Correct compression psi

[mrsimsam]

Hi
I have a rover p6 3.5 v8 , 10.5:1cr on a 1971 engine, I'm getting between
145 and 135 psi across all 8 cylinders is this a good reading, I have a permanent vacum gauge in the engine bay and this reads good / green at idle. Is this an acceptable reading .
Many thanks

[kiwicar]

Hi
yes it is in the right range the real thing is that they are all similar (within about 10%)
best regards
Mike

[mrsimsam]

Mike
Many thanks

[ramon alban]

Hello mrsimsam,

It varies, of course, with compression ratio and altitude.

In your case 10.5 CR multiplied by 14.7 lbs/sq inch atmospheric pressure (say, near the seaside, approx) = 154 psi.

Take off a bit (up to, say, 10%) for reasonable losses around the rings, valves and what the cam is doing, this brings it to about 140 psi.

More than 140 psi, you've got a good-un, but another 10% loss on top of that, giving (say) 125 and things are looking a bit iffy.

Anyway, that's the rule of thumb that was explained to me!

As mentioned - all 8 being similar is also a good thing.

Very happy to be corrected, by the way?

[mrsimsam]

Ramon
Many thanks, I was talking to someone who thinks
That my valve rings( is that correct term) may be gone as even
Though I have decent compression and good vacuum , I get some
Smoke going into the air filter from the engine, my car idles differently with pancake air filter off to when on , is this normal , I'm running a brand new holly carb4160 which was a like for like replacement

[unstable load]

Do a cylinder leak test to check for blow-by or damaged valve seats. Also look at your engine's crankcase ventilation system for blockages and general condition of the plumbing.

[ramon alban]

Many thanks, I was talking to someone who thinks
That my valve rings( is that correct term) may be gone as even
Though I have decent compression and good vacuum , I get some
Smoke going into the air filter from the engine, my car idles differently with pancake air filter off to when on , is this normal , I'm running a brand new holly carb4160 which was a like for like replacement

Hello Sam,

You describe smoke going into the air filter'. But how much and does it matter?

The main point of the breather system is to recycle gasses from the crankcase to minimise emissions.

When the piston rings are unduly worn, then more gasses are able to pass into the crankcase and from there via the breather system to the air intake. Somethimg like this:



It's normal to see some haze or smoke when the breather is disconnected.

If it is too much then your compression readings would be unduly affected.

As per 'unstable load', performing a leak-down test will provide the answer.

Google 'leak down test' for explanations,

http://www.gregsengine.com/cylinder-lea ... sting.html

also YouTube.

https://www.youtube.com/watch?v=N6UDPd7yIjE


It's not really a diy job unless you have the tools, so go to a garage for expert help.

If you have Blue exhaust smoke upon start-up, commonly, the cause is damaged or missing valve stem seals, mostly at the inlet valves, when oil from the rocker gear has drained down overnight and pooled in the area around the valve stem or valve guides.

You can read how/why - here:

http://www.vintagemodelairplane.com/pag ... oke01.html

Regarding the different idle speeds, the air filter provides resistance to the incoming air so if it is removed the engine can gulp more air than when it is fitted.

This will affect idle speed. nothing to worry about, methinks![/img]

[mgbv8]

A 4160 is a 600 cfm carb if I'm not mistaken. This is a little on the large size for a 3500 motor i think. What was it a direct replacement for ??

[mgbv8]

If you are only in Watford I could lend you my leak down tester !!

I pass Watford almost every day.

If you have an air compressor I can show you how to do a basic leak down test ..

Perry