Would running a V8 with no exhaust cause this? (apart from hearing loss)

Ahh the old exhaust theory discussion. Haven't been invloved in one of these for a long time.
As a Physics teacher who loves to build and play with cars, this is something that I've always been interested in.

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Originally Posted by rick130

naa, I disagree. It's not a back pressure thing, it's harnessing reflected waves and balancing velocities with tube diameters.

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I agree entirely. I did some reading years ago before I built myself a set of extractors, and reflected waves are the key. The flames, pops and bangs are caused by unburnt fuel that passes into the exhaust. The reflected waves assist to contain the fuel in the cylinder which results in an increase in torque.
While not the same as 4-strokes, tuned pipes make a huge difference in 2-strokes. My dad used to race go-karts in the 60s and built his own adjustable exhausts. With a short, low volume exhaust, the engines produced huge amounts of torque down low. Increase the length and volume and they wouldn't pull out of a corner but would rev like mad down the straights (without altering diameter).

The Physics is interesting, but the dynamics are complicated!

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Originally Posted by Blknight.aus

ok before you get to the reflecting waves thing and blanacing and all of that, think single cylinders...much like a 2 stroke...( i should have made that a bit clearer when i wrote it)

when you get to reflecting your exhaust pulses to assist with valving and extraction you still need to retain a certain level of back pressure otherwise the gasses just frell off out of the pipes and theres not much to reflect around..

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OK, I think what's happening here Dave is a difference in terms. You're calling it back pressure, I'm calling it velocity. ;)

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Originally Posted by Slunnie

Yes, but in a two stroke the back pressure is a momentary reflecting pulse from the expansion chamber that is about forcing the fuel/air mixture that has escaped into the exhaust back into the cylinder once the transfer port has shut - so in a sence it's almost like a micro supercharger. The 4 strokes don;t have anything like that.

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yep, the 4 strokes do it the other way around they use it to keep the gasses flowing...

pressure rises
flow rises
pressure drops and flow keeps going (enhanced if the pipe is longer)
flow drops.

the bit of pressure that is important is the pressure that gives you the flow to begin with...

try it this way...

blow as hard as you can through a bit of 100mm conduit to blow a sponge ball across a table.

now do it with a bit of 20mm conduit.

the initial pressure and maximim pressure get the gasses moving, the faster you make em move the more they want to keep moving when you drop the pressure supply off (piston hits TDC and the inlet valve opens)

thats one cylinders worth and is the bit before any of your pipes join up...

If you put some extra air in the pipe (pressurise it) its a little harder for the gasses to get flowing, but once theyre flowing they want to keep on going. And since theres 2 things working on raising the pressure and getting the gasses moving (thermal expansion of the exhuast gasses followed by the rising of the piston) a little bit of backpressure isnt detrimental...

then you get all the fun of the fair joining these things together and trying to sort out these pulses so that as the "pulse" from one cylinder is just starting to slow down the next cylinders comes along and shunts the gasses further down the pipe while its doing its pressure+flow increase phase which compliments the scavanging actions of the first cylinder.

Im glad I only need to know the threory of it and dont have to design this stuff..

My exhausts would be harmonic lenghted (for your prefered torque range) bits of pipe poking straight off the exhaust ports out the side of the wings...

All cars would come with ear plugs.

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Originally Posted by awabbit6

While not the same as 4-strokes, tuned pipes make a huge difference in 2-strokes. My dad used to race go-karts in the 60s and built his own adjustable exhausts. With a short, low volume exhaust, the engines produced huge amounts of torque down low. Increase the length and volume and they wouldn't pull out of a corner but would rev like mad down the straights (without altering diameter).

The Physics is interesting, but the dynamics are complicated!

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I raced karts in the late eighties and everyone had a box full of different length exhaust flex to tune the exhaust for different tracks. Peter Dell made a system up where he had a lever on the steering column and a variable length pipe so he could extend the pipe for the twisties and shorten it half way down the straight and over the bridge at Oran Pk. I think it was banned :(

LOL.

