Any jalops know about intake manifold porting?

"BiTurbo228 - Dr Frankenstein of Spitfires" (biturbo228)
04/13/2014 at 19:15 • Filed to: Spit6

I've had a crack at cleaning what will be the Spitfire's 6 throttle bodies, and I've come across a couple of ridges in the intake, either from casting or machining. I've spent the past half-day taking some sandpaper to them to smooth off the transition. Anyone know whether this will be beneficial? I'm assuming it'll promote better airflow, but it's a bit of a black art. Before/after pics after the jump.

You can see the sharp edges on the unmodified intake on the left, and where I've smoothed them off on the one on the left. The light marks are just low points caused by the casting/machining/welding process.

Oh, and shiny clean (but not yet polished) on the left, old and dirty on the right.

DISCUSSION (10)

desertdog5051 > BiTurbo228 - Dr Frankenstein of Spitfires
04/13/2014 at 20:00

Laminar airflow is the secret on porting. Obstructions in the "shortest path" need to be removed as well as obstructions in the sides and top. You want a smooth path, but not polished so as to increase the flow on the shortest pathway. The floor can be a variable configuration depending on the port shape and combustion chamber shape. Quite a complex science. I would suggest doing some searching on your particular head online. It will make your task simpler. Hope this helps.

desertdog5051 > BiTurbo228 - Dr Frankenstein of Spitfires
04/13/2014 at 20:04

Here is basic tutorial on the concept.

https://en.wikipedia.org/wiki/Laminar_f…

Old-Busted-Hotness > BiTurbo228 - Dr Frankenstein of Spitfires
04/14/2014 at 06:28

Intake porting won't do much good if there's a restriction in the head. I'm not familiar enough with the GT6 head to say for sure, but there's likely a lot of unnecessary meat around the valve guide. When I did the head on my Sprite I took about a cup of shavings out of the intake valve pockets and it made a hell of a difference. If you're only doing the intake, gasket-matching should be enough, along with knocking down the casting slag.

BiTurbo228 - Dr Frankenstein of Spitfires > Old-Busted-Hotness
04/14/2014 at 06:56

Gotcha. I've sent the head off to a very respected head-work guy in the British car world, so hopefully the restriction in the head's minimised.

If the Sprite you have is the later 1500 then the cylinder head is the same as the GT6 head, albeit with two extra ports grafted on and I think the 1500 has a set of siamesed exhaust ports whereas the GT6 doesn't.

Old-Busted-Hotness > BiTurbo228 - Dr Frankenstein of Spitfires
04/14/2014 at 06:59

Nope, mine was a 1275, bored to 1293, center main strapped, Piper cam, etc. Little booger would pull all the way to 8000 rpm. Not bad for a stone-age engine. Regrettably sold it when I got divorced.

BiTurbo228 - Dr Frankenstein of Spitfires > desertdog5051
04/14/2014 at 07:08

I see. However, a good number of things I've read suggest that you don't want true laminar flow through an intake tract where there will be fuel suspended in the air. At this point, a bit of turbulence ensures that the fuel vapour mixes well with air and stays suspended in it until it reaches the cylinder.

It's this that made me wonder whether I should have taken off the ridges, as they would promote turbulence just before the injectors. The scuffing I'm doing along the bore should do that as well, but there isn't any info I can find specific to these manifolds.

BiTurbo228 - Dr Frankenstein of Spitfires > Old-Busted-Hotness
04/14/2014 at 07:17

Shame. Those A-series can be built into utter little monsters. Definitely better than the later 1500 engines (although that's due to the long stroke of the 1500, the earlier Triumph 1300s were little monsters as well).

desertdog5051 > BiTurbo228 - Dr Frankenstein of Spitfires
04/14/2014 at 15:44

Good point. I was referring to more modern engines, I guess. Direct injected and port injected. A fine line between a bit of turbulence and restriction that you have to deal with when using a carb. Good luck.

BiTurbo228 - Dr Frankenstein of Spitfires > desertdog5051
04/14/2014 at 16:57

Mine will be on Lucas mechanical injection rather than carbs :) provided it isn't an utter pain in the arse of course...

desertdog5051 > BiTurbo228 - Dr Frankenstein of Spitfires
04/14/2014 at 18:11

Just the word "Lucas" sends chills down my spine. :)