Compression Ratio Increase vs. Power
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Post Fiend
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From: "Islander"
Compression Ratio Increase vs. Power
Down below is a chart that might be of intrest on changing compression and what is gained. As you will see there isn't as big a gain in percentage than you think unless you raise it way beyond pump gas for a daily driver.
http://victorylibrary.com/brit/compression-c.htm
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http://victorylibrary.com/brit/compression-c.htm
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Last edited by "Beemer Nut"; Aug 26, 2006 at 10:02 AM.
Post Fiend
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From: Iowa
This is interesting.
When running a Dyno2000 simulation on a fairly standard warmed up 390, here's what I came up with:
8:1 compression: 373hp
10:1 compression: 417hp
Now if we divide those out, we get about 1.12, or a 112% increase. Their chart shows a 108% increase. Dyno2000 must think there's more going on with compression beyond what they calculated. I would bet Dyno2000 is figuring there's an increase in breathing and scavenging when we lower the combustion volume (space above the piston at TDC).
When running a Dyno2000 simulation on a fairly standard warmed up 390, here's what I came up with:
8:1 compression: 373hp
10:1 compression: 417hp
Now if we divide those out, we get about 1.12, or a 112% increase. Their chart shows a 108% increase. Dyno2000 must think there's more going on with compression beyond what they calculated. I would bet Dyno2000 is figuring there's an increase in breathing and scavenging when we lower the combustion volume (space above the piston at TDC).
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Post Fiend
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From: "Islander"
I ran it thru a Dyno Sim's program for a 414 with 8.88 cr then again at 10.88 cr.
From 374 hp increased to 411 hp or 1.0989% increase vs the chart at 1.08% increase. Would the torque also be rated in "British Pounds" like their money or good old USA ft/lbs? My mechanic friend a Limey will live and die by this chart he gave me. There must be another one but make in the US I would hope?
Coffee's done must work, good morning.
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From 374 hp increased to 411 hp or 1.0989% increase vs the chart at 1.08% increase. Would the torque also be rated in "British Pounds" like their money or good old USA ft/lbs? My mechanic friend a Limey will live and die by this chart he gave me. There must be another one but make in the US I would hope?
Coffee's done must work, good morning.
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Cargo Master
Joined: Apr 2004
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From: Golden, CO
Originally Posted by Beemer Nut
Would the torque also be rated in "British Pounds" like their money or good old USA ft/lbs? My mechanic friend a Limey will live and die by this chart he gave me. There must be another one but make in the US I would hope?
If you poke around the site you will see an affinity for Harleys and Mopars also. His "Panic Tech Papers" go well beyond brand loyalty. Even if you have gears-between-the-ears, bring your math brain; it's gonna hurt! I've thought about posting links just to see if you guys validate the information he presents.
Pain...is...GOOD! Mike
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I'll tell you what's most interesting to me. Everything I've ever heard here is that you must raise the compression ratio to get decent performance. The fact is that you could do all the standard performance mods on an engine and you'd probably be happy with it. In my case, I wouldn't have had to bore my engine and I could have reused the pistons. Granted the Hp could be better for the same mods with the higher compression but if you're on a budget and the engine you've got doesn't need boring then let it go. JMHO. OK....so I'm glad I bored mine...heck after being in this forum I went for every modification I could...LOL.
Tracy
Tracy
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From: suburban atlanta
Raised compression is also for efficiency. I think.
Each cycle does more than a cycle on a lower cr engine. Think Diesel. The cr numbers are up there. And they generally have long strokes to take advantage of the lever arm and time for the combustion pressure to push on the piston.
You can use higher compression for more power, but you also get more efficiency. When you run a turbo mustang 2.3 engine on 'low' setting, it 'tards the timing and limits the boost. Flip to high test setting, and the power goes up because the boost (effective cr) goes up, along with a modified ignition timing.
I had a truck that sometimes hated 87. Was towing a trailer and it was pinging at cruise. And the gas mileage was crappy. Added a bunch of 89 to the tank. Pinging gone and the mpg came back, even towing. So, compression ratio, and in this case probably hot carbon bits, came into play. The higher octane allowed for the compression to take place before auto-ignition that the 87 would have done, and thus got actual power rather than 'opposition' (knock) power for part of the time.
Anyway, compression ratio effects efficiency also, in addition to peak hp and torque.
tom
Each cycle does more than a cycle on a lower cr engine. Think Diesel. The cr numbers are up there. And they generally have long strokes to take advantage of the lever arm and time for the combustion pressure to push on the piston.
You can use higher compression for more power, but you also get more efficiency. When you run a turbo mustang 2.3 engine on 'low' setting, it 'tards the timing and limits the boost. Flip to high test setting, and the power goes up because the boost (effective cr) goes up, along with a modified ignition timing.
