Increasing valve size-stock heads.

[SS_Sean]

Hey, guys. I popped a head gasket again. Am looking to increase the size of my valves on the GN since I'll be taking the heads off. I'm also going to do a port/gasket match on the heads while I have them off. The heads are stock-uncut.

If I'm thinking correctly the stock valves are a 1.71/1.50. What size would you guys recommend for a street/strip car. I drive this a lot on the street, and a little at the strip. Looking for good all around performance increase. Basically same horsepower at lower boost.

I think this should really help wake up that 6262 turbo and cam, as well....

Thanks.

 

[bison]

Id forget about increasing the size of the valves as long as they are good. Have a quality valve job performed then lap in the valves and clean up the bowls. In about 2 hrs you could make a lot of gain. Port matching does about nothing. Almost all the gain is under the valves and the valve job itself.

 

[SS_Sean]

Had a good valve job done earlier this year, and only about 300 miles on the car. Interesting, though. I would have thought it would need to be a port match, smoothing the runners out, smoothing out the bowls, and cleaning up the valve pockets, as well, and increasing valve size. Kind of a trifecta thing...to see the gains...

 

[it's just a six]

Port matching does make a difference if your ports do not line up close.

Especially true when I matched the port on the turbine housing to my exhaust manifold on my Syclone,there is a big mismatch from the factory there.

All the little attention to details will all add up to a higher HP & torque gain.
It's time comsumming but I believe it's worth it. Better if you can do it yourself to save a few bucks.

 

[SS_Sean]

I can do it myself, that's not the problem.

 

[it's just a six]

I am not too familar w/the heads,but if you can install larger valves w/out the chamber really shrouding the valves (impeds airflow) you should see a gain.

Maybe you can use a 1.85 in & 1.60 exhaust? Peeps,chime in here.

 

[WSLN 6]

I did the big valve upgrade a number of years ago, the intakes were 1.77, exhaust was still 1.50. Ported the heads at the same time and ran 133mph with a T-netics 70 P-trim. Not sure where all of the head gain was, but like Bison said it was probably mostly in valve/bowl work.

 

[Pablo]

Having done some flow bench work on these cylinder heads I can vouch for the fact that port matching does next to nothing for airflow.

Actually large changes to the port entry from opening it to putting small obstructions in it showed nearly immeasurable differences in airflow on my equipment

What did show massive changes in airflow was opening the valve throat. This was the number one gain. IIRC I opened the valve throat to 85-90% of the valve diameter and gained roughly 25 cfm. The next largest gain was reducing the size of the valve guide especially on the biased side. This gave me roughly another 10 to 15 cfm.
From there I found it very hard to make any more gains and I tried a lot of things.
From what I've read, using larger valves on these heads only makes for a marginal increase in flow at high lift

 

[SS_Sean]

Very interested posts, guys. Thanks.

Pablo, what do you mean by reducing the size of the valve guide? I'm not quite sure what you mean. Are you talking about drilling out the stock valve guides and going with a smaller diameter?

 

[thessman]

Short side radius of the intake and exhaust and get rid of some of that exhaust guide material in the exhaust port. If its free port matching cant hurt.
Like Bison said... A good 3 angle ,lap and make sure you back cut the valves. Maybe sand the combustion chambers smooth if you are doing it yourself. Steve

 

[thessman]

basic stuff

 

[Pablo]

the guide is the hump for the valve guide that obstructs part of the port above the throat.
Thessman's pic is the kind of thing you are going after.

Here are a couple of pics of my heads:

sorry for the crappy quality, cell phone pics

 

[forcefed3.8's]

The largest valves that fit in stock heads are 1.77/1.5. Little to be gained there. The bowl area sucks on these heads. The exhaust side is horrrible.
We did a minor clean up on a friend's car, mostly bowl work, stock valves, smoothed out ports. The only difference being the head work, we had to add almost 20% more fuel at the same boost to get O2's back in check. 14psi was using the same fuel 18psi with stock heads used. This is just backyard porting with an understanding of airflow.

 

[SS_Sean]

Pablo, thessman, thanks for the good examples.

The largest valves that fit in stock heads are 1.77/1.5. Little to be gained there. The bowl area sucks on these heads. The exhaust side is horrrible.
We did a minor clean up on a friend's car, mostly bowl work, stock valves, smoothed out ports. The only difference being the head work, we had to add almost 20% more fuel at the same boost to get O2's back in check. 14psi was using the same fuel 18psi with stock heads used. This is just backyard porting with an understanding of airflow.

I appreciate you posting, but I'm not sure I understand what your point was? I don't want to jump to conclusions, so could you clarify a bit? Thanks.

 

[forcefed3.8's]

What I was saying is port matching gives little gain as does larger valves. The biggest gain is in the bowl area.

 

[Pronto]

Why did you blow the head gaskets again?

 

[JayC]

The largest valves that fit in stock heads are 1.77/1.5.

Wanna bet?

 

[forcefed3.8's]

Bigger without moving the valves?

 

[bison]

Bigger without moving the valves?

Not likely. Not much point in spending the money on moving them,correcting valvetrain mis-alignmnet, reshaping the area around the valve, and buying new valves unless you are required to run stock heads for some reason. The gain woudl be small on a 3.8 bore. You would be better off leaving the valves where they are doing some good bowl work, a good valve job and modifying the spring seats to take some larger diameter springs if you want to make more hp. Getting enough spring on there to go 140+mph at full weight is a real challenge.

 

[SS_Sean]

Why did you blow the head gaskets again?

I was running in the area of 24psi for a while before I backed it down to 18. Who knows. I could have a boost gauge that's a couple off. It could be a lot of things. Point is I have to get the powerlogger installed and have zero excuse. It's all a guess unless you're logging some data. I'm going to check the deck on the heads, the block and start doing some investigation. I'm tired of running blind.