Is it common for 706 castings to have slightly above 92% throat to valve diameter?

The target from what has been written by knowledgeable people is 89-90%.

I don�t see any way to fix other than install larger 2.00 valves. My throat is 1.745.

 

Which engine are you running hose heads on?
While others may defer, it has been found that 2.00 valves in a stock bore 5.3 cause some shrouding.
This is why Tony Mamo will (and does) use a 1.94 valve to prevent just that.

 

1.94 valves would put me exactly at 90%.
I need to check to see if that size is readily available with 8mm stems and the correct length.

Going on a 3.81 bore.

 

If you can't find them, cut down some 2.00 valves. I doubt it would cost much, a little time on a valve grinding machine.

 

Any reason not to go ahead and put a 2" valve in it?

 

For the same reason Tony Mamo doesn't with stock bore (+/-) 5.3's. Potential shrouding issues.
Mamo knows what he is doing, and is widely respected for his head work.
If he says use a SMALLER valve, there must be a good reason.

 

My findings are different.

 

Care to elaborate?

 

Suit yourself.
I guarantee if Mamo knew that 2.00 valves would do a better job, don't you think he would do it?
With his rep and success rate, I'm not here to second guess his methods.

 

Guys,

One of the members here flipped me a link to this thread....thought I would chime in quickly

My "small bore" specific heads (MMS 205's....not to be confused with the older AFR 205's) run a 1.975 intake valve and a 1.570 exhaust.

Their "standard" out of the box chamber volume starts at 61 cc's and these heads can be milled into the low 50's if you desire.

Im sure you could put in a slightly larger valve but in doing so you start to juggle low and mid lift losses from cylinder wall shrouding versus the small gains up top a slightly larger valve might offer.

Personally, I think the port itself plays a larger role in regards to peak flow when discussing a head of this nature with a small port and a tighter combustion chamber.

You have to keep in mind that we are talking about smaller motors with smaller bores....that's what these heads are designed to be optimal on and a smaller valve with the right size throat can potentially offer better discharge coefficients and slightly better average airflow if your targeting camshafts in the low .600's valve lift or below (the bulk of the market this head is targeting).

BUT.....there are lots of different ways to skin a cat.....some applications and cam profiles might benefit from a larger valve and a different valvejob configuration.

The head I designed will work great with 95% of the applications and complimenting components your likely to consider this head for but that doesnt mean its the perfect head for every application

Although at just under 300 CFM, with this size port and this size valve it moves alot of air and offers alot of airspeed as well....its always going to perform well for the "small bore" target market I designed it for.

-Tony

 

I was hoping Tony could explain why GM made the throat size above 90% on the 1.89 valve heads.

Supposedly, this is not a good thing for throttle response and low lift flow.

 

Just guessing, but it MIGHT be so the HP numbers stay high enough via the additional higher RPM flow.
Only speculating here....

 

That's great and all, but what are your actual findings on this?

There are many ways to skin a cat, and not everyone does things the same way. But to make a claim because this guy or that guy does it a certain way, is reaching a little. If your research merits what you're saying, then so be it, it works for you. Just don't get your panties bunched up because someone else does it different.

 

I don�t think it�s performance related. If I remember correctly, the 5.3L heads had the same throat diameter as a set of 799/243 heads. My guess is that it was done to reduce manufacturing costs.

 

I'm by far no expert in head tech, but do recognize those who are by the results they get, as that IS the bottom line. When someone else comes up with numbers that equal or exceed these results, I'll recognize that as well.
Tony Mamo posted above the how's and why's of what he did to these heads. He is up front, and I appreciate that. He holds no secrets.

 

^^^ This, right here. They can essentially use the same casting for small and large bore heads.

 

Perhaps it's not as critical with the use of the swirl ramp?

 

Man, I sure would like to know.

Hate to spend the money for 2.00 valves and another valve job to get throat diameter as what it should be.

 

I bet this is the reason.

 

Wasn't the 706 head designed before the 243 casting?