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Old 08-12-2005, 02:30 PM
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Worth it to put bigger valves in stock seats???

Hey guys!

I am about done with a home porting (or more like a smoothing) job on a set of 3.8 ford V6 heads from a Tbird supercoupe, factory supercharged, smaller pulley, N2O, bigger cam (212/220 .531/.539 lift 114 lsa), bigger maf TB, injectors, etc etc...

I have been recommended by the "3.8 SC boys" that the intake valve is big enough, and the exhasut valve could be bigger. Exhasut valve is now 1.46 stock.

Some have used 1.60 and 1.625 SBC valves with minimal shimming, pushrod change, etc etc.... but 1.60 require new seats

Machinist says 1.55 is as big as he woud go. But custom 1.54-1.55 valves are also out of the question at $55 each.

I am considering some 1.50 SBC exhaust vaves, because bigger seats are out of the question as well.

I am wondering if it would be worth another $90-$100 for some 1.50 SBC valves over the stock 1.46?

Who thinks its worth $100 to swap from 1.46 to 1.50 exhaust valves?

I have heard conficting opinions on bigger valves without changin to bigger seats, so I figured this would be the place to ask...

So is .040 bigger worth anything???

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Old 08-12-2005, 07:39 PM
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If the the chambers are cut to keep the valves unshrouded and the seats are blended you should see a little improvement over stock.

Little bits add up.

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Old 08-12-2005, 09:32 PM
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Measure the overall diameter of the actual seat insert that you now have. You can have your machinist cut a profile on the existing seat so the 45 degree seat is as large as possible, but still on the hard seat.

You can then use any larger diameter valve, have your machinist turn his valve grinder to 90 degrees, and he can grind the O.D. down for you until they are a perfect, custom fit.

The face can be reground to adjust stem height. Of course this same operation can be performed in a lathe also. The valve face has to be reground in a valve grinder, however.

This is a very simple operation, and one I do all the time to get the valve size that is the most effective for a particular head.

I might add that the seat form is more important than just sheer diameter. The "window" or "curtain" area is larger with a larger valve, and if it is performed properly, it will increase low lift flow dramatically, but often times, people just slam in a set of bigger valves and somehow think that it is going to make them flow better when in many instances, flow is hindered because the job was not done correctly.

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Old 08-13-2005, 12:19 AM
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Thanks guys!

By the way, the valves have already been unshrouded, along with some minor porting, more less smoothing and port matching. I just smoothed out all the sarp edges, etc. I also blended the bowls to the seats.


Are you saying that a 1.56 valve diameter could be trimmed to 1.54-1.55???

I was wonderig if this could be done, is this something that is expensive to have done over the rest of the valve job? I wonder why the machinist didn't offer this suggestion?

We can use SBC valves, and from what I see, there are 1.56 SBC valves available, in addition to 1.50 and the 1.6+ that some other guys have used in these heads...

I am not able to find a 1.54 SC valve, but there is a 1.54 ford valve, and I'm not sure what the demnsions are off the top of my head, but these guys usually use SBC in these heads, as they are pretty close to our valve size, and match up with little shimming, etc.

As for the "window" or "curtain" do you mean the "hole" in the seat? I am realatively new to building engines, and such, so I am not familiar with all the terminology yet... Are you saying the hole in the seat can be enlarged to accomodate the bigger valve also???

These engines, in stock form are built for low end torque in this heavy car, so I need all I can get to make more HP up top, without ruining the low end for the street. All the dyno charts i've seen, the HP starts to drop just after 4500RPM. So I need some help up top... My blower and intake modifications, and the cam will hopefully widen the band, but still looking for a little more towards the top. Loosing a tiny bit of low end isn't too much of a concern, as the factory number was like 312 ft-lbs, and my 60ft times were just around 2 seconds with minor boltons, and exhasut. With all the other modifications and such, I think the low end will be OK...

From the threads I've read on this board on 1.94 vs 2.02 it seems that .080 size differnece talks about low end vs high end on the valve selection, so it seems like .040 .045 bigger should definately help out!

Thanks again!!!
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