1. Engine Compression Theory

2. Four stroke cycle (gasoline engine) u Intake stroke –Piston going down - intake valve open. –Low pressure area is created in cylinder. –This stroke is where volumetric efficiency applies - maximum at w.o.t.

3. Four stroke cycle u Compression stroke –Piston going up - both valves closed. –Mixture squeezed to increase combustibility by keeping molecules tight and by raising temp. –Needs good seal to compress and combust.

4. Four stroke cycle u Power stroke –Piston going down - both valves closed. –Stroke where work is created by expanding gasses. –Power used during only first 25% of stroke. –Only 1/3 of energy is used on piston.

5. Four stroke cycle u Exhaust stroke –Piston going up - exhaust valve open. –Exhaust gasses exit. –Any exhaust left over dilutes incoming a/f mixture with inert gas (exhaust) v Causes poor quality and quantity of charge.

6. Valves u Valves serve two purposes: u Sealing - during compression and power strokes u Opening up of ports - during intake and exhaust strokes

7. Sealing of cylinder u Performed by tight seal of valve face against seat to seal in compression pressures u Most common loss of compression is valve not sealing properly

8. Typical valve failures u Valve face and or seat worn due to excessive mileage u Valve burnt due to lack of contact –Valve adjustments necessary –Foreign material on seat u Valve not closing due to valve spring failure u Valve bent due to piston contact

9. Opening of cylinder u Controlled directly or indirectly by camshaft u Three factors that are related to the opening of the valves are: –Size of the opening –How far the valve opens (lift) –The amount of time that it is open (duration)

10. A problem with valve opening will affect either volumetric efficiency or the quantity of the incoming fuel mixture

11. Factors related to SIZE of opening

12. Big valves and ports u Good for high rpm u Will experience loss of ram effect at low RPMs (related to momentum)

13. Multiple valves u Multiple valve heads will act same as big valve heads u Secondary throttle plates can be used to eliminate loss of ram effect

14. Lift u More lift = loss of ram (momentum) u Lots good for high rpm

15. Head design u Wedge style still most common and not as effective in allowing air fuel mixture to flow u Hemi design very efficient u Porting & polishing increases air flow by creating less restriction and reducing turbulence in the ports

16. Factors related to TIME of opening u Duration - the amount of time valve is open –Long good for high rpm –Short good for low rpm –Need more time at high rpm

17. Typical failures affecting opening of valves u Carbon build up on intake valve stems –Caused by valve guide, seal or stem wear (blue smoke) –Affects volumetric efficiency of cylinder u Flat cams –Intake lobe = loss of v.e. –Exhaust lobe = air/fuel mixture dilution and backfire but no loss of v.e

18. Testing

19. Vacuum gauge testing u Provides quick test for compression problems u Should read 15-20” of manifold vacuum @ idle u Cranking vacuum will be 3-5”

20. Vacuum test results u Dramatically pulsing needle @ idle or while cranking usually indicates compression problems u Extremely low vacuum can indicate valve timing problems

21. Cranking compression test u Used for checking engine seal –Must set up conditions v W.O.T., bat. charger, disable ign., all plugs out –Compare readings - 10% max difference –Typical range: 110 - 150 psi u Wet test is performed when cranking compression is low –Adding SMALL amount of oil to cylinder will help seal worn rings

22. Leak down test u Used to identify location of sealing problem u Cylinder must be at TDC compression u Problem is detected if loss is greater than 20% - leak location needs to be identified

23. Identifying location of leak u Air escaping through exhaust indicates exhaust valve sealing problems v Must loosen valve adjuster u Air escaping through intake indicates intake valve sealing problems v Must loosen valve adjuster u Air escaping through valve cover indicates worn piston problems u Air escaping through radiator indicates blown into coolant jacket

24. Running compression test u Test cylinder seal at idle u All readings will be low because there is less time to fill cylinder u Compare readings - no specs u Will identify compression losses only apparent while running –Broken valve springs –Worn valve guides

25. Snap compression test u Tests for problems with v.e. u Allows rush of air at relatively low rpm nearly filling cylinder which will cause increased compression pressures –If a cylinder has a restricted intake snap pressures will be lower u Compare readings - no specs

26. Valve clearance u Too little will cause sealing problems u Too much will cause excessive noise, wear and loss of lift

27. Hydraulic tappets u Zero lash is obtained with hydraulic lifters u No periodic adjustments u Can fail by pumping up or collapsing

28. Solid tappets u Periodic adjustment necessary to maintain proper lash –Compensates for seat recession in head –Compensate for valve train component wear u Lash will be checked will feeler gauge –May have cold or hot specs u Can be adjusted by turning various adjustment screws or by replacing shims

29. Valve timing control u Advanced camshafts can increase low end performance but will sacrifice high rpm horsepower u Retarded camshafts will increase high end power but will lose low end torque u As timing belts or chains wear, cam timing will retard

30. Variable valve timing u Many variations but all are to get benefits of advanced and retarded timing u Some will vary overall timing u Some will vary only one valve

31. Timing chains u Can use tensioner to maintain minimum slack u Most common with cam in block (OHV) designs

32. Timing gears u Some engines use gear to gear u Most gears are helical cut u Usually stronger than chain or belt

33. Timing belts u Commonly fail by breaking or stretching enough to allow it to jump a tooth or two u Most have service intervals of 60K -90K miles u All will have tensioners to maintain tension of belt –Tension can be maintained with spring pressure, oil pressure or internally maintained by different devices

34. Timing belts u Many manufactures recommend never using a belt twice u If a belt is reused be sure it is installed in same direction of rotation u Belts must be replaced if oil or antifreeze soaked

35. Testing for valve timing problems u Low vacuum and low compression are typical of valve timing problems u Back lash can be measured by watching rotor for movement while turning crank by hand u Erratic timing as seen with a timing light can indicate excessive slack u Belts are often tested for tension by twisting slack side of belt - ½ twist is considered acceptable