How The Motordyne Plenum Spacers Make Horse Power On The 287 HP Engine
The Motordyne plenum spacers alleviate an airflow restriction that exists above the first two intake runner inlets. The inside top of the stock plenum is very close to the top of the lower plenum runner inlets. There is only about 1/8″ clearance between the first two inlets and the plenum cover. This creates a significant air flow restriction that reduces engine breathing and power output.
If the height of the flow area above the runner inlets were increased it by 1/8″ it would reduce the flow restriction (pressure drop) by a factor of 4. If the height were increased once again by another 1/8″ or to a total of 1/4″ it would reduce the pressure drop by a factor of 9. If the plenum were lifted by a total of 3/8″ it would reduce the pressure drop by a factor of 16. If it lifted by 1/2″ it will reduce pressure drop by a factor of 25.
Reduction of pressure drop is asymptotic and inversely proportional to flow area. In other words this means the greatest benefit will be realized within the first 1/8″ of added height. Fluid dynamic theory, dyno data and flow bench test data support this.
As an example, assume the pressure drop above the intake is equal to 3 PSID at high RPM. By raising the plenum even 1/8″ the pressure drop will decrease to 0.75 PSID. That’s a significant inlet pressure recovery that will translate into more horse power!
A 1/4″ spacer will reduce that same pressure drop to 0.333 psid.
A 3/8″ spacer will reduce that same pressure drop to 0.1875 psid.
A 1/2″ spacer will reduce that same pressure drop to 0.120 psid.
Although pressure drop is continually decreasing, the rate of change rapidly diminishes beyond the first 1/4″.
The same applies to gained HP. The pressure drop above the runner inlets is so dramatically reduced the added HP gains are also rapidly diminishing.
The attached plots are for three sets of tests 1/2″, 1/4″ and baseline in that order with three pulls each. The results of the 3 pulls per set were then averaged together. These test conditions were very tightly controlled to provide accurate, repeatable results.
Same tank of gas
Same ambient temperatures
Same engine temperatures
Same ECU starting conditions
Same fan position
Same fan setting
The car wasn’t even taken off the dyno.
So even if the absolute values aren’t correct, the deltas are. These plots show the largest change in gain comes within the first ј “. There is only a 1.664 HP difference between the ј” spacer and the Ѕ” spacer. And whether the car is NA or FI, the reductions in pressure drop will always be proportional to the air mass flow rate. Thus for a given spacer size, FI applications will see even greater gains due to the higher air mass flow rates.
So for NA or FI, nearly any kind of plenum mod is a significant improvement from the stock configuration.