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Thread: bigger exhaust cam than intake

  1. #1
    Junior Member Backyard Mechanic
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    Default bigger exhaust cam than intake

    just wondering if anyone has attempted this and what the results were. or if no one has, what kind of power change would be expected.
    Quote Originally Posted by merc-blue
    Cheap tools take all the fun out of working on a car.
    My Rebuild and Conversion.... '81 Tercel 3A -> 4A-GE 20 Valve - ON HOLD.
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  2. #2
    Toyotard Conversion King Cuts's Avatar
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    Default Re: bigger exhaust cam than intake

    ive heared of people using say a 264 inlet and a 272 exhaust cam, quite a few times ive seen it.

    the results would yeied two smaller power gains at diff stages in the rev range (as opposed to mine where i have both 264 and can feel a kick from them when the come on at about 4200)
    ST205 Group A Rallye GT-Four 307kw atw @23 psi on 98oct, Now on E85.

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  3. #3
    Junior Member Backyard Mechanic
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    Default Re: bigger exhaust cam than intake

    i'm currently running 264 in and 256 out, i feel it kick at that rpm too, its the tvis. just wondering what happens to the curve if i were to go the other way round or say changing the intake back to stock, keeping a 256 exhaust. ive read the g head has poor exhaust flow, would placing the larger cam on the exhaust compensate for that? also in a turbo setup could this help scavenge the exhaust more and enabling faster spool up?
    Quote Originally Posted by merc-blue
    Cheap tools take all the fun out of working on a car.
    My Rebuild and Conversion.... '81 Tercel 3A -> 4A-GE 20 Valve - ON HOLD.
    BAD ASS 1986 MR2 - Finally Moving forward.

  4. #4
    Junior Member Backyard Mechanic
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    Default Re: bigger exhaust cam than intake

    sorry, i did not really ask the question properly first time, here goes my second attempt

    **** hypothetical dreamland ****

    4age (whatever variant you choose) running an intake cam of say 264 duration.
    now you run a paired cam as your exhaust for your baseline.

    what difference would you see in the curve by
    A) swapping the exhaust cam for a less aggessive cam say 256 or even a stock cam
    B) swapping the exhaust cam for a mor aggessivve cam say 272 or 288?

    ***** end dream land ******

    the reason is im entering the 100' shootouts this year even if it kills me, im going to run a smaller cam on the intake for the low end but im wondering what to do about the exhaust in order to get the best low end possible. and no a GZE swap is out of the question, only what can be done easily. I have an underdrive pulley for the alternator and im losing all the dead weight i can, i will be running without an engine lid if i can. im currently unsure of all the rules but its more the "anyone can enter for $25" type of things so maybe you need a complete car.
    Quote Originally Posted by merc-blue
    Cheap tools take all the fun out of working on a car.
    My Rebuild and Conversion.... '81 Tercel 3A -> 4A-GE 20 Valve - ON HOLD.
    BAD ASS 1986 MR2 - Finally Moving forward.

  5. #5
    Olde mechanic Carport Converter oldeskewltoy's Avatar
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    Default Re: bigger exhaust cam than intake

    I've heard all sorts of things........ I've NEVER seen a dyno showing mixing cams is a benefit.

    Anyone have dyno runs showing any worthwhile advantages???
    Information is POWER... learn the facts!!

  6. #6
    www.malicious.com.au Automotive Encyclopaedia ReQuieM's Avatar
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    Default Re: bigger exhaust cam than intake

    US supra forums has a thread on it for the 2jz-gte tho. Results = waste of time. Basicaly you only get the benefits of the lowest cam. IE 264/272 your engine will react like a 264/264

    Thats IIRC, best bet is to check www.supraforums.com/forums and have a look at teh results for your self. Their curves my be of more interest tho...
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  7. #7
    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: bigger exhaust cam than intake

    turbo is a different beast to NA, because there is much higher pressure in the exhaust manifold than for NA...
    in case of turbo, too much exhaust duration can be a bit harmful perhaps?

    most people just think of extra power when mixing cams....
    improving exhaust (porting, manifold, cam) may not increase power that much, but makes everything more efficient.. ie, more exhaust scavenging, assist flow of intake at beginning of stroke, reducing pumping losses on exhaust stroke... all will make car better to drive, but may nto actually increase power since that is limited by the intake cam...

    i don't actually know, but thats what i reckon

    conversely, if exhaust cam makes not much difference... why not just keep a 256 cam, and put in different intakes.. 264, 272, 288, 302.... will the exhaust continue to make no difference, or will it choke up at some point?
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    is a semi-motivated Domestic Engineer 1JZ~lux's Avatar
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    Default Re: bigger exhaust cam than intake

    Subscribed.
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    ......... Carport Converter jeffro ra28's Avatar
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    Default Re: bigger exhaust cam than intake

    as Oc said, exhaust pressure in the manifold before the turbo will be so high, whats the point of using more duration? Scavange? Like scavenge is going to happen if the exhaust pressure is higher then the boost pressure.

