EricTheBald

If I wished to increase the exhaust flow numbers, as a percentage of intake flow, as close to 100% as possible, and I wanted to do it by increasing the physical size of the exhaust valve while leaving the intake stock, what is the maximum size exhaust valve that can be installed while not doing anything more than increasing the size of the opening?

To be clear, I do NOT want to start getting into exotic work like moving the physical location of the entire valve at all. I want to leave it and the stem in the exact same position. and orientation.

What's the biggest valve I can run?
Can it be had in an undercut version?

Injuneer

It might be easier to answer if you explained why you wanted to do that.

EricTheBald

Because the supercharger I plan to use is going to take care of the intake side.

Sure, I'll clean up the intake side but what I want to focus on is getting the burnt stuff OUT.

I want to do better than 75-80%

Mindgame

No reason why you couldn't get a 1.65 exhaust valve in there. I believe that the majority of the extra flow is going to come from the raising of the exhaust port but the 1.65 wouldn't hurt matters any.

-Mindgame

zhevy-1

Get a cam with more duration on the exhaust, that'll keep the valves open long enough to expell gases.

menlatin

yea, i think rasing the exhuast port will get you the most flow, as well as porting the [ EDIT ] out of the exhuast port like i did. 284/237 @ .600. me so appy, and thats with a 70's gm cast iron head with 2.05/1.60. but i mean, there is only o much you can do with an exhuast port. i mean, its like 2-3 inches in total traveled length.

Injuneer

Why write off the intake port on a blower motor? You are just forcing the blower to work harder, which takes more power off the nose of the crank, and causes increased charge temperature with the extra compression required.

Yes, you need to address the larger exhaust flow, but I wouldn't think it would be a good idea to do it at the expense of intake flow.

But then I don't know squat about blower motors, so maybe its a matter of "theory" vs. the real world and I'm way off base.....

OldSStroker

Not far enough to get picked off by a good pitcher, IMO.

Mindgame

Dunno about that...
An article about Mike Moran's quadruple turboed car stated that... While runner volume is a proprietery figure, we do know the heads have a small cross-section and feature very high velocity.

Now why would an engine like that even need a small cross-section (more resistance) and most likely a high degree of port taper? Kind of goes against the bottleneck theory as I see it. I agree with the, don't sacrafice intake flow mantra but I think that the whole point is to make the exhaust as efficient as possible. Look at it this way, most nitrous and blower cams keep the exhaust valve on the seat longer to harness the combustion energy for a greater length of time. When the valve does open there's alot of gasses to get out of there so you need a very high flow port (especially in the lower lift region) to get the byproducts out during blowdown. I think that it is an even bigger deal with nitrous than it is with a blown engine but it's still something I'd want to maximize.
David Vizard has a section in his book on nitrous/supercharged/turboed cylinder heads and he says that a strong nitrous engine will benefit from the sacrafice in intake flow. I've seen people do just that with a bit of success, but I've also seen people gain quite a bit in a good (raised as high as you can get it) exhaust port that just flows like the dickens, with a more 'normal' sized exhaust valve. Tough call, that'd probably take a bit of experimenting/dyno-time to come to a decision on.

-Mindgame

EricTheBald

The intention here isn't really to sacrifice intake flow.

Here's the thing...

I'm going to end up using a powerdyne unit running less than 10 pounds of boost, probably aroud 5 or 6 actually.

I'm going to be keeping the static compression relatively high, about 9-10.

The more I flow on the intake side, the more I have to evacuate on the exhaust side.

By trying to increase the exhaust flow to as much of a percentage of intake as possible I will be able to cut down on overlap AND pumping losses.

WS6 TA

It makes perfect sense. The idea is that the reason that most of us run split duration cams is that the exhaust flow isn’t great enough so it takes longer to get the used stuff out. Add a power adder and all of a sudden you’ve got much more exhaust, so typically a blower cam will add even more to the exhaust side. If you could get the exhaust ports to flow substantially more you could get away with much less exhaust timing and in general have a happier, more streetable and more efficient combination. Basically everything we’re doing here is a crutch for poor exhaust port flow.

The reason it happens is that port work and heads are much more expensive and more labor intesive then a new cam grind. Add to that, that the intake duration has a much greater effect on a cam playing nice on the street and with the ECM, so we focus on intake port flow, and then economics says, save the money that would be spent to get the exhaust to work well and just stick a cam with a longer exhaust duration in, which will idle almost as well as the smaller cam/bigger exhaust port combo, just sacrificing fuel mileage and midrange.

How big can you go? Well, to start with I'd bet that 90% of the heads out there are more limited by their exhaust port then the valve, and second, you can't usually go too big without taking out too much of the exhaust seat for it to live. I suspect that with most heads you couldn't go much over the typical 1.60" without cutting them for new seats, heavily modifying the ports... I'd talk to the machine shop that you're going to have do the work and see what they're willing to try.