BBCorv70

Questions for engine gurus...

I had the cam in my 70 LS5 replaced 20 years ago. The previous owner had installed a solid lifter cam. I asked the shop which did the refresh to install a cam closer to stock. Unfortunately I cannot find any paperwork telling me just what cam was installed.

When the engine was last rebuilt the machine shop owner suggested installing pistons which lowered the compression from 10.25:1 to 9.5:1, ease pinging issues with gas available at that time. I recall carefully checking volumes and accounting for gasket thickness, came up with something around 9.6:1. The heads are original, closed chamber heads.

This engine develops high pressure when measuring compression, all cylinders in the neighborhood of 210 lbs. May I deduce the cam has little overlap if I'm see these pressures?

I get a lower vacuum level than stock, something around 14 inches Hg, going from memory, could be slightly lower. There are no vacuum leaks which I've been able to find. Does the low vacuum tell me anything more?

I'm really beginning to wonder just what cam was put in there, whether it's appropriate for this engine. Is it practical to attempt to take some measurements with a dial gauge, find the degrees when the valves reach 0.050 lift on either side? Has anybody done this with the engine in the car?

540 RAT

Checking the cam you have is a good idea. The compression ratio you refer to is called static compression ratio. The fact is, the engine never actually see's static compression ratio. But, static compression ratio does have an affect the dynamic compression ratio, which is what the engine actually does see. The cam duration/overlap is a primarily factor that determines the dynamic compression ratio. Because cam's duration/overlap determines the all-important intake valve closing point.

So, you don't have to change the pistons to reduce pinging, you can just change the cam. More duration/overlap along with its typically later intake valve closing point, will lower the dynamic compression ratio and reduce pinging. Just the opposite of what some people think, who think you need a milder cam to reduce pinging.

Below is a write-up I did discussing compression ratio, if you'd like to see it:

Most every gearhead believes that he understands compression ratio numbers, and simply takes them at face value. The normal compression ratio that everyone talks about and see’s on spec sheets is technically called “STATIC” compression ratio (SCR). That is always “THE” compression ratio being discussed unless otherwise specified.

And it of course comes from:

The total cylinder/head gasket/combustion chamber volume at BDC (bottom dead center), which we will call “V large”.

Then divide that total volume at BDC by the combustion chamber volume at TDC (top dead center), call that “V small”. So, you have (V large / V small) = Static compression ratio. As the name implies, it is a ratio of the max total volume divided by the small volume.

The 4 strokes are of course:

1. Intake
2. Compression
3. Power
4. Exhaust

And that’s all well and good for textbook learning, but in a real running engine, things aren’t so cut and dried. The “problem” is that an engine never “see’s or feels” the static compression ratio number. So, that makes the static compression ratio more or less a theoretical reference tool.

The difference in a running engine is that the cylinder volume needed to determine a running or Dynamic compression ratio (DCR), is not calculated with the piston at BDC. It is calculated with the piston at the position where the intake valve just closes. It is only at this point, that true compression can actually begin.

Here are the Intake and Exhaust valve timing events at .050” tappet lift (meaning lobe lift or lifter lift, NOT valve lift), per my 540ci BBC engine’s cam card:

In. opens at 25* BTDC (before top dead center)
In. closes at 61* ABDC (after bottom dead center) = 266* duration at .050 tappet lift

Ex. opens at 64* BBDC (before bottom dead center)
Ex. closes at 28* ATDC (after top dead center) = 272* duration at .050 tappet lift

As you can see, there is overlapping everywhere. This is done to optimize engine performance by making use of dynamic intake charge ramming effects and dynamic exhaust gas scavenging effects. So, actual running engine specs don’t fit neatly into the basic idea of the simple and separate 4 strokes. In order to calculate DCR from a useful intake valve closing point, rather than the .050” tappet lift timing shown above from the cam card, you need to use the Cam maker’s advertised tappet lift value.

