Rule of Thumb Accuracy

General engine tech -- Drag Racing to Circle Track

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RevTheory
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Re: Rule of Thumb Accuracy

Post by RevTheory »

SchmidtMotorWorks wrote: Sun Jul 08, 2018 8:32 pm
GARY C wrote: Sun Jul 08, 2018 8:19 pm You can't have it both ways, does the optimal cam make the most power while discarding the boundary "you set" or does it make the best power while taking into account those boundary's?
Two input dimensions is not enough data to sufficiently specify the criteria.

The use of only 2 dimensions is the root of the absurdity.
Unless it works.
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Re: Rule of Thumb Accuracy

Post by In-Tech »

Just like Warp and many have said, this thread is a waste of space and a marketing ploy. I like and respect DV, this gets tiring.

Here is an example of changing VALVE TIMING EVENTS in a tire limited circle track environment, same off the corner power. Of course I could make it undriveable and blame it on the driver.


128 that shit.

Dyno.jpg
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Re: Rule of Thumb Accuracy

Post by RevTheory »

Never once did David say that 128 should be used in a traction limited circle track engine.
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Re: Rule of Thumb Accuracy

Post by SchmidtMotorWorks »

RevTheory wrote: Mon Jul 09, 2018 5:08 am Jon, you pretend to have all this knowledge and can only enlighten us intellectual peasants to a certain extent because you're bound by an NDA. Have you ever considered that the first thing the aftermarket (I know, they're all stupid) does is improve on the output while still maintaining EPA requirements?

Do you think they used a computer program and let it run for a week or did they just use what's known to work? What I find funny is that someone can post up something like David did that works but if you don't like the math, you'll be hell-bent on making sure the thread gets locked down rather than going "shit, that actually works pretty well." I sure hope you don't need 1D simulation if your wife wants to roll her position over.

But hey, if you want to call the equation out because it doesn't work on a lawnmower engine with a 10 inch bore and a half inch stroke or what ever your far out argument was, go ahead and knock yourself out.
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Re: Rule of Thumb Accuracy

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BigBro74 wrote: Sun Jul 08, 2018 10:41 am One of my 2bbl stock car engines (13:1, 4.155x3.5 23 deg. iron head, flat tappet SBC) has an Isky cam recommended to me by Ron Iskendarian for the engine (which also fits 128 rule exactly- go figure). I have the engine modeled up in dynosim so i thought i would try the iterator for cam specs using best area under power curve (3-7000rpm) as my test parameter.
248/252 @.050"
.359"/.359" Lobe Lift
105 LSA
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Re: Rule of Thumb Accuracy

Post by CamKing »

GARY C wrote: Sun Jul 08, 2018 11:03 pm When the 383 first became popular in the early 90's cam company's and tech writers claimed a bigger inch engine needed a wider LSA while DV was proving them wrong by actually testing it.
While others proved that widening the LSA was the correct move, by actually testing it.
Do a little research on the Champion Spark Plug west coast dyno facility.
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Re: Rule of Thumb Accuracy

Post by CamKing »

joe 90 wrote: Mon Jul 09, 2018 12:57 am Nor can you run an N/A cam in a turbo car........but I do.
That's the funny thing about cams. You can be 50 degrees from optimum, and it'll still run. The chance that your cam was optimum for an N/A engine, and optimum for a turbo engine, is very slim.
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Re: Rule of Thumb Accuracy

Post by David Vizard »

David Redszus wrote: Sat Jul 07, 2018 6:52 pm Without seeking to become engaged in a debate regarding the 128 rule of thumb, I thought it might be interesting to create an
excel spreadsheet that would easily calculate the LCA using the "Rule of 128" for any engine displacement and valve size.

I thought it might be informative to see how the 128 calcs would match the known LCA values for other engines.

To this inquisitive end, I am offering the spreadsheet to anyone who might be interested at no charge.
Simply send me your email address and I'll send it to you.

What becomes quickly apparent is that various valve events can produce the same LCA.
And that various combinations of cylinder displacement and valve diameters result in the same "optimal" LCA.

Still, I would be very interested to see how the calculated values compare to other real world applications.
David,

Thank you for being a voice of sensibility, help and reason.
Guys,if you feel you have any need for accurately knowing the LCA then I would suggest you consider taking up David's thoughtful offer.

Thanks
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Re: Rule of Thumb Accuracy

Post by David Vizard »

Erland Cox wrote: Fri Jul 06, 2018 6:51 pm
GARY C wrote: Fri Jul 06, 2018 5:53 pm
Didn't Grumpy come to the conclusion that all SBC's should have a 109 LSA? 128 does change LSA in relation to valve size and can even be adjusted to steeper seats if one knows what size to enter based on how it effects window area.
No but that is what he used and what worked best for him at that time with ported stock iron heads in drag racing.
What does 128 say a modern version of his engines would like for LSA?
Still the same 1,6 - 1,625" exhaust valve with a 2,2" intake instead of 2,055.
And modern less than 18 degree aluminum racing heads.

Erland
Hi Erland,

I spoke to Grumpy over lunch on this point about 1978 ish.

