FrankR
Web Wheeler
Reged: 02/01/04
Posts: 10224
Loc: Columbia, SC
|
Re: Bob's 3.0L Engine
06/23/07 10:22 PM
|
|
|
Yes, this is overkill  - ground the sharp edges and casting/parting flashing to protect against finger cuts and early rusting:
Checked the passenger side exhaust manifold gasket against the head - it looks really good... close at the top as it should be with the gasket hanging vertically on the studs:
Checked the gasket against the exhaust manifold - also quite good:
But, there's always room for improvement, eh? So, a drag racing trick learned a long time ago is resurrected.... Some caveats:
1) Try this at your own risk
2) This is NOT race porting for a high rpm engine - it's only an effort to improve exhaust scavenging to make a bit more torque..... race porting takes a lot more time and effort and requires equipment and skills I don't have.
3) I have no proof that this does anything except make me feel good having done what I could
4) Be prepared to spend several hours
5) Cast iron manifolds will never make as much power as tuned headers (but they can be improved)
Some call this a reversion step - I call it an anti-reversion step, since the idea is to lessen the exhaust reversion pulse. A step will be made between the head port and the manifold opening, which will have the gasket and head port as a barrier wall at the outer circumference of the port opening - so once exhaust gas flows past the gasket, there is a lip that provides a barrier to pulse reversion. A sudden increase in the runner diameter also creates a pressure differential as the exhaust gas expands when it moves from the head port past the gasket opening into the larger chamber provided by the manifold opening. So, theoretically.... there are two barriers to pulse reversion - the physical step and the pressure differential. Since gas flows from high to low pressure areas, the pressure differential should help pull exhaust gas from the head port into the exhaust manifold runner. The exhaust manifold opening is factory designed and produced to be slightly larger than the head port opening to correct potential misalignment, so some step already exists (if everything is perfect), but it's not always correctly aligned and can be enlarged somewhat. Another benefit from better exhaust scavenging is improved cylinder filling on the intake cycle - since the reversion pulse doesn't backflow into the cylinder to the same degree - again..... I can't prove it helps without a dyno or a lot of expensive equipment, but I suspect that it does help to some degree, so here goes:
Tape is applied to the port areas:
Bolts are placed through the gasket and manifold and the gasket is centered (fingers below are your level). Using the gasket as a template, a circle at each port is traced onto the tape:
The gasket is removed and a second circle is drawn freehand about 1/16"-1/8" outside the first circle:
Using a sharp knife, the tape is cut out along the port edges:
A die grinder is used to cut a 45* chamfer all around the port opening to the outer circle:
Guide bevels are cut:
Tape is removed:
A port close-up before it's worked:
Using a combination of grinding attachments, the inner wall of the port is rough ground to transition to the outer edge of the guide bevel. Without a machine shop, it's impractical to perfectly transition the lower area of the port because of the stud boss reinforcement (don't grind much on that area), but it should be blended as smoothly as possible... the most important area is the top half of the port, since more flow is in that area of the cross-section..... so take care to transition the top as accurately as you can and blend it into the bottom area:
Use a ball attachment to reach as far into the port as possible to clean up any rough areas and to blend/smooth around the bend in the port:
Keep working at it:
Then do some more:
Rubbed with a Scotch-Brite pad and finger-gauged for smoothness and transition:
So, that's one port nearly finished. After the others are done, all will be given another final going-over, any flange burrs will be filed and the manifolds will be painted.
Worth it? Maybe. 
Frank
|
|
Previous
Index
Next
Flat
Edit
Reply
Quote
