HP Loss due to reducing compression.
#2835599
10/21/20 12:47 PM
10/21/20 12:47 PM
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Joined: Feb 2006
Posts: 474 Alberta, Canada
451Guy
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mopar
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OP
mopar
Joined: Feb 2006
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Alberta, Canada
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I realize that this is a bit of a loaded question but from your experience how much power will a motor lose from reducing the compression given that you have a suitable cam in both engines, given every thing else in the engine is the same.
Compression from 13.5 to one to 10.5 or 11.00 to one Race gas to premium pump gas..
Thanks
Thanx 451 Guy
512 cu in 71 Demon Super Street Car - NO LONGER - Broke Crank Best ET 9.57 @ 139.96
451 cu in Pump Gas - 71 Demon Super Street Car Best ET 9.99 @ 136.80
512 cu in 69 Charger R/T Best ET 11.39 @ 118.11
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Re: HP Loss due to reducing compression.
[Re: 451Guy]
#2835611
10/21/20 01:15 PM
10/21/20 01:15 PM
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Joined: Jan 2003
Posts: 4,296 Chicago, IL
TonyS451
master
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master
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Chicago, IL
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.I'm guessing it will depend upon the other matched components. ie a cam that works well w 13.5-1 may not work well w 10-1 . Contemplating the same type change, so Im curious to hear what the experts have to say as well.
2 kids and a dog
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Re: HP Loss due to reducing compression.
[Re: AndyF]
#2835639
10/21/20 01:44 PM
10/21/20 01:44 PM
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Joined: Jan 2003
Posts: 14,506 So. Burlington, Vt.
fast68plymouth
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I Live Here
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So. Burlington, Vt.
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given every thing else in the engine is the same. The way to look at it is....... does the combo still “make sense” with the CR lowered 3 points? Are the heads, intake, carb, headers still appropriately sized for the engine displacement at 10.5:1? In other words, are they still the optimal pieces for the combo at the new CR? If the answer is “no”, then expect to take a bigger hit in power than what just the loss of CR, and reduction in cam duration would yield. The other side of the argument is...... how much can you afford to lose and still have the combo be “good enough”....... and does that seem like a reasonable outcome from the CR loss? I feel as though high compression is a fairly effective way to help crutch up a less that optimum combo.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123 Dealer for Comp Cams/Indy Heads
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Re: HP Loss due to reducing compression.
[Re: 451Guy]
#2835735
10/21/20 05:24 PM
10/21/20 05:24 PM
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Joined: Jan 2003
Posts: 11,723 Portage,michigan
B3422W5
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I Live Here
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Portage,michigan
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I realize that this is a bit of a loaded question but from your experience how much power will a motor lose from reducing the compression given that you have a suitable cam in both engines, given every thing else in the engine is the same.
Compression from 13.5 to one to 10.5 or 11.00 to one Race gas to premium pump gas..
Thanks
I have read in many different places that a point in compression is 2-3%. Once you get to a high compression, its a case of diminishing returns. You said you were going to swap to a cam more suitable to the new compression. My guess 30-40 horsepower just from compression.
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: HP Loss due to reducing compression.
[Re: B3422W5]
#2835767
10/21/20 06:38 PM
10/21/20 06:38 PM
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Joined: Aug 2003
Posts: 43,251 Bend,OR USA
Cab_Burge
I Win
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I Win
Joined: Aug 2003
Posts: 43,251
Bend,OR USA
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I realize that this is a bit of a loaded question but from your experience how much power will a motor lose from reducing the compression given that you have a suitable cam in both engines, given every thing else in the engine is the same.
Compression from 13.5 to one to 10.5 or 11.00 to one Race gas to premium pump gas..
Thanks I have read in many different places that a point in compression is 2-3%. Once you get to a high compression, its a case of diminishing returns. You said you were going to swap to a cam more suitable to the new compression. My guess 30-40 horsepower just from compression. I've seen testing done on a SB Mopar bracket motor that lost 30 HP by reducing the compression ratio from 12.5 to 10.6 or so so they could run pump swill with no other changes(timing or jetting)that made a 560 HP motor make 530 HP They should have optimized the timing, jetting and spark plug heat range with the lower compression ratio on the engine dyno instead of doing it later at the track Those changes made it run faster than it had before, that racer is now a big advocate of low compression race motors
Last edited by Cab_Burge; 10/21/20 06:39 PM.
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)
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Re: HP Loss due to reducing compression.
[Re: 451Guy]
#2835813
10/21/20 07:56 PM
10/21/20 07:56 PM
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Joined: Nov 2005
Posts: 15,119 85086
moparpollack
Lil Herman
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Lil Herman
Joined: Nov 2005
Posts: 15,119
85086
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Is a 200 horsepower shot of nitrous oxide an option to make up for the reduced compression?
56 Plaza 63 D100 step side 67 Coronet, 68 Roadrunner, 69 Super Bees, 69 Coronet 500 convertible, 70 Roadrunner Post, 79 D150 360, and a severe case of Mopar a,d,d
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Re: HP Loss due to reducing compression.
[Re: moparpollack]
#2835899
10/21/20 11:03 PM
10/21/20 11:03 PM
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Joined: Jun 2012
Posts: 1,899 MYRTLE BEACH SOUTH CAROLINA
ek3
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top fuel
Joined: Jun 2012
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MYRTLE BEACH SOUTH CAROLINA
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seems to me that 2 things are really important.. volumetric efficiency and burn rate. hi compression requires better fuel to control burn rate..ve changes with compression if you take the piston location into the deal. - a bump in compression will always mean some increase in power no matter what short of perfect ve. the overall goal is to improve volumetric efficiency . rite ?? i have seen a lot of 9 to 1 engines in class rule racing make big power. all things considered , an engine with 13 to 1 that has been worked out like the 9 to 1 will make even more power.... i am looking at the expansion ratio which gives a longer time to apply power during the power stroke as a reason for this.
Last edited by ek3; 10/21/20 11:08 PM.
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Re: HP Loss due to reducing compression.
[Re: ek3]
#2836018
10/22/20 10:52 AM
10/22/20 10:52 AM
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Joined: Feb 2006
Posts: 474 Alberta, Canada
451Guy
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OP
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Alberta, Canada
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Thank you for the replys.
My intension is to purchase a cam that Dwayne suggests for this combo. I also have the option of surfacing the heads to get a little more compression. It will have a flat top piston in it regardless. The max compression I am looking for is mid to high 11.
I guess I have to get it together and see how it works.
Thanx 451 Guy
512 cu in 71 Demon Super Street Car - NO LONGER - Broke Crank Best ET 9.57 @ 139.96
451 cu in Pump Gas - 71 Demon Super Street Car Best ET 9.99 @ 136.80
512 cu in 69 Charger R/T Best ET 11.39 @ 118.11
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