I am having trouble reconciling the compression ration for my 87 firebird with a 305 carb engine. GM calls for 9.3:1 which is all good, but the numbers don't come out right. The head are 52 ccs (measured), the deck height is .025 (measured). The pistons are dished and they measure 12 ccs but that includes the dish, the taper,the volume to the top ring and the deck height volume. In order to get 9.3:1 I need to use a .025 deck AND the 12cc volume plus I need to use an (estimated) .038" compressed height head gasket (6.83 ccs)to come out correct. The problem is the head gasket I removed is a steel shim type (original) which has a compressed height of .022" I don't have an actual volume for this but you can bet it has to be less than a calculated .038". I'm also double counting the deck volume so I'm not sure if 9.3:1 is theoretical like rocker arm ratios, or if GM actually measured. If I subtract the deck volume ( approximately 4.5 ccs) and calculate a rough volume for the steel shim gasket ( allowing on the large side to be fair) I get a ratio of 10.11:1. Can anyone help me find my error? Remember I'm using actual measurements for the piston from the top ring to the deck and for the head. Only the gasket is being estimated and I have to make it WAY big to get the factory numbers. Thanks.

 

9.3:1"-ish".

Production tolerances could give you a lot of variance. They tended to error on the conservative side.

58cc is the "nominal" 305 head chamber volume. 52cc sounds like a 601 head, which wasn't used on 3rd gen f-bodies, and definately wasn't used in '87. Do you have the head casting #'s?

 

These are 081's. I have about 4 complete sets and they all check at 52 cc plus or minus one cc. I have some 601s that check at 58 cc. I wonder if you (or anyone out there) knows what the stock dish on the 305 is supposed to measure. I can't seem to do it right cuz the liquid runs past the valve notches. that's how I wound up cc'ing the whole dis/deck volume. It was easier.

 

I get 9.53:1 based on your numbers. i guestimated the diameter of the factory .022" head gasket at 4.10" (same as the 350 shim)
If you use a .038" gasket the cr will only be 9.14:1

52cc head stock bore size .025" piston deck cleareance -12cc dish volume 4.10"x .022" gasket
Even if I'm off on the head gasket diameter the cr is still not close to 10:1

 

Odd. Typically reported as the other way around. But, then, my 305 Worlds turned out to be 64cc chambers even though "58cc" is cast into them.

12cc sounds close. The typical SBC head gasket is 4.130" (roughly) diameter. Be sure to include the extra diameter when doing your calculations.

 

If you include the piston to deck clearance volume in you stated 12cc volume then you do get 10:1

 

What is the depth and diameter of the piston's dish? Typical 4 valve style pistons reliefs are usually 4 to 5 cc's.
If you can measure the depth and diameter of the pistons dish you'll probabily be able to calculate the volume better than measure it in cylinder.

 

The gaskets, although not perfectly round, are no larger than 4.060 at the widest point. I decided to measure the whole "deck-to-top-ring" volume since it wouldgive me as close to accurate working data as I could generate. Only the head gasket seems to be a problem. in order to keep the CR near factory, the head gasket would need to have a larger volume than it's calculated volume, even if I make the volume larger by allowing the bore to be calculated as round at it's largest measurment. In order for the measured numbers to reconcile, the head gasket MUST have a volume of 6.83 ccs (calculated). It is probably closer to 4.75 ccs
I can't get an accurate depth or diameter from the dish since it is tapered, I can only guess. Since my mearsured "deck-to-top-ring volume is 12 ccs and the deck height volume is approx. 4.5 ccs, I could accept 5 ccs as the dish volume as this would leave 2.5 ccs for the chamfer of the piston top and the gap down to the top ring. I have also seen the 5 cc figure in reference to factory 305 pistons, though I can't seem to lacate any links to support this.
Further confusing the issue are the Monte Carlo SS forums that discuss the lucky 305 owners with factory flat tops vs dished. ??? Based on the conundrum I already face, they 58 cc heads would need flat tops or REALLY small volumed head gaskets to make the 9.5:1 the 305 H.O. was commonly spec'd as.
I probably wouldn't be so worried if I didn't have actual measured volumes that contradicted published data. If I can't trust my math then I can't tell the machinist how much I want decked from my block or how many ccs I need my Vortec heads to be in order to keep 9 or better to 1 . It would suck to spec a cam for a given compression ratio and find out it's all wrong because I made a mistake somewhere. That said I'm also not going to rely on GM as the final word either. anybody know how I can cc this steel shim gasket and try to get some better data? Thanks!

 

O.K. I decided to measure my 350 to use as comparison. Plugging in the the measured data, plus the published volume of the head gasket, i get axactly the published compression ratio. Soooo. I'm guessing I'm doing the math right. Now I'm really stumped.

