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I will never understand why anyone would go to the trouble of installing heads with 1 5/8 valves and then use a header that has in inside diameter of 1 1/2".
Surely you must realize 1 5/8 is 1.625 and has an inside diameter of 1.500
1 3/4 is 1.750 and has an inside diameter of 1.625 like the valves.
It doesn't matter how well you have run with 1 5/8" headers, you probably could have done better with 1 3/4" headers.
Just my $.02
Because the diameter of the valve has absolutely nothing to do with the flow capability of the port.
Years ago I ran the same Hedman 1 5/8 headers, and I swapped them out for 1 3/4 headers mid season and the car still trapped 120mph and still ran the same ETs. The bottleneck was the exhaust runner on the sportsman-II heads.
He's running ported 305 heads with a .479 lift cam. His exhaust flow is going to be around 150cfm max. When you calculate out the duration of the cam and that exhaust flow, it calls for a primary diameter around 1.4"
While I believe in absolute truth, not in this instance.
I have LT's and am lowered. In this pic, the pass side was just a little low - you can barely see it. In the second, you can see what I did to get 3" inters (w/2" primaries) around an 4L65e and is much better, now. It always seems to come down to how much money your willing to spend, and what your own skill level is.
Because the diameter of the valve has absolutely nothing to do with the flow capability of the port.
Years ago I ran the same Hedman 1 5/8 headers, and I swapped them out for 1 3/4 headers mid season and the car still trapped 120mph and still ran the same ETs. The bottleneck was the exhaust runner on the sportsman-II heads.
He's running ported 305 heads with a .479 lift cam. His exhaust flow is going to be around 150cfm max. When you calculate out the duration of the cam and that exhaust flow, it calls for a primary diameter around 1.4"
-- Joe
If you want to believe that......
Be my guest
But, nothing could be further from the truth!
While I believe in absolute truth, not in this instance.
I have LT's and am lowered. In this pic, the pass side was just a little low - you can barely see it. In the second, you can see what I did to get 3" inters (w/2" primaries) around an 4L65e and is much better, now. It always seems to come down to how much money your willing to spend, and what your own skill level is.
While in some cases an 1 3/4" primary tube would work well on a 350, that isn't the case on this build. Stay with 1 5/8" primary and 2 1/2" secondary.
That is what I used on my solid roller 355, which was in my 67 Chevelle. It's been 91 mph. in the 1/8 th. on motor., with 3.89 gears. I shift at 6700.
^^ this lol. when we ran the numbers and after I talked to a couple of exhaust shops I told them my build and what I was aiming for. both shops told me the same thing going up to 1 3/4 inch headers wouldn't do me any better the 1 5/8 same thing with exhaust size that if I wanted to run duals stick with 2 1/2 inch piping I could go bigger but it would just be spending more money for no reason.
The problem with most 1 5/8th headers, (or smaller)they block about 20% of the port
at the head. The flange opening is not large enough for most Hi performance heads.
That is a restriction you don't want if you are trying to make power.
The problem with most 1 5/8th headers, (or smaller)they block about 20% of the port
at the head. The flange opening is not large enough for most Hi performance heads.
That is a restriction you don't want if you are trying to make power.
I totally disagree.
AFR reccomends a PN. 1405 exhaust gasket for their 195cc heads. That gasket has a 1.5"x1.5" opening. At 16 ga. primary, 1 5/8" tube the inside of the tube will measure 1.5" across.
They, AFR also say their Eliminator exhaust ports are raised .150", which tells me off the shelf headers will more than likely wont fit. Because, like you said the pipe will cover the port.
But, when you look at the largest exhaust valve generally associated with a SBC. It falls in at 1.625 on the face. Now, you can put a 1.74 Pontiac valve in the head but it requires the intake to shrink, to allow clearance.
Back to the subject. A 1.60 exhaust vale requires an 80-85 percent seat diameter for excelent flow. That number is 1.280". Now with a 1.625" valve the seat dia. should be 1.300".
You see, the port is the restriction, not the header tube.
As long as the tube does not cover the port, its size is fine. Even a 1.500" tube would work.
And smaller tubes increase velosity while larger ones decrease it, and affect reversion in the exhaust.
Ok, you guys win, I was just using a little common sense.
That seems to be a lost cause in todays world.
BTW AFR 195 ports are larger than 1 1/2"
I wouldn't want mine that way, suit yourself.
I guess you will have to ask them why they recommend a gasket smaller than their ports.
Buddy, not trying to argue. Mearly stating a fact. I could mic. the runners on my Chevelle but really not in the mood for it. However, it doesnt make any difference here about AFR heads or Brodix or whos ever. All that mattrrs is that this thread is 6 months old and the OP, was asking header size for ported 305 heads. Those heads are not junk but not AFR's by a long shot either. I just used AFR as an example.
Only thing I wanted to express is that the throat of the bowl before the valve face, is the real restriction. Not the header primary tube. And you cant open up that area over a certain percentage or there wont be any place for the valve to seat.
I totally disagree.
AFR reccomends a PN. 1405 exhaust gasket for their 195cc heads. That gasket has a 1.5"x1.5" opening. At 16 ga. primary, 1 5/8" tube the inside of the tube will measure 1.5" across.
They, AFR also say their Eliminator exhaust ports are raised .150", which tells me off the shelf headers will more than likely wont fit. Because, like you said the pipe will cover the port.
The roof isn't raised, the whole port is raised. I've got some technical drawings that they gave me when I was making turbo manifolds.
The AFR 195 exhaust port is 1.450" H x 1.550" W, and yes they recommend a 1404 which is 1.5 x 1.5.
I do find it kind of funny that guys are buying expensive 1 3/4" headers just to stick them on TPI motors and run 12s.
