Tuning Engine Compression - Crank It!

The Tuning Secrets Of Cranking Compression

Jeff Smith Sep 1, 2002 0 Comment(s)

Step By Step

The good news is that checking cranking pressure is very easy to perform. Warm up the engine, yank all the plugs, prop the throttle open, and spin the motor over. Don't forget to disable the ignition first. Check all cylinders and look for less than a 10 percent variation between cylinders.

Your results will only be as good as your equipment. Matco sells a great compression gauge in this kit (CT166K) that contains one large 0 to 300 psi gauge, several length adapters, and a compact pressure gauge for tight spots.

You can also get a universal tester by itself (PN CT66). The Matco tool uses this Schrader valve (2b) to capture the pressure in the gauge until relieved

This illustration shows how a later closing intake valve affects piston position in the bore. With an early closing intake, the volume of the cylinder is larger, creating more pressure when the piston is at TDC. With a later closing intake, the piston moves farther up the bore, reducing the cylinder volume.

When considering a camshaft, cranking pressure illustrates why higher static compression ratio is needed when selecting a camshaft with long duration and a later closing intake valve.

Even supercharged engines can benefit from investigating cranking compression. This 6-71 supercharged 355 combines a low 8:1 static compression with a Comp Cams blower camshaft (with an intake closing point of 62 degrees ABDC) and checks out with a respectable 150 psi cranking pressure. The previous combination had a longer- duration camshaft that tested at only 130 psi.

Our first attempt at 87-octane power created this combination of an Xtreme Energy 268 camshaft and a low 8.75:1 static compression ratio to create a weak 140-psi cranking pressure. The best way to enhance this engine is a slightly shorter cam and more static compression using a thinner head gasket.

It’s also possible to improve cranking compression by advancing the camshaft. For example, advancing a cam 2 degrees closes the intake valve sooner and will result in improved cranking compression.

We performed an interesting rocker ratio test and discovered that changing from a 1.50:1 rocker to a 1.62:1 rocker arm was worth 5 psi of cranking compression. This may not be valid for all engines, but it did work on ours.

While we like to think of 2002 as being part of a high-tech age, our beloved internal combustion engine hasn't really changed much in the last 50 years. We're still using pushrods and overhead valves, and the small-block Chevys we play with today are a virtual clone of the ancestral 265. In the early days of hot rodding, things were simpler and hot rodders didn't have toolboxes the size of S-10 pickups, so they used what they had. Take the lowly compression gauge for example. This simple tool can also be used as an engine-tuning tool in knowledgeable hands. Intrigued? We thought so.

First, let's review some engine basics. An engine's compression ratio is actually a theoretical number. This ratio compares the cylinder volume of the piston at bottom dead center (BDC) versus top dead center (TDC). So if we have a volume of 45 ci at BDC and 4.5 ci at TDC, then the compression ratio is 10:1 since the volume at BDC is 10 times the volume at TDC. While this is a useful number, it ignores one crucial variable. The amount of actual cylinder pressure at low >> engine speeds is determined by the intake closing (IC) point.

All performance camshafts close the intake valve 50 to 60 degrees or more after bottom dead center (ABDC). The longer the duration of the camshaft, the later the intake valve closes. It should also be obvious that the engine cannot begin making cylinder pressure until the intake valve closes. Therefore, the distance that the piston travels up the cylinder at 60 degrees ABDC versus 52 degrees ABDC reduces the volume of the cylinder, reducing the cranking pressure.

Keep in mind that we are talking about a street engine here. Early closing intake valves (short-duration cams) tend to maximize cylinder filling at lower engine speeds, while late-closing intake valves (long duration cams) tend to move the peak power point higher in the rpm range. With a given compression ratio, adding a longer-duration camshaft will decrease the cranking compression and therefore low-speed throttle response and power.

This is where the compression gauge comes into play. By checking cranking cylinder pressure, you can use this as a tuning aid. We checked the cranking cylinder pressure on several modified small-blocks >> and discovered an interesting correlation between soggy low-speed performance and low-cranking cylinder pressure. This comes back to long-duration camshafts with late-closing intake valves combined with a relatively low static compression ratio.

Too Little, Too Late
All this originated with our first Agent 87 engine when we combined an 8.75:1 static compression ratio with a Comp Cams Xtreme Energy camshaft. This combination looked good on the dyno and made 426 lb-ft of torque at 4,000 rpm and 390 hp at 6,000, but the engine never ran the way we expected at the dragstrip. Later, we performed a cranking compression test on this engine that delivered a dismal 140 psi. This is a low cranking compression, especially when you compare it to engines like the ZZ4 350ci/345hp crate engine that makes around 190 psi or the current HT 383 with 9.1:1 compression and a big hydraulic-roller camshaft that still thumps with 185 psi (see "Pressure Packed" sidebar).

Then, for the June '01 issue, we tested another small-block with an XE268 camshaft, this time with a 9.75:1 static compression ratio that produced a cranking compression of 175 psi. The main difference between these two engines was the static compression ratio of exactly one full ratio (8.75:1 versus 9.75:1). While we never put the second engine in a car, it appears that it would be much more responsive to throttle and probably much more fun to drive.

From our rather limited research, it appears that building a performance street engine that combines a decent-duration camshaft with enough static compression ratio to create 175 to 190 psi, will reward you with a very snappy engine that is not only responsive and fun to drive, but makes decent power as well. For example, you can run 11:1 compression on pump gas if you use a long-duration cam with a late-closing intake to bleed off some of that low-speed cylinder pressure. This is the main reason why camshaft companies recommend higher static compression ratios with longer-duration camshafts. This is an attempt to improve power at low and midrange engine speeds by making up for the late-closing intake valve with more static compression.

Too Much of a Good Thing
If you're the classic hot rodder, then you're already thinking, "Hey, if some is good, then more is better!" True to form, if you do that, you will be disappointed. Higher cylinder pressure also means that you will need more octane to slow the burn rate in the cylinder. The limit for street engines seems to be around 200 psi of cranking pressure. Numbers higher than 200 psi create excessive cylinder pressure at low engine speeds, which turns the engine into a detonating monster. A classic example of this would be an 11:1 compression engine with a short-duration camshaft that features an early closing intake valve, such as 50 degrees ABDC. This would create excessive cranking pressure and an engine that would rattle its brains out on 92-octane fuel. Our current HT 383 engine actually tested at 195 psi with the original stock cam and later at 185 psi with the large hydraulic-roller cam and does not detonate.

The key here is to give you one more tuning device you can use to dial in your engine to perform better. If your engine checks out with around 170 to 185 psi of cranking pressure, then you're right in the ballpark. But if your engine is weak like our first Agent 87 engine with a cranking pressure of less than 140 psi, then it might be time to make a change for the better. Just crank it and then you'll know.

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