Date: Thu, 18 Jul 1996 08:47:31 -0400
Subject: engine inertia and power loads

Mr. Norman Garrett recently suggested that 8 psi boost applied to his
1.6 Miata was damaging the engine. The suggestion was clear that
Mr. Garrett believed the power loads induced by the 8 psi boost are
causing deterioration of the piston. Further, he seems to suggest
anyone using 8 psi will experience the same problem. This was evident,
to Mr.Garrett, in the increasing engine noise attributed to piston

I would not doubt Mr. Garrett's engine is failing, but I do not agree
with the cause.

My reasons for disagreeing are:
1. Mazda chose to run significantly more than 8 psi when the Miata 1.6
engine was a 323 GTX turbo engine. The piston casting is the same, but
the compression ratio was less.
2. Considerable personal and business related experience of running 8
to 10 psi in the Miata engine for 6 years.
3. Load calculations. 

I have calculated the loads caused by both inertia and burning gas
pressure under 8 psi boost and redline rpm conditions. These
calculations show that the loads induced by the inertia of the parts
(approx 5200 lbs at TDC and BDC, for piston, pin, & rod) at 7000 rpm
exceed the loads created by the burning gasses pushing down on the
piston. (approx 5000 lbs at 8 psi boost) In other words, the inertia
loads already existing in a stock engine slightly exceed the power
loads of 8 psi boost. To arrive at these numbers I used equations from
the Bosch Automotive handbook, weights and dimensions of stock parts,
and cylinder pressure data kindly given to me by Super Flow
Corporation, of Colorado Springs.

The calculations show three other interesting load conditions:

1. The highest compressive load in the conrod occurs at bottom dead
center at redline, until boost pressure exceeds approximately 13/14
psi. Above 13/14 psi boost pressure, the maximum load point shifts to
around 50/70 degrees after TDC.

2. The most destructive load of all is the tension in the
rod/pin/piston assembly at top dead center on the exhaust stroke when
the intake and exhaust valves are open..

3. When the inertia load is high in tension (near TDC), and the power
load is at a maximum in compression, (also near TDC) the two loads
actually subtract from each other to sum into the combined load.

I feel it is relatively obvious that mechanical loads of 8 psi boost
do not damage a Miata engine. In fact, it is fair to say that power
loads don't even tickle its tummy.

Now, heat in the intake charge is a different matter, but, another

I will gladly share more details/info with anyone.

Corky Bell