• Hi Guest!
    If you appreciate British Car Forum and our 25 years of supporting British car enthusiasts with technical and anicdotal information, collected from our thousands of great members, please support us with a low-cost subscription. You can become a supporting member for less than the dues of most car clubs.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Compression. PSI should be...

Share this page

vping

vping

Yoda
Silver
Country flag
Offline
What should the compression be in a 74B engine?

It is an "L" engine which I always assumed had less compression thatn an "H" engine. I have one of those hand held compression jobbies that screws in the spark plug hole and has a guage with a pressure release button on it. It seemed to work well on the '67 so I am going to try it out tomorrow. I am not sure how good or reliable these suckers are.

My '67 is an "H" and has 170 psi in each cyl. I want to check the '74 but want to be prepared in case it is low. I don't want to be shocked or worried if it is.
 
Look for something in excess of 100 PSI, likely 130-ish. More importantly a consistency across all four. Even if it's lower you may be seeing goober'd up valve seats or bore rust. If it's at or over 100 on all four with a "wet" check it'll likely light off when the time comes. After running it thru a few "heat cycles" take another reading. That'n will be a more likely indicator.

Rust will help reseat th' rings. /bcforum/images/%%GRAEMLIN_URL%%/devilgrin.gif

The '67 was a 9.5:1 compression. Don't look for anything NEAR that! /bcforum/images/%%GRAEMLIN_URL%%/wink.gif
 
Will I get the same power out of the '74 that I get out of my '67?
I can't wait to get home later & check it.
 
Vince - IMO, a leakdown test is more telling than a compression check. If you have a compressor and are somewhat handy, it is simple to make one (gut a spark plug, weld an air fitting onto it. Add a pressure gauge and you're done. Find exact TCD, install fitting, pressurize cylinder to 100psi and listen for the hiss/note the air loss. The source of the hiss tells you what might be wrong (i.e intake hiss? suspect the valves. dipstick hiss? suspect the rings and so on...)

A 67 in good shape will likely make a few HP more than a 74, although the latter may have a "big valve" head.

Engine condition is probably more important than the year at this stage.
 
Yep, '67 will feel faster than '74

In addition to the 'dry' compression test, follow it up with a 'wet' test (i.e., squirt a little oil in each spark plug hole before pulling compression)....that'll tell you, if there are problems, where they are - valves or cylinders.
 
A couple more tips:

Make sure the battery is fully charged.
Remove the air cleaners
Hold the throttle wide open (WOT)
Dont crank for more than about 10secs, let the starter cool.
 
I usually count four or five compression strokes. That'll have it to highest reading.
 
Is the WOT so it won't start?
 
No, it seals the rings so you can check the valves.

Pull all the spark plugs, so it turns easier and will not try to start.
 
WOT allows air thru the intakes; closed it would be sucking against the throttle plates.
 
No problem with it potentially starting. Fuel pump not ticking yet so I've covered. I will WOT though.
 
I just got to bet that his carbs will suck air even if they are not open, hehe.
 
So, OK just to weigh in on this....

As advised above, remove the plug and use the trusty old compression tester on each cylinder and record the compression... they 'should' all be within a few PSI. I agree that it is less important that your compression is around 130psi than it is that they are all within a few psi of each other.....

If you have a cylinder that is dramatically lower than the rest, squirt a little 30 weight oil in the cylinder and run the compression test again... if the psi goes up then your rings are worn... If not, then the valves are not seating...

If you perform this oil test (wet) on all of the cylinders and they all go up, then your rings are worn... if not, and if the psi is consistently 100+, then you are good to go... for a while.. /bcforum/images/%%GRAEMLIN_URL%%/rolleyes.gif
 
/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif
 
OK. Wet test performed with throttle full open.

Here are the results and I feel positive about them.

#1 - 156
#2 - 162
#3 - 158
#4 - 160

What say all?

Next up, I'm going to address the gas supply today.
 
So, Vince, what did you have for dry numbers to compare to?
The wet numbers look faily good across the board, but it would help to see how much difference the oil made.
Jeff
 
/bcforum/images/%%GRAEMLIN_URL%%/iagree.gif /bcforum/images/%%GRAEMLIN_URL%%/banana.gif
 
135-140 but I also did not have WOT.
 
Back
Top