T_Racer

iTrader: (1)

My stock manifold, 1983 GSL, has a leak from the last time I replaced the tranny about 6 mos ago. It leaks where the manifold joins the downpipe. I was a minor leak at the time but after driving it some, seems to be getting a little louder.
However, here is my question. I put some new wheels on it and drove it to work. Drives ok, doesn't slide near as much, now needs more power. Anywho, the other night I am just driving down the road to show my neighbor's kid what a "2 cylinder",as he calls it, can do. We are leaving the nieghborhood and the exhaust seems a little louder, but no biggie. The more I get to driving it, it is backfiring on decel more, just little pops in the muffler, and doesn't pull like it seems like it should.
Aren't rotaries very exhaust pressure sensative? As in, with that exhaust leak, could be causing a somewhat slight decrease in performance? It just doesn't seem to pull past 5.5-6K like it did. I know that a stock port doesn't pull that much anyway up high, but it used to pull pretty good to about 7-8K.
I know this is just a small prob., but I don't have the new motor in yet, and would like to prolong this one for a time. I know, I know revving that high is not good for longevity, but isn't it better to die at 8 grand, than to live at 3 grand?

Travis

glewsRx

iTrader: (1)

The bigest thing exhaust backpressure does is open the 5 and 6 ports in a 13b, which you don't have.

j_tso

iTrader: (2)

I don't air escaping in the exhaust area would cause a power loss, just bad for emissions.
Time for a tune up?

ericgrau

iTrader: (1)

leaky exhaust = slight power gain
You have slightly less restriction now.

I once had an FC with a poorly done weld. I think the pre-cat was replaced with a straight pipe. Anyhoo there was a crack that ran half way around the pipe and it was horribly loud. After a little experimentation (and varying levels of success/failure) I wrapped it tightly with a wet mat of fiberglass insulation (it's wet to unfluff it). Then I wrapped a piece of aluminum sheet metal (a.k.a. roof flashing) around the insulation and secured it with 3 hose clamps, being careful not to crimp the sheet metal. I let it sit, tightened it some more, idled the engine to dry out the insulation, and tightened it some more. That quieted the car to normal and it didn't come apart. Wetting the insulation might not be necessary, but that's what I did on my final (successful) attempt. What's more important is to not stick any kind of goop in there, b/c it will burn or flake off (JB weld failed, in spite of its high temperature claims). And don't crimp the metal, or a hole could form at the corner. Though the noise from a tiny hole over insulation is relatively minor.

vipergts_3

iTrader: (9)

any motor needs backpresure to promote scavenging of the gasses left behind after the combustion prosess . if you dont have enough it will cause vacum inefficiency therefore .... loss of power !!
yes the tail pipe does make a difference people !!
http://www.superchevy.com/technical/...t/0505phr_exh/ be sure to read the article in the link , here is a snipet of it in case you dont look
"The exhaust system on a well-tuned race engine can exert a partial vacuum as high as 6-7 psi at the exhaust valve at and around TDC. Because this occurs during the overlap period, as much as 4-5 psi of this partial vacuum is communicated via the open intake valve to the intake port. Given these numbers you can see the exhaust system draws on the intake port as much as 500 percent harder than the piston going down the bore. The only conclusion we can draw from this is that the exhaust is the principal means of induction, not the piston moving down the bore. The result of these exhaust-induced pressure differences are that the intake port velocity can be as much as 100 ft./sec. (almost 70 mph) even though the piston is parked at TDC! In practice then, you can see the exhaust phenomena makes a race engine a five-cycle unit with two consecutive induction events...
Muffler Flow Basics
We select carbs based on flow capacity rather than size because engines are flow sensitive, not size sensitive. This being so, why should the same not apply to the selection of mufflers? The answer (and here I'd like muffler manufactures to please note) is that it should, as the engine's output is influenced minimally by size but dramatically by flow capability. Buying a muffler based on pipe diameter has no performance merit. The only reason you need to know the muffler pipe size is for fitment purposes. The engine cares little what size the muffler pipe diameters are but it certainly does care what the muffler flows and muffler flow is largely dictated by the design of the innards. What this means is that the informed hot rodder/engine builder should select mufflers based on flow, not pipe size."

vipergts_3

iTrader: (9)

ummm.... if you have enough time to do that then why don't you save your money and go get it fixed right ??
i have an 81 with a leak in the same spot and it has no backpressure to speak of and i have no excelleration . got the parts ordered , might as well do it right the first time http://www.motorsportworld.co.uk/fra...s-exhausts.htm
more info on the same subject

ericgrau

iTrader: (1)

'cause it took 10 minutes plus ~$10 to strap it on and be done with it. Retightening it took about a minute. I had roughly 2-3 failed attempts (which were done differently), and 1 successful. Because I didn't know of any solution short of hiring someone to weld it. Thanks to the old welds there was nothing to replace, unless I wanted to replace all the piping from the main cat to the exhaust manifold. My patch job held without any problem at all for the remaining months that I had that car.

And believe or not performance exhausts work by reducing restriction, which is saying the same thing as reducing backpressure: high flow and restriction lead to backpressure; reducing restriction reduces the backpressure that the engine has to fight against. For maximum power racers use a straight pipe from the engine to the mufflers. Glass pack mufflers are also straight through for maximum performance, with fiberglass to reduce noise. This is just one example of performance mufflers; I chose it for simplicity but all performance mufflers are designed for minimum restriction. So in the racing exhausts you have a wide open path from the engine to the tailpipe; almost the same as having no exhaust pipe at all. A leak just means some of your exhaust isn't pasing through your cat and mufflers. Meaning you get noise, pollution and possibly a woozy driver.

I like being an engineering major. Though it still amazes me how scientificamable and complicated the fake stuff you read online tries to sound. The less understandable you are, the smarter you are, right?

JKM

My butt dyno can definately tell when I have an exhaust leak. I have to swap between using the stock cats and the straight pipe a bit, and sometimes I don't end up tightening one of the bolts all the way down, blowing a gasket in short time. Also more recently I had a driveshaft problem that was causing one of the exhuast nuts to rattle on off the car. All of these had the same effect-exhaust leak where the manifold meets the downpipe.

I can tell quite a bit of difference between leaking and non-leaking exhaust. Other than the obvious noise, of course. I dont' have dyno charts, but with the leak the car accelerates pretty quickly when you stomp the gas. It is about the same speed as my Miata. With the exhaust fixed, it gives it just enough extra power to take it from fun acceleration to "HOLY ****" acceleration.

vipergts_3

iTrader: (9)

just because you say you study something dosent mean that you are the 'know all' of that subject . back pressure is a good thing to have , mainly because if you dont have it then ... well your car dosent run and if you are running at open headder or 'leaky' pipes. you have no scavenging happening ...
evidently those of us retards that have good running efficient tail pipes can all rest assured that we can chop off those offencive manifolds and start running open !!! more hp !!
lol . give this a gander . http://www.veryuseful.com/mustang/te...Scavenging.pdf
somebody with a lick of sence will know what i am talking about
https://www.rx7club.com/forum/showth...22#post6289122

trochoid

iTrader: (6)

If you're still running all stock exhaust, the system may be plugging on you and causing the leak to get worse.