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Compression VS. Gas mileage

Discussion in 'General Tech and Maintenance' started by phyregod, Sep 6, 2005.

  1. phyregod

    phyregod !!YTINASNI

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    Ok.. On my newest car, a 1992 Honda Civic CX, I've decided that I like the newly found gas mileage. (54.996mpg the first tank, no shit)

    Now, 70 HP just isn't working for me, So I'm looking to get something like 125 HP at 50 MPG. This will be a project for sure.

    I'm pretty sure I can get those HP numbers out of a vtec head swap, but then my mileage will go down to something like 35.. SO..

    What about raising the compression of the motor? Is that really just free power? You take the same amount of fuel and air and compress it harder, it makes more power. The downside is that it leads to detonation. What kind of compression can your basic 87 octane fuel handle? 10:1? 12:1?

    My thought is to get the D16A6 head, do a p&p, polish the combustion chambers, weld in quench zones, and shave the holy crap out of it. Get the compression up there... With the larger intake, exhaust, a high flow filter, and high compression, I should be able to eek out something like 120hp. I think. The goal is to raise the horsepower without completely jacking the gas mileage. (45-50mpg is the goal)

    Thoughts, suggestions? It can be done... It'll just take some engineering. :)
     
  2. EGProject

    EGProject YEEEEAAHHHHH

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    You could try looking at the aerodynamics - fuel mileage isn't 100% dependent on the motor. Run thinner tires/wheels with higher PSI. Also look at removing the passenger mirror, wing, etc.

    As for the motor, the reason why the CX sips gas, is not exactly due to compression. The engine probably runs very hot (lean), therefore it burns what it needs and nothing less. The cam profile is probably a little towards fuel economy as well. I'm no expert on the CX motor, but I'm just using a little deductive reasoning.

    You could also try getting new plugs, wires, an intake and an exhaust. A little useless on a car that makes 70hp, but I'd assume that they would increase airflow, and let the engine work a little less. But for 120hp? Forced induction would be the way to go - perhaps a tiny turbo, and no intercooler?
     
  3. GSRCRXsi

    GSRCRXsi Super Moderator Moderator VIP

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    more compression needs more fuel.

    what is the compression stock on that motor?
     
  4. theodorcrx

    theodorcrx Member

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    This is what i'm tryin to do also with my 89 crx hf.
     
  5. phyregod

    phyregod !!YTINASNI

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    I think I'm good to go as far as aerodynamics.. I'm certain that it can be better, but I'm not going to be modifiying the exterior of the car much. Lowering the vehicle would probably do a tiny bit of good. Weight plays a major role in start and stop mileage too, but this isn't the focus. Although it may be necessary later to meed MPG demand.. I'm trying for a more efficient engine. More power, same fuel.

    Stock Motor Specs:

    CX:
    Engine: D15B8
    ECU: P05
    Displacement: 1493cc
    Horsepower: 70@5000rpm
    Torque: 91@2000rpm
    Compression: 9.1:1
    Valvetrain: 1.5L 8-Valve SOHC
    Curb Weight: 2094 lbs
    Standard Features: 5-speed manual transmission, driver-side airbag, reclining cloth front bucket seats, 50/50 split folding rear seatback, remote fuel door and hatch releases, tinted glass, rear defogger, 165/70/R13 tires.


    Now, I'm under the impression that raising the compression from 9:1 to 11:1 will make more power on the SAME amount of fuel, is this incorrect??
     
  6. GSRCRXsi

    GSRCRXsi Super Moderator Moderator VIP

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    if you up the compression to 11:1 without changing the fuel, you WILL detonate.
     
  7. phyregod

    phyregod !!YTINASNI

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    So the question remains.. What kind of compression can 87 octane fuel handle?

    I suppose I could use hondata to force the car to run leaner, which will run hotter, but more efficient. Then install a better cooling system.. Maybe a full race radiator or something....

    Also, a lighter flywheel, crank, pistons, rods, valves, etc.. should help...

    I wonder if there is any benefit at all to using an intercooler on a N/A vehicle..
     
  8. 92dxhatch

    92dxhatch Senior Member

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    it sounds like a good idea, but your either going to have to sacrifice milieage or power either way you look at it, you can get the best of both worlds with alot of money and patience, but doing what your talking about, you'll maybe get about 120 hp with only 35-40 mpg if that. i run a modified b7, and i am up at about 120-125hp and barely getting 40mpg, i mean i have to drive like a grandma to get that kinda gas milieage. good luck bud.
     
  9. E_SolSi

    E_SolSi Member of the 20 nut club Moderator VIP

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    remove the resonator from the stock air box
    put in a K&N OEM replacement filter
    run a length of dryer hose from the opening of the air box to right near the cat

    this will suck in hot air and make the car run lean.... i dont know how safe it is... but people on my insight board do it all the time

    use thin tires (less rolling resistance)

    up your tire pressures... i run my insight's tires at 55psi... this gives me an average of 10 - 15mpg better milage on the highway (less rolling resistance)

    remove un-needed shit to drop weight

    make sure all of your shit is clean and working properly (plugs, wires, cap, rotor, cat, O2 sensor, injectors, fuel filter, etc......)

    fuck it... heres some links from my insight board... not all of them will apply but enough of them will
    http://www.insightcentral.net/KB/faq-quicktips.html
    http://www.insightcentral.net/KB/faq-efficiency.html
     
  10. xj0hnx

    xj0hnx I wanna be sedated VIP

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    You could pick up the VX rear difuser too. Not a slam bang aero mod, but it is a decent start towards lowering resistance.
     
  11. wanderinman

    wanderinman Senior Member

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    Energy can neither be created nor destroyed, I forget who's principle it is, but its a biggy. You can't create more energy with the same amount of fuel. At least not almost double the energy. You can gain a few HP through efficiency of course, but you can't deny physics... sorry. Fuel will only give up a limited amount of energy through combustion, I dont think anyway short of a hybrid will allow you to see 50mph and 125hp.