I love these threads, can't be bothered reading all the posts but still love them.

Slunnie, I can see where you are coming from and when it comes to the theory of how it all works then I have absolutely no idea.

But I can talk from a practical point of view. When I was changine over motors in the Rangie I did exactly the same thing running it with just the extractors connected because I wanted to see how it went. And I got exactly the same results as Mark. Had popping and backfiring and was low on torque. Once the exhaust was connected up everything was fine. It still pops on de-acceleration but torque is all good.

Thanks for all the responses - I had trouble following the theory but it sounds like its good news for me - things are going to get better with the exhaust connected :)...................

To give an idea of the drop in torque, it used to idle along nicely in second gear with no throttle. With no exhaust, it 'pig roots' something fierce, even in first gear.

I am not trying to thread hijack but .................

I'm just about to fit extractors to my RRC I was tinking about setting it up like this:

Extractors without a joining Y piece to two exhaust pipes with just a straight through resinator on each pipe would this be a good or bad move?

I don't care about the noise, (or lack of) as long as it's legal.
But I was hoping this would increase the power a bit at low revs ??


And yer I do like the exhaust backpressure threads :-)

IMHO...

extractors are generally tuned to a portion of your rev band...

You dont get something for nothing...

If they give you more low down torqure they steal it from higher up in the rev band and vice versa..

for me the former is good as Im not into sports car driving landrovers they are afterall mearly magrinally modernised tractors.

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Originally Posted by HangOver

I am not trying to thread hijack but .................

I'm just about to fit extractors to my RRC I was tinking about setting it up like this:

Extractors without a joining Y piece to two exhaust pipes with just a straight through resinator on each pipe would this be a good or bad move?

I don't care about the noise, (or lack of) as long as it's legal.
But I was hoping this would increase the power a bit at low revs ??


And yer I do like the exhaust backpressure threads :-)

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re-read my post regarding 90* vs 180* V8 cranks. You need to have both systems connected, either with (IMO a large) balance pipe between both sides somewhere between the last collector (assuming 4-2-1 headers) and the first muffler, or a 2.5" single system (which should give you the best torque characteristics and would be my prefered option on a V8.)

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Originally Posted by Blknight.aus

ok before you get to the reflecting waves thing and blanacing and all of that, think single cylinders...much like a 2 stroke...( i should have made that a bit clearer when i wrote it)
When you get to reflecting your exhaust pulses to assist with valving and extraction you still need to retain a certain level of back pressure otherwise the gasses just frell off out of the pipes and theres not much to reflect around..

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Using laymans talk and broadly speaking:)

And to add to what you are saying
The gas pulse is not a gas body that reflects, it is a shock wave that will only travel along the solid medium of the pipe containing the gas. when the shockwave reaches the end of the solid medium(open air) it will return back up the pipe. It is exactly the same for using explosives, the blast is one thing the returning shockwave is quite another with equally devastating results.
The "tuned length" of an exhaust(as far as exhaust piping goes) is all you need for Torque.(extractors) the rest of the system is an exercise in making the most efficient means of staying legal. there are gains to be made by carefully reducing the systems diameter and then enlarging the diameter back to the equal some of all the header pipes. This does increase the velocity of the gas thru the restricted area and provides gas purging to allow more mass movement thru a part that is only required for legal reasons. eg better without the restriction.
Ther is a book of about 200 plus pages , written buy a mad scientist at a Scottish university. He spent nearly forty years studying the science of gas flow and all that goes with it. Incredible reading that goes way beyond the stuff most of us learn thru trade or life. He put all his theories into to practice to remove the white mans magic aspect of lab tests.
At the end of the book his summary was carefully tuned individual stacks for all out power at narrow rev range(ala Drag). and tuned length pipes joined progressively, maximising shockwave(gas slug) timing to aid in scavenge,, or more to the point not disrupt gas flow with a poorly timed gas slug that opposed mass flow. There are acknowledgments that a heap of other techniques can be implemented but basically they had weight penalties or economic penalty etc.