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Old 07-24-2010, 01:48 PM   #61
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yay for pulling valve cover!
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Old 07-24-2010, 02:46 PM   #62
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Huzzaah


Can anyone figure out how the fuu I saw 150+ compression on all cylinders with my timing that fucked?
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Old 07-24-2010, 04:05 PM   #63
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you mentioned the intake and crank are out of sync with the exhaust, but how out-of-sync is the intake with the crank? exhaust out of sync with the crank will not affect compression numbers. intake out of sync with crank will. if your intake cam is slightly early, methinks it would actually increase the numbers you'll see on a compression test before it starts to decrease in compression numbers given the low engine speed you'd be testing at.
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Old 07-24-2010, 04:39 PM   #64
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well i did see a whopping 180psi cold and dry before i got my compression tester extension stuck. i did think that was rather massive.

i dont really understand this whole intake exhaust thing very well. I stared at this thing for a while but still am not getting how i saw high compression #s. Is it because the intake is allowing more air than normal in?
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Old 07-26-2010, 11:17 AM   #65
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I *believe* that:
Under normal operation, your intake valve should close AFTER BDC. This is because of:
o Our cylinder heads flow poorly compared to how they could have been designed. Dont fault Mazda, they weren't exactly trying to squeeze every single bit of N/A power out of these engines. Remember, these engines were used as powerplants in other cars first, and the Miata was simply designed to accept them.
o Under normal operating speeds, when the piston is BDC, there is still a significant vacuum in the cylinder relative to the air pressure in the intake manifold. This is exaggerated by the poorly flowing cylinder head. Leaving the intake valve open for a few more degrees allows air to continue to flow into the cylinder while the piston begins its compression stroke. In a perfect world, the valve would close at exactly the same time that air pressure inside the cylinder = air pressure in the intake manifold. Unfortunately, air has mass and at different engine speeds the timing to close the intake valve changes. Changes were made to later cylinder heads. In 99, VICS was introduced, which dynamically changed the speed that air passed through the ports for two different RPM bands. In '01, Mazda integrated VVT, which actually allowed for the valves to open/close with a more advantageous and infinitely variable timing across the entire RPM spectrum.

When doing a compression test, I would *expect* that:
o Your engine is spinning too slowly to see an advantage from valves closing after BDC during cranking. This is because:
o With the valve timing being after BDC, your piston will suck in air with the open valve, and then it actually has time to push some of the intake charge back out into the intake manifold before the valve closes. At the slower 'cranking' speed, your cylinder has enough time to equalize with the air pressure in the intake manifold, and then as the cylinder begins its compression stroke, air pressure in the cylinder increases, and some air escapes back into the intake manifold before the intake valve closes.

So, if the above are all true statements:
o If your timing is off such that your intake valve closes closer to BDC than what you would normally expect, then you may also see compression numbers that are higher than what you would normally expect.

It should also be mentioned that:
o At the bottom and top of your piston stroke is when the piston moves the least/slowest, so the first few timing notches off in your intake timing may only be a centimeter or two in actual cylinder compression. The next few timing notches may be an inch, and the next few timing notches may be more than half the stroke of the cylinder. I don't know the actual travel distances, but this is the theory; so your compression numbers may actually look halfway decent with a timing that is off by several notches.
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Old 07-26-2010, 03:33 PM   #66
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Quote:
Originally Posted by fooger03 View Post
I *believe* that:
Under normal operation, your intake valve should close AFTER BDC. This is because of:
o Our cylinder heads flow poorly compared to how they could have been designed. Dont fault Mazda, they weren't exactly trying to squeeze every single bit of N/A power out of these engines. Remember, these engines were used as powerplants in other cars first, and the Miata was simply designed to accept them.
o Under normal operating speeds, when the piston is BDC, there is still a significant vacuum in the cylinder relative to the air pressure in the intake manifold. This is exaggerated by the poorly flowing cylinder head. Leaving the intake valve open for a few more degrees allows air to continue to flow into the cylinder while the piston begins its compression stroke. In a perfect world, the valve would close at exactly the same time that air pressure inside the cylinder = air pressure in the intake manifold. Unfortunately, air has mass and at different engine speeds the timing to close the intake valve changes. Changes were made to later cylinder heads. In 99, VICS was introduced, which dynamically changed the speed that air passed through the ports for two different RPM bands. In '01, Mazda integrated VVT, which actually allowed for the valves to open/close with a more advantageous and infinitely variable timing across the entire RPM spectrum.