 

You've totally lost me here. What the ... are ya trying to achieve. You can replace the pistons with flat tops for probabily less than getting the block decked.

You can calculate the volume of a tapered wall dish by taking the major diameter and minor dia and averageing them for a average diameter. forget the space between the piston and ring. You're making the very hard. Do not try to "spec a cam" based on a cr. pick the cam for the basic rpm range you want. use common sense to adjust. if your using a stock converter do not go wild on duration. Any thing areound 9.3 to 9.8 will get the job done nicely.
Total induction/exhaust airflow is much more important.

 

Sorry I lost you. Let me try to get back on track. I am trying to decide if I would rather invest time in porting my 081 heads, then have a valve job done and of course I'll need new valve springs... Or I could run the set of new in the box Vortec heads that would likely flow better than any work I could do to the 081s.
In order to do this I'll need to make some modifications so I don't lose compression. There is no value to replacing my pistons they are in very good shape. Since my bores show no wear and my piston clearances are way nice I won't need to waste money on having the block bored. My crank was turned and since it was no additional cost to have done, I had them offset grind it to 3.5". This gives me slightly more displacement and reduces my deck height .010". this one simple changes is worth .25 of compression with no other changes. If I have the block decked to 9", still cheaper than .030" bore and a set of pistons, My compression would jump to 9.88:1. Assuming GM's 9.3:1 figure is correct to begin with.

 

Sorry, I cut myself off.
Now, If I were to run a Fel-Pro .015 steel shim (P/N 1094 ? ) with a compressed volume of 3.2 ccs, I would only need to shed 3 ccs from the Vortec heads to retain the factory compression ratio. Several sites have discussed installing Vortec heads on a 305 but usually need angle milling and flat tops plus machining the block, machining the intake.....etc,etc. Angle milling is expensive. I have good parts, the crank is already done, all I need is the decking of the block and a small amount removed from the heads. This will come out to be far less expensive in the long run than angle milling, boring and pistons and leaves my block intact for a time when it actually needs to be bored.
All I need are accurate measurments, which includes ALL volumes, even if it makes the math "hard". Bottom line, GM's published compression ratio doesn't "add up" (pun intended) I believe the 305s with the 52 cc heads are actually closer to 9.9:1 and seem to have the proof to back it up.
As far as cam selection goes, compression ratio is not the only factor I use to select a cam, but "RPM ranges" published by most manufacturers are between inaccurate and wishful thinking, so I don't use that as a criteria .
currently I select a cam like this ( It's OK if people disagree with my method, it works for me and that's all I need): Select a lift where the cylinder head is likely (or known) to flow best. Vortecs are easy because there are so many published flow figures available. They are nice in the .400 to .500 range. I like .460 on the intake and .480 on the exhaust. Then I look at the profiles available in this lift range and discard any that are not recommended for the compression ratio I have. From this list I pick the shortest .050 durations and compare them to the advertised durations to find the highest intensity, then I pick the LSA no less than 111, no more than 114. this has always rewarded me with a strong bottom end, excellent acceleration, more than adequate vacuum and plenty of RPM range.

 

1. if the piston to wall clearance is as nice as your claim ( cylinder wall wear) then the space between the piston and top ring and cylinder wall is soo soo soo small that it is not relevant. You're just confusing your self.
Forget the space between the piston, wall and top ring.

2. do not assume your heads are as they left the factory. Everone elses 081's are around 58cc. Yours have been milled and or have has the valve job redone and or differnt valves installed. It makes a big differnence.

3. I can;t believe ya had a $2 305 crank offset ground. You're a sick man. But there is good medical help available.
See a doctor...

4. you can port 081's to flow more than stock vortecs.
(with 1.94 valves) I typically get mid 230's cfm and 243 best . So can you. lots of other people get more flow.

. you can typically flat mill a small block chevy head down to the intake valve seat edge or spark plug boss threads edge. Which ever is closest. Usually its around .060" and removes about 10cc ch volume. So you should be able to get a vortec head down to around 52-54cc with just flat milling. Check you 081's. is the deck flush with the intake valve seat edge. Would it be if the seat was .050" bigger 1.94" valves. I get 416's down to 52cc with flat milling after a little deshrouding for larger valves. NON stock valves will change the ch size a lot. A "high seat valves job will make the chamber smaller . Sunk seats ( deep valve job will sink the valves and increase the chamber size.

Use the 081's. port them fully and install 1.94" intake valves. You must maintain at least .035" between the piston and cylinder head (including the gasket) or it will smack the head at rpm. .038 to .040 is better.

