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Old 11-04-2009, 11:54 AM
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cams in relation to c.i.

hey guys, I got a question regarding cam specs in direct relation to cubic inch increment. I have been told that a 383 can use a cam with more lift than a 350 and still make power in the same rpm range. I am wondering if there is a direct ratio relationship in both the cam size and the engine displacement size that can be used to figure out the increase in lift you can use. for instance a 383 is 1.094 times the size of a 350. Now say you have a cam that had .475" lift and makes power from 1800-6000rpm. if you were to increase the cam lift by that same 1.094 you would end up with 0.519" of lift. Now could one assume that by increasing to this larger lift on the larger engine by the same ratio you would be making power in more or less the same rpm range? With that said it also becomes a question now as to what engine size the cam manufacturers are specing their camshafts at? I would assume that all cam manufacturers spec their cams to a 350 (the operating range they give). One would also assume that by increasing engine size (say a 350 to 383) and keeping the cam the same that the power curve of the engine would take place at a lower rpm? Is my thinking in all of this correct? It just seems logical that if the engine is larger it will need more fuel thus more lift, but I'm not sure if you could apply the same ratio to duration or if you would increase duration as well as lift to remain in the same operating range. Question, comments, concerns all welcome.

Thanks,
Keith

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Old 11-04-2009, 01:10 PM
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In my experience, this will apply to duration more than lift. When changing from 1.5 to 1.6 rockers for instance, I normally see only about a 12 hp increase on average. A change in duration will be more dramatic. I applaud you for leaning on the math to try to figure this out. Math is definitive. There are no gray areas.

I have never applied math to the relationship of duration to CID as a definitive factor to use in determining cam characteristics from size to size CID, but I'll play with the DynoSim and try to get an answer for you.

Last edited by techinspector1; 11-04-2009 at 01:49 PM.
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Old 11-04-2009, 01:37 PM
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OK, I used file motors I had, changing only the bore and stroke from the file dimensions to 383 dimensions. All else was left the same. The first number is the engine size, second number hp and third number torque....

283 303@5000 348@3500
383 299@4500 405@3500

305 377@5500 396@4500
383 366@5000 440@3500

307 341@5000 385@4000
383 315@4500 424@3500

327 447@6500 415@5000
383 431@5500 445@4500

350 536@7000 475@5000
383 531@6500 491@5000
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Old 11-04-2009, 02:02 PM
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Very interesting Techinspector, thank you for posting that!! So judging by the numbers it seems that increasing an engine in displacemnt but leaving everything else the same results in maximum power at lower rpm. The hp figures for the 383 are not all that impressive in relation to the smaller motor in each case......... I wonder if this is because the cam is too small for the larger 383? It would be interesting to see where the 383 would make max power if a larger cam were used and what the increase would be. Does anyone know what size engine cam manufacturers spec their cams at? For instance the comp cams xe 276 camshaft http://www.summitracing.com/parts/CCA-07-503-8/ has an operating range of 1800-5800rpm. Now is this the operating range of this camshaft in a 276 c.i. engine or in a 406 c.i. engine?
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Old 11-04-2009, 02:04 PM
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Quote:
Originally Posted by techinspector1
OK, I used file motors I had
Was this w/the cam specs of the file engines changed by the same amount that the displacement changed?

I think what will be found, is that the closer the engines are to one another, displacement-wise, the closer that just using the difference in displacement as a ratio to increase the cam specs, the closer it will be to actually holding true.

Some things might respond well to simply sizing them up by the same relationship as the difference in displacement. But the closer the actual engine sizes are, the closer it will be- the error will be less.

But take a look what happens when this is applied to diameters:

A hot 355 can use, say, 1-3/4" headers to good effect. Take that out to an IHRA Pro Stock engine, and you will quickly see that there's no way the 800 CID mountain motor could ever use (or even fit) a 4" header pipe diameter.

Closer to reality, a 1-5/8" header on a stock 305 would equate to almost a (~2.6") 2-5/8" header on a 502 truck engine.

Same 305 w/1-5/8" header would say a mild, DD 350 would need a 1-7/8" set up.

