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pastry_chef · 11-26-2004, 09:14 AM

Awesome work Frank.
It needs one small correction to match up with a camcard.

In dynamic CR calcs
Replace

} else {
ICAx=duration2-180
ICA=ICAx-advance
}

With this...

} else {
ICAx=lsa-(advance/2)
ICA=duration1-(180-ICAx)
}

This will nail the intake closing point which is all that really matters here.
I am assuming that "advance" as entered by the user is in crankshaft degrees per industry standard.

Take care.

MasterGore · 11-27-2004, 07:47 PM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by BVR421:
screw that! now he wants to plant cookies on my hard drive
Id rather use an abacus <HR></BLOCKQUOTE>

Yet another technology that's being criticized for no apparent reason... cookies do absolutely nothing evil whatsoever. They're simply used to add functionality on web sites. A cookie from projectpontiac.com cannot be accessed by another web site, and vice versa. Not to mention that the only information most cookies contain is something along the lines of; new=false, lastseen=112704
(which basically determines whether or not you've visited the site before and when, so only new content since your last visit will be emphasized).

Why are people so quick to criticize things without basis?

By the way, this forum uses cookies... almost all online message forums do.

MasterGore · 11-27-2004, 07:50 PM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by BVR421:
projectpontiac.com seems to down or is it my dumb browser? <HR></BLOCKQUOTE>

I was shocked to discover that my domain name expired on November 23rd. It's my own fault, I forgot to renew it. Luckily it was fixed within a day by my hosting company, they have excellent customer service.

MasterGore · 11-27-2004, 08:07 PM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by Pastry_Chef:
Awesome work Frank.
It needs one small correction to match up with a camcard.
<HR></BLOCKQUOTE>

Thanks! Those changes have been implemented. I was actually asking for advance in camshaft degrees, mostly because I wasn't sure wheter crankshaft or camshaft was the accepted standard. I've adjusted the formula and changed the text on the page to reflect a change to crankshaft degrees instead.

pastry_chef · 11-27-2004, 10:56 PM

Thanks Frank.
I'm afraid I've confused you a little.

The changes I was suggesting would make the correction on their own (a little second guessing on my behalf)

During correction you made one other change in the initialization, that is:
Old
advance=x.advance.value*2

New
advance=x.advance.value/1

previously you were multiplying advance by two, that is why in ...
ICAx=lsa-(advance/2)
I divided the advance by two.

thus with your additional change it is still off a little.

To know when it is bang on use these inputs and compare manual entry intake closure vs. the triple team of duration, lobe sep, and advance entries. When accurate they will yield the same dynamic compression number.

280 adv duration (crank degrees)
110 lobe seperation (CAM degrees)
4 degrees advance (crank degrees)

VS.
66 deg ABDC intake valve closure.

-------------------------------------
(280/2)=140 duration crank degs either side of lobe centerline.
@ 106 ATDC intake centerline (110 - 4 degs advance) there remains 74 degrees left until BDC
thus 140-74= 66 crank degrees of the lobe will occur ABDC.
--------------------------------------

When a 110 lobe sep cam is straight up (no advance or retard), the intake centerline is 110 crank degrees ATDC, the exhaust centerline is 110 crank degs BTDC.

If we advance 4 crank degrees (most typical) the lobe sep is still 110 but intake centerline becomes 106 ATDC and the exhaust centerline 114 BTDC.
(106 crank degs + 114 crank degs) /2= 110 CAM degrees. - most typical of nearly all CompCams

Lobe seperation has the distinction of being the only spec stated in "cam degrees", at least in common practice.

MasterGore · 11-28-2004, 09:55 AM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by Pastry_Chef:
During correction you made one other change in the initialization, that is:
Old
advance=x.advance.value*2

New
advance=x.advance.value/1

previously you were multiplying advance by two, that is why in ...
ICAx=lsa-(advance/2)
I divided the advance by two.
<HR></BLOCKQUOTE>

oops, I actually did modify your corrections to take into account my change of the "advance" variable, but I never clicked the Save button to enter it into the site (I use a CMS because I'm too lazy to manage my site manually). I felt awfully tired last night

Thanks for pointing that out!

pastry_chef · 11-28-2004, 11:34 AM

Bingo!

Verified accurate against Performance Trends compression ratio calculator and correct theory.

buwalda · 11-30-2004, 03:21 AM

Maybe I'm missing something, but I fail to see the 'benefit' of a DCR figure. For instance, is it not going to vary at different rpm? As volumetric efficiency changes thru the rpm range, different amounts of fuel/air will be drawn into to the chamber for compression. So, at which rpm do you choose as a reference? Also, using the 'advertised duration' will give varying figures. If you use a Comp Cams hyd, they usually quote their hyd cam figures at 0.006"; Crane often use 0.004" . Could get a different DCR for essentially the same lobe profile because of a difference in the quoted 'advertised duration' figures. Two identical cams used in identical engines except one has ported heads, will have different DCR's because you would expect the ported heads to flow more air; but the calculator would show the same DCR for each engine.
The very fact that it is called 'dynamic' means that it is changing. And that is exactly what it is doing, constantly: changing. I feel there are too many variables involved to get DCR from a formula or equation.

johnta1 · 11-30-2004, 09:45 AM

The link Diamond Jim gave earlier will explain it the best. Or HERE.