Paraphrased here is what the Master told me:-

"At the high CR we are using there seems to be only small differences on our 331 SBC between 111 and 108 with 109 being marginally the best."

Interestingly head flow trends were moving into the realm of maximizing high flow and be damned about much else. If I put the flow figs into my master cam program (which uses flow) the end result is a 109.6 But, for the 128 Formula (which assumes an intake valve discharge co-effcient of .71 in the 0-200 lift range which is typical for a good 3 angle valve job on a SBC), then the predicted LCA, with a CR correction, for a 15/1 - 2.05 intake 331 is 110.6 for Grumpy's motor.

Just keep in mind that 128 is for a 23 degree SBC so don't go using it randomly on all the various motors that you get involved in.

Thanks
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Re: Rule of Thumb Accuracy

Post by David Vizard »

SchmidtMotorWorks wrote: Fri Jul 06, 2018 7:01 pm
digger wrote: Fri Jul 06, 2018 5:48 pm I'm not sure why people feel the need to make up their own definitions for what a rule of thumb is there are many dictionaries and the like online

"a general or approximate principle, procedure, or rule based on experience or practice, as opposed to a specific, scientific calculation or estimate"

Based on this it's clearly a rule of thumb
DV claimed that the formula was a polynomial fit from data from some great number of tests.

It just happened to come out linear and only depended on a couple of variables (from all that data).
Jon,

You are quibbling about details here that make no difference to the capability, within the boundaries I specify, for the 128 formula. That said you post is subjectively, about 90% accurate.

The curve for the optimal LCA for an intake valve CD of .7 to .72 produced a scatter that produced, when averaged, a near straight line from 102 to 116. From 116 to the other limit of 120 it curved down so what I did was to chop the curve at 116 and make a formula for the 102 to 116 and that was the 128 formula. Without a doubt it is a quick easy test to establish if a regular home enthusiast engine builder has or is being sold the right cam. For thousands of folk it is a near 100% fail safe validation of their cam choice. Please consider that I do my editorial with the enthusiast builder in mind. Saving them money out of their pocket and putting more output into their engines is my intent.

I suspect in that sence I have been reasonable successful.
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Re: Rule of Thumb Accuracy

Post by David Vizard »

cgarb wrote: Fri Jul 06, 2018 7:59 pm Its saying my 421 would want a 105 lobe separation....if I've done the math correct. 2.08 intake valve. I'm not saying it wont work, It just seems weird to pick the LSA before anything else. So what do you do then, take that number and then search the catalogs to find a camshaft that fits your rpm range you wish for with that LSA number? My current cam is on a 109...would be a cool test to swap out a similar cam on a 105 and see what it does.
cgarb,

If you cannot find the LCA from the catalog cams for your engine then it's time to do your own thing and order a custom one. Generally speaking it costs little of no more than a catalog cam.

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Re: Rule of Thumb Accuracy

Post by Warp Speed »

All of this is blah blah blah to me really, but others may find some value, and more power to them!

But a quick question for Mr. Vizard, if your presence here is to simply help the common man, and NOT to constantly promote your self, books and seminars. Why don't you help out with your thoughts in the many cam related threads going on currently, or to any other of the various thread topics?
No matter your views, I think they would be more welcomed, and less harshly criticized, if that was the case......?
I have NO dog in this fight, just some casual observations from the grandstands.
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Re: Rule of Thumb Accuracy

Post by David Vizard »

hoodeng wrote: Sat Jul 07, 2018 3:01 am A rule of thumb as others have pointed out is application specific. If i was building a mud hut my tolerance would be to the nearest cubit or whatever standard is used in mud hut construction, if i am rough machining my tolerance is to the .1mm, if i am honing ,finish grinding my tolerance is .25um.
If you want to know my pin number, it is the last four digits of pi.

David i'm sure this is a bit of a trick question, coming from you.

Cheers.
Hoodeng,

Let me address this part of your post:-

If you want to know my pin number, it is the last four digits of pi. Best of luck with that - as it will not ever be possible to break in!

David i'm sure this is a bit of a trick question, coming from you. No it is far from a trick question. I asked this question with the intent of it surviving negative criticism as a 'rule of thumb' title can easily cause it to become discarded from many, many, builders tool boxes. I needed to make sure that it was seen as the seriously accurate tool it could be within the scope intended. This post is allowing me to demonstrate to a lot of engine builders the capability of such a simple formula. Heed it and it will save SBC builders money on cam expenses (IE stopping from buying a poorly speced one).
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Re: Rule of Thumb Accuracy

Post by Warp Speed »

CGT wrote: Mon Jul 09, 2018 1:16 pm
portinguy wrote: Mon Jul 09, 2018 1:14 pm
CGT wrote: Mon Jul 09, 2018 1:13 pm

Well, you still have facebook to peddle your junk.
It sure seems funny you think you know who I am.

Junk? Jealous POS
Like I said, I've seen how your stuff runs. And jealousy isn't the feeling that came over me.
I just spit Gatorade all over my phone..........thanks! Hahaha
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Re: Rule of Thumb Accuracy

Post by user-30563 »

Both you guys have zero balls. Rather post shit on an internet forum than picking up a phone and telling the person you think I am how much of a idiot he is. Zero class.
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