Imp Engine from Hell! (Long)

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daysineuropa
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Imp Engine from Hell! (Long)

Post by daysineuropa »

Hi Folks,

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
Imp engine
More pictures and descriptions of Imp aluminium alloy OHC engine are here - too big to post http://www.imps4ever.info/development/e ... pment.html

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
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Re: Imp Engine from Hell! (Long)

Post by Steve Simmons »

With funky compression and traces of blue smoke, I would suspect the rings. They may not have bedded in properly. If the engine is barely run then they may not have finished their initial bed-in yet.

The oil pressure drop doesn't concern me that much, but the accompanying loss of power does. I don't figure the aluminum block would expand enough to loosen the rings, but have you tried a compression check both when cold and hot?

I don't know what oil pressure is required for those engines, but 50 PSI at 900 RPM seems really high. 25-40 at 2000 RPM seems fine to me also. 5 at idle can't be good. Are the rocker arms in spec?

What kind of oil are you using? Maybe try something slightly heavier? I don't know the oil path in the engine, so it's hard to diagnose the varying pressures in the engine.
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Re: Imp Engine from Hell! (Long)

Post by Martin Keller »

Hi Mark and Colin,
Sounds like the oil pump is the issue once it warms up and maybe the rings are not fully seated or the grooves in the pistons are just to deep for the rings causing just a little oil to get by them causing the blue/gray smoke on throttle stab and lack of power.
Just a guess on my part but if you can change out the oil pump it is the way I would go to try and get the warm oil pressure up to normal which I would figure to be about 30 psi at 900 rpm once warm. As for the other two measurements of the oil pressure I would think those areas are getting enough for lubrication as they normally are on the return side of the system not like the main bearings which are on the output of the oil pump to my limited knowledge.
Example my GT6 oil pressure runs once warmed up at 850 rpm at about 28 to 30 psi by the gauge in the dash and at 3200 rpm is showing 55 psi. which is all subject to the gauge in the dash.
Hope this helps.

:drive
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Re: Imp Engine from Hell! (Long)

Post by VWNate1 »

Oil smoke upon throttle application indicates cylinder issues , proper bedding in of the piston rings takes 60 seconds , if you did the slow & easy breakin thing , they're prolly never going to seat now .

Does the cylinder head have pressed in cam bearings ? .

Have you taken the oil pump apart to check the clearances and to see of the gears have worn circles in the cover ? .

Have you removed and closely inspeced the oil pressure relief valve ' bullet ' ? BMC uses very soft steel on these so they get a dimple in the nose that allows oil pressure to leak past .

I'm not familiar with this particular engine but I've encountered many low oil pressure engines over the decades .

A friend once grabbed the wrong oil pump to block gasket on a BMC 'B' Series engine , that took a while to find as he kept insisting the pump was properly gasketed when it was obviously the wrong gasket when we took it apart .

I know this is frustrating but there's a simple thing in there , you just need to check all the silly basic stuff that never goes wrong , eventually you'll find it .
-Nate
daysineuropa
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Re: Imp Engine from Hell! (Long)

Post by daysineuropa »

Well, we may have a smoking gun. I asked Colin to strip the engine down as we had supposedly covered all the angles.

It turns out the main bearings that were supplied to meet crank spec were nowhere close.

We took a trip to Ron Williams Automotive in Ventura, armed with crank and flywheel and a short block for demonstration purposes.. Dennis Williams looked on with bemusement at our little sub one liter aluminum lump and then agreed to clean, Magnaflux and measure crank so he can specify how much re grind is needed and we can procure right sized bearings.

Hopefully that takes care of the oil pressure.
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Re: Imp Engine from Hell! (Long)

Post by whitebuffalo »

I do really hope that is the cause, I know this motor has been a huge headache for Colin and I really hope this cures the problems. Then finally Colin can drive the car farther than a few miles :lol:
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