Compression Testing

Tech Question


steve whelan, High Wycombe, Australia, 71 Dodge Challenger 318

HI Rick,
I have just bought a 71 Challenger- it is virtually unmolestedsince the day it was birthed at Hamtramck, except that it has a replacement318, which is all black and has a date coded intake manifold of about 1979and other pollution control devices, plus stickers on the rocker covers etc,so i know its a newer engine. The car has not been touched since it left theStates. I did a compression test (as per service manual) and was blown away(almost literally) by how much compression it had. The lowest on 2 cyl was205psi and the highest was 225 psi! Mostly around 210-215 for the rest. Iknow my gauge is correct as I have used it recently on an 8.5:1 360, whichgave around 115-125. SO what’s happening with this massive compression? Toall outward appearances it’s a stocko engine- 2 barrel factory Holley,greasy engine, sick TF904 etc. This comp suggests a C.R. of around 12.5:1 !Any ideas? I have not driven the car since I had it trucked over frominterstate, but it starts easily and idles normally. Any numbers I cancheck- maybe its a 318 Magnum? I have no idea why the comp would be so high.
Thanks, Steve


Steve, static / cranking compression tests don’t really mean much except asa cylinder-to-cylinder comparison. The camshaft timing has a profound affecton cranking compression – the milder the cam, the greater gauge’s reading.So does cranking RPM, amount of oil on rings, etc.

The only real way to know for sure is to yank a head and measure, look fordomed pistons, shaved heads, etc. Or drive it on pump gas and you’ll soonknow!

A block casting date newer than late 1991 might indicate a Magnumshortblock. but the 1979 manifold could NOT be bolted to Magnum neags.


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