Hi Jeff,
A big single SU being of variable venturi design always provides optimum gas
speed - just the right size venturi - (assuming the correct damper spring)
while coping with an engine breathing requirement that varies enormously from
idle to full power, in other words it has a wide operating range. There is
also the point about the combustion chamber/cylinder only 'seeing' one carb
at a time so a big single is more effect than 2 smaller carbs - I suppose the
twin HS4 or whatever carbs ought to pay off at really high rpm/power demands
but then again so would twin HS6/HIF44. I haven't checked the above
statements in Vizard or anything but I think they are correct.
I think you will find that where the power increases more than the torque,
the rpm that the power increase is made, is further up the rpm range.
I guess the reason DV doesn't look at increasing torque over power is that he
does not see a need to. Remember the gearbox is there to multiply the torque
so as long as you have some torque the gearbox looks after it. This is the
much over looked reason why close ratio gearboxes are such a good thing and I
guess why motorbikes have 6 and 7 speed gearboxes (lots of power, little
torque and spikey at that).
If you are looking for CHEAP torque then pay a vist to www.twminduction.com
and spend some money on a good ram pipe. Remember you can pay a brand new
HS6 carb for #25 and while it is not as good as a HIF6/HIF44 you are spending
a fraction of the price. Also to make the best use of the air you are
getting into the manifold have the manifold coated with a thermal barrier
coating (about #35 over here - cheaper for you www.techline.com (I think).
Not least you don't need to gain torque to accelerate quicker just lose
weight - it achieves the same thing. Increases in torque do NOTHING (except
where there is a power gain) to increase top speed.
Keep reading DV - I don't think you will go far, if at all wrong.
Daniel1312
In a message dated 16/11/00 14:41:36 GMT Standard Time, jboatri@emory.edu
writes:
<<
OK, back to the original question. How does one increase torque
output on a BMC A series engine? In my case, the engine is a 1275
with 40 overbore and a Kent 266 cam. However, I hope that what we
discuss could be applicable to all spridget motors.
One response was that intake gas velocity must be kept high. How does
one do this? Vizard, in his inimitable way, shows the use of long
(15+ inch) ram pipes with detailed discussion, but then mentions that
none of this will really work with twin SUs. I don't understand why.
But, is the only answer to switch to a single carb setup? As close
readers of his book probably know, he spends most of his effort in
increasing flow to increase ultimate power. Does anyone know of other
sources of opinion backed by real data or anecdotal experience?
Also, according to Vizard's charts, it is not necessarily always true
that a modification that increases power at the high end results in
loss of torque at the low end. Several of his charts show that torque
often increases with flow. What does seem to occur frequently,
though, is that that the power increase is proportionately larger
than the torque increase. However, his data are all a little suspect
for purposes of this discussion in that his goal is nearly always to
increase power and let torque take care of itself; he doesn't
systematically study torque. Of course, some would argue that he
doesn't systematically study anything.
Another response was to go with nitrous or increase capacity. The
nitrous idea would be fun but maybe hard on the engine. It may be
hard on the wallet. It does sound fun, though. Increasing capacity is
something for engine rebuild time. I have upped mine a bit (to a
calculated 1312 cc, I think, though the recent thread on cylinder
volume has me second guessing). One downside, in addition to cost and
time off the road, is that as the size increases, the engine does not
idle quite as smoothly - it's the one advantage I've ever heard
discussed about the 948 vs 1275.
A final response was to go with supercharging. Now _that_ is
expensive. But, it sure seems like the way to go for increase torque
and power, particularly as complete, new systems can now be bought
off the shelf (from MM and from the group in Australia). I'd have to
win the lottery, though, to go this route.
Anyway, there it is. I guess I'm looking for sub-$300 DIY torque
solutions. But mainly, I'm simply ignorant to the extreme on this
topic and would love to hear your thoughts on it.
Thanks,
Jeff
|