> Chris - did you ever receive this response? I posted it, but never saw it on
> the mailing list. I recently sent another note that didn't appear - I'm
> wondering if my ISP is having problems!
I got it -- I'm just getting a bit out of my depth here. I thought I'd let one
of the engineers on the list tackle it.
But I'll try. Once agian, purely street experience -- YMMV...
> >According to my "Marks Standard Handbook for Mechanical Engineers", the
> >pumping losses in an engine at high speed account for about 3 psi bmep
> >(Brake Mean Effective Pressure), and 10 psi at throttled idle. If we
> assume that the BMEP is 145 psi, per the TR official shop manual, this
>represents
> >about a 2% loss at high engine speed.
Agreed, basically... (although I think BMEP increases with compression ratio,
which is why "increase compression" is high on my list of modifications)
>>This is the whole
> >reasoning behind superchargers, and is the main reason that turbochargers
> >are the performance mod of choice for today's car manufacturers - they
> >introduce a relatively large charge of fuel relatively inexpensively.
Once again true -- except I wouldn't call them the mod-of-choice (not anymore,
anyway) not all that many turbo models out there -- and there's nothing
inexpensive about forced induction or the modifications necessary to implement
it properly.
(Long bit about why increasing intake valves/port size increases volumetric
efficiency and how increasing volumetric efficiency increases power snipped)
I agree with all that. Burn more fuel/air mix = more power. Increasing VE
by enlarging intake valves/ports gives an advantage at higher RPM's and flow
rates. Granted, but --- Power and torque at lower RPMs depend on the VELOSCITY
of the mixture through the ports for good cylinder filling. Carbs depend
on the vacum "signal" (which is at least related to the veloscity of the
mixture) to mix the fuel and air in proper proportion and atomize that fuel.
In practice--increasing the size of the ports and valves decreases the
veloscity of the mixture through the system. This effect is very pronounced at
low RPM. Bad things happen. Carbs don't mix well. Fuel doesn't atomize
efficiently. Fuel droplets actually drop out if the air stream and puddle in
various places. (Yes, I know PI can alleviate most of there problems.)
I don't care how much you increase the VE of TR engine at 7000rpm -- on the
street - you still have to GET to 7000 rpm. And, to top it all off, you've got
that 3 main bearing cast TR crank to deal with.
The simple pump metaphor for an engine is an excellent theoretical device, but
it has it's practical limitations.
The subject is performace modifications for the street driven TR.
I wrote:
1. Revised exhaust manifold/free flow exhaust system (+ bigger ex valves)
2. More Spark! Timing is everything...
3. Increased Compression Ratio
4. New cam (greater overlap/dwell/LIFT -- this can also differ
intake/exhaust) I say - go mild not wild- new wave street cams have more
lift/duration on the exhaust side)
5. Carburation
6. Bigger inlet valves/port and polish
7. MORE SPARK!
One through three are basically "free" torque throughout the rev range. No
bottom end penalty if done properly.
Chris Lillja
Spit MKIV
Norton Commando
TR4A
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