Re: compression: point of deminishing returns
[Re: Performance Only]
#1545749
12/10/13 12:54 AM
12/10/13 12:54 AM
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Joined: Aug 2003
Posts: 43,252 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,252
Bend,OR USA
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Quote:
There's no one size fits all. It's totally dependent on the rest of the combination.
What fuel, what appication, drag racing, street and strip, roundy round racing,N/A, power adder or no power adder and so on
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: compression: point of deminishing returns
[Re: Cab_Burge]
#1545750
12/10/13 01:02 AM
12/10/13 01:02 AM
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Joined: Jan 2007
Posts: 8,042 Mt Morris Michigan
mopar dave
OP
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Posts: 8,042
Mt Morris Michigan
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4" stroke 4" bore small block using indy 230cc head and indy intake with 1100 dom, .700 lift roller cam. drag race n/a
Last edited by mopar dave; 12/10/13 01:48 AM.
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Re: compression: point of deminishing returns
[Re: mopar dave]
#1545751
12/10/13 05:06 AM
12/10/13 05:06 AM
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Joined: Aug 2003
Posts: 43,252 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,252
Bend,OR USA
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Quote:
4" stroke 4" bore small block using indy 230cc head and indy intake with 1100 dom, .700 lift roller cam. drag race n/a
Which type of fuel, race gas, alcholol or E85? If race gas exactly which brand and octane number, IE VPC10,C11,C12, Rocket 110 and so on? You really need to settle on the fuel and octane before choosing the compression ratio
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: compression: point of deminishing returns
[Re: Cab_Burge]
#1545752
12/10/13 09:29 AM
12/10/13 09:29 AM
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Joined: Jul 2010
Posts: 3,502 SOUTH JERSEY
HEMIFRED
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SOUTH JERSEY
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The curve of a hemi piston slows down the burning of fuel. Many ran that ran 13.5-1 changed to 12.5 which allowed the use of 110 octane instead of 112or 114. Lower octane burns faster then higher grades .
home of the Sox and Martin Hemi Duster
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Re: compression: point of deminishing returns
[Re: HEMIFRED]
#1545753
12/10/13 09:36 AM
12/10/13 09:36 AM
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Joined: Nov 2004
Posts: 4,330 Lynchburg, VA
Leon441
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Lynchburg, VA
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Many comp engines are built at 16.5:1 compression. Like said before it depends on what gas you want to run. You will be wanting 116 or better fuel when you get over 15:1.
My 421 W8 was 16.5:1 and it preferred 118NOS fuel which was illegal in the class I ran.
Leon
Career best 8.02 @ 169 at 3050# and 10" tires small block power.
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Re: compression: point of deminishing returns
[Re: mopar dave]
#1545755
12/10/13 11:51 AM
12/10/13 11:51 AM
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Joined: Jun 2003
Posts: 52,972 Romeo MI
MR_P_BODY
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Romeo MI
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Quote:
k, I guess I was doing that backwards. thought I could pick the cam and fuel to match the compression. 110 race fuel (gas)
Your static compression is just a number that doesnt mean a whole lot... when the valve closes is when it start the compression cycle... my 14.1:1 engine has 170 cranking pressure
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Re: compression: point of deminishing returns
[Re: OUTLAWD]
#1545757
12/10/13 12:33 PM
12/10/13 12:33 PM
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Joined: Jan 2003
Posts: 11,723 Portage,michigan
B3422W5
I Live Here
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I Live Here
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Posts: 11,723
Portage,michigan
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dave......this might give you a little example of cam/compression change had on my 4 inch w5 combo... I went from a 260/266 flat tappet cam 416 with 11.8 compression w5 headed combo that ran a best ever of 10.38 at 129 and change. Swapped in a 422 shortblock, that ended up being 13.3 compression, and added a 700 lift roller that was 273/280@ 50 cam shaft. same heads, headers, carb, intake, gears and convertor. So pretty much the only real effective changes was going from 4.07 bore to 4.100 bore, the compression bump and the bigger roller over the smaller flat tappet. Rest of the combo remained the same. Best with those changes was 9.85 at about 135. So about 5.5 tenths and 5 or 6 mph. The new shortblock of course was fresh, and had 043 ring pack, but it yielded a significant difference. Picked up about a full tenth in 60 foot, suspension remained identical, as did tires/wheels etc. Hope this might help. should probably also add, current owner of car took off 150 pounds,down to 3200 pounds freshened shortblock again, went to a more aggressive roller that is 276/282 I believe,same lift or close, and bumped compression to just over 14 to 1, and picked up additional 5 mph and 2 tenths, with everything else unchanged..same gears,tranny convertor, carb, intake, headers, suspension etc. The car really seemed to like the much more aggressive profile of the camshaft, over the quite mild profile I ran that was easy on springs and was somewhat streetable. All three cams, the flat tappet and the two rollers were all specc'ed by Dwayne Porter.
Last edited by B3422W5; 12/10/13 01:13 PM.
69 Dart GTS A4 Silver All steel, flat factory hood, 3360race weight 418 BPE factory replacement headed stroker, 565 lift solid cam Best so far, low 10.30’s 1/4 1.41 best 60 foot 6.56 at 104.17
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Re: compression: point of deminishing returns
[Re: AndyF]
#1545760
12/10/13 03:50 PM
12/10/13 03:50 PM
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Joined: Apr 2005
Posts: 2,128 Salt Lake City
camastomcat
top fuel
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top fuel
Joined: Apr 2005
Posts: 2,128
Salt Lake City
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Quote:
For a race engine you'll want the most compression possible without hurting power. Sometimes a large dome will reduce power or deep valve pockets will reduce power.
Max power on race gas would be 16:1 compression with a flat top piston but you rarely get that lucky.
I prefer the most compression up to 16.5-1 I can get, without a dome, with a .050 head gasket. That said, I usually need a small dome. Stroke has everything to do with it, and I have 4.5 and 4.75.
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Re: compression: point of deminishing returns
[Re: mopar dave]
#1545764
12/10/13 07:26 PM
12/10/13 07:26 PM
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Joined: Oct 2005
Posts: 46 Macomb Michigan
65 Belv A990
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Macomb Michigan
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You also have to factor in the value of the increased gains vs. the diminishing thickness to your wallet for fuel. If you plan to race a lot, factor in the cost of these expensive higher octane slower burning fuels needed to support that extra compression. Going higher than what the average 110 octane fuel will support (13.0:1 or so with aluminum heads) will add 25-30% or more to your fuel budget which is constant and not a one-time expense like the initial build.
MoPower..... Better to be a racer for a moment than a spectator for a lifetime.
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Re: compression: point of deminishing returns
[Re: camastomcat]
#1545766
12/10/13 10:07 PM
12/10/13 10:07 PM
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Joined: Jan 2003
Posts: 9,881 Weddington, N.C.
Streetwize
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Weddington, N.C.
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David Vizard's book "how to build Horsepower" had an inverted triangle chart that gave you the 'order of magnitude' from old to new CR...I have it somewhere but from say (for example) 9 to 11 was much higher percentage than from 11 to 13 which to some extent is mathematical 11/9ths is a higher proportional increase than 13/11ths. The higher you went the lower the gain, while it's true as you increase the pressure you also increase the speed of the flame travel, but at some point you hit diminishing returns and design limits of the engine itself.
Last edited by Streetwize; 12/10/13 10:17 PM.
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