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Old 04-07-2009, 06:13 AM
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Stock 350 Rebuild

Hello All. I have been lurking thru the forums for the past month or so preparing for my first 350 sbc rebuild. It is standard 3.48 stroke bored .030 over with a set of stock 041 heads and springs. The block has not been decked and from what I can tell so far it will have about a 10:1 compression ratio.

I was looking at the Pure Energy 246PE cam from comp. It will be going into a 4dr 68 Bel Air that weighs around 3500lbs and will be used as a weekend crusier. Will this cam match up well with my setup? I am not looking to burn the tires of of it, just want a smooth running crusier that I can talk the "ole lady" out on the weekend.

http://www.compperformancegroupstore...ory_Code=HFTPE

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Old 04-07-2009, 11:44 AM
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That cam will work better with 9.0:1 than it will with 10.0:1. Are you sure about the static compression ratio?
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Old 04-07-2009, 08:04 PM
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Techinspector...

You know it is funny that I get a reply from you. Normally while lurking thru the forums I dont pay too much attention to the names of the people that post except when I see replies that is good information for a first time rebuilder like me, and I often see that you are the one that has posted that information and I keep it as a note. So I appreciate and respect your reply.

This is what I have - and help me out if I am jacked up here. I used the formula to calculate compression ratio from an online Car Craft article (I am sure you have seen the one I am talking about - it is a popular link here in the forums). Here are my calculations:

Cylinder Head volume = 44.39
Clearance Volume = .17
Piston Dome = .31
Head Gasket Volume = .52
Chamber Volume = 3.91

Compression Ratio= Cylinder vol. + clearance vol. + piston Comp. vol. + gasket vol. + chamber vol. divided by Clearance vol. + piston vol. + gasket vol. + chamber vo

CR = 10.06

What I am not sure about is the piston volume. The formula is piston cc * .0610237. I am using Seal Power Pistons (http://store.summitracing.com/partde...0&autoview=sku) and summit lists the piston head volume as +5.00cc. 5cc*.0610237 equaling my piston dome of .31. Is 5cc the correct number I should be using in the calculation?

Thanks for the help.
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Old 04-07-2009, 08:26 PM
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negative 5cc's. not positive, its for the VR's
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Old 04-07-2009, 08:34 PM
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ap72...

Can you explain why it would be negative? It is a flat piston. I thought a dish piston would make it a negative number, no???
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Old 04-07-2009, 08:36 PM
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Also, negative 5cc's gives me 11.35:1 ratio. That seems kind of high. I am not doubting your reply, I just dont understand why it should be a negative number.
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Old 04-07-2009, 08:50 PM
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Yep, looks like you got it figured out correctly. Same number I get.

You're starting off with a static compression ratio that is too high for what you want the motor to do.

You have two options. Use heads with larger combustion chambers or use dished pistons. Personally, I'd keep the pistons and find some 76cc heads, but the option is up to you. At any rate, you need another 12cc's, either in the heads or the pistons. That will put you at 8.9:1 and the cam you showed here will work great. If you were to stay with the 10.06, you'd have to use a cam with somewhere around 230 degrees duration @ 0.050" tappet lift and the operating range would be somewhere around 3000 to 6500 rpm's. The bottom rpm's would be soggy and you'd need more gear (3.70-3.90) at the rear and a loose torque converter. (3000 stall). I suspect mama wouldn't enjoy it much at all.

Last edited by techinspector1; 04-07-2009 at 09:09 PM.
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Old 04-07-2009, 09:11 PM
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Quote:
Originally Posted by cprice72
Also, negative 5cc's gives me 11.35:1 ratio. That seems kind of high. I am not doubting your reply, I just dont understand why it should be a negative number.
Below the crown is positive. Above the crown is negative.
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Old 04-08-2009, 07:13 AM
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Sorry, I wasn't familiar with the correct connotation, I write my own compression ratio program and use a positive value for a dish, it makes more sense to me to do it taht way but I guess its againt convention.
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Old 04-10-2009, 07:49 AM
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ap72 - no worries man. I was just making sure that I understood.

techinspector - as much as I would like new heads and I understand why this would be a better option, economically (and the coupled fact that this is my first rebuild) I am going to get some new pistons.

Now here are a couple questions that I have. If I purchase dish pistons, will this also increase my deck height (right terminology? the distance from the top of the block to the top of the head)? Also, in measuring my deck height now, I am getting different measurements at times, I have been told that the piston will rock on it's axis where it is connected at the rod. Is this true, or did my machine shop do a bad job of connecting the rods and pistons? Also, what is the most compression that you would run this cam, 9.0:1? 9.3:1?