When doing a compression test, I would *expect* that:
o Your engine is spinning too slowly to see an advantage from valves closing after BDC during cranking. This is because:
o With the valve timing being after BDC, your piston will suck in air with the open valve, and then it actually has time to push some of the intake charge back out into the intake manifold before the valve closes. At the slower 'cranking' speed, your cylinder has enough time to equalize with the air pressure in the intake manifold, and then as the cylinder begins its compression stroke, air pressure in the cylinder increases, and some air escapes back into the intake manifold before the intake valve closes.

So, if the above are all true statements:
o If your timing is off such that your intake valve closes closer to BDC than what you would normally expect, then you may also see compression numbers that are higher than what you would normally expect.

It should also be mentioned that:
o At the bottom and top of your piston stroke is when the piston moves the least/slowest, so the first few timing notches off in your intake timing may only be a centimeter or two in actual cylinder compression. The next few timing notches may be an inch, and the next few timing notches may be more than half the stroke of the cylinder. I don't know the actual travel distances, but this is the theory; so your compression numbers may actually look halfway decent with a timing that is off by several notches.
Wow, that was very educational, thank you! So this pretty much means that a compression test just isn't enough to check if your timing is fucked. I see.
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Old 07-26-2010, 04:12 PM   #67
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Quote:
Originally Posted by fooger03 View Post
I *believe* that:
Under normal operation, your intake valve should close AFTER BDC. This is because of:
o Our cylinder heads flow poorly compared to how they could have been designed. Dont fault Mazda, they weren't exactly trying to squeeze every single bit of N/A power out of these engines. Remember, these engines were used as powerplants in other cars first, and the Miata was simply designed to accept them.
o Under normal operating speeds, when the piston is BDC, there is still a significant vacuum in the cylinder relative to the air pressure in the intake manifold. This is exaggerated by the poorly flowing cylinder head. Leaving the intake valve open for a few more degrees allows air to continue to flow into the cylinder while the piston begins its compression stroke. In a perfect world, the valve would close at exactly the same time that air pressure inside the cylinder = air pressure in the intake manifold. Unfortunately, air has mass and at different engine speeds the timing to close the intake valve changes. Changes were made to later cylinder heads. In 99, VICS was introduced, which dynamically changed the speed that air passed through the ports for two different RPM bands. In '01, Mazda integrated VVT, which actually allowed for the valves to open/close with a more advantageous and infinitely variable timing across the entire RPM spectrum.

When doing a compression test, I would *expect* that:
o Your engine is spinning too slowly to see an advantage from valves closing after BDC during cranking. This is because:
o With the valve timing being after BDC, your piston will suck in air with the open valve, and then it actually has time to push some of the intake charge back out into the intake manifold before the valve closes. At the slower 'cranking' speed, your cylinder has enough time to equalize with the air pressure in the intake manifold, and then as the cylinder begins its compression stroke, air pressure in the cylinder increases, and some air escapes back into the intake manifold before the intake valve closes.

So, if the above are all true statements:
o If your timing is off such that your intake valve closes closer to BDC than what you would normally expect, then you may also see compression numbers that are higher than what you would normally expect.

It should also be mentioned that:
o At the bottom and top of your piston stroke is when the piston moves the least/slowest, so the first few timing notches off in your intake timing may only be a centimeter or two in actual cylinder compression. The next few timing notches may be an inch, and the next few timing notches may be more than half the stroke of the cylinder. I don't know the actual travel distances, but this is the theory; so your compression numbers may actually look halfway decent with a timing that is off by several notches.
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Old 07-26-2010, 05:14 PM   #68
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Quote:
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