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Big Valves


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103 replies to this topic

#41
Ahl

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the mind boggles

Does it really?

#42
Robsbc

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I see Longmans big valves are 35.5mm and Puma's are 36.5mm

#43
Ahl

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I see Longmans big valves are 35.5mm and Puma's are 36.5mm

The mind boggles! B) :lol:

#44
MartinR

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Does it really?

#45
B1ack_Mi16

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Bigger the better B)

#46
Guest_puggti_*

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Bigger the better :lol:

The mind boggles B)

#47
petert

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You guys have lost it. I'm going to post a big valve vrs. standard valve flow chart next week just to end the agony of this thread.

#48
sorrentoaddict

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I would say that the very QUESTION behind its thread IS INDEED impossible, rendering such discussions (relatively) fruitless - here is why:

1) it is not plausible to ask in the first place whether BVH is better than a std. size valve head (or only sliiiightly bigger, like the BTCC MI16), because:

some tuners have spent AGES in developing the flow potential of a STD. VALVE
MI-16 head, which is ENTIRELY different path and results, compared to SOME OTHER tuners, who have spent also many development months into porting & seat & throat shape solutions that would make possible the theoretic advantage of the bigger valve seats - therefore WOULD YOU EVER expect that either one of these tuners would say that the other's concept is better ??? --> NO, simply because there is no way to prove it - when, one sunny day we will see a dyno shoot-out of 2 engines (it will not happen, never, but just imagine it PLEASE), in EXACTLY same spec. as refers to block, intake, Engine Management, and exhaust - only differing in the heads, supplied one by a BV-favoring tuner, and the other by a std-valve-size afficianado.
Now, WHATEVER the winner engine is (if AT ALL possible to say WHAT will be a win, whether a broader & sexier torque characteristic, or some lousy, unusable 1.73 BHP at 8,300 rpm.....), the reason for the better performance will NOT be due to SIMPLY the STD.VALVE SIZE OR BIG VALVE SIZE, but due to the level of success (and suitedness to his own valve size concept) in the design of the ports/seats/throats/port-to-chamber-entrance-geometry, that the very tuner in question managed to develop.

So whichever solution proves to be better, the reason will not be IN THE VERY BV OR SV solution, but of course in the ingeniosity & engineering brilliance of "the author"...

This should be very clear to ANYBODY who conceives engine tuning as something more than just adding-on components & "bolting-on" solutions (it reminds me of that sillly bloke some time ago on this forum, that asked whether he can order online and fit himself the 3-angle seats & big valves :ph34r: B) B) )...

And one more point:

If engine tuning (especially the gas flow part of it) could be based ONLY on logical thinking, I would easily say that on a 83mm bore MI engine, big valve seats would never easily give power, due to the obvious shrouding effects - but only a very shortsighted person would fail to understand that the majority of head design is based on some hard-to-predict parameters, which are not always FULLY justifiable by physics and sound logic.

The argument of Mr. Baker anyway seems absolutely sound and makes sense - that on mildly modified MI engines BVH is the better way to go - but then again it should be only done by someone who has done it more than 3 times - as I am fully convinced that by just fitting BV, without very delicate modifications to suit, it would only drop the flow/power potential of the head (especially at part-throttle and generally low- and mid-rpm-range)..

I believe that now it is clearer why posting questions like "BV MI - to be or not to be ? " are simply a waste of time trying to find the answer - but nevertheless the searching for the answer sometimes makes for the most interesting reading.


cheers


alex

#49
sorrentoaddict

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forgot to add - the "notch" beneath the inlet seat (actually above it when fitted)
is considered to be beneficiary on the MI head (according to some reputable french tuners) only when it is in conjunction with rather long duration cams (more than deg. for example) - why this is so, should be very clear, having in mind the amount of time that the valve head passes away from the seat edge and the consequent decrease of gas entrance speed (if w/o a notch) -
- but I stress that this explanation is what the logic says, while not always it is the true path in head design - more than once it has happened to me that those heads that I believed I had made a mistake while porting - made surprising results.

