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Let's talk port velocity --

Moderator: David Vizard

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ptuomov
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Re: Let's talk port velocity --

Post by ptuomov » Fri Nov 22, 2019 9:54 am

Interesting.

If you go to 1980's and 1990's four valve heads from Germany or Japan, I think you're going to find very low port velocities with this formula.

Take for example 1987 Porsche 928 S4. The port minimum cross-sectional area is about 2.7 square inches. The engine displacement is about 302 sqin with eight cylinders, so 37.75 CID per cylinder. The peak power from factory is at 6000 rpm, making 316 hp. Unless I made a mistake, the formula says 6000*(302/8)/(2.7*360) = 233 ft/sec.

The head itself flows decently considering the port minimum area. It flows 290 CFM with 2.7 sqin MCSA at 28" of water. It's just very big and very slow compared to these numbers listed on the sheet.

Cams are also very short duration, in case that's relevant.

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Re: Let's talk port velocity --

Post by David Vizard » Sat Nov 23, 2019 2:13 am

ptuomov wrote:
Fri Nov 22, 2019 9:54 am
Interesting.

If you go to 1980's and 1990's four valve heads from Germany or Japan, I think you're going to find very low port velocities with this formula.

Take for example 1987 Porsche 928 S4. The port minimum cross-sectional area is about 2.7 square inches. The engine displacement is about 302 sqin with eight cylinders, so 37.75 CID per cylinder. The peak power from factory is at 6000 rpm, making 316 hp. Unless I made a mistake, the formula says 6000*(302/8)/(2.7*360) = 233 ft/sec.

The head itself flows decently considering the port minimum area. It flows 290 CFM with 2.7 sqin MCSA at 28" of water. It's just very big and very slow compared to these numbers listed on the sheet.

Cams are also very short duration, in case that's relevant.
I have no experience with the 4 valve Porsche heads but that trend started way back with the 2 valve hemi style Porsche heads.
At first glance they look like a well designed classic hemi head and one might ask what can be wrong with that. the answer as you may guess is - 'plenty'!

Try this for a quick list cons:-

*Valve angle relative to the cylinder axis is too wide.
*Ports are too big which leads to low velocity and consequently port energy.
*Ports are poorly designed in terms of shape.
*There is no swirl which may be why the heads have two spark plugs.
*The combustion chamber is so far off what is required that they can almost be termed as a disaster volume.

My thoughts here are that with enough welding rod these heads can be considerably improved.

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Re: Let's talk port velocity --

Post by CFM-Z440 » Sat Nov 23, 2019 1:58 pm

David Vizard wrote:
> [quote=CFM-Z440 post_id=835505 time=1573855379 user_id=15298]
> Is there anything that we can use to estimate ideal mean air speed for
> different head types? I'm assuming the 420ft/sec F1 port is very straight,
> with little turn into the bowl, and has perfectly even flow all the way
> around the valve.
>
> I work with 4 valve single's in motorcycles and atv's with peak hp in the
> 9000-10,000 rpm range. The ports are not inclined like I assume the NA F1
> ports to be, but they are pretty straight with most of the turn at the
> bowl, and they have very little SSR. They seem happy right around
> 285-300'/sec mean velocity, between the guides and divider, and I even up
> the local velocity as much as I can, but they are faster on the floor. I'm
> a long ways from 420ft/sec, but I feel like the design isn't worlds apart.
> Could I get it to perform with higher mean velocity if I could get my local
> velocities more even?
> [/quote]
>
> What will eventually stop you achieving the 420 ft/sec F1 mark will be the
> lack of down draft angle and the tight SS turn.But 285 to 300 is a little
> on the low side for a 4 valve head.
> As soon as I can I will post ft/sec figs for a variety of head types so
> you can better judge where you stand on this.
> DV


Thank you for that David. By your formula, I find 380.5'/sec at 9500 rpm, on the 4 valve Suzuki head that I was using for an example. It's a single cylinder, 29.862 CI with a 2.071 sq in MCSA making 70hp at 9500 rpm on methanol.

That sounds much better than 285'/sec. I use pipemax for my guide on cross sectional area. The velocity ratings given in pipemax, are on a flow bench at 28". I was failing to realize that we were taking about different things. It makes more sense now.

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Re: Let's talk port velocity --

