Worrying what some people will put on the web as FACT

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Worrying what some people will put on the web as FACT

Post by damajin »

I was browsing around looking for how other people had made a neat job of dash mounting a brake bias / proportioning valve when I found this:
http://www.rx7.org/Robinette/brake_valve.htm

The suggestion in here to put the valve into the front brake circuit to me just sounds dangerous. It also seem a bit of a waste of time to go to all that effort putting a big brake conversion on the front and then but a proportioning valve in to reduce the power they have!

Am I missing something?

D
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

You can prove anything with facts.

rich
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Re: Worrying what some people will put on the web as FACT

Post by 340truck »

rich_jc wrote:You can prove anything with facts.

rich
Like "back pressure"?
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

I give up
Last edited by rich_jc on Thu Dec 15, 2011 4:39 pm, edited 1 time in total.
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Re: Worrying what some people will put on the web as FACT

Post by 340truck »

:deadhorse: :deadhorse: :deadhorse: :deadhorse:
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

:banghead:

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Re: Worrying what some people will put on the web as FACT

Post by sidewaysrob »

All I can say is 'back pressure' Mr truck! I rest my case! :roll:
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A product of the annalogue age!
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Re: Worrying what some people will put on the web as FACT

Post by 340truck »

Rest what case? As yet no-one has made a "case".
What's in this thread is waffle with absolutely no fact whatsoever.

Where does this back pressure (by definition INFLOW up the exhaust system) come from? 3000 RPM = how many litres of burnt gasses being PUSHED out of the engine by the piston (which has the power to compress a fuel air mixture to 1/9 of its volume) and you're trying to tell me there is a flow AGAINST that!!!!!!

Get real guys - read what you are writing before you post it and think about what happens in your engine.

Look at the title of the thread :agree:
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

edited

still give up
Last edited by rich_jc on Thu Dec 15, 2011 4:40 pm, edited 1 time in total.
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Re: Worrying what some people will put on the web as FACT

Post by canzus »

rich_jc wrote: If you put your finger over the end of a hose pipe, does the water go back into the tap? No. Does the water pressure increase and therefore exit the end of the hose faster? Yes.
Try filling a bucket with both methods. You may learn something during the process...
rich_jc wrote: Instead of proclaiming something you don't understand is a myth, why can't you just accept that you don't get it, and move on?
Dave has a better understanding of the physics involved than you apparently do.

Unfortunately, your manufacturers engineers are not responsible
for the final design of an exhaust system, the accountants are. If the manufacturer
can save $5.00 total on a product, multiply that by the number of units, that's a lot
of money.

By your reckoning Rich, a 1 1/2"(38mm) exhaust would be prefect for a 2.0L, I hate
to tell you this, you're wrong. IMHFO a 1 1/2" tube is on the small side for a single
branch of an extractor, 1 5/8"(41mm) is better, but I've had good results with 1 3/4"
(45mm) branches into a 2 1/2"(63.5mm) collector, into a 2 1/2" exhaust system. I did
this on SWMBOs 2L Pinto in her '72 Capri, which picked up power across the entire
rev range, and got better economy as well...
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

edited

still still give up
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Re: Worrying what some people will put on the web as FACT

Post by Peter-S »

rich_jc wrote: If you put your finger over the end of a hose pipe, does the water go back into the tap? No. Does the water pressure increase and therefore exit the end of the hose faster? Yes.
So on that basis, if I had a hole in my radiator, would drilling it bigger slow the flow?
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Re: Worrying what some people will put on the web as FACT

Post by 340truck »

:D :D :D :D :D
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Re: Worrying what some people will put on the web as FACT

Post by canzus »

rich_jc wrote:edited

still still give up

Rich, you editing your posts isn't going to change the physics involved. Perhaps if you
state why an engine needs back pressure in the exhaust system, we can blow the myth
to pieces. That way, everyone will understand why it's a myth...
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Re: Worrying what some people will put on the web as FACT

Post by 340truck »

Steve -

Those who believe in it will state that "back pressure" is REQUIRED where you have valve overlap (ie inlet and exhaust valves open at the same time) to prevent fuel /air mix being pulled into the cylinder and straight out of the exhaust.
In my opinion, for the millisecond when both valves are open the downward movement of the cylinder is creating a vacuum into which the mix flows and it is conceivable (but unlikely) that there could be a slight inflow of the exhaust gases at this moment.
No doubt the protagonists are reaching for their keyboards as they read the above, to suggest I am conceding their point. But before they do, they should remember where this debate started. It came from somebody (who shall remain specifically anonymous but I have heard more than one person say it) stating that an the possible cause of an engine not starting (and a standard engine at that, no cam mods) was because it had no back pressure!