    I dont know either, however thats my perspective of the matter. Id prefer the setup with the smallest amount of overlap tho. /subscribed.

  10. #10
    Junior Member Backyard Mechanic
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    Default Re: bigger exhaust cam than intake

    im gonna go at it this weekend and drop the stock cam back in as the intake and keep the 256 as the exhaust. Just got the adjustable cam wheels i ordered a while back, they should help me play around with the power a little bit. Ive been fudging around with some math to see "on paper" results but have reached the point where i need to actually DO something. kinda a shame since i just got it running good but its almost all top end. i get some REAL good power after about 4000 rpm (not just from tvis) it pulls and pulls till the rev cut.
    Quote Originally Posted by merc-blue
    Cheap tools take all the fun out of working on a car.
    My Rebuild and Conversion.... '81 Tercel 3A -> 4A-GE 20 Valve - ON HOLD.
    BAD ASS 1986 MR2 - Finally Moving forward.

  11. #11
    Junior Member Too Much Toyota oldcorollas's Avatar
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    Default Re: bigger exhaust cam than intake

    try changing your lobe centres first.. decrease the overlap.. see how that goes.
    "I'm a Teaspoon, not a mechanic"
    "There is hardly anything in the world that a man can not make a little worse and sell a little cheaper" - John Ruskin (1819 - 1900)

    AU$TRALIA... come and stay and PAY and PAY!!! The moral high horse of the world!

  12. #12
    DIY Bloodline Domestic Engineer Talasas's Avatar
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    Default Re: bigger exhaust cam than intake

    I've been wondering lately what the reverse would do on a turbo setup. In my case I have a set of Bigport cams I bought here and the exhaust cam has a broken dizzy drive, yet the intake cam is fine.

    What did you notice change when using a bigger intake cam with a turbo?


    As for the other way around, yes, I have read a few (albeit personal) records of people suggesting that running large cams with a turbo will have adverse effects. Namely this has been to do with exhaust pressures.
    Apart from that there really hasn't been much detail on the matter and I've yet to see any evidence to explain when any of the effects take place. For example it would be good to know exactly what duration and lift is considered "too large" for a turbo setup.
    Chances are it's far more complicated than it seems, I would guess that manifold design will have a role to play as well as the rev range of the engine and the size of the turbo.

    EDIT: and yes, OC's post reminded me, there was mention of too much overlap becoming an issue for exhaust pulses, which vary between an NA and turbo setup for optimum power.
    "In the beginning, the universe was created. This has made a lot of people angry and has widely been regarded as a bad move." -HHGG

  13. #13
    Junior Member Backyard Mechanic
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    Default Re: bigger exhaust cam than intake

    I still have not done the turbo conversion, getting my wife pregnant has slowed that project down a little. Im not allowed to spend as much time in the garage as i used to.

    i got talking to one of my engine guys here, he is primarily a Ford guy (in essence and engine is an engine though) and he said that running the larger exhaust on a turbo'd engine(if setup right) would/should help build exhaust pressure quicker but also did run the risk of getting a reversal in pressure (mainly with a smaller turbo/ restrictive exhaust)

    OC: yeah i may just swap in the cam gears first and play around a little, i am using exactly the same cams as ADSPORT, has in his GZE i will have to dial them in as discussed in another cam thread thats running kinda parallel to this one.

    ughhh, i hate having to re-shim, if i were smart i would have kept a record of what i changed so that i could just put it back the way it was....
    Quote Originally Posted by merc-blue
    Cheap tools take all the fun out of working on a car.
    My Rebuild and Conversion.... '81 Tercel 3A -> 4A-GE 20 Valve - ON HOLD.
    BAD ASS 1986 MR2 - Finally Moving forward.

  14. #14
    Junior Member 1st year Apprentice Grippy's Avatar
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    Default Re: bigger exhaust cam than intake

    I was chatting to a bloke a while ago that was into drag racing a bit and he pretty much told me that for n/a you want big lift with longer duration but for forced induction you want the high lift but not the long duration as the overlap is what messes up the boost pressures.

  15. #15
    Junior Member Backyard Mechanic
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    Default Re: bigger exhaust cam than intake

    Some of the drift cars in Japan frun mixed cams in their 4age's and make around 200hp. Having said that, most of them don't and still make the 200hp, in many cases using approx 270deg both sides. Altering the duration seems to have more effect on where the power is made than how much is made (beyond 290 deg), and getting your overlap/timing correct is crucial. Basically, you need the intake to be open long enough to allow max cylinder filling, but not too long that the piston is not able to achieve max cylinder pressure and the exhaust open long enough to allow quality scavenging, but not fresh intake to escape at revs.

    I have found some specs.
    In/Ex 272/272 deg 192.5hp, stock bores
    In/Ex 272/288 deg 185 hp, 81.25mm pistons
    In/Ex 288/288 deg 196 hp, 81.25mm pistons

    Also, most cams under 290 seem to make peak power at 8000rpm or less, whilst 300 deg cams need 9000 rpm to make peak hp.
    I'm still translating these specs, i'll see if I can work out what the comp ratios/lobe centres are.

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