Warning: long
Denzil,
I recently had a debate with a V8 knucklehead (and 2V fan) over the 2 vpc vs
4 vpc subject in an engine with forced induction. I don't claim that this
is correct but it makes sense to me :)
Assume you have two completely identical engines, one with 4 vpc and one
with 2vpc. This is obviously not a realistic comparison given that the 4vpc
cylinder head would undoubtedly feature twin camshafts and a superior
design, but bear with me
With a 4 vpc engine, there is a distinct disadvantage at low rpm. Because
the gross opening into the cylinder is larger, the velocity of the air
through the ports is lower and the result is poor cylinder filling and
consquently poor low rpm torque. This one of Bernoulli's thereoms. If you
don't believe me, some manufacturers (Toyota) overcame this problem on early
4 vpc heads by artificially restricting the flow of the intake air at low
rpm with a butterfly valve, hence increasing air velocity.
If you replaced the 4vpc cylinder head with a 2 vpc cylinder head, the gross
opening into the cylinder is smaller, hence air velocity is higher. This
results in better cylinder filling at low rpm, and strong low rpm torque.
At higher rpm, the superior flow characteristics of the 4 vpc head will
result in much improved cylinder filling hence better torque (and since this
happens at high rpm, better power)
Looking ok for 2 vpc designs so far, one point to each corner
However when a turbocharger (assume identical turbochargers for both
designs) is added into the equation, the lower restriction placed on the
inlet air by the better flowing 4 vpc head will result in improved
turbocharger efficiency, and this means lower air temperatures, hence higher
density, hence increased pressure (boost) This is the ideal gas law, PV=nRT
So the 4vpc motor will outperform the 2 vpc motor at low rpm and high rpm
(due to the same effect- higher compressor efficiency) provided that both
turbos are producing boost.
This isn't to say that its impossible to build a 350hp turbocharged L20b- it
has been done!
I think revability would be better with the SR20DET because of the
improvements in metallurgy since 1968, but that's not to say it's hard to
build an L20b capable of reving to 8000rpm+. One thing you could do to
improve revability would be to fit longer rods (ie Z20E) This results in
smaller frictional forces in the bores, so less hindrance to high rpm.
The counterflow head would be a disadvantage, but there are plenty of big
power outputs from these designs around.
I should think that if you are aiming for 300+ hp at the flywheel, this
means you will either be running crazy revs or crazy boost (or both!), talk
to a local machine shop about shotpeening nitriding etc for the power you're
aiming for. Forged pistons would be a must.
Remember that a full house (ie race) N/A L20b will pull 210hp+, so 300hp is
a reasonable target (IMHO)
Also, there is a Z18 here in a 510 circuit racer which pulls 327hp with
stock rods, crankshaft, and valves. It uses 125lb valve springs and a
George Fury spec camshaft, extensive cylinder head and intake manifold work
with a Garret T04 on a custom manifold pushing 17psi. This is essentially
the same design as the L series blocks but with a (rather poor) crossflow
head - and this is with a 180cc capacity and 8mm stroke disadvantage!
Nissan L blocks are TOUGH. Let us know how you go with your project!
- Tom
PS It'll be fun to tell the losers that you've just got a L20b ;)
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED]]On Behalf Of Denzil Palmer
Sent: Saturday, 24 February 2001 7:48 PM
To: [EMAIL PROTECTED]; [EMAIL PROTECTED]
Subject: L20B TURBO v SR20DET
Looking at options for my next project, a turbo engine is a must to beat the
local boys in their WRX's etc. The Hot 4's 2001 yearbook has the NIS-016 510
that does 11.4 @ 120 mph with the SR20DET, and it looks pretty innocuous (ie
totally stock except for the wheels and stance) from the outside. It has 260
rwhp at 18psi, which indicates 335 engine hp assuming 22% drivetrain loss.
Diff is a billet axle 3.9:1 R200 LSD with 225/50-15 tyres.
The question is, given a similar size turbo would the L20B engine be able to
produce as much power as the SR20DET? Take it that the L20B would be
injected, with after-market computer, intercooled, mandrel exhaust and built
to the required standard. The fact that the modern engine would be cheaper
isn't the point. The idea is to keep the traditional L-series engine but
with a modern induction system.
Breathing (ie 2 valve vs. 4 valve) wouldn't be an issue would it, as the
turbo force-feeds the cylinders anyway. What about rev-ability and maximum
attainable revs, is the L20B too long a stroke and too slow to rev? Is the
non-crossflow head a disadvantage?
If the 300+hp L20 B turbo idea is feasible, then what needs to be done
w.r.t. engine internals? If anyone has already done this your thoughts would
be invaluable.
Thanks
Denzil Palmer
Queenstown
New Zealand
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