You're confusing it with cylinder pressure I believe.

MasterGore · 11-30-2004, 03:32 PM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by Geoff:
Also, using the 'advertised duration' will give varying figures. If you use a Comp Cams hyd, they usually quote their hyd cam figures at 0.006"; Crane often use 0.004" . Could get a different DCR for essentially the same lobe profile because of a difference in the quoted 'advertised duration' figures.
<HR></BLOCKQUOTE>

The difference that 0.002" makes in valve closing angle is extremely minimal. The actual calculated difference turns out to be negligeable. The only truly accurate way to determine intake valve closing angle is to measure it yourself using a degree wheel and a gauge that measures piston height. All other means (most of them involve basing calculations on the cam duration) are a "best guess" which can be plenty accurate for baseline figures. This is why I added a means to manually enter the closing angle, some people have gone through the process of measuring it themselves.

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by Geoff:
The very fact that it is called 'dynamic' means that it is changing. And that is exactly what it is doing, constantly: changing. I feel there are too many variables involved to get DCR from a formula or equation. <HR></BLOCKQUOTE>

Actually no, the dynamic compression ratio does not change throughout the RPM range, it is fixed. Just because it's called "dynamic" does not mean it changes. I agree that it's a misleading term to use, but it wouldn't be the first technical event that's poorly named

In this case "dynamic" refers to the fact that the compression ratio can be varied independantly from the static compression ratio (ie. with cam timing).

There's a few excellent explanations for dynamic compression available on the web. Jim Hand wrote one, and there's another article that's been pointed out numerous times in this very thread.

Anyways, cylinder pressure isn't measured as a ratio. That wouldn't make any sense. Cylinder pressure is measured in psi.

DiamondJim · 12-01-2004, 05:53 AM

The advance/retard feature does not seem to be working. It has no effect on DCR but it should. -Jim

pastry_chef · 12-01-2004, 08:40 AM

Jim, the advance/retard input is not factored when the box is checked and you enter manual intake closing.

If you know the degree at which the intake closes.
To advance "X" amount of degrees... subtract (-x) from the original number.
To retard "X" amount of degrees... add (+x).

DiamondJim · 12-01-2004, 03:40 PM

OK, Think I've got it. If the box is checked the only field that matters is the manually entered closing event. Pastry_Chef, you are killing me, I'm on a diet!!!! What is that a picture of?? UMmmmm...-Jim

EnergyFX · 12-02-2004, 09:31 PM

So how does dynamic CR relate to what octane fuel you need to run? If you have a high static CR (say 11:1)but a reletively low dynamic CR (like 9:1), can you still get away with pump gas?

pastry_chef · 12-02-2004, 10:22 PM

Lowering dynamic CR by delaying the intake closing point (increasing duration or retarding the intake centerline) can indeed help a little to run higher compression on lower octane.

I believe a dynamic CR of 9:1 is too high for pump gas.

Read this article: http://www.pontiacstreetperformance....mpression.html

nosliw · 12-04-2004, 12:15 AM

wait wait wait...

i've got a 326 in my tempest and my chiltons book is giving me a 10.75 CR while this doo-dad is giving me a little over 7:1

Tom Vaught · 12-04-2004, 07:03 AM

Make sure you have inputted the correct CC volume in the program. A lot of 326 heads had mid 50s cc chambers to get the 10.75 compression ratio.

Tom V.

MasterGore · 12-14-2004, 07:14 PM

And don't forget that Pontiac advertised compression ratio numbers that were 0.5 higher than "real life". At 52.5 cc heads, you're just about at 10.25, which is 0.5 less than 10.75:1.

I'm still working on including a chart of head port volumes for those who aren't sure what numbers to include in that field. But at this point, I'm thinking there's a LOT of information on that one page that may serve to confuse more than help.

Trips4me · 01-19-2005, 04:41 PM

Great job and very interesting reading.. I read the article in Pontiac Street Performance as well and that only raised more questions. I like this calculator and will be using it to do some calulations for my new motor. In the meantime I have a question for the gurus!!

What is the maximum dynamic CR that you can run 93 octane without worry of detonation. I'm thinking that there may not be a "correct" answer to this question due to so many factors but it seems to me there could be a number that would be a good "rule of thumb" that allows one to feel comfortable with the available fuel.

For example... The dynamic CR of my 389 is 6.78:1 and the static is 9.69:1. I mix 105 racing fuel and 93 octane or run straight 100 octane.. am I wasting my time and money??

MasterGore · 02-04-2005, 03:41 PM

<BLOCKQUOTE class="ip-ubbcode-quote">quote:<HR>Originally posted by Trips4me:
For example... The dynamic CR of my 389 is 6.78:1 and the static is 9.69:1. I mix 105 racing fuel and 93 octane or run straight 100 octane.. am I wasting my time and money?? <HR></BLOCKQUOTE>

This answer was a long time coming...

at 6.78:1 dynamic compression ratio, I'd feel entirely safe running even 91 octane gas without fear of detonation. In itself, 9.69:1 is not a really dangerous number. The only time mixing in 105 octane might be of benefit might be during severely hot days. But that's just my opinion based on my own prior experience.

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