A Good Argument against the head drain and TOMEI restrictor.

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A Good Argument against the head drain and TOMEI restrictor.

Postby ZFreak(Dallas) on Mon Aug 03, 2009 8:25 am

So after reading this, the guy has made a good point... a $250 head drain and a $10 restrictor may actually cause more problems than it is supposed to relieve. I suppose our reason would be because we don't run the RB26 oil pan as we use the RB25 oil pan....this may cause the cavitation??? The RB26 has been around for over 20 years and it just "recently" started having these issues? riiiiiiiight.

http://forums.nicoclub.com/zerothread?i ... id=3298988

http://www.skylinesaustralia.com/forums ... 10680.html

If you're looking for soultions here, delete it because there are no
solutions for a fictional problem. It's going to be dripping with
sarcasm, too! What fun. Hang on to your shorts.

That's right. You didn't read it wrong. The Internet spread BS on
this subject has finally reached "urban legend" status and I feel
it's time to set the record straight, vent, shed some light,
whatever. What pushed me over the edge is a new post on Hybrid Z
where the guy (Boosted Z I think?) gets some good #'s on the dyno
but sees oil pressure falling off around 8,000 rpm. The first
questions asked are "Do you have the oil restrictor?" etc... sigh...
Are you serious??? Now we've reached the point where the drain back
issue is so bad that the engine can't even do ONE DYNO PULL without
all five quarts of oil ending up "trapped" in the head?? Give me a
break, man.

What's funny is that the RB26 is, what, 18 years old, but it never
had oil drain back issues until Tomei started selling the stupid
restrictor with a little hand drawn picture of high oil level
without the restrictor and low oil level with it. Actually, it's not
a bad idea, but I'm calling it stupid because of what it started.

A little perspective:
Obviously Nissan doesn't think it's a problem because they never
addressed it in the 33 or 34 engines. Oil returns and pressure
orifice are the same as far as I can tell. At least not any major
changes for what seems to be a catastrauphic problem (sarcasm).
Would someone PLEASE explain to me why Nissan can take a bone stock
R32, R33, or R34 to the Nurburgring and go at full throttle, lap
after lap, eight plus minutes per lap and never have oil drain back
problems that rob the sump of oil, but a guy on an Internet forum
can't do one lap of a two minute circuit or even one dyno pull, have
oil pressure sag, and have everyone under the sun call out for
restrictors and additional drain backs? No, not really; that was

I've seen N1 and Super Taikyu race engines up close and even inside.
Never seen any additional drain backs. Not sure of the Super GT
engines, but they probably use a dry sump with scavenge in the head
and no drain backs, to keep oil off the crank. Saw the HKS Drag GT-R
up close too, and never noticed anything like that. There are
probably thousands of track GT-R's driven in Japan alone that don't
have this problem.

My experience:
Power numbers's? Don't know. Our dyno's here won't hold much over
700 without slipping tires (at least that's as high as I can get
it), and I'm not a big fan of numbers on a piece of paper, so I
really don't care. However, I can tell you that the most powerful
RB26 I've ever built burried a 320km/hr (198.838 mph and don't tell
my mom) speedo on the expressway at over 10,000 rpm. Modified HKS
pump, no aftermarket restrictor, no additional drain backs, oil
pressure holding at a rock-solid 115 psi.

In fact, I've never used an aftermarket restrictor or additional
drain back in any engine I've ever built... I've never had an oil
pressure problem in any RB engine. Ever. I build 'em loose, too.

Maybe some people are having problems. With the above, I doubt very
much it's a "pooling in the head" problem.

Here's why:
Suck air in the oil pump and oil pressure goes to zero almost
instantly. No "falling off in the top end". Some people shift gears
and the oil pressure is magically restored (even though all the oil
is in the head), only to have it fall off again as soon as the rpm's
climb back up.

Oil pressure falling off in the upper rev range can be caused by
several things, the most common of which is cavitation. I would put
money on it that most people running RB Z's are using the steel pans
from the Z31? This not only eliminates the excelent RB26 sump
baffeling, it also precludes you from using the RB26 windage trays.
Foamy oil cavitates. What happens is tiny bubbles in the oil enter
the pump under huge vacuum. A bubble that already exists is much
easier to expand into full blown cavitation than oil with no
bubbles. The bubble(s) expand to many times their own size, taking
up space originaly reserved for oil. Once it hits the pressure side,
the bubble collapses, but the damage is done. Less oil went through
the pump. The faster the pump spins, the more vacuum is created, and
the worse the problem becomes. Oil pressure that falls off as the
revs climb is almost always caused by cavitation. This can also be
caused by a too-small or too-restrictive oil pick up. Moving it all
the way to the back of the engine couldn't possible affect this,
could it?

I watched a video of a guy going around the track in an RB Z. On
braking and turning in only one direction the oil pressure would
zero out almost instantly. Once the car straightened out or throttle
was restored, pressure jumps back to normal instantly. Once again
the common consensus was the 'ol PIHS (pooling in head syndrome). On
decel or only turning in one direction? PIHS or bad baffeling in the
pan/pick up location? You be the judge, but use common sense.

The restrictor is good:
But for the wrong reasons. The right reason is the same as a small
Chevy or any other engine for that matter... The head doesn't have
heavy rotational forces for two reasons: There's no heavy stuff
swinging around, and it's only moving at half the crank speed. It
doesn't need a lot of pressure, especially because the cam to lifter
interface is fed by oil dribbling out of the cam journals. Pressure
in the head is wasted, which is the reason for the factory
restricotr. A smaller restrictor will keep the pressure to the
bottom end higher in the rev range due to decreased VOLUME wasted in
the head.

The additional return is bad:
There are two large returns in the rear of the head and several
throughout the length of the left side of the motor. Due to crank
windage, the ports on the left side of the engine are subjected to a
vacuum because the crank is pulling down on that side of the engine.
The one in the rear, right supplies windage to the head for crank
case ventilation. Air doesn't pull down the left side if it can't
come up the right. Under hard acceleration, oil wants to move to the
rear of the engine. Some people install a large line from the rear
of the head back to the right side of the front sump in the GT-R
pan. Anyone see a problem with this? There's actually two. Under 1g
of acceleration, not only will oil not move forward through this
line back to the front of the engine, it gives the oil in the sump
another path to exit the sump. Reason #2 is that the right side of
the engine is pressure side and not the suction side. Crank windage
at higher RPM can push oil back into the tube if it's not very
carefully baffeled. It could work well in a rear sump if the hose is
almost straight down and entering the left side of the engine. But
this as assuming there really is a drain back problem.

What I have seen/done:
An additional line from the back of the head, under the plenum, back
to the RADIATOR. Additional coolant movement out of the head. Very
common mod on drag engines. Maybe someone saw this somewhere and
mistakenly thought it was for oil and ran with it? Who knows.

With the additional oil restrictor in place, the oil level in the
hed would certainly be lower than without it, but I assure you that
even with the stock restrictor, it's not even close to a problem.
Especially not the problem it's been made out to be.

Anyway. I feel better and will now step off of my soap box. I'm sure
there will be arguments from people who had PIHS cured by these
mods, or whatever, but you really need to stop and think about it.
Especially given the factory stock cars not having problems on the
ring, etc.
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Joined: Wed Jul 15, 2009 3:47 pm

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