6G Celicas Forums

So its time to rebuild one of my 7AFEs because of the all too common oil burning issue, and early signs of rod knock.

SO i want to go fuel economy way since its my DD.

Would a higher CR(12:1ish) make for a better mpg rating when i dont get on it since more torque = less throttle needed?
or is my logic flawed?

your logic (if that's what you wanna call it) is flawed...

more compression = more detonation...
more fuel = less detonation...

do the math

(i'm generalizing a bit... but you should get the idea...)

This post has been edited by Kwanza26: Mar 22, 2008 - 9:39 PM

ah i see.
so no internal mod can really improve economy?

run on two cylinders... =P

tried it!
too rough.. lol

so would after market pistons cams and rods get worse MPG? i am at the same problem's with my 7afe and i am about to rebuild it for my soon to be wifes DD any ideas for a reliable good MPG DD?

those mods decreae mpg....



if you want better mpg get a hot air intake...

higher CR does not need more fuel, it needs a more knock resistant fuel. 10-11:1 is perfectly fine on pump gas with the right timing curves, even 12:1. but the 7afe pcm may not like that much and running timing retarded all the time will hurt fuel mileage. more fuel can be added to help prevent knock by quenching the combustion chamber and cooling things by running rich, but thats not because you need more fuel for a higher CR. honda's run high CR and get great mileage

good example is my GF'd 50CC moped. it came with a dismal 8:1 CR which we increased to 14:1 by simply shaving a little metal in the right place. we checked the AFR by doing plug chops and using a colortune test plug before and after. we did not have to change carb jetting one bit with the nearly doubling of the compression ratio. however we did need to use a higher octane fuel. she netted not only more power but a slightly better mpg number as well. you're extracting more power from a given amount of fuel.

an 1800cc engine is going to burn X amount of fuel at Y rpm and Z amount of load no matter the compression ratio, assuming all things are equal. the difference is that a higher CR will need a higher octane to keep the fuel needs the same, otherwise you'll need to run rich to prevent knock. the pcm does this already at WOT by going very rich to be on the safe side. well it goes way overboard and the afr will dip to as low as 10:1! thats way too rich, the only purpose of going soo rich is to prevent knock.

if you're looking for mileage the best thing to do would be to port the head and intake manifold as you rebuild, a small bump in CR or slight timing advance, some AFM tuning on a dyno, and a slight increase in cam duration. you'll net a good increase in power and should get a slight bump in your mpg as well. of course, only if you can stay off the gas pedal.

good info bitter. fits along with my ideas

just a question is the 7afe nown for nocks?? and do uk were u can get high cp pistons at ive looked and can find anything and does anyone know how gain som hp's easy on the 7afe

cams or turbo?? but thats just a guess

idk i work at bk and im in high school so that means a lack of $$

idk i work at bk and im in high school so that means a lack of $$
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ya i hear ya lol i am in college full time and only make $600 a month and pay 400 in bills.....ya but if u want something bad enough u find a way to get it lol but ya if your looking for a few HP add a cold air or short air and get new plugs and wires if you want more uper end then put a new header on it will set you back $80 on the top end of things run 2.25in exhaust from the newly added header back to a new muffler that fits the 2.25in piping and well you spent well i dont know how much the exhaust will cost but if you get a quote and its more than $250 well your only going to gain maybe 10hp with the header and new exhaust or gain 30whp (ish) with cams for about the same price.. but ya that's how i seen it but i am new to this kinda stuff i am learning as go but when i made my short air ram i swear i gained 4WHP b/c i took it off the other day and drove around and just felt sluggish and i couldn't get second gear scratch any more so ya if you dont already have one order a 90-93 honda accord cold air intake and make a few mods to that to make it fit or make a short air ram out of it up to you hope all this helps lol