How do you pick out the right engine...
09-25-09, 02:04 PM
#2
Well your profile says you have an '88 convertible, so you're going to want an S4 engine, not an S5. It will be infinitely simpler to swap, and have near identical power potential. The wiring is very different between series, which makes swapping in an S5 a major pain.
When looking at used engines, a compression test will be the most important thing. A quick and dirty test can be done by removing the trailing spark plugs and turning the engine over by hand. 3 distinct "chugs" should be heard per rotor, as the apex seals pass over. But this will only tell you that the engine has some compression. In order to measure the actual compression numbers you will need a compression tester, TII bell housing or complete transmission, a TII starter, a battery & cables. You'll want 100 psi on all faces or better.
I also like to remove the turbo & exhaust manifold completely and inspect inside the exhaust ports with a flashlight. Rust from a blown coolant seal would be obvious when doing this, and you can inspect the apex seals for damage at the same time. Of course, removing the turbo can be a bit involved. The seller may not want to let you take too much apart, but if they do, bring some WD-40 or other penetrant. Rusty old nuts and bolts do not come off easily.
Other things to look for would be shaft play on the turbo. ~1mm side-to-side or back-and-forth play is acceptable. More than that, and you're probably looking at some worn out bearings. Check for oil inside the compressor housing too (cold side). The presence of oil could mean a leaking seal.
And if you're getting a transmission with the engine, make sure it shifts into each gear. This might require some rotation of the input or output shafts.
When looking at used engines, a compression test will be the most important thing. A quick and dirty test can be done by removing the trailing spark plugs and turning the engine over by hand. 3 distinct "chugs" should be heard per rotor, as the apex seals pass over. But this will only tell you that the engine has some compression. In order to measure the actual compression numbers you will need a compression tester, TII bell housing or complete transmission, a TII starter, a battery & cables. You'll want 100 psi on all faces or better.
I also like to remove the turbo & exhaust manifold completely and inspect inside the exhaust ports with a flashlight. Rust from a blown coolant seal would be obvious when doing this, and you can inspect the apex seals for damage at the same time. Of course, removing the turbo can be a bit involved. The seller may not want to let you take too much apart, but if they do, bring some WD-40 or other penetrant. Rusty old nuts and bolts do not come off easily.
Other things to look for would be shaft play on the turbo. ~1mm side-to-side or back-and-forth play is acceptable. More than that, and you're probably looking at some worn out bearings. Check for oil inside the compressor housing too (cold side). The presence of oil could mean a leaking seal.
And if you're getting a transmission with the engine, make sure it shifts into each gear. This might require some rotation of the input or output shafts.