Anyway, with milder cams it is believed that the throat transition into the seat should be smooth (for eg. rally engines, where part-throttle drivability rules)

For fulll circuit engines, with very long duration cams, the notch most probably gives better cylinder filling - of course only at very high rpm.

who knows...

#50
phatgti

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its a good thing i blended the seat into the port then whilst using piper 270's, the car is very driveable on and off cam.

good post there sorrento, an impartial opinion, of course Mr Baker is going to talk up big valves, its what he deals in.

#51
petert

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I finally had a std. head flowed and I've put all the figures together on one graph. The 29.5mm exhaust valves had a nice 3 angle seat cut. The 34.5mm inlets are standard, with that nasty single 45 deg. cut and a straight drop into the bowl, followed by the ridge at the bottom of the seat. The 36.4mm inlet has a nice 3 angle seat, blended into the bowl with no ridge. Maximum lift of the std. cams is approx. 0.360".

I'll let you make up your mind.

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#52
Dom9

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But what would the standard valve curve with nice 3 angle seats and blended throats look like on that graph?

Must be very similar to the big valve!!

#53
Mattsav

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Peter,

what shape is the big valve used in those tests, i.e tulip shape as std or flat backed.

The graph is very similar to what I've found with other people's big valve heads, less flow to 0.200" then it starts to gain.

If fitted right a big valve head should shows gains off the seat due to the bigger circumfrence of the valve.

I've found gains off the seat by altering the port shape and using the Longmans 35.5mm valves but some 36.5mm flat backed valves I made up flowed very similar to what you have there.

I've also found that if you use the right seat angles you can get almost exactly the same gains as in your graph without touching the inlet port at all.

#54
petert

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But what would the standard valve curve with nice 3 angle seats and blended throats look like on that graph?

But that's the trouble with the standard setup. There's not enough seat material to make a 70 deg. bottom cut . The throat goes vertical from the bottom on the 45 deg. seat.

I can't recall if they were tulip or flat. Most likely flat though.

Edited by petert, 20 October 2004 - 07:28 AM.


#55
sorrentoaddict

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if the above graph is anything to reckon on, than I guess the discussion about
std. valve vs. big valve MI16 should be put to an obvious end.

Even if (theoretically) you cut a nice 3-angle seat on the std. valve size MI-16, the flow would (maybe) be similar to, but still somewhat lower than the big valve flow values.
It means that even with std. cams, the benefits of the big valve are there (even if, with std. cams, 80% of this benefit is from the beautiful seat angle shape).

And with ANYTHING with at least a little noticable more lift than std. cam, the BV head is sure to make an increased power.

2 things that will never be clarified, however, are:

1) whether the loss of gas speed could possibly deteriorate the lower-rpm delivery, throttle response, and overall drivability (a marvellous feature on the std. MI-16, as opposed to many std. fuel-injected 16-valvers).

2) for out-and-out racing engines the compromises you have to make with limited "cam-timing-playfield" are maybe too big in order to justify the obvious gains to be had in high-valve-opening situations.


For mildly-to-very-advanced tuned road engines, Mr. Baker seems to be fully right.
(As I don't see any arguments against a BV head, except the obvious downside to it - that only very few people out there have the experience to do it PROPERLY, i.e. to really work and not decrease power, as it can so often happen with ill-executed BV conversions).


Cheers

#56
sorrentoaddict

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forgot to add this:

actually, from this graph it can be seen that the BV will work proportionally better as valve lift increases - but, in reality, it is more of an issue that the max. lift or close-to-max-lift is maintained longer, so what springs to mind is that really longer DURATION is what you are after with a proper BV conversion.

And here is the catch IMO: if you use too wild a duration in order to make the obviously very big gains from the BV, then there is even LESS possibility to play with the cam timings, as longer duration means even more likelihood that the intake valves (being MUCH bigger) will "have a meeting" with the exhaust valves at some point.

Consequently (I might be fully mistaken anyway), there will be available only a very narrow range of cam timings that will be safe, thereby (most probably) compromising the power delivery at some rpm-range....

maybe this is the logic behind the thinking of that gentleman that builds crf450's engine.

