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Old 11-14-2001, 12:59 PM   #1
thejaf
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Default STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds?

I changed my head size from 64cc to 76cc to reduce the compression of my 406 engine. Previously it was about 11.5:1, now it should be about 9.7:1 All other things being unchaged, how will this affect the way my exhaust sounds?

Before the head change, the car at idle sounded like a P-51 taxiing down the runway :D I hope to get it up and running again soon, but I'm just kinda curious :)
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Old 11-14-2001, 01:40 PM   #2
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Default Re: STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds? (thejaf)

It should sound less poppy. Higher comp motors that are relatively small have more crackle and less rumble. Hope this helps. And did you change the cam along with the compression change?

Joe
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Old 11-14-2001, 01:45 PM   #3
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Default Re: STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds? (Xracer)

No cam change yet. Just heads for now. I want to see how the new compression does with Lars' quick curve advance.
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Old 11-14-2001, 07:13 PM   #4
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Default Re: STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds? (thejaf)

What heads were the 64 cc's? I'd be interested.
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Old 11-15-2001, 02:28 AM   #5
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Default Re: STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds? (thejaf)

Another stupid question: why would you want to reduce your comp ratio? Doesn't it give you less power? What kind of benefits do you get out of less compression?
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Old 11-15-2001, 03:23 AM   #6
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Well for one thing you generally don't have to run the higher grades of fuel if you lower compression.
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Old 11-15-2001, 10:50 AM   #7
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Default Re: (Simon Alexander)

The heads were "041" heads with matching December 1968 castings. They were ported and polished by whoever built my engine. I sold them to a fellow building a 302 for his 1957 corvette.

I am working with Lars a lot on getting my engine streetable. When I bought the car this past spring, it ran fine on 92 octane fuel, but lacked a lot of low end torque, and got about 7 miles per gallon. Then the distributor sheared gear teeth, so I sent it to Lars to get rebuilt. he told me that a previous owner had wire-tied the advance mechanism in place, thus I had no mechanical advance what-so-ever. :crazy: When we put my changes on the car (rebuilt Q-Jet, dual plane Edlebrock intake, rebuilt distributor) the engine detonated like crazy, even backing off timing. Adding LOTS race fuel helped, but that would limit me to a 30 mile radius from my house :( So, newly rebuilt heads from a 400 SBC were installed, and should lower my compression to the upper limits of pump gas range on a carbed engine (9.7)
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Old 11-15-2001, 11:03 AM   #8
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Default Re: (thejaf)

When you pulled the heads off did the pistons have 10-14 cc domes?

The exhaust note is a function of HP HP = Heat. So higher C/R makes a bigger bang. With aluminum heads,correct cam timing, and advance curve 11.5 would run with 92 octane. I'm sorry I stopped at 10.84 c/r, but i also planned on NO2


[Modified by gkull, 9:08 AM 11/15/2001]
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Old 11-15-2001, 11:12 AM   #9
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It "should" mellow out a bit ie. less crackle and a bit more of a lazy, deep sound.... if that makes sense.
:cheers:
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Old 11-15-2001, 12:29 PM   #10
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Gkull:

When I took the heads off, I found that the block had been "zero" decked. The pistons are TRW flat tops with 2 valve relief's, and they actually came up over the top of the block a hair at TDC. We figured out that I was running somehwhere between 11.5 to 12 CR. I didn't want to drop the money on aluminum heads, and wanted to run a nice quick advance curve. I am waiting on Crane Cams to send me a replacement roller rocker, then I get to tune the car.

It has an MSD 6A under the hood, but I'm not hooking it up until I establish a good timing and idle with the stock system. Because I had no documentation on this engine when I bought the car, I'm trying to get a solid running basline before I go too wild. When I bought it, there was a single plane manifold, screwed up timing, Holley 650 double pumper set REAL rich, etc.

It almost seemed like a previous owner had the engine built, then realized he didn't know how or couldn't run it on the street, so he killed the timing and ran really rich to overcome detonation.

