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Let's talk port velocity --

Moderator: David Vizard

digger
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Re: Let's talk port velocity --

Post by digger »

David Vizard wrote:
> [quote=digger post_id=838190 time=1575409588 user_id=6525]
>
>
> 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.
> [/quote]
>
> Digger,
> Let's walk through this one step at a time so you can see where the use of
> these port velocities comes in handy.
>
> Let's take a well built 4 brl race motor, dyno it and work out the mean
> port speed. Now lets sabotage the carb linkage so that only two barrels
> open. When we dyno it now and work out the mean port speed it will be
> lower. But ask yourself will the port size be optimum with just two barrels
> in operation? Most likely port sizes will need to be reduced to optimize
> in two bal form.
>
> The low speed seen by the Porsche 2 V heads does not mean the velocities in
> the charts are wrong it means the Porsche heads ports are too big!
>
> You have to look at these numbers in much the same fashion as the induction
> tract limiting Mach #.
>
> DV

David,

That’s why I originally said” isn't the slide more along the lines of what they should be for optimum” as opposed to a predictor of what a given engine will peak at.

I agree if you have a given engine and target rpm level it should be used to as a first pass means to identify what the correct size ports should be in order to give the correct port velocity based on the layout/style of head in question.
David Vizard
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Re: Let's talk port velocity --

Post by David Vizard »

digger wrote: Wed Dec 04, 2019 12:04 am David Vizard wrote:
>
digger wrote: Tue Dec 03, 2019 4:46 pm >
>
> 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.
>
>
> Digger,
> Let's walk through this one step at a time so you can see where the use of
> these port velocities comes in handy.
>
> Let's take a well built 4 brl race motor, dyno it and work out the mean
> port speed. Now lets sabotage the carb linkage so that only two barrels
> open. When we dyno it now and work out the mean port speed it will be
> lower. But ask yourself will the port size be optimum with just two barrels
> in operation? Most likely port sizes will need to be reduced to optimize
> in two bal form.
>
> The low speed seen by the Porsche 2 V heads does not mean the velocities in
> the charts are wrong it means the Porsche heads ports are too big!
>
> You have to look at these numbers in much the same fashion as the induction
> tract limiting Mach #.
>
> DV

David,

That’s why I originally said” isn't the slide more along the lines of what they should be for optimum” as opposed to a predictor of what a given engine will peak at.

I agree if you have a given engine and target rpm level it should be used to as a first pass means to identify what the correct size ports should be in order to give the correct port velocity based on the layout/style of head in question.
Digger,
you've got it!
That sounds like what I am trying to put over here.

DV
David Vizard Small Group Performance Seminars - held about every 2 months. My shop or yours. Contact for seminar deails - davidvizardseminar@gmail.com for details.
digger
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Re: Let's talk port velocity --

Post by digger »

If the "port area" is the average i.e. volume / centreline length
What about velocity at other locations? For example
• Throat;
• Choke/ min CSA (if upstream of throat); and
• Port entry.
• Manifold entry
What are your thoughts on these, in particular the choke size?

Clearly you can increase the average port area without increasing the choke size but how does the rpm capability behave? Is there a secondary check for the speed at the choke limiting?
David Vizard
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Re: Let's talk port velocity --

Post by David Vizard »

Digger,

You have some really good questions here but as I answer one set of questions it prompts you you ask yet more. Thats OK but a better way would be for us to talk so I can answer follow up questions right away. When we have done that you can post your appraisal of our talk.

If that sounds like a plan to you give me a tel # and a time I can call you.
Thanks
DV
David Vizard Small Group Performance Seminars - held about every 2 months. My shop or yours. Contact for seminar deails - davidvizardseminar@gmail.com for details.
skinny z
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Re: Let's talk port velocity --

Post by skinny z »

Perhaps a novice question although it pertains to a DV article written several years back.
To reference the article it was about having too little port size for the CID . In particular this was about a 383 SBC and a 170cc Edelbrock Performer head. It was theorized that this particular combination could use a cam spec normally considered too large for the CR (in this case about 9.8:1). The increased velocity of the smaller port (at a given CFM when compared to larger port) would help cylinder filling and recover what would normally be lost by over-camming the engine.
Is this something that one could carry into a build profile and expect the engine to run as it might if it were to have larger heads and more compression? This was suggested in the article and is something I would like to pursue given that I have the pieces to proceed.
Thanks in advance.
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Re: Let's talk port velocity --

Post by GARY C »

skinny z wrote:
> Perhaps a novice question although it pertains to a DV article written
> several years back.
> To reference the article it was about having too little port size for the
> CID . In particular this was about a 383 SBC and a 170cc Edelbrock
> Performer head. It was theorized that this particular combination could use
> a cam spec normally considered too large for the CR (in this case about
> 9.8:1). The increased velocity of the smaller port (at a given CFM when
> compared to larger port) would help cylinder filling and recover what would
> normally be lost by over-camming the engine.
> Is this something that one could carry into a build profile and expect the
> engine to run as it might if it were to have larger heads and more
> compression? This was suggested in the article and is something I would
> like to pursue given that I have the pieces to proceed.
> Thanks in advance.
It's always about the rpm range you are operating in, the best part of starting with a conservative head and cam is that you can always port the head bigger and judge the effect but if you start big you have nothing to compare it to.
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skinny z
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Re: Let's talk port velocity --

Post by skinny z »

This is more of a case where I have the parts and an example to follow. The title of this thread, the word velocity in particular, is what attracted me to post here. If the DV article didn't hit so close to home I may not have paid it much attention.
That said, it IS a question of port velocity and how it would impact a somewhat "mismatched" combination. By mismatched, it's a question of the CR in DV's write up (and which I've more or less dialed in) and the nominally larger than spec cam. I think the point of the article was that targeted CR so as to allow the use of regular grade fuel while still making reasonable power higher up in the rev range despite the smallish heads. The higher velocity aiding cylinder filling at lower RPM.
Personally, I prefer the slightly lower CR. I've experimented with these iron heads on other builds and detonation is always a concern. I'd rather dial back on the compression ratio and have a healthy timing curve than the other way around.
As for further working these cylinder heads, I'm at a point where they are as far as I care to take them. Any additional investment will come in the way of new heads. In which case the point of my question here isn't a concern.
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Re: Let's talk port velocity --

Post by theyoungone »

I'm finding that the Peak Power RPM equation that David Vizard posted correlates to peak torque on my engines and not peak HP. Anyone else experience this?
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