Lotus engines and bike TB's

Jon V. jon at valesh.com
Wed Sep 1 21:41:31 GMT 1999


Nic wrote:

> >Actually, I liked a lot of the ideas presented. I think the motorcycle TB
> >assy is probably too small in the throat, but that was the idea I liked
> >best.
> 
> Think about it this way: If you strip a throttle body of a 1300 Suzuki bike
> that redlines at 12000rpm and mount it on a 2.5l engine that only does
> 6000rpm you are looking at the same flow rate.

Yeah, I was thinking about it two ways... assuming that my Internet
calculator skills are sufficient to the task, a 2L engine that redlines at
6.8-7.2K (about where mine does/would) needs ~23036 CFM to feed it. The
1300, 12KRPM motocycle engine would need ~26327 CFM (These numbers are
pulled from the net, and I didn't check the assumptions that went into
them... just CID, RPM, and (a high) 95%VE), which sounds like it
is a perfect fit and everything is WONDERFUL and I'm just wishing I could
figure out where I'm gonna find a Suzuki without an owner for long enough
to... um... anyway... I'm trying to find the parts...

That was way one, BUT...

That was assuming identical volumetric efficiencies, and I'm not sure that
is valid. The high-winding motocycle would gain more from an easy flow
path at the top end... but it would also be in some ways less likely to
have it, just because street riders are less likely to hit a 12K redline
regularly than I am to hit a 6.5-7K redline.... well... I don't *know*
that really... my ol'dad used to have a Honda with a redline way out in
the stratosphere... like 17K RPM... and he says he used the whole
range.... but then again it was a megaphoned 2-stroke racer when he got it
(stopped three times driving it home sort of thing) and that sort of
breaks most of the rules anyway.

>  I got a set of throttle
> bodies + injectors + FI pump for $40. And they are in mint condition.

See, now THAT is what I consider cheap. I've been trying to find that sort
of deal... I'll need to try harder. Where/how did you get that deal???

> Quick question: Why does most bikes use individual TB assemblies for
> each cylinder while most cars use a single TB? I know how much pain
> it is to keep the sync on a bike right, so what is the real gain?

I think it is the reason most motorcycles don't have 30Lbs flywheels...
they don't need a lot of low end torque, being as light as the are, but a
good top end helps a lot. The individual throttle plates are good for
better air flow (top end) but don't help with torque the way merged
airflow intake systems do. At least, that's what I've mungled my way
into believing... ya'might wanna check the reasoning before you go
quoting it at parties. The current pair of carbs on my engine would,
stock, go into a long flow intake system to get a good air flow going in
the mid RPM range. The previous owner torched that and I in my
infinitesimal wisdom torched what he did in favor of a pair of air filters
from a small ford turbo engine... the result is that I need to unbolt the
air filters to check the oil, but I have no appreciable intake resistance,
as documented by the fact that the old crankcase breather system which
used to attach between the air filter and the carbs and pull a respectable
if not all that impressive vacuum now pulls ZIP when attached to just one
of the carbs.

Most motorcycles seem to have one throat per cylinder one way or another.
My Yamaha 1100 has four 34MM carbs feeding it.

> I'm not sure which route to go with the bike. I can either make the
> Suzuki TB's fit (lots of pain) or simply make a manifold with injectors
> at the ports and a single TB at the intake of the surge tank. Will there
> be any real difference? 

> How about the difference between TBI and multipoint injection?

Multipoint sounds neater. Actually, TBI injection is really just a snazzy
digital way of making a carb that works.... 

> Does the fact that it is an Turbo affect the answers?

Good question! I don't know, but I would ignorantly guess that it makes 
all of the air flow issues for torque irrelevant. Whatever restrictions a
lack of throttle plates would add could be overcome by jumping the boost,
and whatever a long air pipe can do for assuring good air flow at low RPMs
the turbo will do better.


I don't know... but I *like* $40! I gotta find me a deal like that.


-Jon

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