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Forced Induction ?: X-Flow versus JH head



>  Yes, but less RPM = less exhaust gas velocity and less efficiency.

First of all, says who? Since when does higher rpms automatically = exhaust
gas velocity or more efficiency? And, what type of efficiency are you
referring to, anyway? Frictional losses increase with the rpms, for
instance. That certainly ain't going to help with a blanket term called
"efficiency". If the head or exhaust can't flow at 5k rpms, they certainly
aren't going to make more power at 10k rpms.

Practically speaking, a street-driven engine with solid lifters has hardly
any rpm advantage over one with hydraulic lifters. Now, if we're talking
about race engines, that's different. But, there's much more to making power
at sky high rpms than just the lifters.

>  Also expecting the turbo to produce power over a shorter RPM band would
> probably increase turbo lag.

Um... no! Getting a *properly-sized* turbo to produce power over a more
narrow RPM band will DEcrease so-called turbo lag. All turbos start spinning
from zero rpms. However, a turbo that is expected to perform at very high
rpms will be bigger (or bigger A/R ratio) and will take that much longer to
spool.

So, throw in a teeny scroll and that thing will boost *very* quickly. It
will also run out of steam (into the compressor surge zone) more rapidly
than the big unit. But, if you're only interested in sub-6k rpms, who cares?
Okay, consider the boost-generating characteristics of a 1.8T engine with
K03 versus K04 turbo. Or, take a Garret T3 from a Saab 900 and then compare
it with a T3 from a 1st gen 300ZX. See what I'm saying?

>  Having a larger RPM band to work with the and the correct gears in the
> tranny will give you more of a window to drive the motor in rather than
> just a window from around 2500 to 5500 RPM that
> will be partially consumed by getting the turbo up to speed.

This is totally true. However, how fast do you suppose you can spin these
engines of ours? In stock form, our heads are capable of higher rpms than
the bottom ends. Furthermore, the VW ECUs stop all the fun way before the
head spins itself to death. So, again, the performance difference for street
engines is minimally effected by the lifter type.

>  My .02 cents.

We'll need a heftier deposit than that to open an account with this bank of
knowledge. ;^)
--
Scott F. Williams
NJ Scirocco nut
'99 Subaru Impreza 2.5 RS
Mazda 323 GTX turbo "assaulted" vehicle
Golf GTI 16v "rollycar"
ClubVAC: "Roads found. Drivers wanted."

----- Original Message -----
From: Scott F. Williams
To: scirocco
Sent: Monday, February 17, 2003 11:14 AM
Subject: RE: Forced Induction ?: X-Flow versus JH head


"Scirocco" wrote:
> I'd stick with the JH head.  What you would gain from the cross flow
design you would more
> than loose from the hydraulic lifters.

The  hydraulic lifters are only a liability at very high rpms. Spinning the
motor that fast isn't a requirement for a turbo motor. What's the point of
revving higher when increased volumetrica efficiency can be had by turning
up the boost? The cross flow head is superior for turbo use because of:

a) flow reasons
b) turbo fitment
c) *much* less heat transfered to the intake manifold from the blazing hot
turbo and manifold

With all these factors considered, it is fairly evident that the cross flow
design provides far more benefits than whatever minor penalty the hydraulic
lifters impose.
--
Scott F. Williams
NJ Scirocco nut
'99 Subaru Impreza 2.5 RS
Mazda 323 GTX turbo "assaulted" vehicle
Golf GTI 16v "rollycar"
ClubVAC: "Roads found. Drivers wanted."
-