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.


Comments are closed.

This website uses cookies to improve your experience. We'll assume you're ok with this, but you can opt-out if you wish. Accept Read More