Hmmm - You raise some interesting points. In the spirit of keeping an
interesting discussion going, I submit the following thoughts.
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 thrttled 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. If we reduce the pumping losses by
half, which would be a lot, the loss would be reduced to 1%. HP is directly
proportional to MEP X number of cycles completed per minute. Thus, for the
same engine speed, a reduction in pumping losses from 2% to 1% would
increase HP by about the same, that is about 1%. On the other hand, power
output is highly dependent upon the amount of fuel burned. If we can
increase the amount of fuel introduced (in the correct fuel/air ratio, of
course) we will increase the net output of the engine. 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. The
whole point is that the engine will produce more power if the volumetirc
efficiency (the ratio of charge/piston displacement) can be increased. You
can do this by increasing the area of and reducing the losses in the inlet
tract (by porting and polishing, using larger valves, using high
lift/duration cams), which will give an appreciable increase in volumetric
efficiency without increasing parasitic losses, or by supercharging. In
either case, the increase in power output will certainly exceed the 1% that
can be gained from reducing pumping losses.
As for increasing the allowable maximum engine speed to enable us to take
advantage of all this increased power, this can be done by balancing,
hardening and polishing the crank, adding extra strong valve springs etc.,
which is a whole separate issue!
-----Original Message-----
From: Chris Lillja <Chris_Lillja@pupress.princeton.edu>
To: Michael Marr <mmarr@idcnet.com>; triumphs@autox.team.net
<triumphs@autox.team.net>
Date: Monday, February 22, 1999 4:38 PM
Subject: Re: Larger valves in TR
(Every bit of the following is IMHO -- YMMV - street only advice.)
Well, the general idea is leave the intake side alone for STREET use.
Increasing the size of the intake valves or anything else (carbs, ports)
will
will reduce the velosity of the mixture through the intake tract at low
RPM--
this costs low end torque. And unless you increase the useable RPM range of
the
engine, you can't take advantage of the increased top end flow capability.
Increasing the exhaust flow through the valves, ports, and exhaust system
is the first step for a street engine. This just reduces pumping losses.
Less
torque pushing exhaust through the system = more torque at the wheels.
Exhaust
"tuning" really isn't an issue if you run any mufflers at all... (except for
Tri-Y headers- Tri-Y = good)
I'd go:
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 - 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.
4-7 will push the torque/HP peak further up the RPM scale. You could wind up
with less torque than you started with stock at low (street) rpm's.
Especially
if you failed to do 1-3.....
Once again IMHO.... Just gleaned from Grassroots Motorsport over the years.
Have fun. Good luck.
Chris Lillja
Spit MKIV
Norton Commando
TR4A
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