I bought a 383 stroker kit that will give me 11 to 1 compression with 64 cc heads will that change if I go with 58 cc head? IF so can someone tell me what math I would use to figure out what my compression ratio will be?

 

It's just simple arithmatic. In order for the answer to mean anything, the information, going in, has to be reliable. That said, figure the cc's in the cylinder. Divide the cyl. cc's by 11. That will equal the the "clearence volume". In other words, the cc's not in the cyl. This is head volume, piston volume (+ or -), gasket volume, and deck clearence, all together. We divided by 11, because the cyl is 11 parts compared to 1 part. Now take that "clearence volume" number and subtract, 6 ccs (64cc's minus 58 cc's). Refigure the ratio using the cyl. volume from the first step and the new, 6 cc smaller, clearence volume. Pretty basic, Huh? Good luck, and...

RACE ON!!!


ATTENTION ! This post contains a serious error. Where I said to divide by 11, I was WRONG. For an 11:1 compression ratio, you must divide by 10. See my post below, dated 11/1/2003 @ 11:54 PM for clarification. I am sorry for any inconvience I may have caused anyone. I could have just edited and corrected the error, but I didn't feel that was the proper way to handle things.


[Modified by CFI-EFI, 12:31 AM 11/2/2003]

 

I watched the shop check mine with fluid. First they checked the heads cc then the piston/cylinder plus the thickness of the head gasket. My 406 kit was supposed to be 10 to 1 with 64cc heads. It came up a true 11 to 1.

 

CR is static volume divided by total volume.

total volume = static volume + swept volume
static volume = head cc + deck cc + gasket thickness cc + piston cups cc

If you know the swept volume you can calculate the other static volumes (quench + gasket + piston cup) for the 64cc heads; then use that and the swept volume to calculate the CR with the 58cc heads.



[Modified by 65Z01, 7:16 PM 11/1/2003]

 

There are some online programs that will do the calculations for you once you plug in the numbers. I don't know the URL's right off hand, but someone just posted a couple of them a couple of days ago. Try doing a search for it.

Will your CR go up? You bet it will. 6 cc smaller chambers will give you close to 3/4 point increase.

Swept volume/compressed volume. Like most things, there's more than one way to calculate this.

BoreXBoreXstrokeX.7854x16.39+head gasket volume in ccs+combuston chamber volume in ccs. This will give you swept volume (there's another formula using 51.48)

If you want to be really precise you need to add the # of ccs for the valve notches/dish and the piston deck height (above or below the block deck).

Then you add together the volume in ccs of the stuff about the deck which consists of combustion chamber volume in ccs, head gasket volume in ccs, valve notches in ccs and piston deck height. This piston deck height is a little tricky because if the piston sits below the block deck you do one thing and if it sits about the deck you do another; best to ignore it and the notches.

Adding all this together gives you swept volume.

Then you add together just the stuff above the piston and that will be your compressed volume.

Divide compressed volume into swept volume, or, to say it another way, you divide the swept volume by the compressed volume.

To make it simplier, just forget about valve notches and deck height; those two together won't change the final CR that much anyway.

4.155 X 4.155 X 3.832 X .7854 X 16.39 =851.603
Combuston Chambers = 70.00
Head Gasket = 9.0
____________
930.603

Combustion Chambers = 70.00
Head Gasket = 9.0
____________
79.00

930.603/79.00 = 11.779 CR

If you double the head gasket cc to 18 you get:

939.603/88.00 = 10.677 CR

So 9 ccs is about 1.1 point in CR using this bore and stroke combo. So I'm estimating 6 ccs is about .75 point in CR, or so.

I just plugged the #s into Engine Analyzer Pro and it gave me the same result. BTW, the dumb program, which costs a fortune, says I'm making 514 ft/lb of torque at 4250 and 496 HP @ 5750. Now if I can only convince myself to believe that.

Don't be surprised if other programs give slightly different results, but that's close enough for government work. LOL

Jake


[Modified by JAKE, 9:39 PM 11/1/2003]


[Modified by JAKE, 9:47 PM 11/1/2003]

 

Jake,

"Swept volume" is the volume displaced by the piston during a stroke. It is the volume the piston "sweeps". My instructons above are a simpler method. Because 11.0:1 is a "given" in this case, the clearence volume (compressed volume, if you prefer) is by definition, one tenth of the swept volume. By using this (1/10th), you can ignore the unknown, deck height, gasket thickness, piston dome or dish and derive an accurate clearence volume. Once the origional clearence volume is determined, you can simply subtract the 6cc difference in clearence volumes (caused by swapping 58cc heads for the 64cc heads) to get the new compression ratio.
I took the liberty of "bolding" two numbers in your example. What is 3.832? Is that supposed to be the stroke? The 383 uses a 3.75" stroke. Also, you have listed the combustion chambers at 70cc's. His example starts with 64 cc's and changes to 58cc's.
This is not correct. I can't go back and reread my previous post while writing this one, but I think I made the same mistake, above. I think I said to divide by eleven instead of ten. The compression ratio is the ratio of the TOTAL enclosed volume to that of the swept volume. You add the swept volume to the clearence (compressed) volume and then divide that total, by the swept volume. That is why the swept volume is divided by ten for an eleven to one engine. If the swept volume is 100 units and the clearence volume is ten units (1/10th of the swept volume) then the total volume is 110. One hundred and ten (110), divided by ten (10), equals, 11 (110/10=11). I just KNOW, I said to divide by 11 in my first post. My sincerest appologies. I became aware of MY mistake in analyzing yours. Thanks for your help. I think we finally have this straight.

