Riddle me this, riddle me that.... - Team Camaro Tech
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post #1 of 9 (permalink) Old Sep 11th, 05, 02:10 PM Thread Starter
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Riddle me this, riddle me that....

Here's a question that I have never heard or read: How much horsepower is used (lost) when you increase compression ratio? Per whole point. Example: Amount of addition horsepower it takes to facillitate a 12.5:1 engine, vs. a 11.5:1, vs. a 10.5:1 engine. All other things being equal ie. same cam, etc. We all know the various horsepower robbing characteristics of other engine components such as: magneto vs. distributor, high-pressure oil pump vs. standard pressure, various fans, alternators/generators, a/c, etc. But never have I seen the loss for high compression. Don't be mistaken, there HAS to be a tremendous horsepower usage in squeezing (compressing) that fuel/air mixture to a high level. I know we get more power back, but at what cost? Does anyone have a way to figure this out? Thanks.
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post #2 of 9 (permalink) Old Sep 11th, 05, 02:26 PM
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Smile Re: Riddle me this, riddle me that....

Looking back at my old Thermodynamic's classroom days in school, I figure either a T/S or a P/V graph should tell theoretically how much is made AND then compare a set of them at different compression ratio's to an exact set of matching engines that are dyno'ed.

Or was it P/H for Pressure/Enthalpy?)...


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post #3 of 9 (permalink) Old Sep 11th, 05, 02:31 PM
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Lightbulb Re: Riddle me this, riddle me that....

I can tell you how much horsepower is lost when compression is increased without a PHD or PV or PH TY or whatever you were talking about ,
let's see ohh yea (NONE)

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post #4 of 9 (permalink) Old Sep 11th, 05, 02:53 PM
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Smile Re: Riddle me this, riddle me that....


Are you trying to help here or just what??

Now bttt. I would use the Otto Cycle doing this for gasoline S-I engines and go from there!

I used to know how to use the Rankine Cycle to figure out Electrical Steam Generating Power Plant eff's years ago, but I haven't thought of this stuff in years AND years..


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post #5 of 9 (permalink) Old Sep 11th, 05, 07:55 PM
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Re: Riddle me this, riddle me that....

Actually, IIRC, I believe horsepower increases about 3% for every 1 point INCREASE in compression. This is based on the assumption that your fuel and ignition timing will support the increase in compression. Therefore, if you have a 300 horse engine at 10.0 compression, in theory the same engine should make about 309 hp with 11.0 CR, and about 320 hp at 12.0 CR. Not as much power as one would think...The biggest gain I see in an engine with higher compression, is the throttle response.

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post #6 of 9 (permalink) Old Sep 11th, 05, 10:56 PM
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Re: Riddle me this, riddle me that....

As I understand this- very simply- is that some of the static compression is eaten away by valve events. In order to have a desired dynamic compression ratio with a long duration cam you need to have a higher static compression ratio. Remember, your not just compressing air, it is a highly volatile air/ fuel mixture which deflagrates pretty darned efficiently, and is enhanced by the increase in pressure. If the compression loss was a big component, I recon the Diesel would not work so well. If cylinder pressures were a bad thing we would not use roots type blowers and Turbo's, right?




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post #7 of 9 (permalink) Old Sep 12th, 05, 06:41 AM
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Re: Riddle me this, riddle me that....

It's simple ..... you don't lose horse power with increased compression. If you did Flatheads @ 7.5:1 would be out runn'n Small blocks. The amount of energy release as a result of the increased compression far out weights the resistanse to the compression being made... Or to put it more eloquently it's just like Dave Birdwell said!
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post #8 of 9 (permalink) Old Sep 12th, 05, 09:43 AM
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Re: Riddle me this, riddle me that....

Actually, there is power loss...its called pumping loss But like said, the benefits more than make up for it. Its just like that big roots blower sitting on top of a top fueler engine. Many of those can consume 500+hp just to turn them at the rpms and boost levels that they run at. But again, the overall gains outweigh the losses.
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post #9 of 9 (permalink) Old Sep 12th, 05, 11:23 AM Thread Starter
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Re: Riddle me this, riddle me that....

Which is exactly what I was getting at. Just like the roots blower example. I know you get a horsepower INCREASE when you increase compression, I just wondered at what expense. In other words, how much power does it take to GET the other power. I guess what I could do is mock up an engine with one piston in the cylinder, zero gap rings, lightly oiled to stop any pressure leakage, put the head on, run the lash down, and take a torque wrench and put on the snout of the crank and, starting with the piston at BDC, rotate up to TDC, and note the amount of FT LBS it took. Then change out the 10.5 piston to the 11.5, and repeat steps, again noting the additional FT LBS it took. Repeat with the 12.5. Then simply multiply by 8, then convert that amount of FT LBS into horsepower, and I would have my answer on the differences. I think that would work and be the simplest way to figure. Thanks for all the input.
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