When to stop Porting

[brokenujoint]

Im in the middle of building up my 4.6 stroker. short block is done.

Block (53020569 96-98 TJ/Cherokee block)
stock cam, stock head (7120)

question is:

with a 0.020 over bore, 12 wt 258 crank, 677 0.020 pistons, 258 (707) rods, havent decked the block and the head isnt warped. resurfaced both.

how much porting should i do to keep it a DD with good weekend hill climb/mild rockcrawling? Im not gonna race it and i dont need top end. going with a 2000 intake, ported to fit a 68mm Dakota TB, a banks header and 24lb cobra injectors.

I did a very mild gasket match port job. each exhaust port is close to the gasket scribe mark except the two end ones. didnt get agressive. i polished them out and left the bowls completely alone. intake ports are left alone.
Should i get a little more porting done or leave it as is? I see alot of you guys are mirror port/polishing the bowls and really opening up the ports. Are these race applications? My intentions are mostly for more torque and longevity with a nice amount of HP gains but nothing radical.

[Cheromaniac]

It's more important to gasket match the intake ports and the intake manifold runners rather than trying to gasket match the exhaust ports. Clean up and blend the valve throats/bowls, teardrop the valve guide bosses, smoothen the short side radii, and remove any bumps from the port rooves. Most of the flow gains will come from those areas, and you won't lose low rev torque if you keep the port size near stock.

[brokenujoint]

i cleaned up the exh ports and beveled the edge to match the gasket but not entirely. is this bad? for low end torque needs and good midrange is it necessary to clean up and polish out the bowls?

[Cheromaniac]

i cleaned up the exh ports and beveled the edge to match the gasket but not entirely. is this bad? for low end torque needs and good midrange is it necessary to clean up and polish out the bowls?

It definitely helps to clean up the bowls. As for the exhaust ports, it's a futile exercise trying to gasket match them because that would open them up too much. Just aim for a port cross-sectional area of about 85-90% of the valve area.

[amcinstaller]

when you gasket match, do you continue to enlarge the entire runner as well?

[Cheromaniac]

No, only about the first inch. The idea is to leave a slight taper in the intake ports to increase air velocity as it travels down the ports.

[amcinstaller]

perfect, thank you, thats what ive been tryin to figure out

[1bolt]

there's a sticky thread at the top that has all sorts of great info.

If everyone thought of power as something that is "tuned" like a musical instrument, or a Sub woofer enclosure instead of thinking about it as "adding" power it would be easy to explain why you don't want to hog out an intake or exhaust port. You're not linearly increasing power by making mods... you're moving it around!

In very general terms if you Increase the size of the pipe (exhaust or intake tract) or shorten it, the engine breaths easier at higher RPM's (less restriction naturally but also complex inertial "siphoning" effects) but LOOSES power at lower RPM because air has momentum, and longer (and smaller) pipes create a stronger siphon effect at low RPMs that makes air flow very efficient. This is why the rounded runner intake makes more torque through the RPM range, it has longer runners.

It is also why they made the 0331 head with smaller exhaust ports, and smaller exhaust manifold runners... better siphon effect (as everyone knows in exhaust terms this is called scavenging) than the older larger exhaust ports and exhaust header which are much larger, but don't make as much torque.

You are mostly just "moving" power around, once the engines displacement and build qualities (Cam choice, compression, internal friction, oil pressure losses etc.) are set in stone then you have X amount of potential power, and mods for the most part move the peak power around, or "free up" whats already potentially there, but is being reduced by something (like a restrictive air filter or intake plumbing).

Some general advice... If you want low torque don't increase the size of the ports cross section OR the intake runner; and this includes not "gasket matching". You want to PORT MATCH. Gasket matching is a term coined back when casting technology was so bad that the gaskets were often more consistent than either the intake or the head ports, so the gasket was used as the template to get both pieces to line up. Modern head porters do a PORT MATCH which effectively ignores the gasket unless it needs trimmed to fit the ports. Modern castings are very much improved and often line up with so little misalignment between port and runner, that very little material needs to be removed. A gasket match under those circumstances especially with a old crappy non MLS gasket like this:

Can result in removing so much material that you create a pocket that will hurt low torque AND high horse power, this illustration I did a while ago will help visualize why:

By increasing the cross section of the port and then decreasing it again you make a pocket where air will expand which slows it down, the air behind it will slow down as well, the air IN FRONT will be "pulled" by this pocket, in other words slowing the air down. Loss of velocity is always bad for torque (assuming the port was nominally efficient to begin with) I would guesstimate you could drop 10 or even 20 foot pounds conceivably if you make a pocket in such a crucial area.

On the exhaust side... I hate to say it but I think most people make their worst mistakes here, DO NOT bell out, "radius" or "round" the exit of the exhaust port!

This is the worst exhaust I've ever seen, the guy who was building this stroker and put so much time and effort and money into it (including a meticulous writeup on JeepForum -- then he basically went deafeningly silent after he finished, and eventually he quietly sold the engine... I bet it makes less torque than a budget stroker

For the reason why I think that; First go google "exhaust reversion" don't take my word for it. Leave the transition from exhaust port to gasket to exhaust header a SHARP EDGE. Because exhaust flows BOTH WAYS (I know hard to believe but 100% true), and you only want it to flow out well, you don't want it to flow BACKWARD well. You want to make any reversion as difficult as possible and gas flowing backward from the header back into the port will have an EASY time with a radius, but a sharp edge is almost the best possible shape short of an inverted cone (reversion cone).

Also go with smaller exhaust ports if possible (such as an Alabama Cylinder heads 0331 head), and a smaller diameter header primary size with VERY long (longer the better over 2 feet with somewhere around 48 inches being ideal) primaries. The header part is not easy because the aftermarket is just like the OEM's they know that most people ignorantly key on peak horse power numbers, so they are quite willing to move your torque peak higher in the RPM range to get higher peak horse power numbers to feed you. You might very well never rev your engine to this new higher peak, which means you'll have actually LOST USEABLE POWER because the aftermarket part moved it further up into territory you seldom use.. You can also lose fuel efficiency in this deal

[amcinstaller]

a quick google search on exhaust reversion showed me this. i thought it was a good read: http://www.bigcitythunder.com/pages/und ... xhaust.pdf

also alot of anti reversion ideas too.

[1bolt]

Very good reading, another resource would be speedtalk.com forums, there's more know how over there than anywhere else on the entire net.