does the 6.2 need backpressure ?

[Westech]

this is just a thought.. maybe the engine that came with little pipes from the factory are made to handle rapid cooling? O just a thought..... . Yes I have seen a valve in a small block Chevy warp and eat it self due to not running any exhaust. Its not as much as air rushing in but RAPID cooling. look.. I don't care what you do. I really don't. Just trying to help and offer some advice.

 

[watkinssr]

cam overlap has a lot to do with it. A motor with a lot of cam overlap will suck the intake charge right out the tailpipe if run with two little backpreassure. It'll make a lot aof noise, and loose a lot of power.

Comparing a warbird to a drag racer to a street truck is usesless without knowing about the cam....and I have no idea about a 6.2 but there is no one size fits all rule...it depends on the design of the motor.

 

[Tanner]

cam overlap has a lot to do with it. A motor with a lot of cam overlap will suck the intake charge right out the tailpipe if run with two little backpreassure. It'll make a lot aof noise, and loose a lot of power.

Comparing a warbird to a drag racer to a street truck is usesless without knowing about the cam....and I have no idea about a 6.2 but there is no one size fits all rule...it depends on the design of the motor.

The convo is about short exhaust pipes or stacks on an engine & having cooling air rush back up into the pipes when the motor is shut off... the cam has zero affect on a stopped engine.

'Tanner'

 

[axemurdock]

Well I got the same size pipe as the muffler and welded in place on both pipes , but before I did I welded in three 3/8washers in a circle pattern in both sleeves for backpressure. Heard that trick from an old biker that used that trick for straight pipes on harleys. So anyway the truck sounds great , not loud but rumbles nice. And it did open up the flow and it does go faster.

 

[papabear]

Well...unless you are on a farm or some other piece of private property....straight pipes are not desired...and you even stated you wanted to get muffled again when ya have the funds.
I do not subscribe to the back pressure/valve bending theory as i ran straight pipes on several vehicles when i was young...2 or 3 years ago. But it could be true...I don't know.

One of our members has a well muffled CUCV and it sounds like it's flying apart under normal operation....if he tried straight piping that rascal I would be forced to ban him from the compound!!!

I say buy another muffler and go back to stock and none of these concerns should even apply.

 

[watkinssr]

The convo is about short exhaust pipes or stacks on an engine & having cooling air rush back up into the pipes when the motor is shut off... the cam has zero affect on a stopped engine.

'Tanner'

The OP was asking about backpreassure. Didn't see anything in the OP about a stopped engine.

 

[militarytrux]

What does ASE mean? Assumed Smart Enough?

 

[Westech]

Good way to start at SS.

 

[DeuceNewb]

post a video of how it sounds

 

[OL AG '89]

running for 47+ yrs w/o backpressure

my wifes been running for 47+ years with no backpressure, never seen her bend a valve.....

 

[nhdiesel]

Watkinssr had it exactly right. The answer depends on the engine design. If you take an engine with a lot of valve overlap and open the exhaust, you will lose a lot of power and the engine will run poorly. On most stock street engines opening the exhaust will do nothing except make lots of noise. Considering how bad a non turbo diesel sounds WITH mufflers, I would never want to drive one without mufflers. I have enjoyed driving turbo diesels with straight exhausts, because the turbo acts as a muffler and does a goo job of quieting the exhaust. Inline turbo diesels also sound better than Vee non-turbo diesels, in my opinion. With a commercial inline turbo diesel, you get that big-truck sound. With an unmuffled 6.2 you just get an old junker with a bad exhaust sound.

Jim

 

[docfarmall]

6.2 with straight pipes is load wont hurt it a bit i had one ran the pipes out right behind the cab only thing was it was load as heck . Nope straight wont hurt it at all.

 

[Cucvnut]

its loud not load

 

[M543A2]

I keep hearing the back pressure question and have to wonder why it is a question. The object of good headers and a cam with overlap on a racing or street performance engine is to clean the by-products of the last combustion cycle out of the engine cylinder. The header length is designed to create a negative pulse, or vacuum, at the exhaust valve while it is open. The cam overlap that allows both intake and exhaust valves to be open at the same time for a short time allows the header negative pulse to draw air through the cylinder briefly to clear out all of the waste gas possible. The rule on any engine is: EXHAUST BACK PRESSURE MEANS THERE ARE RESIDUAL COMBUSTION BY-PRODUCTS IN THE CYLINDER WHEN IT STARTS THE NEXT INTAKE CYCLE. Those residual by-products take up space that could be used for a new combustion charge, therefore reducing the available volume of new fuel/air mix. This means less power.
Some engines will normally have higher back pressure that is unavoidable, like ones with turbos on them. The back pressure is the result of the pressure needed to spin the turbo compressor wheel. This problem is somewhat offset because the turbo forces air into the cylinder on the intake stroke, helping reduce the affect of the remaining exhaust gases.
A normally aspirated engine with a long exhaust system and restrictive mufflers will have some back pressure. It is an unwanted result if you want performance. The less back pressure you have on a naturally aspirated engine, the more power you can get because the new incoming fuel/air charge is larger because it has a more empty cylinder to fill.
You can even run a diesel or other engine with no exhaust manifold, but the advice about the system being extremely short allowing cold air to the exhaust valves on shut-down is correct. Normally on a truck or car, one runs the system back past the driver resulting in a system long enough to protect the exhaust valves from rapid cooling.
In summary, run the glass packs if you like and do not worry.
Regards Marti

 

[nhdiesel]

Too many people believe myth, rather than facts. Myths such as the back pressure claim, if its cold turn your lights on to warm the battery (particularly funny one), the old myth about not leaving a battery on a concrete floor, etc.

