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Discussion Starter #1
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
 

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Trained (ex)Professional, , 1953-2018 RIP,
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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%).
 

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Discussion Starter #3
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?
 

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

Steve
 

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Richard Jemison
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Static Compression

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.
 

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...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
 

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would it be safe to assume i need a valve job?
It would be safe to assume nothing. Keep in mind a compression test doesn’t 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.
 

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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.
 

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Discussion Starter #10
If your engine runs OK, ..... nothing that needs to be done.
unfortunately the engine runs terrible. but i like your optimism

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?

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
 

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

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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.
 

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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.
 

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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?
 

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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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Absolutely. Can be fouled plugs, bad rotor (burned) or cap (cracked or dirty inside). Those are the first things I look at.
 

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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.
Whoa, whoa - You're right, dribbling in a little oil will raise the readings if worn rings are the culprit. So yes, that's a diagnostic: if compression is low without oil, comes up with oil, blame the rings.

However, that doesn't mean:

1) you should always dribble in some oil before measuring compression.

2) if the readings are high with oil, then everything is fine.
 

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I neglected to say anything about what it means if the readings do come up. I didn't say that the rings would be fine if they did come up as you imply I did in your second point. I think you misunderstood my comment, and just assumed that.

When I said, "If you don't do this, you can end up with low readings", I meant that it may be the case if the rings are worn, and it will result in those low readings.

It has been standard practice to use a little oil in the cylinder to see if that does happen. It is a check to see if the readings do come up, meaning that perhaps the compression rings are faulty; or if the values don't come up, valves are most likely dodgy.
 

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As others have stated the running terrible is not caused by low compression in your case. 110 is not that low and they are even, so solve the other problems first. There are several tech articles about extreme wear in piston engines and the conclusion is that they can be run until the compression is so low they will no longer idle. I've seen engines run with numbers as low as 60 and once warmed up they run fine, of course they don't make the power they should.
If this was my engine I'd leave well enough alone and find the real culprit to poor running then only if I needed more power would I open the engine up.
 
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