Port Size VS Valve Size

[leeuk]

Hello Guy

I am continuing development on a Ford CVH head because now im going to be fitting a 2.1 block to the CVH head

What i need to know is there some sort of rule of thumb that what port size would be the maximum to a 43mm inlet valve ?

The reason being is that other people have fitted the 2.1 block before and have had problems with peak power being to low around 5k

the reason im thinking is 1600cc ports not flowing enough air it does not matter if i lose bottom end power my aim is maximum top end power

Regards
Lee

[Guy Croft]

On your 2.1 liter use a port of smallest diameter 34mm minimum and 35-36mm preferred.

Big port = more top end power. Smaller port = better mid range torque, for sure.

GC

[superbike]

Hi all

A very interesting subject port vs valve size.

A 43mm valve with a 34mm port gives a ratio of around 80 percent. That same 43mm valve with a 35.2mm port gives a ratio of 82 percent.

I have rencently been testing these ratios on my ducati work. The best ratios for the tested ducatis i have done are 77-78 percent with a cam lifting not over 11.5mm. One ducati in particular has 40mm inlet valves with a 33mm port giving a ratio of 82 percent wich i have proved to be way to big unless your lifting the valve to over 13mm (road cam only goes to 11.5).

I have some dyno charts here wich i could post of 2 ducatis of exactly the same spec except one has 33mm ports and one has 35mm ports. The 35mm ported one gives less power everyware to the tune of 15 HP .

This subject keeps me up all night but i enjoy it very much

[Guy Croft]

Remember of course that at valve full lift - near coincident with highest piston velocity, the controlling section that governs filling speed is the port and it's optimum size is very dependent on cylinder size (and naturally stroke, rod length etc). True to say that the bigger the engine the bigger the ports need to be and if you make them too big on a small engine (relatively speaking) you're going to get a big 'hole' in the torque curve - ie: an undriveable engine that you have to 'rev to death' to get it to fly..

GC

[superbike]

Remember of course that at valve full lift - near coincident with highest piston velocity, the controlling section that governs filling speed is the port and it's optimum size is very dependent on cylinder size (and naturally stroke, rod length etc). True to say that the bigger the engine the bigger the ports need to be and if you make them too big on a small engine (relatively speaking) you're going to get a big 'hole' in the torque curve - ie: an undriveable engine that you have to 'rev to death' to get it to fly..

GC

Guy im wondering if there is some sort of equation on how we can size our ports based on cylinder size/rod length/rpm ect ?

I know the mach index has been spoken about .

chris

[Guy Croft]

Chris, hi

No, there isn't really. I have been working on this for some time but due to lack of velocity data from real engines I don't have sufficient confidence in the results yet to publish, sorry.

GC

[vcg]

Hi,

As my rule, I try to convert my heads and fit inlet valves that are 55% of the piston diameter. I do inlet ports 80% of inlet valve. I fit exhaust valves that are 80% of inlet valve, and do ex ports that are 95% of ex valves.

This is just a general rule that I apply and I am only a hobby race engine builder. Professionals have have many secrets per type of engine that take years to develop. Most secrets I have personaly found out after examining race heads that worked impressively have to do with throat-seat radii, and port straightening techniques. Of terribly great importance is whether the head is straight port or curve port.

Straight port heads with straight inlet manifolds and sidedraught cards will always make better race engines. Curved port heads with downdraught manifols and carbs can make good rally engines with great torque spread and wide band, coming from good atomization and preatty steadier rate of vacuum filling due to the constrains I thing.

In the late sixties Alfa GTA dominated circuits using radied throat seats, huge valves, small ports, small lifts, huge durations.
Fiat dominated late seventies rally world with an engine tuned to make 240bhp where it could make 300bhp. Small duration, huge lift for the 16V accounted for a fantastic torque curve, instead of great bhp, the reason for this choice was the car's very big weight ofcourse.

Take care,

[Guy Croft]

As my rule, I try to convert my heads and fit inlet valves that are 55% of the piston diameter.

This rule won't work, 85mm bore TC would give valve 46.75 which is way too big. I dropped back to 45mm as per works engines because it gave same results as 46mm on dev tests of my NHRA engine. On the SOHC 55% of 87mm bore would give 47.85mm...


I do inlet ports 80% of inlet valve. I fit exhaust valves that are 80% of inlet valve, and do ex ports that are 95% of ex valves.
The inlet port idea doesn't seem a million miles out..

Of terribly great importance is whether the head is straight port or curve port. Straight port heads with straight inlet manifolds and sidedraught cards will always make better race engines. Curved port heads with downdraught manifols and carbs can make good rally engines with great torque spread and wide band, coming from good atomization and pretty steadier rate of vacuum filling due to the constrains I think.

Steady! The golden rule is bends always cause losses. Loss of inlet massflow, reduced torque.

In the late sixties Alfa GTA dominated circuits using radiused throat seats, huge valves, small ports, small lifts, huge durations. Fiat dominated late seventies rally world with an engine tuned to make 240bhp where it could make 300bhp. Small duration, huge lift for the 16V accounted for a fantastic torque curve, instead of great bhp, the reason for this choice was the car's very big weight ofcourse.

