This was my original post in reply to Chris's note about larger exhaust
valves. Chris just posted a response, but my original note was not included
in it's entirety, so I thought I would repost for those of you interested in
this topic. I'd be interested to hear other opinions, and I assume Chris
would too.
-----Original Message-----
From: Michael Marr <mmarr@idcnet.com>
To: Chris_Lillja@pupress.princeton.edu <Chris_Lillja@pupress.princeton.edu>;
triumphs@autox.team.net <triumphs@autox.team.net>
Date: Monday, February 22, 1999 6:54 PM
Subject: Re: Larger valves in TR
>
>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
|