I'm just wondering, since I dun really know. If I go and swap some high compression pistons that'll push me from like 8.8:1 to 12.3:1 (or thereabouts) and drive the car with the same acceleration as normal and all that, would I see and increase in gas mileage for the increase in efficiency? Thanks a lot, I'm really new to this engine building stuff.

 

Well with the higher compression you'll need to flow more air to make power.....more air is goign to mean you need more fuel....so yes, your gas mileage will go down.

Please note that this is a VERY simplistic explination, and there are tons of variables that can increase or decrease your fuel economey.

 

Ah, thanks for the response. I've been wondering about this because I know lots of the hybrids run silly high 13:1 and 14:1 ratios, but I didn't know if it would be quite the same converting a stock low CR engine to something with a higher CR.

 

Hybrids are a totally diffent ball game.....a straight fuel motor is alot more complex and alot simpler at the same time.

 

Wouldn't it all depend? I increased my compression approx 11.0:1. I get lower gas mileage in general (on streets) but on the freeway, it has gone up a few mpg.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by SVOboy &raquo;</TD></TR><TR><TD CLASS="quote">I'm just wondering, since I dun really know. If I go and swap some high compression pistons that'll push me from like 8.8:1 to 12.3:1 (or thereabouts) and drive the car with the same acceleration as normal and all that, would I see and increase in gas mileage for the increase in efficiency? Thanks a lot, I'm really new to this engine building stuff.</TD></TR></TABLE>

higher static compression yields higher mpg for the "same part throttle" operation

 

if the combustion chamber is properly configured (shape, burn bias and flow bias) and the valves, ports, etc. match, higher static compression will allow you to run less spark advance, since the higher pressure creates more heat. this allows the spark to ignite the fuel later, and still burn it as completely (in less time).

the piston doesn't have to do as much work on the compression stroke to fight the stronger downward thrust created when, to burn the same amount of fuel, the spark had to ignite the intake charge sooner. you reduce pumping losses and free up power, which also increases efficiency.

of course, being that the engine can produce more power, it can usually burn more fuel if you wring it out, but for under the same loads and rpm, it should get better mileage than a lower compression engine.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by xEnderx &raquo;</TD></TR><TR><TD CLASS="quote">Well with the higher compression you'll need to flow more air to make power.....more air is goign to mean you need more fuel....so yes, your gas mileage will go down.

Please note that this is a VERY simplistic explination, and there are tons of variables that can increase or decrease your fuel economey.</TD></TR></TABLE>

No you don't need to flow more air!! It's just compress the same air at higher rate.

 

Hmm, thank you all for the replies. I'm wondering mainly because I'm considering a mini-me d15z1, and if I just do the mini-me I'll have a 12.3:1 cr, as opposed to the full engine swap which would leave me at 8.8:1.

 

12.3:1
8.8:1

 

Sweet, thanks for the advice.

 

A lot of bad physics on this thread, so let me help with a thermodynamic primer and the effect of compression ratio. Here is a long .pdf on the subject. Particularly see slides 9 & 10. That is the key to your gas mileage question.

http://www.mtggraphics.com/big...s.pdf

 

I do love your site, bigmoose, but I cannot open the pdf. :-(

 

Anyone else have trouble with the .pdf?

Here is a .ppt Power Point, lets see if that works.

http://www.mtggraphics.com/big....ppt

 

I got it, drdisco69 sent it to me on aim in picture format, love it, thanks a bunch. I think I've got my answer, :-)

 

Here is another take at the .pdf file. I "crunched" it with a different encoder. It's also 200K smaller. Let me know if you can open this one.

http://www.mtggraphics.com/big...2.pdf

 

Opens right up, thanks, good read.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by BigMoose &raquo;</TD></TR><TR><TD CLASS="quote">Here is another take at the .pdf file. I "crunched" it with a different encoder. It's also 200K smaller. Let me know if you can open this one.

http://www.mtggraphics.com/big...2.pdf</TD></TR></TABLE>

Hope I don't scare you when I say I read most of your posts. You've always got something helpful/informational to share. Without people like you, I have a feeling this place would sadly go down the toilet. Thanks again.

 

Thanks Peter, your comments are very kind.