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NXS 01-06-2005 06:17 PM

Exhaust port question
 
Well...I'm thinking about pulling my heads back off and giving the exhaust runners a decent port job. I have vortecs and have heard(and seen flow charts to back it up) that the exhaust is a weak area as far as power is concerned. I noticed that the factory gaskets and manifolds are massive compared to the stock ports.

my question is:

If I "over compensate" what effect will I have? Driveability is not a concern, nor torque under 2500 rpm. I am looking for max power around 62-6500 rpm.

I am thinking that thru the mufflers if i leave the mild port job like i have on the intake and open up the exhaust port to "race" it will be ok, not hurting my midrange too bad.

What do YOU think? Has anybody tried this before?

-Thanks for the experience!

Max Keith 01-06-2005 06:40 PM

Basic Porting 101
 
As with 95% of the production heads out there, the best gains are to be had with a good bowl blending, and if possible to raise the roof of the exhaust port as much as you can, which with water jackets and other stuff to get in the way, if you can go up 1/4 inch, thats a lot.

The idea is to take some of the angle out of the turn at the bowl, but undercutting the floor side of the bowl and to slightly decrease the angle between the bend in the runner and the exhaust port. You will want to take out a little more from the port side than at the bend, and keep the roof in as straight of a line as possible to that bend.
Some gain can be had by slightly widening the port as well. As a general rule, doing any grinding on the floor will only take you backwards.
If you have some good pictures of ports to look at, you can see these are the basic manouvers.
This is a very basic instruction on porting, and not to be construed as total gospel in all occasions.

Just remember; A little bit is good,but a bunch aint always gooder.

The basic idea is to try to equalize the flow in the runner, so that you have as much flow at the port as you do the valve throat. Meaning that you want them to have as close to the same cross section as possible, and to keep that uniform all the way from one end to the other.

NAIRB 01-06-2005 06:40 PM

Clean the bowl area up, reduce and narrow the guide boss, radius and smooth the short turn radius. You can use the round port header gasket for a template to make a quasi- D port exhaust. I would just use the smaller round port header gasket, scribe it and just grind the roof area larger, and concentrate on the port roof and sides, but staying away from the exhaust port floor. If you want, you can make the roof area a little larger than the stock round port header gasket, but I wouldn't go as large as the bigger round port gasket.

Be careful, Vortec castings are thin, and hogging them out leaves them very fragile, and you risk grinding through the port.

You can pickup an easy 20 CFM if you are careful, and you won't have to remove alot of material.

NXS 01-06-2005 06:52 PM

Re: Basic Porting 101
 
Quote:

Originally posted by Max Keith
Just remember; A little bit is good,but a bunch aint always gooder.
Most certainly the case, unless of course it's good looks(hard to have too many of those;) )

About the port floor there is a problem(I can't take pics b/c my camera is on "vacation to florida"). You said to raise the ports, which seems to me to be the best thing to do, allow the exhaust a straight shot out like on the 14,000 jap bikes but the factory manifolds and exhaust gaskets are lower. I have read that the exhaust needs to be lower !?

I've also been reading from Joe Sherman and he said to make the ports not like funnel, but a waterfall, which I did(even though I gave the wrong advice some time back to make it like a funnel).



Quote:

Clean the bowl area up, reduce and narrow the guide boss, radius and smooth the short turn radius.
Already done and installed. I am thinking about pulling them off.
Quote:

Be careful, Vortec castings are thin, and hogging them out leaves them very fragile, and you risk grinding through the port.
Which I did to a set of vortecs about 5 years ago(just the intake port by the pushrod.



Anyways is what I'm really asking is if the exhaust flows "too much" in relation to the intake what is the downfall?

-Thanks

willys36@aol.com 01-06-2005 06:52 PM

Of course if you had a hemi you wouldn't be having this problem!

NAIRB 01-06-2005 07:01 PM

The stock Vortec exhaust port isn't so bad. It's on the small side.

On most any small block Chevy exhaust port, the floor area is a low flow area. If you create a velocity probe and run some air through the port, you will see what I mean. The roof area is the busiest part of the port, so that's where you need to work.

Anytime airflow has to negotiate a turn, it almost always helps to widen the port area where the turn is negotiated, thus widening the port in the bend or radius area helps.

Chevy exhaust ports are easy to improve, and are easily accessible. I can do a full race port job on 8 exhaust portsin 3 hours. The intakes are tougher IMOP.

Max Keith 01-06-2005 07:05 PM

Exhuasting work
 
Yeah, just the opposite of Fords.
Takes about that long to do the intakes, the exhaust a whole lot longer.

Lots of room to widen out SBF exhaust ports, the difficulty is finding headers big enough to cover the opening.


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