Post by ptuomov » Sun Nov 24, 2019 9:06 am

David Vizard wrote:
> [quote=ptuomov post_id=836509 time=1574434448 user_id=9433]
> Interesting.
>
> If you go to 1980's and 1990's four valve heads from Germany or Japan, I
> think you're going to find very low port velocities with this formula.
>
> Take for example 1987 Porsche 928 S4. The port minimum cross-sectional
> area is about 2.7 square inches. The engine displacement is about 302 sqin
> with eight cylinders, so 37.75 CID per cylinder. The peak power from
> factory is at 6000 rpm, making 316 hp. Unless I made a mistake, the
> formula says 6000*(302/8)/(2.7*360) = 233 ft/sec.
>
> The head itself flows decently considering the port minimum area. It flows
> 290 CFM with 2.7 sqin MCSA at 28" of water. It's just very big and
> very slow compared to these numbers listed on the sheet.
>
> Cams are also very short duration, in case that's relevant.
> [/quote]
>
> I have no experience with the 4 valve Porsche heads but that trend
> started way back with the 2 valve hemi style Porsche heads.
> At first glance they look like a well designed classic hemi head and one
> might ask what can be wrong with that. the answer as you may guess is -
> 'plenty'!
>
> Try this for a quick list cons:-
>
> *Valve angle relative to the cylinder axis is too wide.
> *Ports are too big which leads to low velocity and consequently port
> energy.
> *Ports are poorly designed in terms of shape.
> *There is no swirl which may be why the heads have two spark plugs.
> *The combustion chamber is so far off what is required that they can almost
> be termed as a disaster volume.
>
>
> DV

I have no experience with air cooled 2-valve ancestor-911 semihemi heads, other than observing them making a lot of power turbocharged.

Most Porsche four-valve heads have 27.4 degree intake-exhaust included valve angle and intake ports significantly larger than what your slides indicate as normal for 4V heads.

I’m not arguing what velocity or port side makes the most power normally aspirated. I am simply arguing that the slide is inaccurate as a description of the 80’s and 90’s production 4V heads. They have much larger ports than the slide indicates.

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Re: Let's talk port velocity --

Post by CFM-Z440 » Sun Nov 24, 2019 6:32 pm

[quote=ptuomov post_id=836849 time=1574604395 user_id=9433]

I have no experience with air cooled 2-valve ancestor-911 semihemi heads, other than observing them making a lot of power turbocharged.

Most Porsche four-valve heads have 27.4 degree intake-exhaust included valve angle and intake ports significantly larger than what your slides indicate as normal for 4V heads.

I’m not arguing what velocity or port side makes the most power normally aspirated. I am simply arguing that the slide is inaccurate as a description of the 80’s and 90’s production 4V heads. They have much larger ports than the slide indicates.
[/quote]

I could be wrong, but I don't think the technology had caught up to David's formula, back when those were produced. I work with motorcycles and atv's and I don't think that most of the motorcycle manufacturers caught on to the importance of air velocity until somewhere around the late 90's. Or at least it seems to be after that by the time the production parts were taking advantage of it.

Some of the sport ATV heads I work on, produced as late as 2005, were still way too big. It's common practice with those, to fill in parts of the intake runner with epoxy, to get the air velocity up. I've seen about a 10% gain in torque through most of the usable rpm range, and even a couple extra horsepower on a single cylinder 450cc ATV with the only difference between the 2 engines being the epoxy in the intake port. The airflow of both heads were the same.

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Re: Let's talk port velocity --

Post by ptuomov » Mon Nov 25, 2019 8:25 am

I wasn’t arguing about what makes most power. I was just making the point that most 4-valve production heads now in cheap used cars that are being hot rodded are much larger than the slide indicates. Also, they gave silly short camshaft duration.

As a side note, Just observing from the sidelines, many of these 80’s and 90’s 4V heads seem to work better with about 25% more displacement, 10% more rpms, and longer duration camshafts...

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Re: Let's talk port velocity --

Post by digger » Thu Nov 28, 2019 12:20 am

ptuomov wrote:
> David Vizard wrote:
> > [quote=ptuomov post_id=836509 time=1574434448 user_id=9433]
> > Interesting.
> >
> > If you go to 1980's and 1990's four valve heads from Germany or Japan, I
> > think you're going to find very low port velocities with this formula.
> >
> > Take for example 1987 Porsche 928 S4. The port minimum cross-sectional
> > area is about 2.7 square inches. The engine displacement is about 302 sqin
> > with eight cylinders, so 37.75 CID per cylinder. The peak power from
> > factory is at 6000 rpm, making 316 hp. Unless I made a mistake, the
> > formula says 6000*(302/8)/(2.7*360) = 233 ft/sec.
> >
> > The head itself flows decently considering the port minimum area. It flows
> > 290 CFM with 2.7 sqin MCSA at 28" of water. It's just very big and
> > very slow compared to these numbers listed on the sheet.
> >
> > Cams are also very short duration, in case that's relevant.
> > [/quote]
> >
> > I have no experience with the 4 valve Porsche heads but that trend
> > started way back with the 2 valve hemi style Porsche heads.
> > At first glance they look like a well designed classic hemi head and one
> > might ask what can be wrong with that. the answer as you may guess is -
> > 'plenty'!
> >
> > Try this for a quick list cons:-
> >
> > *Valve angle relative to the cylinder axis is too wide.
> > *Ports are too big which leads to low velocity and consequently port
> > energy.
> > *Ports are poorly designed in terms of shape.
> > *There is no swirl which may be why the heads have two spark plugs.
> > *The combustion chamber is so far off what is required that they can almost
> > be termed as a disaster volume.
> >
> >
> > DV
>
> I have no experience with air cooled 2-valve ancestor-911 semihemi heads, other than
> observing them making a lot of power turbocharged.
>
> Most Porsche four-valve heads have 27.4 degree intake-exhaust included valve angle
> and intake ports significantly larger than what your slides indicate as normal for 4V
> heads.
>
> I’m not arguing what velocity or port side makes the most power normally aspirated.
> I am simply arguing that the slide is inaccurate as a description of the 80’s and
> 90’s production 4V heads. They have much larger ports than the slide indicates.

isn't the slide more along the lines of what they should be for optimum ? maybe that's for David to comment.

