Quote:
Originally Posted by Aurora40
Just curious, why do you use the quantifier "better"? A large displacement LS motor is physically smaller than a small-displacement Northstar.
And in the Corvette vs XLR, the base 'vette motor makes 430hp to the XLR's 330hp. The XLR gets worse economy at 15/24 EPA vs 15/25 EPA for the automatic 'vette.
The Northstar block was initially designed for a max displacement of 5.4L (I recall reading this ages ago, I can't back it up with the actual article). If you did the math, that would put it at roughly 390hp. Still a lot less than the 'vette, and presumably fuel economy would drop further.
I fail to see how it's "better" simply because it is OHC or because it uses less "displacement". What is the downside associated with displacement?
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You're playing "devils advocate" ain't cha Bob?
OK...I'll play along
Isolated anecdotes aside, generally...
- the flow thru of a 4-valve design is superior to the (practical) two-valve
- OHV means less valve train weight, allowing for reduced spring tension which relates to less wear for any target peak rpm.
- The burn characteristics of the pent-roof chamber in the DOHC (e.g. LT5) allows for very fast burn rates which allows compression ratios of 11:1 on 87 octane, and higher (12:1) ratios on 93 octane. Higher compression ratio = better power and efficiency (apparently).
- Intake and exhaust cam phasing (DOHC) can be independently and dynamically controlled "on the fly" to optimize output/efficiency for any load/rpm circumstances
- Sustained high rpm operation (again due to simplicity of the valve train) favors the OHC/DOHC in actual practice (but you already know that).
So, from a cubic inch to cubic inch, or hp/volume, or a dynamic load capability point of view, it is hard to argue the virtues of a single cam, push-rod approach as being anything but a little archaic in comparison.
After the 2-valve flow has been optimized, the only door available to the daily-driver, push-rod motor to keep up the horsepower
is displacement.
If I give you physical size in favor of the push-rod as a "given", I really don't have to explain further why the OHC/DOHC is a
better design, do I?
(Jess pullin yer tail a little, Bob
)
P.