Plenum & IH Pull

[Paul Workman]

[QUOTE=32valvZ;302776]

Originally Posted by jss06c6

Im guessing Pauls gains of 70-75 hp was because he had his heads ported as well?

Below are two Dynojet "before and after" my porting and then final tuning by Marc Haibeck, prior to some later minor improvements to the IH/Head junction.

I don't have a Dyno graph of my motor with only top end* porting. I wish I did**. Instead, I have to derive an estimate of rwhp by subtracting the known gain achieved by others after the heads too were ported (that being between 25-35 rwhp) to approximate what my top end only porting was.

The Dyno graphs show the before (stock) peak starting point, and peak after that establishes the full porting (and cam phasing) improvement - the bulk of improvements up to that time: starting at 336 avg rwhp and ending with 432 rwhp with HEAD porting included. So, est. of "400" rwhp before porting the heads is reasonably accurate, in my estimation.

Dyno graphs and HP numbers and estimates are popular for most - salesmen especially. But, where rubber meets the road means more to me than HP bragging rights, and actual force at the wheels, i.e., TORQUE over the entire rpm range, or POWER UNDER THE CURVE gives a better indication of the worth of any modifications - to me, anyway.

*TOP END included the plenum and IH brought out to 36mm, and later 37.5mm

** Port matching into the heads was a lot more than just matching the junction diameters only.

Instead, I took to heart the recommendations of many knowledgeable people and good ol' physics and extended the port matching in the heads from the IH junction tapering all the way down to the valve bosses (to achieve a taper that was no more than ~ 2% (think of a the velocity stack design on a Webber single bore carburetor)

Unfortunately, I didn't dyno the motor before proceeding to finish porting the heads too, because the "seat of the pants" improvement after finishing the heads was very disappointing, compared to the improvement over the top-end only achievement. And, based on experience and study witnessed by others too with even minor improvements to porting, I'm certain doing the long taper has significant benefit over merely grinding the opening of the smaller runner to match and little more. And too, I suspect the longer taper mitigates (somewhat) any anticipated gains from pulling the heads to complete the tunnels. (I'll put it this way: Given the same circumsances as I had at the time, I don't think bothering to port the heads justifies the trouble or expense UNLESS the heads have to come off for some other mod or reason [my reason was a burnt valve]).