MI16 head flow / lift curve

[kyepan 291]

Hi there,

I don't suppose anyone would have the pictures to go with this thread from guy crofts site! I'm after the CFM on a standard MI-16 head against lift, and if anyone has some modified ones too! 
guy croft link
i believe it was @petert who did the work but i can't seem to get the search to play ball on here! so thought i might ask kindly! Glad the forum is still running, i'd be mortified if it ceased to exist. 

New engine build coming this year touch wood. 
Cheers

JB

[petert 630]

No problems at all. I’ll post data later.

[petert 630]

Thanks for the link. I recall the converstion now. Unfortunately, Guy measured his at 10" and mine are at 28". It can be converted, as he said, but it's difficult to compare with any accuracy. The other heads were all done at 28" however, same test bench. I have attached the raw data and a chart made from that data.

 

 

[petert 630]

The other variable is, I can't recall of mine which were done on Ø83 or Ø86 bores.

[petert 630]

Here's another interesting chart, comparing the flow of exhaust manifolds. What's important, is never to let you sell snake oil on the exhaust ports. They flow so well out of the box.

[wicked 105 3 Cars]

Nice data! Is that mi16 manifold the 405 or 309gti16 version? Surprising to see that the gti6 manifold is worse...

[SRDT 232]

Mi16 must be the 405 unit, The GTI16 manifold doesn't fit on a RHD car so it's unlikely to find one in the UK or Australia unless it's from a LHD Gr.A 309 GTI16.

The wedge needs to be short so that the GTI6 manifold can fit, to be fair it should be compared with a Mi16 manifold cut and welded to fit in a 205.

[petert 630]

It was the 1.9L XU9J4 exhaust manifold and the XU10J4RS exhaust manifold with the wedge plate shown. I doubt the wedge plate made any difference to flow. Although I didn't try that by itself. The GTi6 pipes probably just flow worse because they're longer. Regardless, they're enough to make 250+ hp. With bigger pipes I'm sure it would do better. Whilst the exhaust flow looks poor, it's actually very good. Attached is also Honda 4Piston vrs stock S2000 head for comparison.

[petert 630]

If I were doing it again, I'd use Ø35.5 and Ø30 valves. The larger Ø36.5 valves limit large camshaft choice, as the valve to valve clearance is insufficient.

[Hedgetrimmer 0]

Hey @PeterT.  I have just bought an Mi16 engined 205 and its having an engine refresh.  Do you still make the extend oil pick up as i am thinking of fitting a GTI6 sump.  I am UK based currently.  Thanjs

[kyepan 291]

On 1/29/2026 at 9:51 PM, petert said:

If I were doing it again, I'd use Ø35.5 and Ø30 valves. The larger Ø36.5 valves limit large camshaft choice, as the valve to valve clearance is insufficient.

Thanks for all of the above, very very interesting, sorry for the slow reply! How restrictive in duration terms are we talking roughly?

This curve with the 36.5 mm valves looks like it flows way better, had the bowls and ports been shaped on your line and had the seats been 3-4-5 angle, valves back cut?
 

Also converting the scale of valve lift to mm....it's not quite flattening off at over 12mm of lift!! been spending quite a bit of time looking at lift curves and seeing more lift does not equal more flow much of the time... as it reaches the point where the valve area is close to the port area (my terminology might be a bit off there). What i mean is you have the ratio of port area (imagine a cross section of the port) which flows x ... vs valve area, which is the circumferance of the valve x lift (so effectively the sides of the imaginary cylinder). Been watching quite a few of david vizard's videos. 
 

Edited March 19 by kyepan

[petert 630]

Big, like 260deg @ 0.050” on the inlet and 250deg @ 0.050” on the exhaust.