Porting a Crossflow Head

[Anonymous]

Hey everyone,

You may have read my previous posts about planning to make a 200 hp or more XD falcon crossflow. Well if not, I'm planning on using a holley 390cfm 4 barrel carb with vacuum secondaries, extractors and 2.5 inch exhaust, mild crow cam, and an XE alloy head with the larger valves from the XF and all the usual gear.

Would i need to improve the flow of the alloy head, and how extensive would these modifications need to be? And would the improve flow be a big drag on low down torque?

 

[xctasy]

A7M, you reply dude!

The head flows brilliantly in stock form. Any porting, without proof fropm a flow-bench, will most likely amke it worse. Alloy heads are not that easy to modify because the casting is nice and thin around the intake ports, and much smaller in port area, than the 76 to 80 cast iron heads.

The cam is the key. These cams are designed around the likely head to be used. Crow, COME, Wade all do a dumy run on a computer, and then validate it via there own, or consulatant work from a garden variety engine. A good cam will rasie the valve enough to get the intake and exhast CFM needed for the power the customer is looking for.


Just another thing , though. If you use the smallest volume intake runner possible, then gasflow for the highest cfm, then you'll gain torque everywhere. More efficeint gas flow gives more torque and power. As soon as intake runner volume goes up, the gas speed drops, and you often lose mixture motion and power, and fuel economy at the low end, with the benefit of better high end power.

Fuel economy is best using the stock intake port, with just a de-dag of casting slag, and no smoothening with a emery or a grinder.

 

[Anonymous]

When porting heads that already have a great design, don't think volume think VELOCITY.
Just smooth the contures and ridges and shape the valve stem boss. Smooth the inside of the intake manifold (as far as you can reach) and port match it to the ports.

Nothing fancy, just look for areas of turbulance and correct it as much as you can.

This works for any head, but it really works on one that is a good high speed head to start with.

John

 

[MustangSix]

The alloy heads are already smooth with none of the casting flash and machine marks that you find in an iron log head. They desparately need port matching. My EFI runners and injector bosses were as much as 3/16" off from each other and they don't have a way of indexing as you assemble them. The only other area to blend is directly under the seat. Other than that, unless you have access to a flow bench, I'd leave them alone.

 

[Anonymous]

Thanks guys, I will probably go with matching up the intake manifold runners and port sizes, and i might ask my machinest what he thinks about giving it a touch up.

So how about the Crow 14771 cam:

At .05; 200 intake, 205 exhaust

Total valve lift; .458 inlet, .471 exhaust

111 degree lobe separation.

Would that be suitable for 200 hp?

I found a COME cam with similar duration specs, but with more valve lift. Would this become a reliability issue?

 

[aussie7mains]

Heres some ideas:
The head is just fine, best head is the one with 1.85 valves, others can tell you which PN this is.
Dont waste money on porting the head, but instead spend the money on a Ultraflow limited sprint intake, everything else in single carb is poop, and 500 holley two barrel, vac secs never give the power they should , too lean metering.
That cam is too small try the Crow 14770 or spend the bucks and go mechanical 14773.
Your 2.5 exhaust is good, make sure its low restriction including the tri pipe joints.
Otherwise 200bhp shouldnt be too hard.
Bottom needs no work.
A7M