    PS Adding a turbo wont work, turbo's add more air, which allows more fuel to be blown up, which gives you the horsepower gain... its not magic.
     
  12. jimboburgess

    jimboburgess Go fast, Go Cheap

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    Reduce Unsprung and rolling weight, not only thin but light weigth tires the 13" steelies aren't too thick but they are heavy, find VX aluminum wheels, the VX was the epitome of gas misers in it's gen.
     
  13. jamesA

    jamesA Well known pissed off telephone guy VIP

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    The general cutoff, from my experiences is you can use 87 octane from 9.8ish and below. Any car that has 10 or 10.2:1 has required premium from the factory.
     
  14. phyregod

    phyregod !!YTINASNI

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    Efficiency is the key. You CAN eek out more power from one gallon of gasoline. The old chevy pickups pushed 125 horsepower.. Big fat V-8s.. They got like 9 mph.. Now, we can get 125 out of our lil honda motors at 35mpg... I'm certain we do not have the most efficient motors possible. Just got to work on it.

    Our fuel injectors are still stone age. If you've ever seen a humidifier work, it turns water into a gas that lazily floats around a room... when our injectors emit a mist that fine, I'll be impressed.. I'd be willing to bet that if we could spray fuel that fine, we would get 30% more power out of our engines. Combustion would be so much faster and more efficient... emissions would drop to damn near zero because the fuel mist would burn much more completely.

    Injectors are only one area. The combustion chamber relies on complete chaos. The more chaos, the better the combustion. This is how today's engines work.. Thats why vortec heads got so popular, because they cause more chaos. Someone needs to get inside the combustion chamber and figure that out.. bring it under control. Maybe a little less fuel near the spark plug would cause the mixture to ignite faster.. Maybe we need to move to a 4 plug per cylinder system and light it from the corners instead of the middle.. Hell, go to 5 plugs per cylinder..

    Adjustable compression.. nuff said there...

    hell, vtec just came out not long ago.. we've been building combustion engines for over 200 years and just now figured out how to somewhat control valve lift??

    Then you have the gasoline itself. I'm sure you could add this or tweak that and make it more explosive and give it the ability to be sprayed into a super-fine fog-like mist.

    There is Sooo much more to be done.. If I had a bagillion dollars I'd bet I could get an engine running with damn near zero emissions, 200 HP and 50mpg.. I hope to someday look back on this and laugh as I just filled up the 1 gallon tank on my non-hybrid 500 hp sports car.. and know that that gallon will take me a month to finish. yeah, its a dream.. we won't get there, but we could damn sure take a few more steps in that direction.

    [/soapbox]

    Back to the real world.. I'm going to have to figure out the math on compression vs power vs octane vs fuel cost...

    something like 1 liter air/fuel mixed 14.7:1 and compressed at a ratio of 9:1 =100hp and costs 3.00/gallon for 87 octane
    whereas 1 liter of air/fuel mixed 14.7:1 and compressed at a ratio of 13:1 =135 hp and costs 3.50/gallon for 94 octane...

    And what of this VX rear diffuser? I know nothing of this... :eek:
     
  15. confusatron

    confusatron Senior Member

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    I've never heard this but was wondering about this too. My new Si-R motor is listed at 10.4:1 compression. It's running great on 87 octane, but should I be running 93?
     
  16. confusatron

    confusatron Senior Member

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    9 mph? holy shit those trucks were SLOW! :lol:
     
  17. VTECin5th

    VTECin5th Administrator

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    :lol:
     
  18. jamesA

    jamesA Well known pissed off telephone guy VIP

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    Well fuck it. if it's running "great" on 87, why bother spending the money? You're just running the engine with retarded timing all the time now.
     
  19. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Soooooo........

    I was going to do a giant "classic Mike" multiquote with a bunch of responses, but I'm getting tired and I have to be at work in 7 hours... and I still have to do laundry and make dinner.

    Higher compression = higher efficiency
    Yeah, you might need more fuel at WOT, but you're also making more power. A 12:1 compression engine will be more efficient than a 10:1 compression engine, assuming that both are tuned to the same "level" of quality. The 12:1 engine will take less pedal effort to make the same amount of torque.

    Less mass (static and rotational) = good (duh)

    Better aerodynamics = good
    You can't really change this all that much though.

    Small turbo = good
    Yes, you will eat more gas at WOT, but that's because you'll be making more power. A small turbo that spools up almost right off idle will give you 1-2 psi of boost pressure with light throttle. The slight positive boost pressure takes your engine's volumetric efficiency above 100%, and you actually get an increase in fuel efficiency. Keep the right foot out of it and you'll get better gas mileage than you would if you stayed all motor.

    Leaner = high combustion temps = bad
    You'll get more NOx emissions, but most places don't sniff your exhaust at inspection. The bad part about high combustion temps is engine life- go too high and you'll start to wear down your engine, or you'll detonate and melt a piston. You CAN run air/fuel ratios in the 17s at light load and partial throttle. Most modern cars run in a very lean state at partial throttle cruise. The Insight runs leaner than 20:1 at light loads. If you want to pursue something like this and are worried about the life of your engine components, you could always send the pistons out to be thermal barrier ceramic coated. The coating will keep your pistons cool, and you'll lose less thermal energy out through the piston, which means more thermal energy to PUSH the piston rather than heat it, which means you'll be more efficient and make more power. You can coat the combustion chamber too- that helps even more. Oh- coatings = lower chance of detonation = can run higher compression levels too = even higher efficiency.

    Thinner tires = less drag
    Yup, no brainer here too.

    K bed time.
     
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