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

[Tom Di Iulio]

Fred, I knew this thread was over my head and you've confirmed it. No
worries, I'm still smarter than Mad Dawg.......of course, so is my
border collie!


Tom,

If I understood Ken, his interest was in building an engine with forced 
induction that would produced power at lower rpm and would therefore use

less expensive components. I guess that would equate to something like 
an 8.0:1 compression ratio that could be boosted to a modest 10:1 ratio 
that would produce peak power at lower rpm than a non boosted engine 
with a higher compression ratio that would require high rpm for peak 
output. I didn't understand him to mean equivalent compression ratios.

I can only go by my experience by saying with 8.5:1 compression ratio 
and about 7.5lbs of boost, my torque curve was almost flat from about 
3200 rpm to 5700 where dyno testing was terminated. The horsepower curve

was an almost straight line to 635hp at 5700 rpm with no signs of 
tailing off. This was on 93 octane fuel. To me, this is power at low rpm

and would fit Ken's criteria of low cost components. Superchargers and 
turbochargers are routinely sold and installed on stock or factory built

engines that don't seem to create any stress problems.

I would say to Ken that an engine with forced induction can be built to 
produce good power at low rpm and with less attention to expensive 
components.

Fred T.



Tom Di Iulio wrote:

>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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