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Confused by civic hatchback CX/VX

Discussion in 'Engine Building' started by imzjustplayin, Apr 3, 2008.

  1. imzjustplayin

    imzjustplayin New Member

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    I'm confused by the mileage figures between the Civic Hatchback CX and the VX.

    Here are the engines on wikipedia:Honda D engine - Wikipedia, the free encyclopedia
    And this is the comparison between the two cars: 1995 Honda Civic Specs and Features - MSN Autos

    The VX has D15Z7 engine which gets 47/56MPG City/Highway while the CX has the D15B8 engine which gets 40/45mpg City/Highway. The CX has 70hp while the VX has 92HP. The largest difference I noticed was that the CX has 2 valves per cylinder while the VX has 4 valves. The VX also has VTEC, so when under I think 3K RPM it runs on 12 valves and above that on 16 valves. In 12 valve mode, the engine keeps the "unused valves" slightly open as to not have fuel collect behind it but otherwise it's effectively shut. The CX is running on 8 valves all the time, which I thought would actually yield better fuel economy since it wouldn't be able to consume so much fuel at once. I believe this means that on the D15Z7 you've got 1 intake and 2 exhaust at under 3K RPM which is most driving. I can sort of rationalize why the VX would get better highway MPG since it does have more HP, though in my experience, vehicles with more HP always get less MPG compared to their lower powered brethren. (Of same engine type/block/litre)

    So everything that has been mentioned, could the reason why the VX gets better mileage than the CX is because it has twice as many exhaust ports as intake ports? The NonVTEC version of this engine gets worse mileage than the one with VTEC-E which is what the D15Z7 engine has and what is subsequently in the VX model.

    If this is true, wouldn't this imply that performance is hampered by exhaust restriction? And that improving exhaust flow would improve performance somewhat while improving fuel economy considerably? It would seem to make sense since it would rob the engine of momentum by having to force the exhaust out of the chamber. So then why would people make the impression that the exhaust system on the civics are adequate?
     
  2. OKCZombie

    OKCZombie Banned

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    who in the hell ever said the exhaust system is adequate? you could spend 1500 on an exhaut system and I would point it that it could be better and that might be adequate
     
  3. AHHVTEC

    AHHVTEC Well-Known Member VIP

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    the vx was a special made motor meant for good gas mileage.it have the vtec e motor witch stands for economy..thats why is gets more miles than the cx..
     
  4. imzjustplayin

    imzjustplayin New Member

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    The only "special" thing about the VX was that it has a VTEC-e head, the block is still the same amongst all the other D15 engines.
     
  5. AHHVTEC

    AHHVTEC Well-Known Member VIP

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    that and it had a different intake manifold
     
  6. imzjustplayin

    imzjustplayin New Member

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    Which was what, more or less efficient? Either way, I'd imagine that having a better intake manifold would mean more HP but not necessarily more fuel economy while having a good exhaust manifold should give more HP and fuel economy, but then again what do I know?
     
  7. AHHVTEC

    AHHVTEC Well-Known Member VIP

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    every part of the d15z1 was tunned to make the motor efficient, from its small intake manifold and so on..every gen civic that was made had some form of a special model that was made for gas mileage and the 92-95 usdm years was the vx
     
  8. ernesttex1

    ernesttex1 New Member

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    P28 Ecu

    HEY OKCZombie, where the hell is my P28 ECU I paid you for via PayPal over a month ago? WTF!!! I been emailing you and emailing you with no response? WTF!!! What seems to be the hold up??? STOP avoiding me and send me a email!!! Or better yet, how bout a fukcing refund!!!
     
  9. haao13

    haao13 New Member

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    why the hell did you post that on this thread? :shrug:
     
  10. |Chaz|

    |Chaz| Well-Known Member VIP

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    Why would you buy something from someone with little to no rep?

    I'm assuming this is one of the first times that he has seen that guy post in awhile so he jumped at the opportunity to confront him.
     
  11. ernesttex1

    ernesttex1 New Member

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    sorry haao13, chaz is 100% correct

    Sorry haao13, chaz is correct. OKCZombie ripped me off and will not return my emails. Sent a PayPal ($70.00) for a P28 ECU in Feb 2008, guy ripped me off!!
    Took my $$ and never sent ECU, now will not rerturn my emails!!!

    So my mssg to Zombie is:
    Hey Zombie, I can give them proof, or we can take care of this properly.
    You gave me no choice but do what I did by posting here. Either you can email me and refund my money today, or I post all the proof I have concerning this transaction on every forum board you frequent. YOUR CALL!!!
     
  12. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    The CX's engine is also a LOT simpler than the VX. The difference between the two cars is a lot more complex than just the valve configuration.

    The CX model was made to reduce cost to have the cheapest Civic possible. The VX was built specifically to maximize gas mileage. It has the lightest chassis in the entire 5G Civic line, it has the lightest wheels (makes a huge difference), it has an engine with TWO optimized rev ranges because of the VTEC-E while the CX block only has one (so it's easier to go for max mileage)...

    You have to take ALL these factors into consideration, not just the valve count on the engine.
     
  13. AHHVTEC

    AHHVTEC Well-Known Member VIP

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    i thought the vx was 15lbs heavier than the cx, making the cx the lightest
     
  14. imzjustplayin

    imzjustplayin New Member

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    How much does the CX weigh compared to the VX? I doubt the weight difference is what you're making it out to be. The '96 civic and the '01 civic are a difference of 100lbs which equates to in the real world about 3mpg. Having optimized rev ranges don't matter when fuel economy tests are done in the lowest range. Have you implied that the 8 Valve CX has the intake valve open more than the intake valve of the VX in 12 valve mode and since it uses only 8 valves, has larger valves to boot. If you were to remove the extra exhaust valve in the VX, making it an 8 Valve yet retain the Vtec-e part, in the low range RPMS (where VTEC-E is effective) would you say that the CX produces more power in that same power band?
     
    Last edited: Apr 4, 2008
  15. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Who knows? What I'm saying is that it's not that simple. There's a lot more to it than just one valve being open longer or another set of valves having more area. Sure the weight might not make that much of a difference, but the goals for the cars were totally different to the engineers.

    The optimized rev ranges DO matter. The CX is the bargain basement model right? So the engineers have to make the car work for the entire rev range, not just for the mileage test. As a result, the single cam profile might not be as fuel efficient in that lower rev range for the mileage test as the two cam profiles of the VX.
     
    Last edited: Apr 4, 2008
  16. imzjustplayin

    imzjustplayin New Member

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    So wouldn't that imply that if you were to use the cam profile in the VX used for 12 valve mode that you'd end up with less top end HP than with that of the CX?
     
  17. phunky.buddha

    phunky.buddha Admin with a big stick Admin VIP

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    Probably, but you can't say for sure.
     
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