|D3VE-A2A Specific Porting Guide|
The level of porting needed on your heads depends on your intended use. There are several stages of porting, starting with the "bowl blend" and graduating into full on passage modifications to generate large cross sectional area for big inch engines or high RPM use. I would call this last one stage three. Stage three includes all prior porting operations.
Port cross section or cc's isn't as critical with exhaust ports as it is with the intake ports. This doesn't mean that you should hog out the passage for GP though. Port shape is far more important than port size and in some cases big will dissappoint you big time.
With the right intake flow and state of tune 180 cfm exhaust flow can and does allow big power. I have seen 175 cfm exhaust flow support 650+ horsepower N/A. All depends on your choice of parts and how these parts work together as a team.
I highly recommend that you use good 3 angle seats having the seats cut first, as the port size and shape is dependant on the valve seat diameter. If you are having larger valves installed, have the seats roughed in "First" then do your porting. You must use care to avoid damaging the new seat angles though. I like working on clean iron and would have them tanked and clean regardless. When you are done porting the machine shop can retouch the seats if you knick any of them. Just be careful as a high speed grinder and burr can get crazy if you are not paying strict attention to what you are doing. If you have a junk head, by all means practice on it first to get the feel for your specific equipment.
Level 1 porting consists of blending the bowls at the area where machined meets cast surfaces. You just blend the sharp edges left above the valve seat back into the as cast bowl surface. The bowl (venturi) diameter using a 1.76" valve and seat should be approximately 1.475". Exceed 1.5" and flow seperation at the seat will occur. This is especially true at the back of the bowl where the "divot" is located. The area above the seat should be about 83% of the valve head diameter. If you want good flow above .6" lift you can make the venturi slightly larger. This area should be radiused from the throat cut angle, which is essentially the fourth angle , to the as cast bowl. The blend should be rounded as it leaves the fourth angle and graduates into the as cast bowl surface. The area above this venturi will be larger than the venturi with the iron ford castings. The right side of the bowl being the exception.
Level 2 porting would generally consist of reworking the bowl area above the valve and seat. Also included is working the valve guide boss and the areas adjacent to it. I have heard them referred to trenches and like the term. This also includes removal of the thermactor boss and the necessary shaping of the boss into a directional vane. As a rule stage 2 work is done from the bowl side of the passage. You may want to do some vane shaping from the exit side of the passage though.
Working your port to this level can net you 180 cfm at .600" lift.
A full on passage rework can get you near 200 cfm, however there is alot of extra work for that 10 to 20 cfm and with out a flow bench you can not be sure if your shaping is hurting or helping flow at this point.
The stage 1 porting will get you from 125 cfm at .6" to about 140 cfm. Rolling the critical short turn will get you to about 160 cfm, this includes the thermactor boss removal / shaping. Stage 2 work involving.raising the trenches and streamlining the guide will get you to about 175+ cfm. This is a very realistic and possible plan tor getting good flow with an almost stock exit size and shape.
With careful shaping of the short turn you can generate flow in the 195 to 205 cfm area.
This last few cfm isn't going to make or brake you in the hot street or boat state of tune these heads are used for. 175 to 185 cfm is fine.