Torque v Power

Discussion in 'Modified Shizzle' started by zedders, Sep 27, 2016.

  1. It seems to me that:

    VW a/c engines were designed for torque at the expense of power.

    This means the HP figures are misleading in terms of driveability.

    This also means there is plenty of scope to improve the max HP and acheive impressive gains at the expense of driveability. Still driveable, yes, but...

    It also seems to me that most engine builders building non-standard engines fail to understand that when their customer asks for a torquey engine for a bus, that's what they want.

    It also seems to me that cams with "torque" or "fast road" in their sales gumph, nevertheless kill what are usually relatively not much larger cc than stock engines under 3,000 or so rpm.

    Discuss. :)
    davidoft likes this.
  2. Correct.
    zedders likes this.
  3. 'Tis the same in many walks of life, Classic Racing being my area of knowledge. Many times have I been faster on a 120hp torque monster than the 150hp dyno queen.
    Cov1987 and zedders like this.
  4. It also seems to me that if your max power is at 6,000 rpm (100mph), driving at 50 - 70mph you are driving where your engine is rather inefficient, I imagine it's going to get hotter than it might otherwise?*

    *Unless you're still in 3rd, driving at your engine's peak efficiency.
  5. Is this a 'my torque is bigger than your torque' thread? (from the man with the biggest torque)
    zedders likes this.
  6. Peak torque, rather than peak power, is a good indictor of peak efficiency (at least at full throttle, where you measure it on a dyno).

    All petrol engines lose efficiency as they are throttled, which is shown very nicely on a VE map for fuel injection control. This is never done on a dyno power run.

    If you have control over your ignition and fuel inputs, you can tailor parts of the operating range to be more efficient (as opposed to maximum power / responsiveness).

    It's interesting to watch cylinder head temperatures and AFR when messing with spark advance. More advance cools the burn up to a point and then it gets hotter again (detonation). Spark advance also affects the AFR - if you add advance and the exhaust shows leaner, you are burning more of the fuel, and so are more efficient. Leaner burns slower, so needs more advance, so you are chasing an optimum point. If too retarded, the mixture doesn't burn fully in the power stroke and so less of the energy is converted into work and more into heat.

    At cruise I'm trying to get the AFR and spark advance optimised according to keeping the CHT low on the basis that it's good for the engine and should indicate an optimum burn.
    zedders likes this.
  7. Torque is cheap
    paulcalf likes this.
  8. I've been idly reading up on building high torque engines ready for the time my 1600 dies (can't be long!)

    I think part of the issue is that there is so much conflicting advice around that making sense of it all gets confusing. Also people don't seem to make the distinction between what's best for bus and beetle that often, or the difference between 1/4 mile time and driving a loaded camper in the real world.

    My understanding (and to be fair it may be well off, happy for any advice) is in a daily driven bus you are looking for plenty of low down torque - this is what gets you away from the lights, or up hill without resorting to second gear.

    More capacity generally equals more torque. Better breathing (via more aggressive cam and larger valves) increases torque, but also moves it up the rev band, so it may not be where you want it. As horsepower is only torque times RPM divided by a constant, this approach increases horses.

    It would appear if you are on a budget, then it would seem that large b+p's with a near stock cam and heads (with a good port and polish) would be the way to keep it really drivable. If budget is higher, then even larger capacity and stroker crank, but still keeping a relatively mild cam and valve size to again keep the torque low down.

    Is this right?

    Still no closer to working out what to aim towards, other than bigger than 1600!
    zedders and paradox like this.
  9. Torque is about gas speed in the inlet tract. If the charge is moving fast, it fills the cylinder better and more mixture equals a bigger bang equals more torque.

    To get high torque at low rpm, you need a small diameter inlet tract so the gas is moving fast at low revs. The trade off is that at high revs, the small diameter acts as a flow restrictor and reduces torque.

    Cam choice should match the base characteristics of the inlet tract - there's no point having a cam that works best at high rpm and a small diameter inlet tract.

    Without a turbo or fancy variable inlets / cams, you can't have torque everywhere, so every engine is a compromise.

    More capacity is always good as, even if your torque is biased to the top end of the rev range, you may well still have enough at low revs. The trade off here is that the relative efficiency of he engine at light loads will be lower, so more fuel used.
    77 Westy and orangefeeling like this.
  10. I found torque was expensive!
  11. The standard cam has no valve overlap, quite the opposite in fact. It closes too soon leaving some exhaust in the chamber and doesn't scavenge. It's great for emissions and great for torque, but useless up the revs where it needs overlap and not great for mpg.
    But as far as I can work out all cams are performance cams. Even the ones claiming to be stock aren't, because even when the lift and duration is stock, the timing is different.
  12. I'm hoping to have some exhaust valve rockers made with greater ratio to better expel the gas. Bolt on and see what happens. :)
  13. If you want them cut from solid I may be able to help. Can't do casting though.
  14. The way t1 ones are done is to move the pushrod cup closer to the pivot point, but that's too difficult. So the idea is move the large pivot hole by drilling off centre and bushing back to size. Rough in head calc says if I move it just 0.5mm towards the pushrod I'll get in the region of 1.5mm more valve lift. That should leave the adjuster hitting the valve near enough in the right place and in fact compensate for pushing the valve further causing the adjuster to get nearer the edge of the valve stem. I feel lucky. :)
  15. Is it not easier to weld up the pushrod cup and recut it .5 closer with a dome shaped milling cutter?
  16. It would have to move over a mm for the same lift. They are 1.3:1 already. They are made of "hard stuff" so welding might not go well. Then the oil hole might be difficult to reinstate and the dome hole accurately placed. You can do all that?
  17. ...and the fat pushrods might hit the tubes...
  18. I have a load of rockers in the shed. I suppose I should have a proper look before making suggestions.
    zedders likes this.
  19. I don't have any to look at either so I'm guessing from memory too. I suspect the t1 ones are cut from the same castings as standard, but are cast with the pushrod cups filled in.

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