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

[F&D Terry]

Ken,

Ken,

I believe any device, such as a supercharger, that increases the intake 
charge could be compared. The engine doesn't know what makes the boost, 
only that there is more of it. On that note, my supercharged engine was 
both bored and stroked but the only claim to reliability I could make 
would be the two days on the engine dyno followed by several trips to 
the chassis dyno. These instances would have place the engine under 
maximum stress but only for a short periods of time.  The engine was 
later completely disassembled and there was no evidence of damage to the 
components.

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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