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Impact Bumpers

Leicestershire

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  1. Yes. knackered air cooled 911 motors frequently deliver 90%.
  2. Look at it in another way: a compression check measures how much pressure an engine can make. a leakdown test measures the engines ability to maintain pressure. worn valve guides, valve seats, scuffed bores, missing head studs and general wear can often get missed by a standard compression check - or at least show a reasonable and consistent reading on the gauge. Every engine spec has a different compression value. A 2.7 cis engine with 8.5:1 pistons will show much higher gauge pressure than a hot rod 3.0 with a 10.5 :1 CR and hot cams with lots of overlap.
  3. Yes. With a leakdown test you can usually find the fault. If it didn’t pinpoint the area it would certainly let you know that further investigation is required.
  4. A very tired 911 engine with broken head studs, worn valve guides etc will still show good cylinder compression.
  5. Silkolene have a useful product selection guide on their website.
  6. My own SC is an early ‘81 model year registered December ‘80. Fuel filter, side repeaters and the hard fuel lines on the original CIS confirm it’s an ‘81 model car. The original camshafts were retained with the large hex bolt and spring washer as fitted to early motors and the first batch of 204 bhp engines before changing over to the hex head bolt with thick washer. This engine has the large ports and valves. Possible reasons: My engine was a parts bin special using up old stock heads - unlikely. it had been meddled with at sometime in the 30 years before my ownership- possible. the vast majority of information on the web originates from the US and is of questionable accuracy - ??
  7. I was referring to the numbers quoted for the port sizes - 49mm inlets??? SC's 34,36,39mm - trying to clarify what year, model or market has what. 3.2 has (I think) 41mm - somebody please confirm SCRS 41mm
  8. anything is possible especially when these cars are knocking on 40 years old but my own car car is an '81 with 39mm ports.
  9. Don't believe everything you read!
  10. Who knows. Perhaps porsche got in wrong in the factory manual. Either way, if you set at the average of the porsche 1.25 or JD's value 1.26 your engine will be fine.
  11. Brick acid is popular with many of the hand car wash places for cleaning alloy wheels. Always a risk using the road side 'hand' car wash. The chemicals are too harsh, industrial strength hot pressure washers and sponges going from a mud plugger range rover to the next victim. Fine for the daily drive but not a classic. Also check your brake calipers (acid makes these go black) and all yellow plated parts around the engine.
  12. The gains with cam timing change with the cam profile. Maximum bhp gain is with the cam timing retarded. If the cam timing is set at maximum retard (for the spec of cam) and the timing chain stretches then the cam timing will be retarded out of spec and performance will suffer. Most engine builders will time cams at mid range.
  13. It’s a very fine balance. For reference SCRS heads had 41mm ports. Its only a few fractions of a mm from perfection to disaster. I am sure that many ‘DIY have a go heroes’ will have gone with the theory that bigger must be better.
  14. Torque ultimately makes bhp. Port size changes where the torque and power are made.
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