Compression

Gpaw16

New member
I am needing the basic compression on a clone for that matter a stock one also.
Got clone in package deal. Was ran in a Jr class. All I know about it.
Thanks
 

alvin l nunley

Site Supporter
I'm a real big fan when it comes to checking the compression. But, sorry, I don't know what a clone can be expected to produce. Another thing, the compression release skewers the results.
Whatever the number, you can get a feeling for good or bad just by checking it often. After each race I would think. If the engine seems down on horsepower, during the race, checking it would be a good idea. I like to think that compression is "The Holy Grail". Comparative readings, to me, is what's important.
 
Too bad Monty Python did not make a movie ...Where
I'm a real big fan when it comes to checking the compression. But, sorry, I don't know what a clone can be expected to produce. Another thing, the compression release skewers the results.
Whatever the number, you can get a feeling for good or bad just by checking it often. After each race I would think. If the engine seems down on horsepower, during the race, checking it would be a good idea. I like to think that compression is "The Holy Grail". Comparative readings, to me, is what's important.
Monty Python Where art thou holy compression Lord of PSI ?? the lost episode...... I heard it was leaked.
 

flattop1

Dawg 89
True with the down fall of the flathead and animal most do not have a starter .
If you do , then your golden .
 

knighty

Member
if doing a cranking test remove the exhaust pushrod first. give it 10 brisk pulls, best remove the carb first to prevent fuel being drawin in.......lowest acceptable for me is about 70-80psi........best I have seen is around 130psi.......most are around 110-120psi after 10 pulls.

For leakdown, at 100psi feed pressure I regularly see 2%........have seen many at 1%.......if its around 4-5% you have a leaky valve or rings.........after rebuild they are regularly around 4-8%.......then dyno it, measure again and its back at 2% as everything has sealed up and settled down.......so dont panic if the reading is too high after rebuild........when they are leaking over 10%-20% after rebuild, i stop and remove the head again and re-lap the valves to get it below 10%.......the dyno run will bring it back down to 2-3%........a hard track run brings it back down to 1-2%

For the record, there is no correlation between high cranking pressure and low leakage equalling massive power on the dyno.....its just an indication of general health. When doing the leak-down test get used to placing a 2 ft length of garden hose to your ear, and then the other end down the ports, and the breather........you will soon deduce where the leakage is coming from (valves or rings) and then you can go about fixing it.
 
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Big Chris

Member
Be careful when comparing compression numbers, as there are two types of compression testers.

The first type have the schrader valve in the part that threads into the spark plug hole. Quality gauges like Snap-On, Mac, Craftsman, & Actron are built like this.

The second type have the schrader valve at the gauge (which is at the end of the hose). These are the low quality gauges like Harbor Freight & Petrol Gas.

The low quality gauges with the schrader located a the end of the hose, often read between 20-30 psi LOWER than their high quality counterparts..........

Ask me how I know.
 
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