Jim

 

[rlwm211]

Put a pair of glass packs on, muffle the noise, slightly, and live happily ever after!


RL

 

[watkinssr]

I keep hearing the back pressure question and have to wonder why it is a question. The object of good headers and a cam with overlap on a racing or street performance engine is to clean the by-products of the last combustion cycle out of the engine cylinder. The header length is designed to create a negative pulse, or vacuum, at the exhaust valve while it is open. The cam overlap that allows both intake and exhaust valves to be open at the same time for a short time allows the header negative pulse to draw air through the cylinder briefly to clear out all of the waste gas possible. The rule on any engine is: EXHAUST BACK PRESSURE MEANS THERE ARE RESIDUAL COMBUSTION BY-PRODUCTS IN THE CYLINDER WHEN IT STARTS THE NEXT INTAKE CYCLE. Those residual by-products take up space that could be used for a new combustion charge, therefore reducing the available volume of new fuel/air mix. This means less power.
Some engines will normally have higher back pressure that is unavoidable, like ones with turbos on them. The back pressure is the result of the pressure needed to spin the turbo compressor wheel. This problem is somewhat offset because the turbo forces air into the cylinder on the intake stroke, helping reduce the affect of the remaining exhaust gases.
A normally aspirated engine with a long exhaust system and restrictive mufflers will have some back pressure. It is an unwanted result if you want performance. The less back pressure you have on a naturally aspirated engine, the more power you can get because the new incoming fuel/air charge is larger because it has a more empty cylinder to fill.
You can even run a diesel or other engine with no exhaust manifold, but the advice about the system being extremely short allowing cold air to the exhaust valves on shut-down is correct. Normally on a truck or car, one runs the system back past the driver resulting in a system long enough to protect the exhaust valves from rapid cooling.
In summary, run the glass packs if you like and do not worry.
Regards Marti

The problem arises when the cam is designed to run with a restrictive exhaust, the cam overlap is too great. So some of the intake charge, instead of being in the cylinder when the exhaust valve shuts is in the tail pipe. The result is less fuel in the cylinder to make power, since the fuel system is only putting out so much. You can compensate by increasing the ammount of fuel/air going in to begin with, but with the price of diesel/gas this doesn't make sense. The higher the RPM, the less this matters.

The correct sollution is to run the right exhaust for the cam you are using....it would be interesting to see someone throw one of these things on a dyno and get some idea of how it beforms with different exhausts etc...It might make more power with glasspacks, it might not.

 

[Tanner]

The OP was asking about backpreassure. Didn't see anything in the OP about a stopped engine.

Westy commented on the air rushing back in up the pipe & causing bent valves due to rapid cooling -

Quote:

"What pipes and mufflers are for is to reduce noise and rapid cooling of the valves. Have you ever cooled off a frying pan in the sink when it was hot and it warped? Well I have and that is what valves do if cool air can get to the valves after the engine is shut off. Thats why you see drag racers put balls or cups over the pipes after the engine is shut down. YOU NEED EXHAUST PIPE ON THAT ENGINE!! a couple feet may not be long enough to prevent rapid cooling. YOUR ASKING FOR IT!

Love Westy. "

So - armed with this information, I'd say that the main mission of all SS members should be to preach this message to the Harley faithful & get 'em to muffle those bikes... Wouldn't want them to bend the valves on their hogs -

Axemurdock - you can get a cheap turbo-flow muffler for a few bones at your FLAPS -

'Tanner'

 

[als804]

I cut the mufflers off my m1008 so the pipes let out a foot after the cab. Sounds like crap, disgustingly loud. Do I need mufflers on it to run good , do I need back pressure like a gasser? Ill weld sleeves on so I can attach the tail pipe section so it'll exit in stock location , near rear bumper and curved out. It shouldn't be that loud I'm guessing but will that be enough backpressure for it?

Yes.
At low RPM range, the backpress help exhaust gas to be discharged. Yes I know it is weired. Backpressure is related to how well it emits. But this regard is at high rpm range.
At low rpm you definitely need Backpressure

Loss of back pressure can effect torque/HP on a N/A engine, but for a turbo charged engine backpressure is not good at all rpm range.

Typically on a N/A engine, loss of back pressure will lower low-end torque, but increase upper end torque, resulting in more HP on the top-end.

 

[Sharecropper]

Yes.
At low RPM range, the backpress help exhaust gas to be discharged. Yes I know it is weired. Backpressure is related to how well it emits. But this regard is at high rpm range.
At low rpm you definitely need Backpressure

Loss of back pressure can effect torque/HP on a N/A engine, but for a turbo charged engine backpressure is not good at all rpm range.

Typically on a N/A engine, loss of back pressure will lower low-end torque, but increase upper end torque, resulting in more HP on the top-end.

Dude -

This thread is ELEVEN years old.
But since you brought it back from the dead -

NO - you do not need back pressure. This topic has been beat to death for years. This subject has caused irreparable harm to close lifelong friendships. I know of one divorce and two shoot-outs this silly backpressure debate has caused. I promised my current wife and both my near-miss friends that I would never be drawn back into the debate again. Don’t even think about trying to get me to respond further on the backpressure subject, as I will not. This tape will self-destruct in five seconds. Good luck Jim.