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Learning how to build Lotus Seven replicas...together!
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PostPosted: April 20, 2016, 10:50 am 
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horizenjob wrote:
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A third goal is just to have the cylinders all exiting via their own pipe. I just like the way that sounds.


Do individual pipes sound different than putting them all together? With a V8 when you group them into each side it sounds different because the firing is not even, but if you put them all together I think it becomes even and wold basically sound the same as 8 individual pipes, except you're likely to face 4 of them more directly.

Since the sound of each cylinder happens at a different time, using one muffler makes sense. The pulses on a V6 just don't overlap much. Putting a little more effort into building one muffler shared by 6 gets you way ahead of anything you can do building 6 different units...


That's a very interesting thought, Marcus. I haven't really looked at putting all six inlets into one muffler, but I've thought about it. I think you'd have to have two chambers though in the case of a V8 or V8 since the pipes will be on opposite sides of the car. If you take the 'bustle' notion as the basis, I don't think two chambers would be that hard to do. It would just be a subdivision on the muffler into two pieces internally. In both cases, have the inlets on one side and the outlets on the opposite side. That would give you quite a bit of space crosswise to make something effective and useful.

And, yes, individual pipes do sound different. You don't see it much on road racing cars, but here's an oldie from my home region. The Huffaker Genie Mk 10 of the 1960s.
Attachment:
Huffaker Genie Mk10.jpg


Cheers,


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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

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PostPosted: April 20, 2016, 2:29 pm 
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I was talking to a friend of mine yesterday about the noise levels on my 7 and he remarked how quiet it was.
I told him I thought it was loud, but maybe that could be because I start the car in an enclosed garage and then drive away.
Anyway he said he had a friend that had a db meter and he would call him up and see if he could get it or the friend and come by today, take a few driveway readings.

So let you know what happens if they turn up. 8)

Al

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PostPosted: April 20, 2016, 7:00 pm 
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Nice day for a bike ride, 86 degrees and sunny, so my friend and his friend with the meter did make it over.
So I got some readings of the 7 and the AMX as well since they were here.
I don't know much about the types of db meters, they brought a Reed, how good or how pricey I don't know.
I don't have any insurance on the car yet, not until the end of the month, so no drive by readings.
My exhaust is homemade headers thick pipe for the headers 13ga, all under the hood including reducers.
From under the reducers it is about a 100 degree bend for 14 inches to the front of the muffler case.
Then a 21 inch muffler and 37.5 inch side pipe from the back of the muffler case that end in a 90 degree bend with a slight turn down.
In the driveway with the meter just above the console it registered 68db on startup with a fast idle of 2000 rpm.
After I kicked it down to 850 idle it read 64db.
Still at idle we took a readings at:
12 feet=55db
25 feet=50db
50 feet=44db
We held the meter 12 inches from the drivers tailpipe at idle and it was 78db.
From the passenger side we got 81db, my only guess for the slightly different reading is the headers are not symmetrical.
I can't prove it so I don't know, just a guess as everything else is the same.
I brought the revs up to 3500 and took a reading at the tailpipe 12 inches out and the reading went up to 85db.
Then while I held it at 3500 another reading was done at 25 feet, the reading dropped to 72db.

And since they were here I fired up the AMX and at 800 rpm the reading from inside the car was 73db.
12 feet from the back it was 60db.
25 feet back it was 54db
Both rear pipes were 80 db at idle from 1 foot out.
I never took any readings at higher rpms.

On impulse I started up my Passat and at idle it showed 44db just standing about 4 feet behind the car .

So that is what I got this afternoon, not sure how much it proved other than my hearing must be a bit wonky as I thought it would all be a lot louder.
On the other hand I have no problem carrying a conversation with the wife at 50 miles an hour.
Anyway it was fun, let me tinker and get the cars ready for summer.
Who knows maybe I can get the guys over again some time in the summer and do some blast down the highway readings.
All in all I have been happy with the exhaust as it is, it does have a kind chorkle sound at lower revs, I suspect a cross over might cure that to some extent.
I have been to lazy to look at it though.
Also it does highlight the lumpy cam at certain rpms and that isn't bad at all.

So a decent day, in fact a rare day as there didn't seem to be a single person home in the neighborhood, so very quiet, kind of unusual. :lol:

Al

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PostPosted: April 20, 2016, 8:10 pm 
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Your results demonstrate the importance of taking measurements at distances representative of those used at the track (or whatever facility). Measurements taken in close proximity or inside the garage just aren't representative of free-field and will inevitably be conservative.

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PostPosted: April 20, 2016, 8:50 pm 
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With sound level readings, every time you double the distance, subtract 6dB. It works for your 12, 15 and 50 ft readings. But 3500 rpm under no load, is not representable of real world conditions.

As far as driving comfort goes, when I ran my noise tests a few years back I had 90dBA in the cockpit coasting at 45MPH. 2 noise sources of 90dB each would add only 3dB. Therefore, getting the exhaust level at cruise or moderate acceleration much below 90 is a waste of effort IMO. YMMV

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PostPosted: April 20, 2016, 10:21 pm 
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Well to be sure I don't drive my car at a rural or urban tracks so it wasn't a concern of mine, also as much as I see youtube videos of people driving around in cars not insured for the road we are not allowed to get away with it where I am, so I have to wait until the car is insured for under load tests.
I basically suddenly had access to a meter and figured why not do something with it.
This is what I did and this is what I got, perhaps it was a waste of time, I did comment "not sure how much it proved".
But I had a good day and met a new friend.
As far as tracks go we have a stock car track and I will be mostly over there this summer.
As it is a bit remote we are not concerned with noise levels, though I did run a lot of cones in parking lot ventures back in the 80's.
They pretty well dried up and probably because of the noise, insurance and lack of track creativity more than anything.
Either way it was a play day for me.