(this is just a theoretical opinion, as I myself have never "touched" a Big Valve MI-16 head before).


cheers

#57
Shevy

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Peter,

what shape is the big valve used in those tests, i.e tulip shape as std or flat backed.

The graph is very similar to what I've found with other people's big valve heads, less flow to 0.200" then it starts to gain.

If fitted right a big valve head should shows gains off the seat due to the bigger circumfrence of the valve.

I've found gains off the seat by altering the port shape and using the Longmans 35.5mm valves but some 36.5mm flat backed valves I made up flowed very similar to what you have there.

I've also found that if you use the right seat angles you can get almost exactly the same gains as in your graph without touching the inlet port at all.

Matt,

Would my B/V head that you fixed, benefit alot from a cam running more lift than the Kent PT1603 I've got fitted, looking at that flow chart it would seem to be the case ?
Also, how easy would it be to test the flow on my head ?

Cheers
Kev

#58
Mattsav

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At the moment i've got 2 problems:

1) i cant find the paperwork from when I plotted the profile of your kent cams

2) My laptop has gone to be fixed, I've stuck the drive into another PC but the flow bench software has flatly refused to open the data files for some reason.

If only I was organised......

Off the top of my head the 1mm duration of the PT1603's is either 260 deg or 230 deg, 260 wouldn't work with hydraulics so it has to be around 230 degrees (simialr to the 273 deg Catcam Inlet.
I wrote the measurements down it but didn't have time to pay much attention to it, I want to find it and have another look as 230deg@1mm seems to be far to little for a 290deg cam (they were definatley 284 @0.1mm as I double checked incase the dial guage had moved.)

As soon as its turns up i'll work out the exact measurements.

I obviously couldn't resist flow checking your head and if at some point PissyWorld decide they can fix the laptop then i'll send you the graph's. Its very similar to what Peter_t has, looses flow up to 0.200" then gains. It was something like a 7% gain under the graph up to the lift of the cams but i'll double check when I can.

The interesting thing with the mi16's is the poor low lift flow.
The Vauxhall Ecotecs (1.4 & 1.6) manage almost the same flow up to 0.200" but with a pair of 31mm valves.

Puma may or may not agree with me (probably not ;) ) but i'd say the tulip shaped valves are a better bet for the mi16.

In fact I keep coming across engines where people manufacture flat backed valves but the ports seem to like the a more tulip shaped valve. The saxo VTS heads repeadedly show an increase in flow if you go from a flat backed valve to a more rounded shape due to the angle of the port, whereas the Ecotec prefers a flat backed valve as the ports are at a different angle.

Or maybe i'm full of crap who knows :D

The next step for me is to get the car back on the rollers now its got a pair of Catcams in it and then try a head with the 35.5mm valves (as I've got a set)

#59
smckeown

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The next step for me is to get the car back on the rollers now its got a pair of Catcams in it and then try a head with the 35.5mm valves (as I've got a set)

yes half the country's mi16 drivers are waititng big time ...so hurry up ;)

#60
PumaRacing

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There's a general rule that horizontal ports with abrupt short-side bends (Mini, MGB, Pinto) like flat backed valves and the more downdraft the port is the more tulip shaped the valve can be. However, you can make flattish backed valves work very well in downdraft ports if you get certain details of the seat and back angle right and you save a lot of valve weight compared to a tulip valve which shows up as reduced valve spring poundage and consequently reduced parasitic power losses. There's actually a "magic number" for the angle of divergence of tapered sections which leads to best flow and this same number works very well as the angle on the back of many valves.

Every valve I design has the port angle taken into account in order to determine the optimum shape for the valve but this shape might not always be the best for flow at a given lift. There are many other factors to be considered. Also the back angle on an inlet valve is always different to that on an exhaust valve.

As to reduced flow at low lifts with a bigger valve - back to the flowbench I think because there's no excuse for this to happen if the valve shape and seat shape are correct.