This same thing occurs if you use most any diameter- be it venturi, throttle plate or whatever.
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Old 11-04-2009, 02:13 PM
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Quote:
Originally Posted by sexypizzaman
Very interesting Techinspector, thank you for posting that!! So judging by the numbers it seems that increasing an engine in displacemnt but leaving everything else the same results in maximum power at lower rpm. The hp figures for the 383 are not all that impressive in relation to the smaller motor in each case......... I wonder if this is because the cam is too small for the larger 383? It would be interesting to see where the 383 would make max power if a larger cam were used and what the increase would be. Does anyone know what size engine cam manufacturers spec their cams at? For instance the comp cams xe 276 camshaft http://www.summitracing.com/parts/CCA-07-503-8/ has an operating range of 1800-5800rpm. Now is this the operating range of this camshaft in a 276 c.i. engine or in a 406 c.i. engine?
Camshaft listings are typically targeted at the most common engine size unless specified otherwise. SBC is usually a 350, BBC is 454, SBF is 302, etc.
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Old 11-04-2009, 02:26 PM
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Quote:
Originally Posted by cobalt327
Was this w/the cam specs of the file engines changed by the same amount that the displacement changed?
No, nothing was changed except the bore and stroke, just to see how the displacement change would affect the results.
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Old 11-04-2009, 03:22 PM
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Quote:
Originally Posted by techinspector1
No, nothing was changed except the bore and stroke, just to see how the displacement change would affect the results.
OK, so what happens if the cam duration and/or lift specs are also changed by the same ratio that the displacement changed between the engines?

I realize this would be a lot of work for no real practical application, but I believe this is what the OP was getting at in his first post.
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Old 11-04-2009, 03:30 PM
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how do u post stuff

im new and im trying to find out how to post can u help me
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Old 11-04-2009, 03:40 PM
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im new and im trying to find out how to post can u help me
You gotta love the irony! LOL
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Old 11-04-2009, 04:27 PM
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Yes cobalt you are bang on with what I am trying to figure out. I know it would be very hard to find camshafts with the exact ratio increase of both lift and duration to test, but it would be interesting to see if the results would be maximum power at the same rpm for both engines. The reason I even brought this question to the table is I need to find a camshaft for my 383 that I am rebuilding and from all the cams I look at they list the operating range, now assuming this operating range is for a 350 if I were to increase the cam specs by the given ratio I am assuming the operating range would be almost identical? I want power to 6000rpm max but if I buy a cam that lists an operating range from 2000-6000 rpm, for a 383 it might only make power to 5500rpm, I hope you can see where I am going with this. I think in the overall scheme of things it is safe to say that an increase in displacement needs an increase in breathing to operate in the same range.
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Old 11-04-2009, 04:59 PM
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Quote:
Originally Posted by sexypizzaman
Yes cobalt you are bang on with what I am trying to figure out. I know it would be very hard to find camshafts with the exact ratio increase of both lift and duration to test, but it would be interesting to see if the results would be maximum power at the same rpm for both engines.
This IS interesting.

I'm thinking that an additional 10% duration of a 350-sized cam would likely put you (w/a 383) above where you would want to be. I do not know what a sim program would say on the subject, though.

With most cams, lift would tend to increase as a result of the additional duration. But this shouldn't cause the torque output to be shifted upwards nearly as much as an increase in duration does.

But w/a 10% duration increase (comparable to the difference in size between a 350 and a 383) will put a 230 degree @ 0.050" cam at 250-plus degrees A 0.050".
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Old 11-04-2009, 05:28 PM
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It is only indirectly the cubic inch, more specifically the stroke.. the longer the stroke the larger the duration
A fellow who grinds many types of high end cams shared a formula with me
The odd thing about this equation is that you only input the stroke
never the bore, yet it is the bore that changes the duration.

Without seeing and playing with the formula I sure that makes little to no sense and he asked that I keep it confidential.

I am going to change it up some to respect his wishes and allow some of you number crunchers to play with it and get an idea of how bore, stroke and rpm play into the duration..

( Square Root of (Stroke x RPM / xxxxx.xxxx) ) x 360 = Intake Seat Duration.

where x is a constant. it is a number around 35,000 specific down to one ten thousandths
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Old 11-04-2009, 08:55 PM
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Quote:
Originally Posted by NXS
Square Root of (Stroke x RPM / xxxxx.xxxx) ) x 360 = Intake Seat Duration.
where x is a constant. it is a number around 35,000 specific down to one ten thousandths
Doesn't work out.
SR (3.75 x 6000 / 35000) x 360
SR (22500 / 35000) x 360
SR (0.6428) x 360
0.25355 x 360
91.27
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Old 11-04-2009, 09:04 PM
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b/c I changed it up, he asked me not to share it. it's just so you can get an idea of how bore/stroke/rpm all play into duration...

see the duration is calculated only by rpm and stroke 100%
0% bore or engine size
yet if you play with it enough you'll see that it is infact the bore that is affecting the duration.. strange.. I'll try and work a few out to show you what I mean...

BTW, your math is wrong....
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