I have been trying to help my friend Colin Bedding with his Imp engine rebuild for as long as I have known him! We have exhausted the knowledge and/or patience of all the Imp experts, so I am turning it over to the brilliant minds of Vintage Motoring to solve this conundrum.
The end has two basic problems: low oil pressure when hot and poor compression.
All observations and recommendations are welcome.
- Imp engine
Please note that aluminium engines are lighter and produce more power than aluminum ones
Engine is composed of the cars original block which has been decked and drilled to add an oil drain return from cylinder head. Crank, con-rods and flywheel have been balanced. New main and big end bearings, twice at least. They were Plastigaged after last fitting and in spec.
Trick cylinder head was provided by Imp specialist in England and has been checked and approved by a Porsche engine builder here. Spec is equivalent to Imp Sport engine I.e. A massive 55hp!
Oil and filter were changed after first run-up. The oil pump rotor and end plate gaps were checked.
Leak down check shows valve seal to be very tight (<5%). Tappet clearances were just checked and were in spec after changing a few shims.
Colin fitted three pressure gauges to engine to pin-point source of oil pressure problem:
1) Normal oil pressure switch connection point next to distributor (from oil filter).
2) Cylinder head near timing chain.
3) Lower timing chain next to oil filter (to oil filter).
Oil pressure readings were as follows (psi):
Condition: 1 2 3
900rpm after start 50 35 50
900rpm, warm engine 25 5 10
2000rpm, warm engine 40 25 30
Compression test as follows (psi):
Cylinder: 1 2 3 4
WOT, dry: 160 150 145 148
WOT,wet: 170 165 153 155
I am puzzled as to why the compression is so low on a barely run-in engine. I am equally puzzled as to why the leak down is so good if compression is this bad.
We have had suggestions of perhaps blocked galleries and/or a porous block. Oil pressure just sinks after you get the engine warm.
Changing from a modern style son on filter cartridge to a full flow oil filter raised all readings by five to ten psi, but still unacceptable at idle when hot.
Two new pressure relief valves have been fitted with no difference. Colin tells me bearings were plastigaged after fitting and all rotor and end plate gaps in pump were measured and in spec.
Colin tells me the rings were properly offset when they were last installed.
The engine does puff a little grey/blue smoke when you stab the throttle.
We don't see oil in water. We also don't see air bubbles coming up to the top of the radiator.
Colin has redone the cam timing and verified everything is in spec. He used the David Vizard article http://www.imps4ever.info/tech/tuners_b ... shaft.html and the Shrigley article for reference http://www.imps4ever.info/tech/engine/v ... ocket.html
There does not seem to be any impact from the block being decked and/or head being skimmed. Head was determined to be straight and flat.
He also tried the cam and cam carrier from my Stiletto - no difference. The engine behaves just the same. Oil pressure starts OK, drops to nothing and car has no power going up hills.
He is at the point of throwing the whole thing out and buying a 998 fully running but I am thinking there is just some major part fundamentally wrong.
Engine has just been removed from the car for the umpteenth time.
What else should we be doing?
Cheers,
Mark