Thanks for the help!
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Old 04-10-2009, 12:48 PM
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First, let's get the terminology nailed down.
BLOCK DECK HEIGHT: The distance from the centerline of the main bearing bore to the block deck where the heads bolt on. Nominally, on a small block Chevy, this measurement is +/- 9.025".
PISTON DECK HEIGHT: distance from the crown of the piston to the block deck where the heads bolt on with the piston at top dead center (TDC). The most accurate measurement here will be taken at the 3 Oclock or 9 Oclock point on the piston as you're standing at the side of the block. Measuring at Noon or 6 Oclock can allow the piston to rock a little on the wrist pin and give you an erroneous reading. This measurement will usually be anywhere from 0.045" to 0.000" with the piston down in the hole. If the piston is popped up out of the hole at top dead center, this measurement could be anywhere, but would normally be no more than 0.025" above the deck. It would be very unusual to find a negative piston deck height such as this. It might mean that the block has been cut excessively or that the previous builder used a custom piston with a taller compression height piston.
PISTON COMPRESSION HEIGHT: distance from the centerline of the wrist pin to the crown of the piston. Nominal spec on a 350 SBC will be 1.560". Cheezy rebuilder pistons for a 350 will be in the area of 1.540". The manufacturer produces these pistons with a shorter compression height so that a rebuilder can cut the block decks for a shorter block deck height and have the piston deck height remain the same as it was before the rebuild. For us hot rod types, these shorter compression height pistons are to be avoided. Occasionally, you will find a piston with a compression height that is taller than stock, for instance 1.561" or 1.565". These pistons are desireable (as long as everything else about the piston suits what you need for the build) because it means you can run a tighter piston deck height without cutting the block decks to get there.
SQUISH: the measurement from the piston crown to the underside of the cylinder head with the piston at TDC and with the head gasket in place. (add piston deck height and compressed gasket thickness to arrive at squish). Many professional engine builders will recommend 0.035" to 0.045". It is desireable, in my opinion, to use a piston with a large, flat squish pad area on the crown to facilitate this mating of crown to head. Dish pistons with the dish cut all the way around the crown and leaving only a thin ring of material around the perimeter of the crown (right above the top ring area) have insufficient area on this flat ring to "jet" a good squish of fuel/air mixture across the chamber toward the spark plug. In my opinion, Keith Black hypereutectic pistons are the most desireable in this respect. They use a D-cup rather than cut the dish all the way across the crown. This D-cup leaves a generous flat area on the crown to mate up with the underside of the head. There are, of course, other manufacturers who configure their pistons in such a manner.

Dish pistons, in and of themselves, will not alter the piston deck height. It is the piston compression height (centerline of wrist pin to crown) that will alter the piston deck height (piston crown to block deck).

Ideal static compression ratio for use with the cam you linked will be 8.9. I would run it if the SCR worked out anywhere between 8.7 and 9.2, as long as you pay attention to the squish and get it right. In my opinion, getting the squish tightened up to 0.035/0.045" so that you can run a decent dynamic compression ratio and still operate without detonation on pump gas is the most important part of engineering the build. Other hot rod junkies like myself may have a little different take on it, but that is my opinion.

With a stock +/- 9.025" block deck height, Victor Reinz #5746 composition head gasket (4.100" X 0.025"), +/- 64cc heads and these pistons,
http://kb-silvolite.com/performance....tails&P_id=154
the parts stack would add up to 9.001", static compression ratio should work out to around 9.1 and squish would be 0.049". Close enough in my opinion. All this applies only if the block deck height measures 9.025". Less block deck height would mean increased static compression ratio, tighter piston deck height and lower squish. It is also desireable to cc the chambers in the heads. You don't have a clue what previous owners have done to them. They may have been over volume when produced and measure out at +/- 66cc's or they may have been cut and measure out at +/- 58cc's.

If the KB's are too expensive for you, find another piston with a 18cc dish and a good squish pad or wide ring.

You can't assume anything on an engine build. You must measure everything yourself and do the calculations if you intend doing it correctly.

Last edited by techinspector1; 04-10-2009 at 05:51 PM.
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Old 04-12-2009, 12:18 AM
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Thanks for the reply. I found a set of those pistons on summit and will be ordering next week. From my calculations, depending on which gasket I run and the piston deck height, it will give me anywhere between 8.92:1 and 9.13:1 CR which should run fine with that cam it looks like. Thanks for the help, I appreciate it.
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Old 04-12-2009, 01:00 AM
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Quote:
Originally Posted by techinspector1
Below the crown is positive. Above the crown is negative.
Unless you're using an online compression calculator... some of them ask you to put in negative numbers for piston dish, some ask for positive numbers since it adds volume. You just have to read the fine print, or try it both ways until you get the logical answer.

But you're right... when doing it with the paper/math method, dishes are postive numbers.
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Old 04-12-2009, 02:01 AM
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Thanks Curtis, yeah, just another of my idiosyncrasies. I insist on figuring it myself with a pencil, paper and my desktop calculator. The piston manufacturers all use the negative figures when talking about domes and use positive figures when talking about dishes, so that's where I take my cues from.
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Old 04-12-2009, 02:46 AM
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Compression ratios

If I remember correctly, most 041 heads we have seen come through our shop are close to 68cc combustion chambers, putting you in the 8.68 range with the 18cc dish in the KB pistons.
I made a little Excel file (actually Lotus 123 back in the day) to figure compression ratios that does a pretty good job of it. You have to enter DISH piston cc's as a NEGATIVE number and DOME piston cc's as a POSITIVE number.

Also, the H.G.V. figure it is asking for is in thousands of an inch. Seems like I would have gotten that labeled correctly in the last 18 years.



The Excel spreadsheet the photo is of. Just unzip it and put your numbers in the DARK GREEN boxes and it will tell you the compression ratio.

MrChevyCR.zip

I think I have these attachments figured out, I think you can just click on the .zip file and save it to your harddrive and unzip it.
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Last edited by MrChevy; 04-12-2009 at 02:48 AM. Reason: Misspelling, of course.
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