I had a truck that sometimes hated 87. Was towing a trailer and it was pinging at cruise. And the gas mileage was crappy. Added a bunch of 89 to the tank. Pinging gone and the mpg came back, even towing. So, compression ratio, and in this case probably hot carbon bits, came into play. The higher octane allowed for the compression to take place before auto-ignition that the 87 would have done, and thus got actual power rather than 'opposition' (knock) power for part of the time.
Anyway, compression ratio effects efficiency also, in addition to peak hp and torque.
tom
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Along this same line, seems that compression ratio determines what octane fuel is required. For instance, I finally ran a compression test on my '89 4.9 FI SWB HiBoy and found the readings thusly from #1 back: 166, 166, 169, 165, 168, 165. So now I believe this engine might want a higher octane than "At least 87", but can't figure out or find exactly where the octane requirement would change with increase in CR. The only example in my books is a compression test "for instance of 150psi", (plus or minus an acceptable variance). But that does not really tell me anything useable when it comes to thinking about fuel grades required, without knowing all the esoteric capacities, bore and stroke of the cylinders, clearances, cam timing, etc.
The only online reference I could find for compression ratio and fuel octane is this one, which indicates a radical change in octane required for a modest change in compression ratio. But then, final compression ratio and cylinder PSI tests are not exactly the same thing:
The only online reference I could find for compression ratio and fuel octane is this one, which indicates a radical change in octane required for a modest change in compression ratio. But then, final compression ratio and cylinder PSI tests are not exactly the same thing:
http://www.eaa1000.av.org/technicl/engemp/engemp4.htmDoes a higher compression ratio increase the horsepower output of the engine? Yes, because the thermal efficiency is increased. Compare the Lycoming O-320-A2B and the Lycoming O-320-B2C. The only apparent difference between these engines is upping the compression ratio from 7 to 8.5. Since the bore and stroke are identical between engines, this was presumably done by reducing the volume of the combustion chamber above the piston at top dead center. This results in a 10 horsepower gain. The down side of this is the fuel required changes from 80/87 to 100LL.
Last edited by codejunkie; Sep 13, 2006 at 11:28 PM.
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Post Fiend
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From: "Islander"
Compression test numbers do not determine fuel grade, BMEP numbers determine grade.
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Post Fiend
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From: Iowa
Wait a minute there! There's a lot more to it than that. The following determine octane requirement:
Dynamic compression ratio
Combustion chamber dynamics (quench chamber or open chamber)
Ignition timing
Combustion chamber condition (sooted up or not)
Engine speed
Engine temperature
Air temperature
And many other factors...
Dynamic compression ratio
Combustion chamber dynamics (quench chamber or open chamber)
Ignition timing
Combustion chamber condition (sooted up or not)
Engine speed
Engine temperature
Air temperature
And many other factors...
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Post Fiend
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From: "Islander"
Thats true rusty I was lazy on the first reply.
All the above besides the intake ram affect at its resonant rpm which will increase the intake charge density hence higher BMEP numbers closer to the detonation zone. Too much to think about at 12:30 AM.
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All the above besides the intake ram affect at its resonant rpm which will increase the intake charge density hence higher BMEP numbers closer to the detonation zone. Too much to think about at 12:30 AM.
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FE "Freakin Expensive"
Joined: Dec 2003
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From: Smith Mountain Lake, VA
Well I think anything over 10.5 to 1 for a street driven engine is just a waste anyway!
Hell My Next Big Inch build will only be about 7.5 to 1......LOL... Geeezzzz
Russ
AKA- RJ
Hell My Next Big Inch build will only be about 7.5 to 1......LOL... Geeezzzz
Russ
AKA- RJ
FE "Freakin Expensive"
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From: Smith Mountain Lake, VA
Originally Posted by Beemer Nut
Yea right 7.5 to 1 with 14 pounds boost added, don't BS a BS'er we know you Russ.
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Oh I got a Hankerin for another Bike over the weekend..I still have my Harley down for paint ( 2 years now) and my Cousin just bought one of those Hayabusa's ...Damn is that thing QUICK... I dont know what they run in the 1/4.. But I hit the throttle in 4th gear about 90mph and it stood up on the back wheel and trying to Dump me off?? LOL... GEEEZZZ there fast!!
He's only had it 2 weeks and wants to put a Hair dryer on it? Freakin Guy is NUTS!
Russ
OOOoops......
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Post Fiend
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From: "Islander"
I'm old school, old truck, older wife, old bikes maybe i'll live longer on them that's two out of four above mentioned. I can trade the wife in for two 27 year olds. Didn't model T's have 4.5 to 1?
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FE "Freakin Expensive"
Joined: Dec 2003
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From: Smith Mountain Lake, VA
Yeah Carl I do know the Fealing!! Bro!! I try and change them out every now and then for new stuff...But nobody wants the OLD stuff?? LOL.. I told the wife..I was going to trade her in for 2 25vers..and she just laughed..told me I couldnt handle that anymore!
I told her.. But I'd love to die Trying!!! lol..
Russ
I told her.. But I'd love to die Trying!!! lol..
Russ