I dunno about the Alfas and actually I don't have the power figures for many 131 16v either. I doubt they capped the power at 240bhp. That said they used a lot of different setups, I mean a lot. As for the 16v Fiat cams, check out the curves. I have examined more 16v works profiles than most incl 073, 225, 215 in addition to those shown, they were not stunning in terms of lift or duration (or anything else) compared with today's top profiles - or even my 8v profiles come to that. Were their torque curves fantastic? Well, they were obviously good enough but the reality is that with a head (131 16v) that flows 133cfm @ 10" 'out of the box and capable of around 150cfm (or more), any competent engine builder could pick a works profile and get 225+ bhp from even a standard 131 16v head on one of those engines. Did they win because of their torque curves? A lot of their success came from the budgets they had for planning, travel, prep and support and the calibre of the drivers who worked with the works teams. Look at Vauxhall and DTV - by comparison their budget was absolutely 'shoestring'.

Now. If you don't tune for high bhp you are going to get poor engine breathing where it really matters and a lousy working powerband, because although keeping the port size down and restricting the cam type can give lovely driveaway torque and maybe reasonable peak torque - the top end torque - that governs the peak power (power is a function of torque and speed) - will be hopeless and limit not only the power severely but curtail the powerband. You need a minimum of 2000 rpm from peak torque to peak power fro stage or tarmac and more is better. In those days, pre sequential gearbox they needed a wide powerband and I don't mean below 5000. Nobody rallies below 5000 rpm unless a) they can't drive, or b) they are pulling out of the parc-ferme.
Whether you get it - well that depends utterly on the extent and effectiveness of head prep and the cam choice follows that.

GC

[amichie]

What ratio of intake port area to valve area would you recommend for a 4 Valve head?

Considering that my application is a Toyota 7m-ge 3 litre 24 valve in-line six in a heavy road car that needs strong performance in the low to mid range.

[Guy Croft]

Andrew, hi

I don't know exactly in the case of 16v units.

They all have a 'controlling section' that governs the air fed to the valve region and that is the two barrels formed by the splitter (which is there of course to permit fitment of 2 inlet valve seat inserts..). Just taking an example that I know from memory, the XE Vauxhall 2 liter 16v, it would typically run 34mm inlet valve and I'd go for a mean dia in the port section thru the splitter region of 37mm. That gives a port to valve area ratio in each barrel of 63% and you cannot go bigger because you'll break thru. The combined splitter area is 1145 sq mm and the pport outer face is about 48 x 30mm or slightly bigger giving an area of 1248mm. The fact that the port section outboard of the splitter is about the same area as the foregoing shows what an effective port regime it is. You don't need to make the entry section massive to get it to flow..

It's confusing, because to an extent, the amount of 'enlargement' in the port can depend on the position of the inlet valves relative to the port entry face. You frequently encounter quite pronounced divergence in the port because of the valve spacings (mainly a function of the bore size/engine length), and it's necessary to generate optimum (as fas as is possible) curvatures on those regions. It ends up looking like deliberate 'enlargement' which it isn't really, except by default..

Now, that is a well downdrafted head - ie: ignoring the divergent region in the port, it is (or at least can be made to be) of relatively linear section from outer face down to the short side rad and the included angle betw port axis and valve is quite acute. Thus the viscous losses in the port as a whole are reasonably limited compared with a head that has sidedrafted ports - or at least - ports with less downdraft. It is quite a plausible argument in the case of the latter to make the port much bigger than an XE style one in order to reduce the relative effect of those losses on the flow. I can think of numerous examples of this, Honda Vtec, Suzuki Swift, many Peugeot 16v units.

The Honda is typical of the latest 'big port' designs, a powerful unit, at 1600 or 2 liter, but it does really need the variable valve timing to develop a wide torque curve. I have not done much with these yet although I will have 4 x rally ones ot prep quite soon. I have to admit I have no real idea what the design philosophy is behind such huge ports. It cannot be for high air velocity based on pressure differential betw cylinder and atmosphere, of that I am certian, because the vel would be very low compared with something like an XE. Yes they flow ridiculous amounts of air on the flowbench, but that is not always the same thing as great bhp. It may be something to do with pressure waves but, as I say, I don't know enough about it to remark qualitatively.

So you can begin to see that to offer a 'rule of thumb' for the size of the port relative to the valves is almost impossible, which is where the flowbench comes in.

GC

edited 16th Oct by GC due to typo

[amichie]

I have made some measurements on the intake port at the head surface and the intake runners both in the manifold and in the plenum of my Toyota 7m-ge engine. As far as i know Toyota commisioned engineers at Yamaha to design this 4 valve head.

Anyway the inlet ports are oval shaped and are approx 1401 sq mm in area at the head to manifold mounting surface.

The manifold has runners that are about 100mm long and they merge smoothly from the oval shape at the head to a circular shape at the plenum end.

At the plenum end of the manifold the circular ports are approx 44mm in diameter (1520 sq mm).

The plenum has circular shaped extensions of the runners that are about 150 mm long that lead up to the plenum chamber.

Given that the engine is a 3 litre six, using two 32mm intake valves and is an undersquare design (Stroke 91mm bore 84mm) these ports and runners seem to be way too large.

I am considering filling the ports or making venturis/chokes that fit inside the manifold section.

The only other observation regarding the port design is that the divider between the inlet valves has a very blunt edge relative to the images you provided above.