I picked up a copy of PHR's May 2005 issue which contains a write up from
David Vizard.

I'm only into the first two pages (pg. 56 and 57) and I'm already questioning
a few things.

1 PSI = 27.680 in. H20 @ 4° C (inches in water)

David states 1 PS1 = ~ 20-25 in. H20

He also makes a strong comment that a piston sweeping the bore only creates
a 1 PSI depression at the intake valve (Page 57, Column 2).

Is he confusing this with the pressure loss across the intake runner from entry to the intake valve?

Vizard also states that the exhaust gas scavenging creates more "Peak Suction"
than the piston sweeping the bore?

Quote (verbatim):
Here are my quick thoughts:

- If this is true, why only minimal amounts of overlap ground into the cam?

- How would any VE calculations and DCR values work?

- Why does a compression test reveal upwards of 175 PSI with hardly any
exhaust flow activity. There certainly isn't enough air flowing down the
exhaust system to suck air into the cylinder during cranking.

Once the pistons start flying under throttle, that cylinder pressure is going
to drop much lower than atmospheric...certainly more than what exhaust
gas scavenging can pull in during overlap.

I`m really wondering what is going on. Even a quick reply of `Yes Vizard is correct`, or `No, Vizard is dreaming` would help me sleep at night.

Thanks again.

 

Thanks for bringing this up on a friday night.

I just don't know what to think, but I'm imagining a vacuum gauge screwed into a spark plug hole, the rockers taken off and the engine turned over. That seems like quite a suck to me. Wouldn't the effects of that still be influential with the valve wide open? (atmopheric present). hmmm... damn.

Overlap effects overcoming inertia of the air. Getting the intake charge started, which is free work done. The increasing area of the cylinder as the piston moves down affects the fill cycle. The intake valve closes well after bottom dead center so it appears to me besides the obvious system approach going on the number one factor in cylinder fill is using the charge inertia to your advantage. The proper balance of flow vs. velocity and intake valve closing. Then there is the whole hemholtz resonator thing and intake tract ram tuning. My head hurts, its been a long day. Enjoy your weekend!

 

Tino,

Do you work for the press?

You just put a major spin on that by misquoting Mr. Vizard.

The article is speaking primarily about inches of water, and makes the loose correlation to psi for the layman reader who may have no concept of the units of in. of H2O.

"20-25 inches of water (about 1 psi) is the limit........"

As for the other comment, you must reread the paragraph. First off he is talking about max effort race motors which must only run at full throttle. Therefore the head flow will be maximized without any concern for part throttle operation.

The piston traveling down the bore only has the potential to exert a suction of ~1PSI on the intake port because the entire time it is traveling down the bore air is flowing in! If you pulled the piston down and then popped open the valve then yes, there would be more than 1 psi.

I'm going to retype this for the sake of others reading who don't necessarily have the article in front of them. I added the italics to emphasize a few things.

"On the most successful single 4-barrel drag race motors, a well tuned exhaust will exert a suction of 6-7 psi on the cylinder. Because this occurs during the overlap period much of this suction is applied via the open intake valve, to the intake port. From this it can be seen that the exhaust system draws on the intake port as much as 500% harder than the piston going down the bore. This demonstrates that under these circumstances, it is the exhaust that is the primary element of induction, not the piston traveling down the bore. With such a system, the charge in the intake port can be traveling into the cylinder at 100 feet per second even though the piston is still parked at TDC! In practice then the exhaust phenomena make a race engine a five cycle engine with two consecutive induction events."

Who’s cam, what cam?

For OEM cams it is minimal when compared to race cams because of emissions, AND the motor has to operate at part throttle.

For race engines, I don’t agree with your very general statement that there is ‘only minimal amounts of overlap ground into the cam’. For race cams it is optimized and balanced against the other design criteria. Think about what would happen if you opened the intake valve too soon and there was still positive exhaust pressure. You may end up pressurizing the intake tract with exhaust gasses and that I’m sure you can tell is bad for a number of reasons. Now think about leaving the exhaust open too long. You start drawing air a fuel out. Not necessarily a concern for a dragstrip car, but definitely a concern for anything that will be range limited like road race car, or even a street/strip car (which also has the emissions concerns).

