Help with manifold matching

[nwticerider]

I am having some problems finding a manifold to get close to the 200cc heads I am using.
the airgap rpm that came on the engine is not even close. Checked by scribing a line on the gasket and then removed the manifold.
There is enought material to seal, but not enough to hog out and back up into the runner. (creating a ballon)

The head port matches a 1206 quite well. very similiar port to a RHS 1.3" X 2.25"
390 stroker 9.2:1 sbc with a extreme marine 276 cam 222 and 230 @ .050 , holley 770
the boat cruises at 3800 and WOT is 5300.
Keep in mind the pump acts like a 5300 stall convertor and it is not run under 3100 except to idle.

Other than a rather pricey victor cnc I have not been able to find a manifold that is close that i can easily match .
missing the top 1/4 inch + of port and a huge step into the head must be down on power .

Would a large runner and plenum of a single plane kill the small cam?

looking for suggestions, comments, and or recommendations.

Thanks
the ice is coming off soon.

[user-30257]

Run it.

[BILL-C]

You can buy a .090 thick version of that gasket from Cometic that will raise the manifold up and center it better on heads. Then you will have a little more material to work with while port matching. There is only a few hp to be had, but it sounds like you are dying to make some chips. Go for it and know that you did your best.

[496s10]

If the heads are 200cc import casting with dual pattern intake mounting the vortec intake's usually are a closer match out of the box.

[Newold1]

You can also have a welder make a couple of passes over the top of the runner top of the runners on the manifold at the junction of the runners on the manifold and the entrance to the heads and then grind out the top of the runners on the manifold to match.

[turdwilly]

Fel-pro 1266 same port opening dimensions but is .120" thick that will get you closer

[groberts101]

Fel-pro 1266 same port opening dimensions but is .120" thick that will get you closer

Ding ding ding.. think we have a winner folks!

The other thing I would propose to do is to measure the actual gasket opening sizes and you will see some variations. Go through a pile of new in package gaskets and you can see tons of shift in actual openings compared to the rubber seals located around each port. This is why I never "gasket match" based on stampings used for the port openings. Otherwise you end up with varying sizes based on the mass produced stamped gaskets. Hand selecting is always best when you have the option as different production runs seem to make a difference too. Who knows.. maybe one set is done on Monday and another is done on a Friday.

Another trick is to use the next size gasket down, say a 1205 vs a 1206.. and port the gasket itself all the way out to the printoseal.. or whatever they call the Viton type sealant used around the port openings these days. I have found those rubber seals to be MUCH more consistent in sizing than the actual stamped openings themselves.

PS. I should point out that you cannot quite reach a 1206 port opening size when porting the 1205 out to those printoseals but it's pretty close. Kind of a hybrid sizing.

[nwticerider]

Thanks for the replies.

I believe most of the flow is on the bottom of the port and it matches there so I am going to run it.

Just a bit of radiusing in the mismatch, clean up the casting slag, and radius the plenum divider.

If it was for a high revving all out effort this would be more of a concern. And the super vic would be on it.

Regards,
John

[groberts101]

Thanks for the replies.

I believe most of the flow is on the bottom of the port and it matches there so I am going to run it.

Just a bit of radiusing in the mismatch, clean up the casting slag, and radius the plenum divider.

If it was for a high revving all out effort this would be more of a concern. And the super vic would be on it.

Regards,
John

That is incorrect thinking. As the manifold port turns down into the heads entry.. it steers and bunches up some of the flow towards the roof of the manifolds port exit. Put a pitot tube on that entry floor and you can "see it" . Same thing happens on the outside of the turns of the manifold leading into the heads entry as well.

I too would probably just run it and not have second thought about it.

[JoePorting]

Matching up the ports is going to have no effect on power. The bottle neck is the abrupt angle between the bowl and the low port. There is no way to change that in a low port head, so you are stuck with what you have.

[MadBill]

Looks to me like the minimum cross section area might be the port exit in the manifold, whereas the head is probably designed with its (larger) MCSA at the pushrod pinch. If it was me, I'd want to honor their intent by opening up the top of the manifold ports as much as is consistent with leaving the sealing beads fully supported. Prior to above work, I'd pick a gasket thickness that left most of the mismatch at the top, with ~ 0.020" -0.030" manifold overhang at the bottom and left untouched.

[user-17438]

its a boat with a maximum rpm of 5300 rpm. throw it on and run it, any gains you may get will most likely be above that rpm, right around where that cam will fall on its face.

Enjoy the boat and run it in my opinion.

[KnightEngines]

Measure the cross section at the pushrod pinch in the heads.
Compare that to the cross section at the port exit in the manifold.
If the manifold port exit is larger cross section than the pushrod pinch then leave it alone.

Using a thicker gasket to raise the manifold up a bit is worth it.

[steve316]

I always match; if it helps great, in your case I would just raise the roof. Use straight edge to keep it flat.

[Newold1]

Which RPM air gap manifold part number are you planning to use. There are several and the 7501 Edelbrock is best for torque and in a jet boat Torque is King!