Head porting question...(throat size)
#1
Head porting question...(throat size)
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?
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?
#2
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.
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.
The following 2 users liked this post by TrendSetter:
DualQuadDave (05-31-2024), mac1183 (03-20-2024)
#3
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.
#4
UPDATE:
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 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:
The following 4 users liked this post by Utinator:
#5
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.
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.
#6
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.
Maybe it's a credit to the GM head engineers that they work so well in spite of breaking these so-called rules.
#8
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.
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.
I have been using the flow numbers that Katech posted in their video as a comparison.
I don't expect to beat a CNC machine, but I want to make sure I'm somewhere between stock and a CNC machine. Here's what I got from the Katech video:
Intake
Lift stock CNC CNC/vj CNC/vj/2"
.200 140 140 146 144
.300 184 186 193 192
.400 216 205 229 231
.500 219 227 242 251
.600 225 245 250 261
.700 232 257 259 268
Exhaust
Lift stock CNC CNC/vj
.200 95 97 101
.300 123 128 133
.400 151 161 162
.500 169 176 180
.600 177 186 192
.700 182 192 200
#10
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.
#12
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.
I was thinking that I might need to increase the CSA over the short turn. It might be restricting the flow volume.
#13
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
Here's some before/after pics:
stock, high mileage vs wire brushed
clean cut vs lapped
#14
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.
#16
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
Here's some before/after pics:
stock, high mileage vs wire brushed
clean cut vs lapped
No idea why the factory did it; I'd imagine cost had something to do with it.