421 Serval Compression

[theputz]

So I wanted a little more from my 421 Serval this year so had the cylinders ported and went from 24cc domes to 20cc domes. My compression only increased 10 lbs. I went from 150 lbs last year to 160 lbs this year. I expected more from the new domes. The builder that did the work this year did provide a thicker base gasket which I assume is matched to his domes. The squish measures 0.050 with this new setup, right where he said it would. Two questions:

 

1. With only 160 lbs compression do I need to bother with race gas. I fully intended on running race gas with this new set up but also expected to see higher compression numbers.

2. Should I drop 0.010 thickness on my base gasket essentially bringing the squish down to 0.040 and raising the compression?

 

I know that a compression reading only says so much but I want to ensure I am pulling everything possible I can from this motor and not leaving anything on the table.

 

This motor only has two seasons on it so don't think the top end is shot. Everything looked solid when I sent the cylinders off earlier in the year.

 

Any advice you can provide is appreciated. 

[trickedcarbine]

As the motor was ported that will in a sense lower the compression if there was any exhaust work. Reason being, by raising/widening the exhaust more volume is able to escape.

 

160lbs is actually pretty sweet. It is enough compression to take advantage of race fuel without being to harsh on engine internals.

 

Do you know what the squish is like with the base gasket change? That can also lower compression.

[registered user]

theres been several publications regarding this stuff. i dont even use a gauge because frankly i dont give a shit what it says. heres what mull says

 

THE TRUTH ABOUT COMPRESSION TESTING

by Mull Engineering

We would like to take the chance to answer some general questions and myths about compression testing an engine. Compression testing is a method of using a pressure testing device to measure the “cranking” pressure developed in the combustion chamber during cranking speeds of kick or electric starting. These engine speeds are generally between 100 and 400 rpm. This is usually accomplished by removing the spark plug and installing a pressure tester in place of the park plug and cranking the engine over. This pressure can be somewhat indicative of the condition of an engine and is often used to trouble shoot an engine.

We get a lot of questions about what the pressure should be for a given engine and especially, what is safe for certain fuels. Well, hate to break the news but if you are going to rely on compression testing to determine your fuel selection, you are asking for trouble. The reason is that cranking compression numbers can vary greatly from actual running compression due to many variables such as volumetric efficiency or air fill into the cylinder at cranking speeds, and the wet fuel that is added by the carburetor as rpms come up to operating speeds which does not compress as easily as air. These do not even consider the high pressures of burning fuel in the combustion chamber. The only way to correctly determine the right fuel for an engine is by comparing the swept volumes of an engine when the piston is at top dead center and bottom dead center. This is expressed as the compression ratio or CR. There are also other considerations like head design, piston design, squish clearance, etc, but we will keep it simple for this discussion.

Checking compression is a great way to check the condition of an engine and determine if an engine is in need of service providing additional tests are performed. This is only applicable if there is a base line measurement to refer to, either from a manual, or from testing a given engine when it is new and considered a valid value. Regarding four strokes, this form of testing is also a great way to determine if valves are seating properly and at the right time. This form of testing, however, may not give accurate data as to the piston ring seal, wear on the piston rings, or piston condition itself. Bottom line is that many other things can be going wrong in the engine and the compression test will not show it! This is why experienced technicians use this form of testing only as a guide and usually along with other tests. This testing might be compared to testing a child’s temperature to determine illness, it is simply not enough data to give the full picture.

However, by compression testing, one can easily determine, if an engine has lost substantial compression and if an engine is in need of service. Many people will look to perform a compression test at home and this can be done with just as much accuracy as any service shop, given the right equipment and knowledge. Contrary to popular myth, an actual pressure gauge bought about anywhere, can be rather accurate. This, however, is only looking at the dial gauge portion though. There is also a hose extension and the adaptor component that will screw into the head in place of the spark plug.