Pcadams135, did you follow all that? I'm sorry I misled you above. Good luck, and...

RACE ON!!!

[Edited in an attempt to get the numbers in the quote to line up as origionally printed. I don't think it worked.]


[Modified by CFI-EFI, 12:07 AM 11/2/2003]

 

Well, my point was that there are currently too many unknowns to do the math, since we don't now know swept volume.

CR = CompressedV/ TotalV
For simplicity say that:
CompressedV==HeadV + DeckV + PistonV + GasketV
TotalV==HeadV + DeckV + PistonV + GasketV + SweptV

If we know SweptV then we can ignore DeckV, PistonV and GasketV (and whatever other static volumes are involved) to calculate the sum of these.

Once we know this we can get the new CR.

 

We have everything we need. We DO have the swept volume. It's a 383. That makes the bore, 4.030" and the stroke, 3.75". The information we have been given has to be accurate for the answer to mean anything. But, as it is, we DO have sufficient information.

Damn, maybe this is more complicated to others, than I thought. Here goes. Follow along with your calculators.

I use the formula for the volume of a cylinder as pi * r squared * h.
r (the radius) is half the bore. 4.030 * .5 = 2.015
r squared = 2.015 * 2.015 = 4.060
pi = 3.1416.
h = 3.75 (stroke)
3.1416(pi) * 4.060(r squared) * 3.75(height or stroke) = 47.830 cubic inches.
47.830 * 16.3870 (CCs per cu in) = 783.790 CCs, swept vol per cyl.
783.790 / 10 = 78.379. This is the cleareance or compressed volume. This includes the 64 cc heads, gasket volume, deck clearence volume, and piston dome (-) or dish (+) volume. Let's test the numbers.

65Z01's formula: "CR = CompressedV / TotalV"

CompressedV = 78.379
TotalV = 78.379(compressed volume) + 783.790(swept volume) = 862.187
CR = CompressedV(78.379) / TotalV(862.187) = 0.0909
OOPS! the formula is backwards. Lets try, CR = TotalV / CompressedV

CR = TotalV(862.187) / CompressedV(78.379) = 11.000


We have proved that the numbers are correct and work. Time to swap heads. The CR is 11:1 with 64 cc heads. If we swap in 58 cc heads (6 cc's smaller) the new compressed volume becomes 78.379 - 6 = 72.379. The swept volume remains unchanged, but the total volume becomes 862.187 - 6 = 856.187. Plugging the new numbers into the formula, yields:

CR = TotalV(856.187) / CompressedV(72.379) = 11.829

There it is! A 383 with an 11:1 compression ratio, using 64 cc heads, becomes 11.83:1, with a swap to 58 cc heads.

Whoever quessed, "about a .75 increase", wasn't far off. I think we've got it solved, finally. Again, I appologize for the mistake in my earlier post. I got my tongue wrapped around my eye teeth, and I couldn't see what I was saying. Sorry! Good luck all, and...

RACE ON!!!


[Modified by CFI-EFI, 9:27 AM 11/2/2003]

 

Sorry about that...duh...383=displacement==swept volume.

 

No problem. It only took 4 days, with a few mis-cues, along the way, but we got'er done. Good luck, and...

RACE ON!!!

 

See what I mean. I told you there were other ways to calculate CR. More than one way to skin a cat, ya know.

Some of this I've never seen inprint before.

Explanation: The numbers I used were as an example, not for his engine. He has to plug in his own numbers once he knows them. I used my numbers because I know them; I don't know his. 3.832 is an off-set ground 400 crank's stroke.

I'm sure most of you have camshaft catalogs or other books that give the procedure for calculating CR.

Lots of folks get confused by the term swept volume, thinking it means only that area that the piston travels. In fact, it is the entire volume that the cylinder would hold with the piston at BDC. It includes everything from the valves to the piston deck, including the valve notches.

In other words, if the heads were bolted on the block, with the piston and gasket installed and the pistone at BDC, how many ccs of liquid could you pour in there. That's the total space that gets compressed as the piston rises. All of that volume is referred to as swept volume; of course a rose by any other name . . . . .

That, too, explains why boring and/or stroking and engine and leaving everything else the same increases CR. Don't ask me how many times I've had to argue that simple point.

Compressed volume is that area which exists when the piston is at TDC and includes the volumes of the valve notches, deck height, head gasket and combuston chamber. When the piston is at TDC, how many ccs of liquid will the cavity hold?

Some folks even go to the trouble of calculating the dead space between the piston deck and the top ring. Talk about fanatics for precision!

As someone said earlier, the math is simple. Plug in all your numbers then divide the swept volume by the compressed volume. Bingo!