Al

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PostPosted: April 21, 2016, 7:41 am 
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Thanks for sharing that Al. Not a waste of time at all and very helpful. I don't think Chuck meant you are wasting your time.

Enclosed areas and testing closer than typical does not result in conservative indications. They are more likely to be higher due to reflection in an enclosed area, so scaling for distance would also be higher.

Typical up close testing is at 45 degrees to the pipe. Most meters have a "peak hold" function which is useful for reving.

Any reduction is worthwhile imho, even though wind noise and other sources can become the primary contributor.

I've ordered some copper to build silencer inserts from. I looked at just buying some inserts off the shelf, but they are crazy expensive and are plain steel in the size I need. This will be a variation of the washer and tube design I've described before.
These will be adjustable while running for testing and removable.

Most all the products I've seen for exhaust noise reduction come right out of Phillip Smiths book, originally printed in 1962.

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PostPosted: April 21, 2016, 8:04 am 
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MV8, by conservative I meant higher (in the context of higher being a worse case from the noise Nazi perspective), so we are on the same page.

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PostPosted: April 21, 2016, 8:50 am 
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Al, I do apologize if I came across that way. It was not my attempt.

In a previous life, I did some sound level measurements as part of my job. One thing I was trying to say that if you have measurement A at a distance B, there are ways to calculate what the sound level would be at distance C if you could not measure it at distance C. I think it is great that people like you are starting to get some test data on exhaust noise. :cheers:

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PostPosted: April 21, 2016, 9:34 am 
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rx7locost wrote:
...3500 rpm under no load, is not representable of real world conditions...

This. One time I started my car with no exhaust at all and marveled at how quiet it was. Even driving on the road was surprisingly quiet - at no load - but tip into the throttle and all hell broke loose. With the engine under load it gets way louder, so any noise test is useless unless you replicate the testing setup at the track: typically 50 feet away from where you come by at full throttle. Anything else is just wasting time.

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PostPosted: April 22, 2016, 6:40 pm 
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I made some locost hangers. The grommets are pretty standard. These were the cheapest I could find on ebay at around $2 each for a dodge application. The rod is 5/16 which is thick enough to be stiff and strong but thin enough to bend fairly easily with hammer and vice. These are not welded yet but look so much better than the universal type on the other side which is going away.

Still have not put the bigger wheels on.


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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: April 28, 2016, 8:52 pm 
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You won't find any like this. Built the exhaust tip insert from brazed copper, about 6 inches long overall due to the bend in the tip and the exhaust pipe is 2 inch. There are no holes drilled, nothing blocking the center tube, or packing. I made it to work on a range of frequencies and the restriction makes the muffler more efficient also. It is not reversable. If it were reversed, I doubt it would do anything.

Idle dropped to 67.6 db from 71.9 db (-4.3 db) and reving produced a peak of 85.7 db from 96.3 db (-10.6 db). Noticeably quieter.

I didn't fully insert it but made it to fit flush with a single screw through the side to hold it. Nothing was holding it in during testing but the fit.


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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: April 29, 2016, 12:42 pm 
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I'm not sure how that works, but good on ya for figurin' it out. I guess it's the copper. Watch out, Bret Farve, eh? You could be the "Copper Muffler" spokesman on TV. :lol:

It's completely off topic, but you know my wife bought one of those copper-infused, non-stick pans, which I thought would be total baloney. But, I'll be damned, the things actually works. I know 'cause I'm the breakfast short order cook and I've been using it for about a month now. Who'd a thunk?

Cheers,

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Damn! That front slip angle is way too large and the Ackerman is just a muddle.

Build Log: viewtopic.php?f=35&t=5886


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PostPosted: April 29, 2016, 9:42 pm 
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Thanks for the interest. I'm sure some people are finding it hard to believe and maybe this is a joke. It isn't.

I used copper so it would not corrode like steel, but I don't know how well it will hold up to the heat.

Speaking of football, I flew into Midway at Chicago and the Atlantic ramp was crowded with Dallas, Clemson, and Tenn Titans aircraft there for the NFL draft. Lots of eye candy also. :shock:

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Miata UBJ: ES-2074R('70s maz pickup)
Ford IFS viewtopic.php?f=5&t=13225&p=134742
Simple Spring select viewtopic.php?f=5&t=11815
LxWxHt
360LA 442E: 134.5x46x15
Lotus7:115x39x7.25
Tiger Avon:114x40x13.3-12.6
Champion/Book:114x42x11
Gibbs/Haynes:122x42x14
VoDou:113x44x14
McSorley 442:122x46x14
Collins 241:127x46x12


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PostPosted: April 30, 2016, 12:55 am 
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That's a pretty massive reduction from the insert. Depending on the type of copper, that may not be a bad material: Copper expands more than steel with heat. Jam it in cold, and it will be less likely to come out hot(? Not sure how tube heat up VS inserts). I have seen inserts come out before and guys get red-flagged at autoX. I would need a more secure fit than friction. Sure hate to lose a run (or day) to that.

Currently healing after surgery. Hopefully start poking around in the shop again in a few weeks. Very curious to see what my levels are at 50ft, and what a insert or 2 does to them.

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