Exhaust Port : V-Twin Forum: Harley Davidson Forums

aharp · 10-27-2009, 04:12 PM

How cruicial is throat-to-valve on the exhaust side? Are flow numbers more important than this ratio or no?

hightop · 10-27-2009, 05:38 PM

I like 80-85%, but that's just me. Can't give you any theory or anything. Maybe someone else will chime in.

route66paul · 10-27-2009, 08:35 PM

It is ALL crucial. As the compression gets higher, the exhaust get more crucial.

claydbal · 10-27-2009, 08:46 PM

Quote:

Originally Posted by hightop

I like 80-85%, but that's just me. Can't give you any theory or anything. Maybe someone else will chime in.

hightop, your posts always seem intelligent. and i have followed.

tell us more about yourself please.

hightop · 10-28-2009, 06:00 PM

Claydbal, I work at an indy shop up in Md., mostly build motors but do get a chance to do some wrenchin', and we've got 2 250i's, one in a room and one in a trailer for the road, but I don't do a lot of tuning unless it's a carb, so I'm pretty ignorant about the EFI tuning end of it.

I'm surprised there's so few posts to aharp's question. Sometimes you can get jumped on when you voice your opinion about questions like this, but it's been pretty quiet lately.

I like to use a smaller throat on the exhaust, as I said above, because it allows me to put a radius below the seat, the exhaust port seems to like it, not so on the intake side, IMO.

There, that oughta get somebody going!

aharp · 10-28-2009, 07:04 PM

Quote:

Originally Posted by hightop

I'm surprised there's so few posts to aharp's question.

Kinda what I was thinking. But I have a feeling that those in the know had to figure it out the hard way, the way I'll have to

wfolarry · 10-28-2009, 07:22 PM

The size of the throat is more important than the ratio. Some guys like a big radius so they'll use a bigger valve to get it. Others only want 3 angles so they don't need as big of a valve to get there. It boils down to what the head porter thinks works best.

claydbal · 10-28-2009, 07:26 PM

some of us just dont say too much, its the way we were trained around here.

try and help someone, or answer a legit question, and get ready for circumcision. it just aint worth it.

aharp · 10-28-2009, 07:47 PM

So the situation I have is I have an exhaust port that actually lost flow by opening up the throat. The throat-to-valve ratio was already out of whack and opening up would just make it worse but I did anyway to see if the numbers would follow. I sometimes suspect my flowbench but had an identical stock head that flowed better. This port was not opened up, just cleaned up the casting marks and polished it out. The way it's looking is to get the exhaust to flow the percentage of the intake I would have to install a bigger seat and valve. But how could installing a bigger seat and valve pick up flow when opening the throat on the stock seat killed it?

wfolarry · 10-28-2009, 07:55 PM

Does the port sound turbulent [noisy] on the flow bench? The right seat cuts could get the flow back. What size is the valve & throat?

aharp · 10-28-2009, 08:15 PM

The port doesn't sound any noisier than the stock one, hard to find a turbulent spot with the pipe on there. The throat is at 1.45 and the valve is 1.625 so it's right at 89% which has shown good for other heads I've done. The other thing is that the port is already huge and shape-wise doesn't look like it could be leaned on much (at all?). I'm afraid that opening the throat I've made the seat too thin on the stock seat, being this is probably the hottest part of the entire motor. This is a "play" head so it doesn't much matter if it doesn't work but do I try a bigger seat with a 1.650?

nw_guy4_fun · 10-28-2009, 08:34 PM

Measure the stocker and see what that ratio is

aharp · 10-28-2009, 09:02 PM

Hey Don and Larry,
It's about 87%. The seat over hangs into the combustion chamber a bit, could a simple radius cut and maybe knock the seat down a bit with a top cut get me the flow I'm looking for? I'm looking to pick up about 10 on the exhaust.

wfolarry · 10-28-2009, 10:22 PM

1.450 is a little big but it can still be saved with the right seat cuts. What are you using to cut the seats?

aharp · 10-28-2009, 10:31 PM

Mondello seat cutters. This is actually on a 110 head. Put in a 2.10 intake to bring the throat-to-valve back down and cut the seat and picked up bunch at low lift and some at the top. Trying to get the exhaust up to 79% of the intake now but having some trouble like I said. Thanks for your help guys, this isn't the type of info head guys usually just hand out