[DeTomaso] HP limits for 351C Block & Rotating Assembly, Turbovs Non-Turbo

[Tom Di Iulio]

Okay, I'm gonna jump in water that's way over my head but......
When dealing with a supercharger, it seems to me if you're slamming air
into the cylinders, you're effectively raising the compression ratio by
adding more volume into the chamber. 
I'm thinking a supercharged engine with a 8.5 to 1 compression ratio
sure as heck isn't that same ratio when the blower is running full tilt.

I understand that a high-compression engine is running under higher
stress all the time, but under load, the low-compression engine with a
blower is also experiencing similar stress (perhaps at a different time
in the combustion cycle). 
And to take it a step further, the octane demands of a low-compression
engine with a blower are going to be as high (say 104-118) as a
high-compression engine wants all the time to run properly. 
Okay, the water is filling my lungs......am I stating this correctly? 


Before starting my supercharger project, I read a considerable amount 
comparing the costs of making power with and without a supercharger. As 
I recall, when the cost of the supercharger was included, expenses were 
about equal. However, forced induction surfaced as being less stressful.

I'll try to express what I read. Power made with a non boosted engine is

achieved with a high compression ratio exerting high forces at the top 
of the power stroke. Forced induction packs more charge into a cylinder 
but at a lower compression ratio. With less expanding gasses in the high

compression engine there is a shorter downward force at higher stresses 
than the the lower compression engine having more gasses to expand for a

longer period of time on the power stroke. Unfortunately I am unable to 
express the theory as simply and eloquently as what I read. I guess as 
another example of forced induction being "easy" on an engine would be 
the Indy engines that run successfully under maximum load.

Fred T.


Ken Green wrote:

>    I re-read the discussion in Maximum Boost by Corky Bell on an
engine's ability to handle HP on a naturally aspirated versus
turbocharged engine, and he seems to make some valid points.  Basically,
he states that the design point for rods is TDC after the exhaust stroke
because the rod is in maximum tension and the power stroke places less
stress on the rod than at this point.  He goes on to say that with a
normally aspirated engine, you have to increase displacement or RPM to
substantially increase power.  A large bore isn't an option with a
Cleveland, so we're left with stroking and/or RPM.  Both of these
increase the stress at the design point.
>   
>      He also looks at the peak stress on the power stroke, and it
looks like you can double the HP with turbos and only increase the max
load on the power stroke by 20%, but doubling the HP with natural
aspiration would double the stress (or worse because RPM really
increases stress quickly).  This probably is an over simplification, but
it does suggest that a lower RPM stock stroke 600 HP turbo motor would
place less stress on the block and rotating assembly than a stroked
and/or higher RPM 500 HP naturally aspirated motor.  
>   
>      If this is correct, it may be a cheaper to build a reliable 6,000
RPM 600 HP turbo motor then a higher RPM or longer stroke 500 HP
non-turbo motor because you don't need a high dollar rotating assembly,
valve train, heads, etc.
>   
>      Does anyone have experience with the reliability of a stock short
block turbo motor?  Can Dave, Dick, ect. comment on this please?  It
seems like a carefully thought out motor could be built with a stock
Patera long block, and provide great performance and reliability at a
reasonable cost.  Of course, none of us seem to be satisfied anything
reasonable .
>   
>  Ken
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