Downdraught angle vs seat angles?

[4v6]

More a question of downdraught angle vs valve seat architecture really and how it affects the required/optimal valve and valve seat architectures.
Its something that has had me wondering about for a while now and I dont seem to be able to find any specific references to it.
For example is there a specific or general rule regarding the lower angles you'd use depending on the port angles?
Say an inlet port with a 30 degree downdraught angle and how would that dictate what bottom cut, seat angle and top cut angles for optimum flow, if at all?
Any answers to these questions would be much appreciated as always.

[Guy Croft]

Good question.

I think there is a general rule for this. We should assume, however, that more downdraft is accompanied by more inclination on the valve angle, meaning the downdraft angle is accompanied by quite an acute angle betw the valve stem and the port axis. Without that there is little virtue in having downdraft at all. The reason for having, that said, is not so much to get a bit of help from gravity as to maximise the flow all around the valve. You can visualise what I am referring to in the sketch below. In the case of pipe 'A' one would expect the flow to be evenly distributed all around the valve periphery and that is true. When the relative orientation is altered as in 'B' it's a very different matter. In an actual port there are going to be curvatures and those in the proximity of the valve head are going to determine entirely the distribution of air as it heads for the valve.

More downdraft gives better distribution and the converse is also true. In fact some heads with no downdraft at all exhibit such an extreme bias to one region of the valve (invariably adjacent the short radius) that one wonders if the valve even needs to be circular at all.

The more the design of the inlet valve tends towards 'penny on a stick' the worse the flow distribution is when the air hits it. However that is not to say that kind of design doesn't work, the opposite can be the case. The more the downdraft the more the design of the non-firing side of the valve design is critical and valves for that type of head should have quite a pronounced head angle. The angle I'm referring to is the one shown on the cad dwg. A head with good downdraft like the Sierra Cosworth needs 30 deg of head angle, whereas a semi-sidedrafted one like the Fiat 8v TC works best with a head angle of 15-18 deg. In test I have found that a few degrees either way has little practical effect on the torque curve even if the flowbench says one is better than the other, but there is a massive difference betw 28 deg and 30 and if you put a 30 deg head in a Fiat TC and it will be a disaster, because the intrusion of all that metal just messes up the already poor flow distribution. And I have been confronted by designs like that. If you find a downdrafted head like the Ford ST170 (Zetec type) shown below with a valve with shallow head angle they got the design utterly wrong and they are not the only ones, I am sure Honda got the design of the inlet valves wrong on the sidedrafted VTEC 16v to name but two.

A fully sidedrafted head like the Peugeot 205 design will not respond well on test to any head angle at all, in other words they are a design that really does call for 'penny on a stick'. To discover that I did very extensive flow testing with every possible angle to determine the trend. 15 deg head angle was worse than a near-flat head and 18 deg was even worse.

MORE TO FOLLOW ON THIS AND SEAT ANGLES

G

[4v6]

Thank you for that Guy, I look forward to the next part!

[4v6]

Adding on to this.

I've been doing some flowtests this week on an old 1982 Audi 100 cylinder head in readiness for some similar modifications to a 1983 head which at first sight is identical.
It does however have some differences in that its very poorly developed in terms of a short side radius, basically it hasn't got one.
The port floor is flat and simply stops at the insert leaving a lip at the bottom of the insert for the air to find its own way around. Luckily the 83 head is more filled in at the port flor and has enough material in place to produce a nice radius, it also flows better as standard.

My efforts however have not been focussed on that, I've been more interested in the intake seats and angles with a view to understanding how much they might be worth.
These heads have as stock items, an inlet valve of 38mm diameter with a 45 degree seat.
The matching seat in the head is 1.7mm wide which continues out into the chamber until it meets the surface at about 1.5 to 2mm high.
It looks like its a result of a 45 degree cutter going into the surface of the chamber while they cut the seat angle.
The bottom cut of the seat is 75 degrees and about 5mm in length before the insert runs into a 90 degree angle.

Stock seat.

P1040367 (1000 x 750).jpg (164.49 KiB) Viewed 8602 times

I did a bare port flow test to start as usual and then a full lift test to 11mm as seen in the first graph. Bare port flow was 141cfm @28" depression.

Test 1)

Test one in blue is the stock valve seat.

Test 1 (557 x 608).jpg (115.52 KiB) Viewed 8602 times

Test 2 involved altering the seat angle to 30 degrees in the head and valve which I expected to give a gain, it did, although initially it appeared to be a disaster at low lift where it should have been stronger.
That was caused simply by the fact that the cutter created a sharp edge in the alloy which choked off flow until it was removed.
The graph demonstrates it clearly with the flow dipping under the stock flow figures until it recovers as the valve clears the obstruction.
Test 2)

Low lift flow dips under, shrouded by the lip created by the cutter.