VE and DCR are significantly different concepts, but the common element they have is that they deal with IVC. Exhaust system design and overlap deal with IVO. Opposite ends of the stroke. VE certainly can be effected by IVO and exhaust systems. Remember, they are only calculations. A whole thesis could be written about VE calcs, so I’m going to stop here unless you have a specific question about them.

Think about RPMs. A compression test is so slow that you are definitely getting 100% VE (minus whatever slips past the rings, thus the bleed down part). A compression test is a diagnostic technique, a running motor is vastly different.

A couple of general comments.
-If you don’t understand something from Vizard, reread it until you do. Don’t get turned off and try to pick it apart, clearly he is the expert not you. I'm not saying that he is the holy gospel, but i haven't seen any writings of his legitimately disproven.
-Context is everything; Vizard is writing in generalities, but does set up some specific background for you. For instance, in this article he is only talking about serious race motors, so don’t take any of it and apply it to OEM engine design. Make sure you read all the details and stop and absorb the information if you need to, or else you will certainly draw inaccurate conclusions and will not comprehend the article. There are also some suttle but key words in his writing that are easy to misinterpret.

I should have just posted “Yes, Vizard is correct…..”

-brent

 

Thanks for clearing that up Brent.

I think my biggest problem was reading the line:

When Vizard is stating that the exhaust is the "primary element" of induction,
he means that exhaust gas scavenging "begins the cycle of induction", as opposed to the piston beginning the cycle of induction?

I misread that to imply that the exhaust gasses pull in more charge per overlap
period than the sweep of the piston during the intake stroke.

Understand my confusion?

As for the suction on the intake port, your explanation makes more sense.

Because the intake charge is 'static' sitting behind a closed intake valve with
a certain amount of tuned pressure from the previous cycle, the exhaust
scavenging sees more of a pressure differential due to interia.

Once the intake valve opens, the charge begins to move from the depression
caused by the exhaust gas leaving the chamber, combined with the pulse tuned
pressure that the intake charge had built up behind the closed valve.

so...

Since the inertia has already been overcome, the piston is already getting chased
by the intake charge and only sees about ~ 1 PSI of suction.

Correct?

Good thing I didn't write the editor and slam Vizard!

For the record, I have read many articles from David and I have great respect
for him. I was just very confused from that line and made me believe it was
an error.

Sorry Mr. Vizard if you are reading this!

 

Bingo, it is the main thing happening around TDC. Also take note of the part where he describes a race motor as a psuedo 5 stroke, having 2 types of induction.

Almost, except that you design for intake tuning IVC, not IVO.

Yup.


-brent

 

Certainly. I was implying that as the intake valve begins to open, we're
hoping for a reflected pressure wave to arrive in a timely manner to help the charge
fill the cylinder.

Thanks for steering me back onto track quickly Brent!

 

Yes, DV is wrong when he says that 1 psi is 20 inches of water but that's the only part I really see wrong in any big way. I am sure that this is a misprint. It is a little less than 28 inches of water like you said above that quote.

They use 28 inches water drop because it is very close on intake to what the port really sees in an average sort of way. Anotherwords it mimics a running engine fairly well on the intake side.

The exhaust "scavenging" can either help or hurt the process of pulling more intake in and that depends on the rpm and exhaust size and tuning. Exhaust overlap generally hurts race engines that are more developed and we end up going to ultrwide LSAs to try and kill overlap as much as possible on a real engine. it is a necesary evil in order to get the valves open enough but you can only have so much overlap before you kill power up top. If you have a great exhaust compared to your intake then the overlap won't hurt you as much ans you can tighten the cam up or run less exhaust duration or both.

 

They use 28 inches of water because minute flow differences show up when porting at that high depression.

 

[QUOTE=racer7088]Yes, DV is wrong when he says that 1 psi is 20 inches of water but that's the only part I really see wrong in any big way.
QUOTE]

Oh boy here we go again.

No no no, DV isn't wrong at all and it wasn't a typo either. Tino misquoted the article and you failed to read the posts in this thread.