One of the most common mistakes is selecting a tester in which the head adaptor does not screw into the cylinder head with the same amount of thread as the spark plug that came out. This lack of thread can be compared directly to adding extra volume in the cylinder head and will not represent the same pressure numbers as if the spark plug were reinstalled. Obviously selecting a tester with the correct length of threads will eliminate this problem. You must also be careful not to use too long of an adaptor as well because of the opposite effects as well as potentially causing internal engine damage from mechanical contact from moving parts in the engine and the tester adaptor.

The next and most common problem we see is when a tester is selected, it should have a “Schrader valve” in the very tip of each adaptor. A Schrader valve is the same valve used in the valve stem of a tire and is a one way or check valve device. What this does is isolate the volume in the cylinder head from the volume in the tester adaptor and extension hose. Without this valve in place at the very tip of the adapter, the extra volumes in the hose and adaptor will act to increase the cylinder head volume and give a false (low) reading. The Schrader valve must be in good condition and work correctly and it is common to replace them regularly.

The method for compression testing an engine is rather elementary but certain things must be considered. First, you want to turn the ignition switch off so you don’t get surprised. Install the tester adaptor in place of the spark plug and either hold the throttle wide open to allow the most air to the engine or you can simply remove the carburetor entirely. Then either kick or electrically crank the engine over until the compression gauge will not go any higher. This process generally takes around 50-200 revolutions of the engine to complete the test. Excessive cranking while the gauge is still climbing is sign of leakage somewhere in the engine.

When the test is complete, the gauge should hold the tested pressure at the gauge indefinitely. If not, there is certain leakage of the tester, either the mentioned Schrader valve, or other unions should be inspected. It is NOT a requirement to purchase an expensive compression tester but one should certainly be selected of decent quality and include the considerations listed above.

Regarding four strokes, it should be mentioned many engines use a form of compression release to lower cranking compression to make an engine easier to turn over and start. This compression release is usually turned of by mechanical means right after starting. This mechanism will directly affect accurate compression testing. You may need to refer to your manual for an estimated compression value with the compression release in use, or you may need to render the release mechanism ineffective for testing. If the engine uses electric start, it may be impossible to turn over without the compression release so derated numbers from a service manual will be required.

[theputz]

Squish is 0.050 with a base gasket of 0.050. Changing the base gasket would affect the port timings slightly though would it not?

[registered user]

http://www.2strokeheads.com/tech2.htm

[trickedcarbine]

A bit yes. That's a big base, but if everything is where it should be and you have good squish, run it.

 

For the record you checked those numbers?

[possum]

who would put .050 base gasket in? Did someone add a thick gasket to raise ports a few degrees and call it ported? Then just cut domes to match.

[theputz]

I must be wrong on the base gasket thickness. I will double check tonight.

[dirtydownunder]

any way to find out how ported it was, ask for durations ? maybe it was just a clean up or something minimal.

i did pretty much the same  had the cylinders ported  halfway between serval n cheetah cub and went from 24cc to 20c, kicking 170psi.   12.6:1 c/r, .045" squish

and it was a huge difference from standard serval, still running pump gas too, but stock timing advance.

[theputz]

sorry got my mm and inches mixed up. My new base gasket is 0.0196 and the old one was 0.0118 (a difference of 0.0078). By using this thinner gasket I could drop my squish from 0.050 to 0.042. How much would this impact my compression? Would this affect port timing much?

[sprinklerman]

sorry got my mm and inches mixed up. My new base gasket is 0.0196 and the old one was 0.0118 (a difference of 0.0078). By using this thinner gasket I could drop my squish from 0.050 to 0.042. How much would this impact my compression? Would this affect port timing much?

I'd rather run the more loose squish on a play bike. . 050-.060 is good and safe

Did you measure compressed thickness or new?

But really, these are questions best answered by the person you had port the cyls. Not random people on the Internet.

[Redline Racing]

I have answered all these questions in emails. We raised cylinder to 190* with a little flatter exh shape. We provided a .020" base gasket. With 24cc domes we usually see 11.8-1 compression with 20cc domes it will be about 14.5-1. This is plenty of compression @ a 190* exh. A compression gauge is great for a reference point but not very accurate.

[trickedcarbine]

Yahtzee.