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-   -   engine compression? (https://www.alfabb.com/bb/forums/engine-repair-diagnostics-rebuilding/393554-engine-compression.html)

JimPunkrockford 02-17-2015 01:35 PM

engine compression?
 
sorry for the new thread, but i'm having trouble "searching".

what should my compression be on a 1968 1750 spider motor?

thanks in advance

papajam 02-17-2015 01:58 PM

IMO, anything north of 160-ish is good. More important than actual numbers though is that all the cylinders are within about 10% of each other (some say 15%).

JimPunkrockford 02-17-2015 10:17 PM

Quote:

Originally Posted by papajam (Post 6070690)
IMO, anything north of 160-ish is good. More important than actual numbers though is that all the cylinders are within about 10% of each other (some say 15%).

thanks. so i am sub 110 but all within 10% and no blue smoke.

would it be safe to assume i need a valve job?

GTA R 02-18-2015 01:01 AM

Sub 110 ? Are you doing the test with the engine hot and with the throttle wide open ?

Steve

Alfar7 02-18-2015 06:02 AM

Static Compression
 
Quote:

thanks. so i am sub 110 but all within 10% and no blue smoke.

would it be safe to assume i need a valve job?
Not at all. Static compression readings reflect several engine conditions but the chance of having all the valves leaking "evenly" is less than miniscule.

Since the motor isn`t smoking that says the oil control ring and 2nd ring are still wiping oil off the liners adequately.

With your discription my suggestion is to check your engine`s cam timing.
If the cam timing is off it will likely reduce the static CR.

One of the "urban myths" scattered by many sources was the added power from advancing the intake camshaft and retarding the exhaust cam a couple of degrees.
This might help but only at high RPM. In fact moving the cams in such a manner increases an already excessive amount of overlap killing low and mid-range torque/HP. The stock cams are not not going to reflect much power change at all due to their mild profiles.

Advancing cam timing needs to be clarified. Advancing the intake cam (clockwise) opens the valve earlier and retarding the exhaust (counter clockwise) closes the valve later. Both increasing overlap, reducing torque and reducing engine efficiency and reducing the static CR #s you get when checking the engine with a guage.

To be clear these 1750 engines came with camshafts timed at various points LC wise depending on whether Euro carbed models, or USA versions with variable intake timing. If it was a spica car it had the vvt unit and the timing was to be at 114 int (100 when advanced above 1700 RPM) and 102 exh.

Alfa issued a TSB changing weber carb`d versions to 104 / 104 in the `70s.
The Euro 1750 used a larger lift version of their standard lobe design, and LCs were set at 99.5/99.5.. The drivability of these cams improved as well when reset to 104/104.

If your car has Carbs now, set the cams to 104/104 LCs and recheck the CR. Should be a bit higher and drive much better.

If it still suffers the Spica curse, set the cam timing on the exh side to 104 and make sure the intake side is not less than 114. If it`s a bit outside the 114 mark (cam mark to the intake side of the cap mark) that won`t hurt either.

If there is leakage issues the only way to determine where the compression is going is with a leak down test.

rogerspeed 02-18-2015 08:00 AM

Quote:

Originally Posted by Alfar7 (Post 6071754)

...If it was a spica car it had the vvt unit and the timing was to be at 114 int (100 when advanced above 1700 RPM) and 102 exh...

Richard, you are confused. No 1750 SPICA motors came with VVT.
As far as I know VVT start with the 1982 l-jet 2.0 spider

ljayr 02-18-2015 08:01 AM

Quote:

would it be safe to assume i need a valve job?
It would be safe to assume nothing. Keep in mind a compression test doesnít tell the whole story of an engines health and people often get hung up on the compression number. Typically if all compression values are within 10% then the engine passes the compression test. If you suspect you have a head problem I would do a leak down test.

101/105guy 02-18-2015 08:03 AM

Roger....
Back away from the keyboard slowly......

MitchW 02-18-2015 08:11 AM

It may be that your gauge may is reading low. I've got 2 gauges and they read about 15psi apart. If your engine runs OK, you're not using oil and the relative readings are close then there's probably nothing that needs to be done.

JimPunkrockford 02-18-2015 09:27 AM

Quote:

Originally Posted by MitchW (Post 6072002)
If your engine runs OK, ..... nothing that needs to be done.

unfortunately the engine runs terrible. but i like your optimism

Quote:

Originally Posted by ljayr (Post 6071954)
If you suspect you have a head problem I would do a leak down test.

i guess that would be the logical thing to do. then i can narrow this down. is there a thread about how to do a proper and comprehensive leak down test?

Quote:

Originally Posted by Alfar7 (Post 6071754)
With your discription my suggestion is to check your engine`s cam timing.
If the cam timing is off it will likely reduce the static CR.

this has also been suggested in the past, i guess a leak down test first would be in order. but how would i go about checking cam timing? is there a thread or a book or something?

as always, all of this help is greatly appreciated

alfaparticle 02-18-2015 11:03 AM

Quote:

As far as I know VVT start with the 1982 l-jet 2.0 spider
It was introduced in 1980 on Spica 2L Spiders.

101/105guy 02-18-2015 12:13 PM

As always,
pictures of the offending motor would settle this.....

And as far as running poorly,you can spend the next year (some have !) going from suggestion to suggestion, changing parts and spending $$ or you can do what a competant mechanic would do.....
Trouble shoot each system in a logical order.
There are several books with this M.O. and it will save you time, money and grief.

ljayr 02-18-2015 12:38 PM

Quote:

is there a thread about how to do a proper and comprehensive leak down test?
I dont know, but there are some good YouTube videos on the subject. I would recommend that if you are not familiar with the leak down test to get a friend or find a mechanic who is. A miss diagnosis could be very costly.

Alfajay 02-18-2015 12:48 PM

Quote:

Originally Posted by GTA R (Post 6071458)
Sub 110 ? Are you doing the test with the engine hot and with the throttle wide open ?

JimPunkrockford's answer to GTA R's question (which was asked way back in post #4 ) would take us a long ways toward answering the original question.

Readings of 110 psi would be low for a warm engine with the throttles open. But if cold/closed, 110 doesn't sound so bad. So, which is it?

Del 02-18-2015 01:55 PM

Don't forget to dribble a little oil into each cylinder when doing the compression check. If you don't do this, you can end up with low readings.


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