headers to a Viper, a logical next step is
3" exhaust with high flow cats(or cat delete).

 

[Gerald]

I would think to a certain extent that smaller diameter, ahhhhhhh who knows! I give up. I know what I want to think in my head, but I can NEVER put it down on "paper" the way I think.... Damnit this scares me!

 

[HP]

I would think to a certain extent that smaller diameter, ahhhhhhh who knows! I give up. I know what I want to think in my head, but I can NEVER put it down on "paper" the way I think.... Damnit this scares me!

Happens to me all the time Gerald, there are exceptions to everything.
What diameter headers and exhaust, have a lot to do with what other mods
you have, and where you want the power to fall on the power band.
The companies that manufacture, or sell headers for Vipers, can guide you the
best diameter to use on your car. Plus, you can't lump all stock Vipers in
one group, some years respond better to exhaust changes. We all have read
posts, where no power change was noted after exhaust mods. I can't recall
any posts where someone has lost 'power', although there torque/HP peaks may have shifted.

 

[Torquemonster]

Yes - exhausts are a fascinating subject and a science in itself....

A certain pipe diameter however does have a certain cfm capacity and that has a direct impact on hp potential for a given engine efficiency.... therefore a small pipe will make hp gains impossible beyond a certain point. There are charts available showing what diameter can flow how much hp (unboosted).

I often wonder how well an expansion chamber coupled with a venturi collector would have..... aim to find out one day - but if you look at the exhaust system on the British TVR V8's they use something like that with cross bank cross-overs - looks like a siders web - but man do those things sound good - and go well too.

I agree that backpressure is not the desireable thing - but speed of flow and best shape/size combo to flow pulses with minimum backpressure and heat loss... trouble is the best size/shape changes with each rpm rise.

For an engine that runs a big cam but needs to make good bottom end too this is why I like the anti-reversion headers (yes they do work Joe117....grin) coupled with stepped primary pipes and merge collectors - all ceramic coated. This combo (non equal length - but smooth equal diameter bends) gives the best compromise for an engine that is required to operate over a wide rpm band.

They might make slightly less peak power than a tuned length setup but will make more power across the cams power band - and therefore accelerate the car better with street gearing. It also means headers can be made to look good - not look like a pigs nest to fit an exact length - for there is only a perfect length for a given rpm - not for a wide range of rpm.

NOTE: I do not think the AR design is necessary on a typical Viper engine however as they rev too low and run cams with insufficent overlap to need it... but a hot one that rpm's to 7000 would pick up a huge amount of (otherwise) lost bottom end. For all Viper non blown engines I've seen on this forum - a traditional stepped setup into merge collectors would be the ticket with sizes varying depending upon hp - and a heat coating like ceramic or a wrap. If that lost power there'd be some head scratching going on!

 

[joe117]

I understand scaveging and the flow volume vs. velocity vs. tube cross section physics well enough.
I understand reflected, high and low, pressure waves in an exhaust or intake system.
I never could see how a tube manifold running to a fairly long pipe, as in a street system, could do anything much more than remove restriction when compared to a stock header that was probably designed to be castable in iron with performance secondary.

And as far as the 488 V10 being much bigger and flowing more than a typical V8, that's true for the part of the system after the header. The header tubes themselves flow about the same in a 350 V8 as in a 488 V10.

 

[Torquemonster]

very true - which is why Vipers like smaller primary pipes.... if they were a V8 they should be running 2" primaries at 500hp.

I'm not sure why longer pipes are better for torque, i do understand why short pipes are better for top end.

The first 3-4" are the most critical - if you can get a nice straight shot out of the exhaust port for a few inches before you force the gas to turn, and then keep the turns not too tight and in a spiral that minimises backpressure - I think that would be worth a lot more than length considerations. I've seen headers made to perfection except they run a tight 90 degree turn straight off the flange - what a waste of time - might as well run stock manifolds and a big bore pipe to it.

 

[Gerald]

I think we need to get Treynor and Torkie in a room together. I think Einstein would roll over in his grave.. Is there anything these guys don't know? Sheeeeeeez....

 

[HP]

I thought this was exhaust flow/velocity 101,
Torquemonster might just have opened up a can of worms with reversion and pulse frequencies.

 

[Torquemonster]

sorry - gee - i hope I don't come across as a "know it all"! ouch!! i don't know infinitely more than I do - I just find some of these threads very stimulating.

now - on a more serious note - I think if we fitted a magnet on every cylinder, earthed it to the chassis, then connected up a harmonic vibrator to match the exhaust pulse on each port - then we could run a pap smear on number 2 and we should have 100mpg and pick up an extra 200hp! what do you think Joe?

what do you mean "NO!?"

 

[HP]

Torquemonster, you're going to have to post your name in your sig.
Actually, your posts are entertaining(and informative), but I bet your brain gets tired by the
end of the day

 

[Torquemonster]

nah - just a grunt and gentle squeeze, and out it comes like molten cheese!

I should rename myself the talkmonster!

truth is i type fast so it looks like more time spent than it is... otherwise i wouldn't get any work done.... hey there's a new concept...

work!

mutter mutter back to it....

 

[Torquemonster]

just for you Hugh

hey that rhymes!

updated my profile - i came out of the closet.... gee i feel so relieved now - like a huge weight has been lifted.

 

[joe117]

torquie,
You better watch out with the kidding around about the magnetic harmonic vibrator. It look's like these guys believe all of the stuff you write.
Next thing you know some unscrupulous fellow will be selling them on ebay.