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I am about to start porting a pair of stock, untouched, 862 heads. So far, they have only been cleaned. Before grinding anything, I started by measuring the current (stock) throat size. I'm getting about 1.745", which is about 92% of the valve size (1.89"). Most head porters don't recommend going over about 90-91%...maybe 92% for a race only head. If I blend the valve seat into the bowl like I normally do, it's gonna go way over 92%. I measured a junk 706 head that I use for testing (see previous dimple port video), and it was out to 1.752". That's about 92.7% of the valve. This would be fine with a 2" valve, but I plan to use the stock valves.
I used an inside diameter, snap-spring tool to "measure" the throat width. Then, I measured the tool with a dial caliper. I know it's not the best way to measure something, but I can't get anything else in there. That's how I'm getting the throat measurement. I tried a dial bore gauge, but it won't fit in there. Am I measuring wrong, or is the stock throat size really that big on the 706/862 heads?
youre going to get better results with a mostly stock head and 2" valves than whatever you try to do to the port keeping the stock valves.
ls heads have always used real large throat %. gm must not have gotten the memo from the internet considering how far they have strayed from the 'norms' with the ls3 heads also.
I have been getting about 10-15cfm gains with the stock valves and stock worn out valve job. I plan to get some valve seat cutters eventually. I was just surprised at the throat size. Now I know why the 2" valve is such a good upgrade.
I just dropped off the 862 head that I was porting, at the machine shop. I opened all but one intake port to 90% for a 2" valve. I left one intake throat stock, but put a new valve job on it with the new valve seat cutters that I ordered recently. The machine shop will flow the two inner ports. One will have a 90% throat and a 2" valve. The other will have the stock throat with a stock valve. When I get the head back, I will share the results. So, we will know how much (cfm) the valve job is worth, and how much (cfm) the 2" valve is worth. I ported this head the same way I have done the last several cathedral port heads I have ported. The only thing I did differently this time, was the valve job and throat size.
Just for comparison, here's the last 706 head I ported:
I have noticed the same thing with the 706/862 heads. The factory throat size is around 92%.
like you said, all the porting gurus say that’s a no no. Here are some 706 heads I ported. They had 2.00 intake valves, 90% throat, and blended into the bowl. BTW, I am NOT a professional porter, just something I like to do from time to time. It’s good calming therapy for me.
Even stock 706 heads don't get the credit they deserve, as even they are still WAY better than most stock SBC heads.
Maybe it's a credit to the GM head engineers that they work so well in spite of breaking these so-called rules.
SURE, be a HOG.
REMOVE the Venturie at the valve seat. Kill the Valve Owerlap flow. Kill the Low Lift flow.
When the "hog" method is used, the Camshaft will need to be changed.
Lance
I have noticed the same thing with the 706/862 heads. The factory throat size is around 92%.
like you said, all the porting gurus say that’s a no no.
Here are some 706 heads I ported. They had 2.00 intake valves, 90% throat, and blended into the bowl. BTW, I am NOT a professional porter, just something I like to do from time to time. It’s good calming therapy for me.
Hey, those are good flow numbers.My best so far is 241cfm with stock valves and stock, beat up valve seats. My goal is to beat a stock 243 head. I'm not sure if that will happen with stock valves, but the 2" valve should definitely out-flow the stock 243. For what I can get a 706/862 head for, it's cheaper to port and 2" valve upgrade than buy a stock 243. The last 243 I ported peaked at 271cfm, but I didn't touch the valve job. Now, I have tools to re-do the valve job, so I will be doing that from now on.
I have been using the flow numbers that Katech posted in their video as a comparison.
Forgot to mention the heads were flowed on a 4” bore.
I had someone do the valve job for the 2.00 valves. The bummer was that now the installed height on my intakes are 1.770 instead of 1.800.
Wow, I got 1.855" (Int.) and 1.9" (Exh.) installed height. I had to cut the Intake valve seats deeper/wider to fit the 2" valve. I cut the exhaust valves to get the pitting out. I guess I removed more material than I thought. My install height gauge is a cheap Amazon tool. It might not be 100% accurate. I'll probably measure one of the rec port heads that I have with a caliper to see if the install height gauge is accurate.
I get my 706/862 heads flowed on a 3.9" bore. That's the smallest the machine shop has. I was disappointed with the 2" valve peak flow. The low lift was a great improvement, but I expected more at peak. It seems that the valve job did nothing. Comparing this new 1.89" valve chart to the one I previously posted above, they are about the same. I'm pretty satisfied with the exhaust flow, though. This was my best exhaust flow yet.
2" valve same head, same valve job, different valve size, different port.
just a little sanding, and I blended the valve seat into the short side. It was mostly just smoothing everything out. Those 862 heads are really rough.
I was thinking that I might need to increase the CSA over the short turn. It might be restricting the flow volume.
Does anyone put a back cut on the exhaust valves? I noticed that the stock intake valves have one, but the exhaust valves don't. I was just wondering if it would make a difference. I have been cutting the 45 degree sealing angle on the exhaust valves just to make sure they seal properly. I have the tools to put a 30 degree back cut on them too. I just haven't done it because I wasn't sure if it would help. I don't want to spend the time if it's not going to help. My exhaust flow numbers are already pretty good. If I can get more with a back cut, I will.
Here's some before/after pics:
stock, high mileage vs wire brushed clean cut vs lapped
I haven't seen anybody bother with a backcut on exhaust valves; all I've seen is Charles Servedio and Eric Weingartner talk about a backcut on intake valves. Check those guys out on Youtube, they have good stuff.
Does anyone put a back cut on the exhaust valves? I noticed that the stock intake valves have one, but the exhaust valves don't. I was just wondering if it would make a difference. I have been cutting the 45 degree sealing angle on the exhaust valves just to make sure they seal properly. I have the tools to put a 30 degree back cut on them too. I just haven't done it because I wasn't sure if it would help. I don't want to spend the time if it's not going to help. My exhaust flow numbers are already pretty good. If I can get more with a back cut, I will.
Here's some before/after pics:
stock, high mileage vs wire brushed clean cut vs lapped
A backcut on the exhaust can increase/ aid reversion, so I never do it on any engine.
No idea why the factory did it; I'd imagine cost had something to do with it.
03-19-2024, 06:50 PM
Head porting question...(throat size) 
