Compression Recommendation for a 1965 BB

[Joel T.]

Hi All;

I'm getting ready to rebuild a 1965/396, using a "restoration" block. As I recall, stock compression for the '65/396 was 11:1. Given that I will need to get a new set of pistons, should I stick with the stock compression or drop down a bit to say 10.5:1??

I want the car to be street-able.... i.e. Sunday afternoon cruising...

Thanks,

Joel

[Mark K.]

Re: Compression Recommendation for a 1965 BB

For what it's worth, I have an original (unrestored) 435HP 67 also with the same compression ratio (11:1). Even though the compression on our engines is nominally 11:1, the overlap on the camshaft pretty well ensures that the effective compression while cruising is well less than 11:1. I never need to use octane booster (no detonation) and the octane from high test at the pump (93RON) is enough for me to explore those occasional 6500 RPM moments.

[Duke W.]

Re: Compression Recommendation for a 1965 BB

Specified compression ratios are about as real as advertised horsepower in that era.

With the OE cam, a TRUE CR of about 10.25:1 is okay on premium unleaded fuel without having to excessively retard the spark timing. This is likely achievable with the OE type pistons and selection of an appropriate head gasket thickness to achieve the desired value or range.

You must start by measuring block deck height. Then use cranktrain dimensions to determine deck clearance. Combine with piston volume and head chamber volume, and you can accurately calculate actual CR with a CR calculator using different gasket thicknesses to achieve your objective.

Anything else is just GUESSING! You have to make the requisite measurments and run the calcs if you want any semblence of accuracy. Too high and you could have a detonation problem. Too low and you are leaving part of the torque curve on the bench.

I've posted a link to an online caculator that I used many times.

Duke

[Terry F.]

Re: Compression Recommendation for a 1965 BB

Duke,

If you are trying to build a nice engine in your restored corvette, is it worth trying to match the CC in each chamber of the cylinder head? I am a total amature at this so bare with the question. Thanks, Terry

[Mark #28455]

measure installed piston height

With a "restoration" block, it had to be decked (if it has "full length" new broach marks). The reason stock engines had lower compression was that few original blocks were truly the correct deck height. Most were a tad too tall and lowered the compression. If you rebuild your stock rods, this will help as the piston will sit a little lower in the bore.

You need to mock up your engine assembly and accurately measure how far above or below deck the piston really sits. Then you can calculate the actual compression ratio of the engine. Fel Pro makes composition BB head gaskets in 0.039", 0.041", and 0.051" which are easily available. There is also a 0.061" out there but a lot harder to get. Don't just get the thickest one hoping it will be the "best".

Mark

[Duke W.]

Re: Compression Recommendation for a 1965 BB

For a precision restoration build, my objective is to run as much compression as you can get away with and not have a detonation problem, which means you want the CR to be as high as possible and near equal on all cylinders.

For a NHRA build the point is to maximize compression within the rules, so the objective is somewhat different, but requires a similar approach.

If deck height is equal on both sides then the same compressed thickness head gasket can be used on both sides. If unequal, different thickness head gaskets can usually be found to minimize the difference.

For the head chamber I recommend measuring them and grinding the chambers as necessary so that the difference in volume from smallest to largest is no more than 1 cc. For an NHRA build you equalize the chambers to say 0.2 cc and then mill the head to get them down to minimum size - whatever the rules say, so you end up with as much compression as possible. Same with the block, you deck it to get down to minimum deck clearance. The point is to get as much compression as possible by bringing dimensions and volumes down to the minimum allowable.

On a restoration engine you need to manage CR to a maximum value that will allow the engine to operate detonation free on unleaded premium without having to resort to excessively retarded timing.

The above attention should keep the range of max to min compression at no more than 0.2, which should be okay.

As far as performance is concerned the single biggest improvement you can make is to pocket port/port match the heads/manifolds and run mutilangle valve seats. All this has been documented in many "how to" books for both SB and BB including the Chevrolet Power Manuals. Such work will gain in the range of 10 percent more top end power and 500 more useable revs without any effect on idle quality or low end torque.

Most of the vintage engines are well engineered, but low cost mass production prevented head ports from being optimized, and that's were a restorer can really improve engine performance with no compromises or deviation from engine visual appearance or operating character.

Duke

[Terry F.]

Re: Compression Recommendation for a 1965 BB

Thanks! I asked the question because I heard there can be considerable variation from each cylinder head chamber. I might measure mine for the fun of it and see what I get. Terry

[Duke W.]

Re: Compression Recommendation for a 1965 BB

Yes, the biggest variation in CR is usually due to side to side deck height differences. It's not unusual to see .010" and that's a quarter point right there.

The crank throw radius, rod length, and piston compression height are usually held to +/- a thou or less, so they are usually not a source of great variation.

Head work usually increases chamber volume a bit (like bevel grinding chamber overhang), so it's a good idea to cc them and get them within a maximum variation of 1 cc. Once cc on a typical SB is about one-eighth of a point on a SB, less on a big block.

Since the CR is the ratio of a large volume to a small volume, a small change in the small volume can have a significant impact on CR.

With a SHP cam that has a fairly late closing inlet valve, you need a decent CR or the engine will lack torque and have unimpressive power. A lot of SHP engines have been seriously compromised over the years by installing "9:1 pistons" and a thick head gasket which probably yielded a CR of no more than 8.5:1, but then most "engine builders" never make the proper measurements and run the numbers to actually calculate the as-built CR much less actually try to manage it to a specified value or range.

I guess if you want to run regular unleaded it's okay, but the engine will be a dog compared to one built with a proper CR. A back to back drive in a '70 and '71 LT-1 would give a good indication. With current unleaded premium most vintage engines will run just fine if CR is held to about half a point below spec on SBs and a little more on BBs, and this is usually manageable with the OE pistons and proper head gasket selection.

Duke

[Terry F.]

Re: Compression Recommendation for a 1965 BB

Is a three angle valve job standard these days? Thanks for the info. I would like to get about 95% of the max compression ratio on my engine. I will take my time in rebuilding it. Can I micro polish my crank my self?? Terry

[Duke W.]

Re: Compression Recommendation for a 1965 BB

Multiangle valve seats with proper widths for performance applications is a time consuming task is is not part of a "standard" rebuild.

You should get "How to Hot Rod Your Big Block" Chevy, an old Chevrolet Power Manual, and maybe some other books. There are lots of titles out there on performance engine building, many of which are specific to engine architectures like SBC or BBC.

The Chevrolet Power Manual has a photo showing hand polishing crankshaft journals, but I think you're better off having a machine shop do this with the shaft chucked on a lathe.

Once you're up to speed, put together a definitive plan including what replacement parts you want to use for anything that needs replacement, what outside services you will need for measuring, machine work, etc. and be sure that you select a machine shop you can "manage" i.e. they will do only those operations that you specify. This can be tough as in some cases, no matter what you say or do they will do it "their way" and you could end up with a compromised engine.

Duke

[Clem Z.]

do not forget a back angle cut on the valve head

as this helps low lift flow a lot.

[Terry F.]

Re: Compression Recommendation for a 1965 BB

I have seen those books mentioned on the board. I will get them. There is a definite limit as to what I will do to this engine. But, what do, I want to do right. It helps to know a little about what you are talking about. Also, I care about the engine so I just what to apply a higher standard than factory assembly. Terry