For my Comp Cams steel billet solid roller cam, the advertised duration specs are given at .015” tappet lift. But my cam card does not provide the actual intake and exhaust timing events at that .015” tappet lift spec. So, I manually measured and calculated the piston/crankshaft position at the intake valve closing point based on .015” tappet lift of my actual engine. I did this during engine assembly mock up, where I could also take into account valve lash, rocker arm geometry, and rocker arm ratio. By doing it this way, I ended up with very precise numbers, which were used to get the most accurate final results. But, to get numbers this precise, it required that I determine the actual DCR after the fact, rather than determining it before buying any parts. So, I had to make some careful calculations earlier, in order to end up as close to my target as possible.

I ended up with my intake valves closing at 80.5* ABDC (or only 99.5* from TDC, rather than the theoretically ideal of BDC or 180* BTDC). This position had the piston 2.805” from TDC.
And considering that my stroke is 4.250”, this means that my piston had traveled 34% up the cylinder before the intake valve had closed, and compression could finally begin. I have a fairly large bad boy street/strip cam, and the larger the cam’s duration, the later the intake valve will close.

Then to do the calculations for DCR, it’s from the total cylinder/head gasket/combustion chamber volume at the point of intake valve closing. Call that value “DV large”. Then divide all that by the combustion chamber volume at TDC, the same value that was used above to calculate the SCR, which was called simply “V small”. So, you have (DV large / V small) = Dynamic compression ratio. As the name implies, it is a ratio of the large volume divided by the small volume. It is of course the same process that is used to determine SCR, except for the DCR, the large volume (DV large) is a much smaller value. And since the TDC volume (V small) was used for both SCR and DCR calculations, you can see how changing that TDC volume will change both types of compression ratio’s. They are linked by that “V small” value.

After crunching all the numbers, I ended up with an actual running engine compression ratio, or dynamic compression ratio (DCR) of 7.43 to 1. So, my two compression ratio numbers are:

Static Compression Ratio (SCR) = 10.75:1, which is the one seen on spec sheets
Dynamic Compression Ratio (DCR) = 7.43:1, which is the one the engine actually see’s/feel’s

You can see that the dynamic compression ratio is a far cry from the more commonly referenced static compression ratio of 10.75 to 1. This 7.43 DCR is what the engine actually see’s/feel’s and is what primarily determines its octane requirement. And as you have seen by now, the cam and its intake valve closing point, is the primary factor for determining an engine’s DCR. Change your cam, change you DCR.

If your Hotrod is on the ragged edge of detonating/pinging, you could switch to a cam with more duration, which will reduce your DCR and make the engine less sensitive to the octane it requires, because of a later intake closing point. That is just the opposite of what some folks might think. Because they’d likely think if their Hotrod was on the ragged edge of detonating/pinging, they’d need a milder cam. But, that would be going the “wrong” direction. Because a milder cam with less duration, would close the intake valve sooner, increasing the DCR. And that would make the engine even “more sensitive” to the octane it requires.

As an example, my cam has a 108* LSA (lobe separation angle), and the narrower this is, the sooner the intake valve closes, thus upping the DCR. And my cold cranking compression checked out to be 175 psi. But another very similar BBC engine with the same displacement and the same SCR, but with a wider 112* LSA, checked out to have only 165 psi cold cranking compression, due to its later intake valve closing, and thus lower DCR.

General approximate guidelines for DCR, though not absolute, are that a DCR of 7.5 to 8.5 will make best power for a street engine running 91 octane or higher. And the lower the DCR is in that range the better, for avoiding detonation problems.

Note: Race engines using race gas, can tolerate higher DCR's up to about as high as 9:1.

As you can see, my 7.43 DCR came in quite close to the conservative 7.5 DCR number I had been targeting. I wanted to stay at the lower end of the recommended range so that my engine could tolerate California’s winter blend of pump premium, which has been known to fall below the octane number that we see with the summer blend. Call it adding a bit more margin of safety. Because detonation can cause ugly failures that you must avoid at all cost.