Most of the small differences occur based on when, during the calculations, the numbers are rounded. The more decimal places, the more accurate and consistent the different ways of doing this become. But if you round each number that's plugged in, you get, what in the engine building arena is called tolerance stack.

For example, using the constant .7854 gives you cubic inches - To get the CID of an engine it's bore X bore X stroke X .7854 X 8= CID. Using 16.39 converts cubic inches to ccs, but since 16.39 is only carried to two decimal places and is rounded, tolerance stack creeps in.

A couple of weeks ago I came across an article on this CR subject in a mag and it uses a different constant that eliminates using .7854 and 16.39; I sat down with my little Texas Instruments and calculated my engine using both methods; the difference was miniscule and all due to rounding.

Just trying to help.

Jake

 

CFI-EFI, I read your response with interest. I just can understand how you came up with the swept volume being 78.379.

I see that it's 10% of the swept volume, but how did you arrive at 10% to use?

With 64 cc heads and a head gasket that has, say, 9 ccs of volume (taken from the FelPro package for my gaskets) you end up with 73 ccs. We we need to come up with 5.379 ccs from somewhere.

Assuming a flat top piston with no valve notches and a zero deck height (which ain't likely) we wouldn't pick up any ccs.

Assuming a flat top piston with 3 ccs of notches and a zero deck, we're getting closer, but still not there.

Assuming a dished piston with, say, a 12 cc dish and a zero deck, we're now over the top.

Now if we begin to play with the deck height we open another can of worms.

Take me to school.

Jake


[Modified by JAKE, 6:34 PM 11/2/2003]

 

Jake,

You son of a gun. Just when we get this all scienced out, you pop up with something like this:
YOU are the one that's confused. Swept volume means the volume swept by the piston. What you are describing in the quote is total volume. The total volume is the swept volume plus the compressed volume. If you disagree, what do YOU call the volume swept by the piston? On to other issues.
In your second sentence in this quote did you mean to say "can't" instead of can? The compressed volume being "10%" of the swept volume (my definition of swept volume) is simply a by product of it being, one tenth (1/10th). The one tenth is derived by the problem at hand. We are told that the origional engine has an 11:1 compression ratio. That means that the unknown compressed volume is one eleventh of the total volume. If all the volume (total volume) divided by the compressed volume is going to be 11, then, the compressed volume must be one eleventh of the total. Ten tenths (swept volume) plus one more tenth, (compressed volume) makes 11 units. The compressed volume has to be one 11th (11:1) of the total. If we had been solving for a 10:1 compression ratio, we'd have divided by 9 inorder to get an equal 10th unit. Is this explaination clear? I know what I'm saying, but I'm not sure I am conveying it well.
None of this last quote matters. WE'RE not playing with anything. We were given numbers and asked to solve a problem. WE are not building this engine. We don't know or care where the various cc's came from. We are told that the heads are 64 cc's, and we know, by figuring it out, that the compressed volume is 78.379 cc's. That's IT!
I hope I have been able to make myself understood. Follow-up questions, allowed. Class dismissed Good luck, and...

RACE ON!!!


[Modified by CFI-EFI, 12:51 PM 11/2/2003]

 

I'm an IDIOT! Yea, and I admit it too.

You know how it is when you do something a certain way for so long that you think it's the correct way? Well that's what happened to me.

You guys were right and I was wrong.

Swept volume is the piston travel from BDC to TDC, the volume of that area. Compressed volume is all the remaining space above the piston when it's at TDC, which includes the valve notches, deck height, gasket volume and combustion chamber volume.

So you add swept volume and compressed volume together and divide that total by the compressed volume.

For so long I'd just added SV and CV and called it SV that I forgot "That ain't right".

So I'm an idiot.

Sorry for any confusion this may have caused. Now I'll go sit in the corner with my dunce hat on.

Jake

 

Oh, Okay. I just read your last post and I now see the math you're using. You are taking the assumed CR to be accurate and working off that.

Well, as I said there's more than one way to skin a cat.

In fact I just received Air Flow Research's new catalog and on the back cover they have another, never seen before by me, method of calculating CR.

SV = 3.1416 X bore X bore x stroke/4
CV = (chamber volume - dome volume + deck clearance volume + gasket voulme) X .061.

So in skinin' this cat we've all got to remember to make sure it's dead or we'll get all scratched up.

Can we put this to bed now?

Jake

Jake

 

NO !, Jake, you're NOT an idiot. You just made a mistake. Boy, I'm glad *I* never did that. :rofl:
BINGO !!!
We have to. There is no other information given. It is an, "If this, Then what", type of question.
G' Night

Has anyone else noticed that the inquirer seems to be nowhere to be found?

Good luck, all, and...

RACE ON!!!

 

[SNIP]

Has anyone else noticed that the inquirer seems to be nowhere to be found?

Good luck, all, and...

RACE ON!!![/QUOTE]

Yea, I did. Strange set of affairs. Anyway, it served to clear the fog from between my ears.

Jake

 

Hopfully, it was a benefit to others, too. I know it did ME good to go through it. Peace, good luck, and...

RACE ON!!!