6.If the cylinder walls are that good all you need is a light honing and slide new (or good used) flat tops in) cheap no boreing required. You can do it at home.
7. try camming your 305 as if it was a 350 ci SBC using Vizards method of determining needed overlap, LSA and duration required to fit what you're trying to achieve. think 108 LSA for max performance from you 305. if you build a proper exhaust system it will work very well.
Not saying you have to use 108 LSA but that is what will make the most power and torque. (with long tube headers)
A 305 needs overlap to breath because of the small restrictive bore size and limited usable valve size.
Compression does not start until the intake valve is closed.
reguardless of the compression ratio or what you think the cr should be, (therory) close the intake valves as soon as ya can.
You'll make more torque. the motor will act like it has more cr than the mech numbers suggest. take dynamic compression theory and calc and throw them out the window. if that theory worked in the real world you could just delay intake valve closing and buidl as much cr as you want and run 87 octane and make power and not detonate.
Too bad it does not work in the real world. I tried it. LOL
got the broken pistons to prove it. So did pros like Crower, General Motors , Fueling Vizard etc etc.

Work within realistic daily usable cr's for the octane you can get and make the motor move more air. Tune the cam to help the exhaust system pull air thru the motor. (overlap)
With cast heads any thing over aobut 10.3:1 will be real touchy.
A motor that is running in even mild detontion does not make as much power and will not last long. your beaututifull cylinder walls and rings will get destroyed even thou the pistons don;t break. How much power will you have then?

RPM costs money. more than you can afford.

8. measure the dish diameter and depth as I discribed and add 4 to 5cc for the 4 valve reliefs. it will be much more accurate than your water method.

9. so now the pistons are actually .010" below deck at TDC right. and the stroke is 3.50"?

10. Don't go and destroy a perfectly good set of vortecs by shaving them to the max for a stupid 305.

Sorry for the lecture but your bypassing way to much oppertunity to make real power improvements by fussing over the cr so much.
Do not assume anything. Do not read Hot Rod mags about angle milling heads. LOL
measure twice , cut once. When in doubt build a 350. LOL

 

No apology is needed for the the lecture man, it's all good. I have built lots of 350s, no fun anymore. Vortecs, just about any decent cam and boom, great street motor.
I also goofed on the deck height. I have increased the stroke by .020" so I get .010" closer to the deck. The deck height would now be .015".
I am sick, but no help needed. The crank was going to be turned anyway. In fact I have all my 3.48" cranks ground to 3.5" cuz it doesn't cost any more $$$.
I did calculate the dish of the piston per your advice and it comes out at 5ccs. I also through out the extra volume below the piston and used your 58 cc figure for the heads.Of course this only makes things worse...thanks alot (LOL) It is now impossible to get the compression ratio to come out right. I get 9.4:1. I don't kow about this head thing either. I have measured every single chamber on all my 081s. They all come out 52cc and I have one pair that is for sure original. Now what the heck do I do? I have a set of 601s that come out 58cc.
I don't want to do a max shave on the Vortecs, I'm only in need of a couple ccs. I just don't think I could get the same good flow numbers as you from my 081s. I'm only so-so at porting.
Last I have a cam that has 206/212 @.050" duration, .431" I and .480" E lift with 109 LSA. I think this should allow the overlap you mentioned I need.
I'm going to the garage to re-measure the 081s. I'm doing something wrong here.

 

How much can you shave off stock 601's to improve CR? This is for a circletrack claimer motor.

 

GM says don;t shave the head any more than .040".
Otherwise the deck gets a little thin. It may fail.

You can physically shave them right down to the edge of the intake valve seat, or the sparkplug boss thread edge. Which ever comes first. If you use a 2.02 valve with its larger diameter seat you won;t be able to shave the head a much as with a 1.94" valve seat.
I got a set of 416s down from a nominal 58cc to 52cc max flat milled.
These heads do not lend them selves to angle milling well because of the position of the sparkplug hole. You're pretty well stuck with flat milling them.
You can expect to be able to decrease the chambers of you heads by about the same all said and done (6 to 8cc net after some deshrouding for larger valves)
The finished size will depend on what cc they are now.
I was able to build a 12.65:1 compression 4valve relief flat top 350 using minimum deck clearance/ thin gasket and max milling 416's. Makes for a spunky claimer motor. Use 110 octane fuel.

 

Thankyou for the help. I was reading the port and polish thread. It said about putting an epoxy over some holes? which holes??

Also, I have heads off a 83 camaro Z28 601 heads with original 1.84/1.5 valves. An engine friend preaches high CR for a lowbuck restricted app. I know if I installed 1.94/1.6 I would be adding CC's. Is the trade off worth it? Are these heads 53 or 58 cc to start??????

I run a stock 2 brl 313 PP intake, roch 2 bbl, shorty headers, 3/8 mile oval 3000-6000 RPM.