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cracked exhaust manifold

Started by Tinkeys, October 18, 2011, 06:44:31 AM

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Tinkeys

Exhaust manifold cracked  after new exhaust system fitted !
Just one of those things ,fix one thing something else needs attention !

chetbrz

With your skill level you should be able to weld on a new flange.  All part of the hobby.

Chet...
http://www.1948Plymouth.info           Web Master - Forum Administrator - AACA member

wellery

Tony

Too much back pressure from the new exhaust? it will always find the weakest point the bugger.

Cheers

Wayne

Tinkeys

Re welded exhaust flange no leaks after 20 min idle see what happens when we go for a drive later on !

Gary 30U

Tony, how did things work out after some use?

Your doing such a fine job on this car I was just hoping it went well.

Gary

Tinkeys

Hav'nt driven car since repaired manifold ! Its ready to go but weather is stormy and I dont like to take her out in bad weather fingers crossed she will be good !

Tinkeys

Driven the old girl about 60 klms, not a hint of exhaust leak ! manifold looks patched ,so eventually will get another for car shows car seems just a little more lively ,I did not think leaking exhaust would make that much difference .
I also got hold of a new stainless exhaust deflector from USA , I am going to start dressing up the old girl !

wellery

Tony think of it this way !!!!!


The Exhaust Pulse

To gain a more complete understanding of how mufflers and headers do their job, we must be familiar with the dynamics of the exhaust pulse itself. Exhaust gas does not come out of the engine in one continuous stream. Since exhaust valves open and close, exhaust gas will flow, then stop, and then flow again as the exhaust valve opens. The more cylinders you have, the closer together these pulses run.

Keep in mind that for a "pulse" to move, the leading edge must be of a higher pressure than the surrounding atmosphere. The "body" of a pulse is very close to ambient pressure, and the tail end of the pulse is lower than ambient. It is so low, in fact, that it is almost a complete vacuum! The pressure differential is what keeps a pulse moving. A good Mr. Wizard experiment to illustrate this is a coffee can with the metal ends cut out and replaced with the plastic lids. Cut a hole in one of the lids, point it toward a lit candle and thump on the other plastic lid. What happens? The candle flame jumps, then blows out! The "jump" is caused by the high-pressure bow of the pulse we just created, and the candle goes out because the trailing portion of the pulse doesn't have enough oxygen-containing air to support combustion. Neat, huh?

Ok, now that we know that exhaust gas is actually a series of pulses, we can use this knowledge to propagate the forward-motion to the tailpipe. How? Ah, more of the engineering tricks we are so fond of come in to play here.

Just as Paula Abdul will tell you that opposites attract, the low pressure tail end of an exhaust pulse will most definitely attract the high-pressure bow of the following pulse, effectively "sucking" it along. This is what's so cool about a header. The runners on a header are specifically tuned to allow our exhaust pulses to "line up" and "suck" each other along! Whoa, bet you didn't know that! This brings up a few more issues, since engines rev at various speeds, the exhaust pulses don't always exactly line up. Thus, the reason for the Try-Y header, a 4-into-1 header, etc. Most Honda headers are tuned to make the most horsepower in high RPM ranges; usually 4,500 to 6,500 RPM. A good 4-into-1 header, such as the ones sold by Gude, are optimal for that high winding horsepower you've always dreamed of. What are exhaust manifolds and stock exhaust systems good for? Besides a really cheap boat anchor? If you think about it, you'll realize that since stock exhausts are so good at restricting that they'll actually ram the exhaust pulses together and actually make pretty darn good low-end torque! Something to keep in mind, though, is that even though an OEM exhaust may make gobs of low-end torque, they are not the most efficient setup overall, since your engine has to work so hard to expel those exhaust gasses. Also, a header does a pretty good job of additionally "sucking" more exhaust from your combustion chamber, so on the next intake stroke there's lots more fresh air to burn. Think of it this way: At 8,000 RPM, your Integra GS-R is making 280 pulses per second. There's a lot more to be gained by minimizing pumping losses as this busy time than optimizing torque production during the slow season.



So in other words the crack in the manifold allowed for a fluctuation in pressure thus allowing an easy escape path for your exhaust gas, and also creating a back pressure problem (this more than likely increased the crack area/size) - so now with everything being equal the new exhaust adn the manifold gap sealed it has created the appropriate pressure (more like a vacuum) to allow the exhaust gas to be expelled more easily from the cylinder and the manifold down the exhaust system thus allowing more air/fuel mixture into the cylinder. A cleaner burn with fresher air to allow a more complete combustion – thus creating a little more power and running smoother (in layman terms)

Hope this helps

Wayne

Tinkeys

Thank you Wayne
It all makes very good sense and logical I dont know how you got that info out of your head in 1 go !
I am very impressed with your k owledge , you certainly explaind it better than I have ever done !

Tinkeys


wellery

Tony

Thats great !! - if I ever crack a manifold I know where I can take it to fix it.

all the best

Wayne