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Re: Let's talk port velocity --

Post by RevTheory » Fri Nov 29, 2019 10:07 am

Are you measuring port velocity at the valve lift that will be seen on the engine or retainer-to-guide contact on the flow bench?

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Re: Let's talk port velocity --

Post by David Vizard » Mon Dec 02, 2019 10:18 am

RevTheory wrote:
Fri Nov 29, 2019 10:07 am
Are you measuring port velocity at the valve lift that will be seen on the engine or retainer-to-guide contact on the flow bench?
Rev,
Don't you already have the IOP porting program? If so you will see it gives the mean port velocity at each lift increment flow is tested at. This gives a means to figure out a valve lift that is compatible to the mean port area.

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Re: Let's talk port velocity --

Post by David Vizard » Mon Dec 02, 2019 10:49 am

Digger,

Ref your post:-isn't the slide more along the lines of what they should be for optimum ? maybe that's for David to comment.

The numbers quoted are more along the lines of what is typically shown by each type of cylinder head configuration. If any one of the lower groups has a very well conceived port configuration peak power can happen at much higher mean port speeds.
An example here is the 61 CID Ford Anglia engine I did back in 1972. Although a 2 valve pushrod design this road race unit produced, on 96 R+M/2 fuel (CR 12/1)139 hp at 9,800 rpm and 136 hp at 11,000. If it was necessary to hang on to a gear between two or three corners in a complex then it could turn a useful 12,000 rpm.

Port velocity on the down drafted intake port at 9,800 rpm was 345 ft/sec.

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Re: Let's talk port velocity --

Post by David Vizard » Mon Dec 02, 2019 3:47 pm

Back to the comments asking if I had done ABA tests to prove the numbers and concept of the mean port velocities.
I did a quick review of Ricardo but did not get around to Charles Fayette Taylor.
Rather than repeat what many others have written here is the link to a good but short description of this very gifted gentleman.

http://web.mit.edu/hmtl/www/taylor.pdf

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Re: Let's talk port velocity --

Post by digger » Mon Dec 02, 2019 11:13 pm

David i look at it this way, using the equation to predict the peak hp rpm

RPM = Vel x 360 x Port area / CI

Is never going to work reliably unless you make the assumptions regarding how optimum the cam, intake manifold etc are hence the word potential should be used.
Take a given engine and do nothing other than a manifold length change you can change the rpm where peak hp occurs by well over 1000rpm.

In any case lets’s take the example from here with a cam change and a few other bits but same CI and head

viewtopic.php?f=49&t=59241

RPM = 360 (say) x 360 x 2.4 / 47.4 = 6600 for the race it peaked 7300 and the street 6300

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Re: Let's talk port velocity --

Post by David Vizard » Tue Dec 03, 2019 9:17 am

Digger,
You are going to have to disagree with thousands of tests done by Sloan, MIT, GM, Ford, Weslake, Ricardo, etc.
Remember these are recorded results from engines considered successful by main stream research establishments. I am passing on these numbers as a guide to what is seen in general for a given type of engine.

Of course you can move the delivered velocity figures up or down the scale by changing camshafts but if the figures fall much below those quoted here it is an indication that the ports are too big or under utilized.
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Re: Let's talk port velocity --

Post by digger » Tue Dec 03, 2019 4:46 pm

David Vizard wrote:
> Digger,
> You are going to have to disagree with thousands of tests done by Sloan,
> MIT, GM, Ford, Weslake, Ricardo, etc.
> Remember these are recorded results from engines considered successful by
> main stream research establishments. I am passing on these numbers as a
> guide to what is seen in general for a given type of engine.
>
> Of course you can move the delivered velocity figures up or down the scale
> by changing camshafts but if the figures fall much below those quoted here
> it is an indication that the ports are too big or under utilized.
> DV

David,

The equation states that the peak hp of a given engine combination is soley dictated by the port size. If that’s what they say then I disagree and the example shows an engine with two different peak hp rpms with the same cubic inch and port size. you only need 1 example to show it doesn't always work.

As a concept the only way I could possibly see this being reliable is with at least the following assumptions which in reality are rarely ever true:

• Optimal cam;
• Optimal inlet configuration; and
• Optimal exhaust configuration.

I’m sure one could cherry pick examples where it does work to as I have some where it does work. I find it a useful equation but has to be used correctly.

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Re: Let's talk port velocity --

Post by pastry_chef » Tue Dec 03, 2019 6:55 pm

For the linked example engines
viewtopic.php?f=49&t=59241

Could we agree the port velocity factor would not be the same between?
Mike R

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