DV can legitimately say "25 in of water column is about 1 psi." But to say the converse as you guys are spinning his statement, "1 psi is about 25 in of water column" is incorrect. You are the ones drawing illegitimate conclusions and putting words in his mouth.

Or another example, its perfectly ok to state that "1300' is about a quarter mile." But to say "A quarter mile is about 1300'" is incorrect because as any drag racer knows a quarter mile is 1320'.

Take whatever you want from the article, but there is NOTHING incorrect in the article in regards to water column and psi.


Brent

 

Let's add fuel to the fire.

In your eyes, why did David use an approximation instead of a determined value?

Is he accounting for temperature?

If 1 PSI = 27.680 in./H20 @ 4° C, then would 1 PSI = ~ 24 in./H20 at 20° C (which may be closer to ambient air temperature in his test)?

 

The density of water would vary less much less than a single percent, so no thats not it.

The reason he did use an exact value is simple. He was discussing a range, 20-25 in water. He then added a footnote for people who don't regularly use flow benches and have no concept of what the unit of in of water column actually represents. He added the approximate value of a common unit that most people are familiar with. That is why he used the word about and put it in parenthesis.

So instead of:
20-25 inches of water (about 1 psi) is about the limit if decent power is to be achieved.

You wanted him to write:
20-25 inches of water ( 0.72 - 0.90 psi ) is about the limit if decent power is to be achieved.



It would have been ridiculous for him to write that since he was making an opinionated general rule of thumb, not discussing actual hard values. People use the word about all the time to indicate that values are not exact.

This is from an american dictionary, maybe the canadian one says something different.....
Definition: a•bout, adv.
1. Approximately; nearly: The interview lasted about an hour.

I will bet a million dollars that if someone has Vizards phone number and calls him up, he will know that 20-25 in. of water doesn't equal exactly 1 psi. How on earth could it since 20-25 is a range and not a single value.

We need to set up a new advanced reading comprehension forum.....

Ok, sorry about the long tirade, but he used the word about and I think it made perfect sense to most readers.

-brent

 

It seems as if you took that as a stab toward David...hopefully not?

I'm simply asking if the range of 20-25 inches of water has anything to do with
the ambient air temperature (since the source I quoted referenced 4 degrees C.).

 

No, i understand. I just can't believe we are overanalyzing this one sentance of the article to this degree.

Water is most dense at 4 degrees C. The crystaline structure of water (ice) is actually less dense than water. This should be no suprise since ice floats. As water warms up it also expands. The density of water at 20 degrees centigrade is 0.9978 g/cc versus exactly 1 g/cc at 4 degrees centigrade.

Therefore, temperature has nothing to do with it. He stated a range of 20-25 because it was just a general rule of thumb he was giving and was based on his opinion.

Please, no more wood on the fire

-brent

 

Try telling that to a kid in a candy store!

I guess it's frustrating for you guys to answer all of these relatively easy
questions.

For the boys on the other side of the fence, it's like gold to see a response.

I agree, let's let David have the spotlight and leave his approximations alone.

As for me, I'm asking all the silly questions until I understand...so grab a few
fire extinguishers and sit tight

 

Brent,

I responded to what was written. 1 psi is NOT 20 inches of water. DV knows this and was misquoted and you corrected it but the poster was asking about something totally different really and that was whether DV's approximation was that close which it was not. I KNOW David Vizard. I HAVE his phone number and I HAVE his cell phone number. 20 inches of water is around 1.5 inches HG and .75 psi and is what Holley carbs uses to rate 4 bbl models at. 28 inches water is about 1 psi and that is what we flow heads at.

Just try to understand what is being said before worrying so much about protecting a magazine article. David Vizard is not a fool and he knows all about pressure drops. If he used it as an extremely GROSS approximation then that is cool too although usually he doesn't always take people to be that stupid. I have talked to David several times and am always very glad when I see him at PRI and when he comes to our dinner every year as he has a very good understanding of the ICE. So don't get your panties in a wad. just discuss the topic at hand if you can and don't worry so much about Primedia!

BTW DV is doing fine and has his own program now in NC and the next time I am up there he is going to give me the big tour. If you are there you can come too unless you are already there!