Test 2 (557 x 608).jpg (116.31 KiB) Viewed 8602 times

Lip from cutter visible more easily on right side of seat. A slight 60 bottom cut is just forming due to the formtool.

P1040372 (1000 x 750).jpg (128.56 KiB) Viewed 8602 times

Test 3 (yellow trace) was a simple exercise in removing that newly generated lip and blending out into the chamber.
I applied a 15 degree top cut with the neway to remove it and fettled the rest in by hand.

Lip removed. Neway cutter with a 15 degree angle did most of it then blended by hand.

P1040377 (1000 x 750).jpg (109.35 KiB) Viewed 8602 times

Quite a good improvement I felt for such a small amount of work.
However its apparent that the high lift flow has started to suffer with it dropping below stock levels.
Test 3)

Result of that lip removal.

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Test 4 (turquoise trace) involved the addition of a very small 45 degree angle to the bottom of the 30 degree seat where it meets the 75 lower angle. Its no more than 1mm wide but it yielded the results in the test 4 graph.
Seat width is at 1.5mm at this point (60 thou).

45 angle barely visible at around 1mm width and just creates a transition between seat and bottom cut.

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Test 4)

Test 4 (557 x 608).jpg (117.39 KiB) Viewed 8602 times

Test 5 (purple trace) was an exercise in putting a 20 degree back cut from the valve seat on the valve back to the stem and blending it in.
Another good gain but in my suspicious mind I wondered if maybe I had slipped up so I re-cut another valve with just the 30 degree seat and re-tested it.
The results were as previously found, identical in fact so the 20 degree backcut was working as indicated.
Test 5.

Addition of a 20 degree backangle on the valve, seat width still at 60 thou or 1.5mm.

Test 5 (557 x 608).jpg (119.43 KiB) Viewed 8602 times

Test 8 (red trace) is the result of just the barest radius applied to that sharp lip on the insert at the short side.
Test8.

Red line is the result of a minor short side radius applied.

Test 8 (557 x 608).jpg (121.1 KiB) Viewed 8602 times

I was just wondering if there is any reason why this seat angle combination couldn't be used?

For some reason I have a nagging doubt about either its longevity or maybe it would be deficient in some other manner?
Other than that it appears to be a vast improvement over stock.
I have another 3 seats I can alter to test different setups if anyone has a combination to suggest.
Comments welcome.

[timinator]

Hi Tony, Using your first picture as a reference it appears that you could widen where the ssr joins the bowl or valve pocket. Use the profile of the ssr to grind where the port wall meets the bowl wider to increase the area. Then from the new radius you can blend up to the bottom of the 90deg. cut and out to the bowl wall. I call this widening the shoulders of the bowl. This could slow down the flow on the short side to allow the air to turn better and bring more flow to that side of the valve. With your flat floor it will force most of the flow to the spark plug side of the valve. Try a small radius on the flat side of the exhaust valve to pull the flow around the margin of the valve to help bring it under the valve.

The big cu. in. engines of the 1960's used 30 deg. seats to build flow early to boost VE with their small valve lift. They had little valve overlap though. If you plan to have much overlap you will be increasing the amount of reversion by increasing the flow at low lift.

I would suggest trying a 35 deg. top cut enough to remove the ridge where the chamber meets the valve seat. A 45 deg. seat 1.5 mm. wide with a 55 deg. cut 2 mm. wide. Leave the 75 deg. and begin the 90 deg. cut where the dia. is about 80-85 % of the valve dia. Make the 90 deg. cut at least 2.5 mm. deep. Do not worry about what it looks like where you were working on the ssr. It is more important to turn the flow on that side than the progression of the 55,75,and 90 deg. cuts.

Clear as mud now as to what to do - right?

Tim

[4v6]

Hi Tim, thanks for that, makes perfect sense to me.

Throat is already at 86% of valve diameter, so in the ballpark there.

Your reversion heads up is a good one, so with that in mind Ill start flowing the port backwards, ie, using the inlet to flow air out of to test out a couple of ideas with the valve face and see if reversion can be reduced.
What I plan to do here is to put an angle on the front of the valve at the periphery (just slight) to see if it diverts airflow into the chamber wall rather than let it roll around a nice friendly radius, so if I do a series of lift tests in reverse flow direction I should see a drop in flow if I get it right.
Could also ensure the margin at the face is left sharp to discourage reverse flow.
Valve lift on these engines is 10mm although with a racier cam they can go as high as 11mm.

The short side on this particular head is very very poor, but with the smallest of a radius applied to it, it improves a good amount.
I understand what you mean when you speak about widening the shoulders of the ssr as thats something I've done in the past and it does improve flow on these a little.
These heads already have a very strong bias to flow around the ssr even at full lift and I'm not sure i can improve on that situation.
Will report back when I've done more work on this.
Thanks again.

