Even-ing up the mixture and other thoughts

[GraemeSmith]

So I'm currently grinding in a new exhaust valve on No 4. Compression went to pretty much zero - just like that.

With the head off it was pretty apparent the edge of the valve had started to erode. Fortunately the valve seat was OK. Further digging - in the (EMPI) head - there was a significant casting "ridge" in the exhaust port (not present in the No 3 end). And the port itself was narrower than the gasket by a fair amount. Not at the No 3 end. So I'm thinking restricted exhaust flow got the valve hotter than optimal. EGT's were not suggesting anything however.

So I've ported and polished the No 4 end of the casting to get rid of the ridge and to "gasket match" the gasket and exhaust pipe. Some matching polishing in the other end to try and keep things even. No real metal removal required at that end.

Just waiting for the mail man with the new exhaust valves.

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So....... Let's go street car racing a moment. Look harder at the inlet side of things. You know - no street car mechanic would allow this mismatch between the red manifold (rectangular holes) and the aluminum (round) ports.

Street wisdom would have it that the mixture being delivered though the rectangular inlet - hitting the flat edges of the round holes in the ports would tumble and be restricted and inlet flow would be compromised. On the street - you would remove aluminum from the round holes till they were a rectangular match for the inlet manifold. Remove restriction, improve flow, more power. Usually this simple mod, with matching polishing of the inlet casting, would get 10% more power.

So let's take this a little further - could I try differential porting - making one hole match and not the other - to improve the differential mixture the engine suffers from and so even up the CHT and EGT's? Power improvement would be less of a factor (though there would be some). Getting all four cylinders to burn equally and so smooth the engine out would be a better longer term aim.

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Two thoughts against:

- Over porting can actually decrease the inlet mixture speed.

- Perhaps the mismatch of the rectangle to round holes is a deliberate "design" way to increase mixture "tumbling" to get a more even mixture?

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Anyone?

[Area 51%]

Check out Great Plains intake manifolds. The ports are a very good match and they have a divider cast between the ports. They have a shape that doesn't require the fuel/air mixture to come to a stop, then back-up to get into the rear cylinders. The geometry is the same as the supplied intakes. I have a set on our yet to be run engine.

Need a picture?

[GraemeSmith]

Pic - no - had a look online - I'll give them a try. Lot cheaper than my time to port the inlets to match the Sonex intake and maybe booger up a head.

[bvolcko38]

I was frustrated by big differences between front and rear egts. Then on recommendation from this list I put on the Great Planes intake manifolds. Egts evened our dramatically. I had to trim about an inch off the GP intakes. Money well spent.

[dirkverdonck]

Hi,
I did the same thing, cut about 3 cm and bolted it on.
Also, it is easier to remove the rear spark plug!

[GraemeSmith]

Also, it is easier to remove the rear spark plug!

That alone has to be worth the price! What a PIA it is to check plugs with the Aeroconversions manifold - having to remove it just to get at the plug

[Bryan Cotton]

Also, it is easier to remove the rear spark plug!

That alone has to be worth the price! What a PIA it is to check plugs with the Aeroconversions manifold - having to remove it just to get at the plug

Odd, I can remove my plugs with the manifolds on.

[Bryan Cotton]

Regarding evening up the mixture, here is some interesting reading. Lutorm's blog:
https://blog.familjenjonsson.org/blog/2 ... ew-intake/

More discussion:

viewtopic.php?f=7&t=3725

[GraemeSmith]

Regarding evening up the mixture, here is some interesting reading. Lutorm's blog:
https://blog.familjenjonsson.org/blog/2 ... ew-intake/
More discussion:
viewtopic.php?f=7&t=3725

Thanks!

[Onex107]

Also, it is easier to remove the rear spark plug!

That alone has to be worth the price! What a PIA it is to check plugs with the Aeroconversions manifold - having to remove it just to get at the plug

Odd, I can remove my plugs with the manifolds on.

Me too.