I was wondering if anyone could help me figure out my compression ratio?
I have a 350 olds bored .20 over with the stock dish type pistons. I have # 6 heads with 3 angle valve job milled .015. I was going to go .030 but my head said they heads looked like they had been milled before. So I guess lets just say they are milled .015 since i am not sure if they had milled before or not. I also used felpro blue head gaskets. I am not sure what else you may need to figure this out. I am not sure what the cc on the heads were stock.

Thanks in advance, Mike

 

Sorry for the bad sentence's, I am on my work computer and trying to type quick and be discrete about this.

 

http://users.erols.com/srweiss/calccr.htm
Try here in case you left out some info in your post.

 

How do i figure out my compressed gasket thickness? Here is the part number from summit on the Gasket, FEL-8171-PT1. Does anyone know the combustion chamber sized on # 6 heads milled .015 with a 3 angle valve job? On the CR calculator when it asks for Piston Dome Volume in CC's use "-" for Dished Pistons what exactly is it asking for? I found this for the pistons, Compression Distance 1.596(in). And how would I enter it? Last but not least when it asks for Piston Deck Clearance If ABOVE deck use "-" what would I put? I did not have my block decked.

Thanks, Mike.

 

This should be calculated while the engine is apart. Otherwise, our guesses will be inaccurate.

piston to deck is how far below or above the bores the top of the piston sits\ at the highest point of its rotation.

 

This one is .045.
Stock head is 64cc. 4.057"X.015" volume is 12.7cc. 64 - 12.7= 51.3.
Most accurate way is to cc the head.

Piston Dome Volume is the volume that the top of the piston adds to the combustion chamber sixe. If it has a dish or valve reliefs you use the volume of those holes. The manufacturer of the piston can tell you what these numbers are. If the piston is perfectly flat the value is 0(zero). If the piston has a dome, the volume of the dome is -(minus).

Piston Deck Clearance is the distance from the flat surface at top of piston to the block deck. Piston manufacturer specs again.

Without all this info it's just a rough guess.

X2 J-

 

Yeah I know it's late in the game to be getting a completely accurate number here. I called my machine shop and found out my piston dish is 220 thousands, and I figured out my gasket numbers. Now I need my combustion chamber volume in cc and piston deck clearance. I know both of those will be guesses. That is why I am asking you all so i can have an educated guess on those. My block was not decked if that helps and i used stock type pistons bored 20 over.

 

You need dish volume not dish depth. Call the machinist again for dish volume and deck height or find out what the brand and part # of the pistons are so you can figure it out.
Piston deck clearance should be .0415.

 

Unfortunately, the .015 cut is not known for sure. Most clean up cuts will be .005-010" but you can not be sure at this point. If you can measure the short, lower outside head bolt boss (like by the number...) it should be 2.200 from the deck to the top of the boss's surface, from the factory.
This is not a 100% guarantee.

#6 heads were 68-70 cc's from the factory, most at 70 cc's. Even if you you did have a clean up of.015", you would only be to 68, but that is before the valve job. A 3 angle valve job will easily ADD 1 or 2 cc's to the chamber volume, depending on how aggressive the machinst grinds the seats in the heads, and the top 30 or 15 cut will usually add a bit more.

You could easily be at 72-73 cc's, and not know it.
No way to know until you have them measured.
JMO
Jim

 

Apparently I can not do the math here today. Stock head is 64cc. 4.057"X.015" volume is 12.7cc. 64 - 12.7= 51.3. So what would it be with the bore being 20 over on the block.

 

NO IT IS NOT.
Sorry...
Jim

 

WOW, don't use that.
TRY... for every .006 cut, remove 1 cc.
You still do not know how much volume was added with the valve job.

I do not know WHO started the 64cc figure, but they were not measuring factory, uncut 350 Oldsmobile heads. FACT.
Only 64-67 330 heads are in that ballpark, ALL #5,6, and 7 heads are 68+ cc's to start with.

Jim

 

Thanks. I found out I have the Keith Black Silv-O-Lite pistons. The part number is 1633.

 

Where do I find the piston volume with those? The machine shop said they were 8.4 to 1. I know my heads will change that.

 

The tech page says 64cc. If most of that info is incorrect, which it seems to be, maybe that page should be corrected or deleted.

 

Also what is the stock deck clearance?

Thanks, mike

 

You probably can, it's me that can't. I think the math is right if it was a gasket thickness volume formula but it's not. Don't know where my head was on that one.

 

Your math is off bigtime, 4.057x4.057x.015x12.87=3.177cc, not 12.7. And a #6 head is typically 68cc not 64.
.006 off the head surface normally decreases cc by 1, .015 would equate to roughly 2.5cc like Jim said.

Like J said all this would be a guess but I'll bet you have about 65cc chambers(after being milled .015), 14cc dish pistons that are .025 in the hole and at least a 10cc head gasket.

If you do the math I get, swept volume=725,+65+14+5+10=819/94=8.71.

I think this may be a bit optomistic but without checking who would know.

 

It must be the cold affecting my math. I have to blame it on something.
Being that my calculations were way off were the definitions at least correct?

 

No problem, just looked up that piston, 2.92x2.92x.220 dish. That comes to 24cc. Your compression might be 8.0:1, maybe. You'll need to mill a whole lot more than .030 off those heads to get where you need to go.

Definitions? CC is borexborexstroke(or thickness)x12.87.

 

I meant my definitions that you quoted(Piston Dome Volume & Piston Deck Clearance)

 

Those are some low numbers. I already have the motor in the car and driving. It feels ok. I guess I will drive it for a while then pull the heads back off and have them milled more. I hate having to this over but at least I have learned from this one. So what would I need to mill off those heads to get around the 10's? And would it be worth doing?

 

No.

 

Yes, except the original block deck height would still be a guess. There is no hard fast figure that I know of that's an absolute. You can't say they'll all be 9.325 or 9.330 etc.

 

Fully agree with you. Tolerances + or - all add up in the absolute equation.

 

Lesson learned. Next build will definitely be flat tops or something better than the dish pistons.

 

One last question for you guys. Would adding OAI with the lower compression help out a lot? I am looking for something that will help this motor out without me having to get back in to it and open it up. The motor only has about 100 miles on it. I will post some pictures of the build also.
Thanks, Mike

 

Turbos and S/Cs work great on low comp engines. Don't think you wanna go that way but I thought I'd just throw that in.

 

Nitrous.

 

When using cast pistons "Just say 'No' to Drugs."

 

and artificial resperation.

 

Ok I thaught that those options would probably be to much for cast pistons. But what about ram air? Would that work?

 

Ram air has no boosting effect until well over highway speed limits.

 

So im S O L.

 

Your original post was about what your CR was. Cam and CR need to work together. Noone asked about your cam. If its stock or close there to you will be ok.

 

Well any guru's want to to guess the hp and torque? There is a dyno down the road from where I work that I plan on putting the cutlass on in. They charge $45 for the first pull and $25 the second and so on and so on. Im sure it wont be great but it will be nice to know for sure.

 

You can do a small shot of nitrous(75) or 4-5lbs of boost with cast pistons.

 

I have the comp cam high energy 268H part number is 249-CL42-229-4.

 

317 HP my guess.

 

So what would be the correct nitrous kit if i was to go that route?