So, this is what I have so far, wonder how many horses I should expect?

1. 400 sbc, bored out 0.030
2. TRW flat top pistons w/2 relief
3. mystery dual pattern long duration solid lifter cam with about 0.45" lift
4. Crane Race Gold 1.5 roller rockers
5. Large headers with 3" collectors
6. Eldebrock dual plan Performer manifold
7. Rebuilt Q-Jet from a 1969 350/350 Corvette
8. Quick curve points distributor with MSD 6A to be added later
9. 76cc "3973493" heads with Z28 valve springs
10. Bock zero decked, with estimated compression of 9.7

So, what do you expect i should see at the flywheel?
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Old 11-15-2001, 12:32 PM   #11
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Quote:
It "should" mellow out a bit ie. less crackle and a bit more of a lazy, deep sound.... if that makes sense.
:cheers:
Yes, I think so :) So instead of being the loudest Corvette at local get-togethers, I should blend in a little more :chevy It was definitely a unique exhaust sound, very distinct and loud "bangs" and pulses of air at idle. Only thing it reminded me of was the old WW2 fighters, like the P-51 Mustang :yesnod:
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Old 11-15-2001, 02:12 PM   #12
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Default Re: (thejaf)

Quote:
Yes, I think so :) So instead of being the loudest Corvette at local get-togethers, I should blend in a little more :chevy It was definitely a unique exhaust sound, very distinct and loud "bangs" and pulses of air at idle. Only thing it reminded me of was the old WW2 fighters, like the P-51 Mustang :yesnod:
It might blend in a bit better - there again, it might not ;)
I love the sound of high comp engines with open pipes. Two mates ride Commandos, one with a standard engine that makes a sort of "domp domp" sound on idle, and the other runs a lumpy cam & high comp which makes a "blat blat" sound, with really powerful exhaust pulses coming out the back. Lovely :D Both exhaust systems are identical & "silenced" in the same way that Monza pipes are.
When I were a lad, a friend bought a Triumph 500 which he tuned to the max (11.5:1) & it made a really healthy snarling sound. He then fitted a small bore 2 into 1 exhaust pipe (no muffler) which stopped about 3" in front of the rear tyre (it was so tuned that it wouldn't idle, but sitting in traffic he couldn't rev it for long as it would melt the back tyre! He had to roll it backwards & forwards while he kept the engine going! Got good grip from the tyre though ;) ). The small bore pipe made it howl when it got on cam & it sounded just like a WWII fighter in full dive. We regularly used to see pedestrians stop & start looking up in the sky :lol: Couldn't get away with that sort of antisocial behaviour these days... :reddevil
:cheers:
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Old 11-15-2001, 05:51 PM   #13
Chuck Harmon
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Default Re: STUPID QUESTION: How will a change in compression ratio affect my exhaust sounds? (thejaf)

Yeh, the sounds do change with compression and cam, but it is hard to describe. If you love and know cars though, you know the difference. Perhaps a sharper sound, more clear. Like listening to music live instead of over a desk top radio. Each firing of a piston is a little more distinct. Decibels don't have much to do with it. A loud, un-muffled low performance motor (even with headers and 4 barrel) leaves me cold. However, even if a performance motor is hobbled by quiet stock type mufflers I can still hear the animal trying to get out! The engine becomes more vibrant as rpm's increase in a high performance motor.

A lower compression motor can still sound sharp and crisp if it uses very high flowing heads, intake, etc. Even though the air is theoretically being compressed less, since more air has filled the cylinder the cylinder pressure is as high or higher.


Chuck
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Old 11-15-2001, 07:23 PM   #14
The Dude
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Default Re: (thejaf)

Quote:
3. mystery dual pattern long duration solid lifter cam with about 0.45" lift.
Get a new cam and eliminate the mystery. .450" lift sounds pretty small for a 406 unless you have smog concerns.
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Old 11-15-2001, 07:23 PM
 
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