[Guy Croft]

Some good work here. But I will assert straightways that multi-angles will not work. If you try to prove me wrong good luck!

I am too busy working to write much at the moment - sorry.

1) I need a photo of head showing port layout.
2) The optimum valve seat angle will be 20 or 30 top cut, 45 seat and then 70 or 75 throat into parallel (90 deg) lower seat region.
3) The insert top cut should invariably match the head angle on the valve behind the contact face.
4) Yes a rad on the central region of the seat insert on the SSR side often works. Depends on the distance from port floor on SSR side to valve contact face. Do it on heads with a 'deep drop' SSR like an 8V Peugeot and it is a disaster.
5) Multi-angles give far too much flow separation on small valves like this. To be honest I seriously question if they ever work. Air nor more likes changes of angle than curves (unless the latter is aerodynamically perfect - almost impossible to achieve in a small thing like an engine port)
6) A fully radiused throat only works on steeply downdrafted heads which have a very short seat insert & distance from contact face to port floor.

G

[4v6]

All taken very much on board Guy, I'll see what happens if I just use the angles suggested on one of the other ports.
Its all a bit hit and miss for me as this is the first time I've delved into seat angles in any depth.
Ill take some photos of the layout also for you.
Thanks again for your input, I appreciate it indeed.

[timinator]

One of those things I read a long time ago was that air flows smoothly up to about 9 deg. of change of direction. Don't ask where I read this it has been a long time. I included the 55 deg. cut to see if some pressure increase on the spark plug side of the valve might help build flow on the ssr side. If GC has tried this and it does not work with small valves then enough said. Same goes for the 35 deg. cut. It would, I hope, try to keep the flow straighter coming into the chamber and keep the pressure higher than inviting the flow to turn out towards the chamber wall. The advantage of two valve heads as opposed to four valve heads is induced swirl. Forming the structure in the intake track to accent this swirl flow may be of more advantage than other changes. How to do this is why one needs a flow bench and lots of time.

I can't give you the exact quote. One of the people that I think has a good grasp of port flow said something to the effect of:
the port will have areas of good flow and bad flow, fix the bad flow.

For the most part I like to see stock heads that have run on the road for a long time to see where the gas has kept them clean. Also where reversion has made them dirty. These things can indicate where the bad flow is.

[4v6]

As requested Guy, some photos of the ports and seat areas in stock condition.

Port downdraught angle at port floor is 20 degrees, at roof 25 degrees.

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P1040473 (1000 x 750).jpg (94.63 KiB) Viewed 8544 times

I hope these will suffice, if not please let me know and I'll get some more for you.

[Guy Croft]

Tony can I have a shot from this angle?

Trying to get a handle on what the port outer shape, splitter and downdraft really looks like.

G

[4v6]

You certainly can Guy, also its a two valve head, sorry for not making that clear.
Hope these are a little more useful.

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P1040487 (1000 x 750).jpg (92.87 KiB) Viewed 8481 times

[Guy Croft]

It's not really a downdrafted port is it Tony?

Looks like more like a long curve with downward trend. And there is a fairly big curvature on the short radius.

For optimum results in the critical 3mm lift + range I would be for using sharp 70 deg angle below the contact face. The SS may be near-correct, I would smooth it and test. The most it might want I would say is lowering fractionally in the central 15mm wide bit and shaping into a curve of slightly bigger radius but I certainly would not 'hog it out' right down to the seat insert.

What is the head BTW?

G

[4v6]

Yes you're right Guy, its not got any particular steepness to it, it just tends to look like it.

I'll source a 70 degree cutter for that lower angle shortly, as at the moment I only have a 60 and 75 I can use.
You're absolutely right when you refer to hogging the ssr region out, Ive tested that in the past and its not helpful at all.
Smoothing it as you suggest seems to be the best way I've found so far with a little widening at the shoulder areas where the ssr meets the port walls gains a little flow also.

The head is a 1983 Audi 100/200 2.2 litre item, quite poorly cast it appears.
I've not done any further work as of yet on valve angles as I've been collecting rebuild parts for the engine itself.
This head is the spare item I was given to further refine my tests on but its quite poor compared to even others of the same year that I've worked on previously.

I shall report my further findings on the valve angles suggested as soon as I've tested them.

Thanks again for your valuable input.

Best wishes, Tony.

[timinator]

Hi Tony, how goes the testing? Good I hope. Not sure about how much effort you want to put into changing your intake port but would you consider filling the floor with two part epoxy? Just line of sight for your intake looks like filling the floor up about 1/4 inch(6-7mm) might help your ssr. Keep the same corner radius and fill down to the beginning of the ssr. Was looking at your pic P1040484 and it looked similar to a port I saw modified in this manner. Lot of work or fun which ever way you see it.

Tim