On top of that, I wanted to run a lot of ignition timing advance at low rpm, for crisp and quick throttle response. And staying at the lower end of the DCR range, allows me to do that without issue. It’s also no secret that larger engines, say upper 400 cubic inches and above, are big enough that they can absorb a low DCR and/or big cams with ease, so that you won’t even notice it.

BOTTOM LINE: The critical compression ratio that really counts, is the Dynamic Compression Ratio (DCR). OEM’s of course design their engines based on DCR. That’s why a lot of high performance, high rpm, factory stock engines with more cam duration and/or wider LSA’s (which results in a later intake valve closing), are running higher SCR’s, because that brings the DCR back into the desired range.

This lowering of the DCR, due to the late closing of the intake valve, is the reason why aftermarket Hotrod and Racing cam manufacturers spec a higher static compression ratio for their larger cams, because that gets the DCR into the proper range.

NOTE: HP = (Torque x rpm)/5252.

Little engines can make big HP, if you spin them to a high rpm. And in order to spin them to a high rpm, you need a large duration cam for the engine to breathe. But of course a large duration cam means a later closing intake valve, thus a lower DCR. So, you adjust the static compression ratio (SCR) to set the DCR to right where you want it. That allows you to have a very high performance engine that runs on ordinary pump gas. Here’s an example of just that:

The 2011 Yamaha YZF-R6 (600cc in-line 4 cylinder Sport Bike)

Its short 1.673” stroke allows it to rev to a 16,000 rpm redline, with only a 74.4 ft/sec average piston speed, while still being under the OEM limit of 80 ft/sec.

And it’s large duration cam that allows it to breathe enough to rev to 16,000 rpm, would have lowered the DCR unacceptably, except for the amazingly high 13.1 to 1 SCR which brings the DCR back up to an acceptable level. And the DCR is still set low enough so that even with the 13.1 SCR, it can still operate safely on ordinary pump premium gas.

After reading this, you may never look at the commonly referenced static compression ratio (SCR) the same way again. What is REALLY the most important compression ratio, is the Dynamic Compression Ratio (DCR). Because that is one of the primary factors determining how well your engine will run, and what its octane requirement will be.

540 RAT

U.S. Patent Holder

Member SAE (Society of Automotive Engineers)

Member ASME (American Society of Mechanical Engineers)

To see my entire 130+ motor oil “Wear Protection Ranking List”, along with additional motor oil tech FACTS (with over 35,000 “views” worldwide), here’s a link:

http://540ratblog.wordpress.com/

63mako

Im guessing you should have higher vacuum with your static compression and cranking compression. Cam timing, ignition timing and curve, A/F ratio can effect vacuum. Did you change your vacuum advance can when you swaped the cam? Did you retune everything for the new cam and has it been changed since? You can narrow the duration and lift down with a degree wheel and dial indicator but your cranking compression looks like the cam is close to what the engine needs as far as advertised duration. A roller cam upgrade and proper tuning will go a long ways if your leakdown test is good. A good dyno tune by a shop good with carbureted cars and timing curves would be worth pursuing.

BBCorv70

Thanks 540 Rat and 63Mako, lots of good information to digest.

I don't know what I have for a vacuum advance bulb, most likely a stock replacement. I've owned this car for 30+ years, engine had a major overhaul in the mid 80's. That's when the compression ratio was reduced to 9.6:1. The engine was refreshed again in the mid 90's while I had the body off. I had the cam replaced as part of the refresh. It had a solid lifter cam installed by the previous owner. The problem I run into now is I have no record of what cam was installed. I am thinking about buying a few tools so I may use a dial gauge to get a rough idea what's in there for a cam. Go from there.

I'm approaching work on this car one thing at a time. The cam investigation isn't top priority yet. The car runs pretty well but stops pulling early when I get on it.

Thanks for all of your input. I never knew the relationship between static compression, dynamic compression, and cam timing. Very interesting.