Rule of Thumb Accuracy

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David Vizard
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Rule of Thumb Accuracy

Post by David Vizard »

Here is my question:- How accurate does a calculation have to be before it falls out of the ‘rule-of-thumb’ category?

I am posing this question in to establish whether or not my 128 LCA formula for 23 degree SBC with a 10.5/1 CR actually qualifies for such a title.

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Re: Rule of Thumb Accuracy

Post by cgarb »

If this is a serious question David... a calculation or formula must be accurate. A rule of thumb is a 1975 Camaro will pick up roughly .2 tenths of a second when you put on headers and free flowing exhaust. Where would the space program be if the calculations had to be roughly close? This is also my opinion...so take that for what its worth. I never got the whole picture on your 128 formula, I tried to read along but the 40 pages off pissing and moaning took away my willingness to try and follow along. I respect your experience and your work, so please don't take this negatively...you asked and I answered.
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Re: Rule of Thumb Accuracy

Post by Sparksalot »

I take this as not asked in a engineering sense as it's not asked as an engineer would posit such a query.

I have an engineering perspective beginning at NASA in 1963 so my response could go wildly astray from what you are truly asking. Please flesh out your question. Oh, plenty of racing was mixed in that half century BTW. Slide rules to FE analysis, what a ride!
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Re: Rule of Thumb Accuracy

Post by statsystems »

David Vizard wrote: Fri Jul 06, 2018 12:09 am Here is my question:- How accurate does a calculation have to be before it falls out of the ‘rule-of-thumb’ category?

I am posing this question in to establish whether or not my 128 LCA formula for 23 degree SBC with a 10.5/1 CR actually qualifies for such a title.

DV

It has to work for what it was developed to do. That's a rule of thumb.

As a rule of thumb I like the intake valve to be 52-54% of the bore diameter. Sometimes it's damn near impossible. But it's still my rule of thumb. 9 times out of 10 I if it doesn't work it's because some other factor in the equation jacked up my rule of thumb.
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Re: Rule of Thumb Accuracy

Post by Atom »

Difficult to answer.
My personal take on it is that rule of thumb is not about the accuracy perse but about being created empirically.
In my book a rule of thumb could be more accurate than a physics based formula.
But that doesn't mean it's no longer rule of thumb.

The problem with a rule of thumb is that it doesn't tell you why.
And it won't help you to get a deeper understanding of the subject.
These 2 points are probably why folks take issue with you LSA formula.
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Re: Rule of Thumb Accuracy

Post by mag2555 »

I good friend who has since passed use to say this at the drag strip before making is first time shot, " I think I am within 15 feet of home plate" in regards to changes he did to the motor or car and what he predicted his ET would be.
Building off of this statement I would say if you to within 15 to 20% plus or minus of what your shooting for your doing very well!
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Re: Rule of Thumb Accuracy

Post by ptuomov »

Rule of thumb also needs to be very easy and quick to compute. I think that's more of a defining characteristic than accuracy.
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Re: Rule of Thumb Accuracy

Post by jed »

A rule of thum has a hit or miss accuracy. That's a rule of thumb.
When doing machine work I would like something closer than a rule of thum.
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Re: Rule of Thumb Accuracy

Post by ijames »

Back in school a few engineering professors told us that when someone said much greater than or much less than, that meant at least a factor of 10, and that to really be useful a rule of thumb (or simple approximation) needs to get within 10% of the best available most accurate computation. A cruder approximation might still be used as a sanity check but you wouldn't use it to design anything. So if a difference of 2 degrees in the LCA changes the resulting hp by 10% then a rule of thumb needs to get within 2 degrees of the fully optimized LCA. My NSHO, YMMV, etc, etc :D .
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Re: Rule of Thumb Accuracy

Post by Walter R. Malik »

In my opinion ... a "rule of thumb" is where something which experience with that particular situation has given a good result 9 out of 10 times throughout the past.

It could involve almost anything.
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Re: Rule of Thumb Accuracy

Post by novadude »

Walter R. Malik wrote: Fri Jul 06, 2018 9:52 am In my opinion ... a "rule of thumb" is where something which experience with that particular situation has given a good result 9 out of 10 times throughout the past.

It could involve almost anything.
Well said.

From the results demonstrated, I would suggest that "128" fits the rule of thumb category - not sure it jumps into the realm of pure science.
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Re: Rule of Thumb Accuracy

Post by David Redszus »

When asked about computational accuracy, Einstein is reportedly to have said; "plus or minus x10".
An order of magnitude in either direction is close enough when describing the dimensions of the universe.

I would consider the accuracy of a "rule of thumb" to be plus or minus 10%.

But I don't consider rules of thumb, I have smaller fingers than my thumb. :)
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Re: Rule of Thumb Accuracy

Post by Carnut1 »

Rule of thumb is old English law that states a man may not beat his wife with a stick larger in diameter than his thumb! As for 128 after 3 seminars it is slowly soaking into the granite like structure of my brain. Thanks, Charlie
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Re: Rule of Thumb Accuracy

Post by Brian P »

The underlying assumptions involved in the "rule of thumb" have to be clearly attached to the rule itself. It has to work at least somewhat outside the boundaries of how it was originally developed. If it works on a Chevrolet, it should work on a Ford or a Mopar or a Toyota. If it works on a 350 cu-in engine, it should also work over some reasonably wide range of boundaries outside of that.

I know that when I first saw this, I never saw the underlying assumptions, and I plugged in the dimensions of a handy 2-valve-per-cylinder motorcycle engine that I happened to have in my shop at the time, and the outcome was not even remotely plausible.

A "rule of thumb" that only applies to a Chevrolet 350 cu in small block with a certain type of cylinder heads and a certain type of intake system etc ... is not useful to anyone working on anything else.

Another cam timing "rule of thumb" is to set intake and exhaust lobe-centers at 105 degrees. That's the one I use for a first crack. It might not be the absolute optimum (which can only be found by repetitive dyno testing) but it's a setting that will allow the engine to start and run decently enough to allow further experimentation ... and it probably won't be more than a few degrees different from what is eventually found. If the engine wants something radically different from that, it probably has another problem that you haven't identified yet.
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Re: Rule of Thumb Accuracy

Post by 6.50camaro »

True story, I have a sister-in-law who had a fellow living with her ,that during a remodelling project cut off one of his thumbs with a circular saw .6 months later she really wanted this guy to move out . But still felt bad about the accident. She asked her daughter how long she thought she the mother should keep letting the guy live there because it the accident . In all seriousness the daughter replied "I don't think there is a Rule of Thumb in matters like this".
IMO a rule of thumb needs to to be simple easy to remember, fairly accurate, repeatable with constraints developed for. Dan
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