As has been hinted, they should look at the simple laws of physics first.

Nature abhors a vacuum

Flow will be along the path of least resistance

And finally, a quote from a famous film,

Theoretical physics can prove that an elephant can hang from a cliff with it's tail tied to a daisy but use your eyes and ears to decide if it's possible
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Re: Worrying what some people will put on the web as FACT

Post by canzus »

Dave; I've been arguing the back pressure myth for 35 years. Every single time I'd ask
why do you need back pressure? I never got the same answer... Here is a good
question for the entire group to try and answer, it's a tough one, but not too tough if you
think about it a bit... Thinking caps on...

When using a set of extractors(headers in the US) it is possible to scavenge the exhaust
from the next cylinder in the firing order, where does this scavenging effect start??

When this is correctly answered, we'll move on to sizing an exhaust system...
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Re: Worrying what some people will put on the web as FACT

Post by Major_Tom »

Rich isn't saying there's backflow along the exhaust.. I think what he's getting at is thatmaybe a pipe of some sort helps the flow of gas overall.. like carb trumpets..
I'm the one who leaves all those shoes in the carriageway.
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Re: Worrying what some people will put on the web as FACT

Post by canzus »

Major_Tom wrote:Rich isn't saying there's backflow along the exhaust.. I think what he's getting at is thatmaybe a pipe of some sort helps the flow of gas overall.. like carb trumpets..
Which is why I asked Rich to explain himself...

;)
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Re: Worrying what some people will put on the web as FACT

Post by Major_Tom »

I dunno. Does it? I know nothing about it. Draught over ships funnels, factory flues, carb trumpets.. all helps gas flow in one way or another but why should it? Seems the path of least resistance isn't always as simple as venting to the atmosphere as quickly as possible.
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Re: Worrying what some people will put on the web as FACT

Post by rich_jc »

Christ, here goes again. One more time, and then end of. Canzus, if you have not "believed" in this for longer than I have been alive, I have no hope. I went to college with a girl who didn't believe in gravity, and no matter how hard me or my other phyicist mates tried, we failed badly, because in the end, she didn't want to believe in it, just like you and Dave don't want to.

"Okay, lets simplify. What is "back pressure"?

If you exhale a full breath, it is easy. If you then exhale through a straw, it is harder, as flow is restricted. As you blow, your cheeks will bellow. The pressure used to fill your cheeks, as a direct result of the air flow restriction, is called "back pressure". The smaller the straw, the more pressure in your mouth. In en electrical circuit, it would be the resistance of the wire.

"Does back pressure mean gas particles move backwards back into the engine?"

No, and like an elephant dangling from a cliff, is a ridiculous conclusion to come to. Electrons in a circuit do not flow backwards just because of high resistance.

"Why should an engine need an exhaust (of any size, as anything is a restriction by definition) to run well?"

If the exhaust flow is too open, your engine will spit out exhaust gases much more easily than it can induct new mixture. This will make the inlet system become a restriction, unless it too, is freed up sufficiently enough to match the free flowing exhaust. (Note, pinto head exhaust ports are noticably smaller than the inlet ones, and are rarely touched on anything under a stage 2 tune up). So while one piston is easily chucking out gas, one is compressing, one is igniting, and one is struggling to pull in new mixture. Work being done by inlet and exhaist piston is very unbalanced

Back to the straw, it is easy to keep exhaling out of a straw, while also inhaling through your nose (like a trumpet player will do) therefore flow can be kept constant. This is a lot harder to accomplish without a straw (try it!)

Imagine your battery going flat quicker than the alternator can charge it up because there is no resistance in your electrical system.

Or think of a sink with the tap running and no plug in. If the plug hole is too big, the sink never fills up, too small, it will overflow. Just the right size to match the tap, and the sink will keep a constant level.

"Can shock waves travel up the exhaust system?"

Yes, wave motion can be opposite to direction of travel of its medium, and travel at supersonic speeds. When gas flow expands, it will slow, and send shock waves back opposite to direction of flow. Think of a train driver stomping the brakes. Train keeps moving, but carriages "caterpillar" jolt all the way to the back.

"Can this help with valve overlap/ loss of intake mixture problems?"

If tuned correctly, yes.

"Does an engine run like cack with no exhaust system fitted?"

Try it.

Okay, done now. End of. Merry xmas. Don't bother replying to me, I won't read it.

I don't think my friend Briony has understood Newton's laws as yet if that is any consolation to you. But she is happy, and we all get on with life.

I hate you all, as you have made me sound like former board member TeslaCoil. :banghead:

Rich
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