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Old 12-30-2015, 09:28 PM   #1
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Question Compression tested my 1996 1.8l miata

okay so ive been doing a ton of research to see what needs to be done before just slapping a turbo in my miata. i figured i would make sure everything was healthy on the motor as it sat.(stock) before doing any modifications. i checked the timing and it was spot on 10 degrees exactly! while i was under the hood doing a tune up *Plugs wires etc.* i figured i would do a compression test! What i found wasn't exactly what i was hoping for

this is what i came up with going from front to back
Cyl1: 170
Cyl2: 150
Cyl3: 165
Cyl4: 180

i am aware they are supposed to be withing 5% of each other but since there not now what..... should i just say screw it and see how long it lasts? or is it going to instantly fail and be a waste of time to install the turbo. *i only plan to run about 5lb. of boost* also the motor has 170,000 miles but then again Greg from TheCarPassionChannel has a turbo miata with over 200,000 miles and he is fine.

Yes i am new here so try to keep the roasting to a Minimum also any help is greatly appreciated!
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Old 12-30-2015, 10:34 PM   #2
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Is it your only car?
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Old 12-30-2015, 10:39 PM   #3
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Smoking?
How much oil does it use?

Would suck to destroy a 1k turbo with metal in oil. Maybe someone can chime in with if thats is likely.
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Old 12-30-2015, 10:51 PM   #4
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no i also have a 1997 jeep wrangler! and i usually add oil when i hear the lifters making noise on start up about 1qt every month and a half
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Old 12-30-2015, 10:56 PM   #5
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not much smoke though most of it puddles up in spark plug holes i need to do a valve cover gasket
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Old 12-30-2015, 10:59 PM   #6
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Quote:
Originally Posted by Ca11 Me Bubbles View Post
not much smoke though most of it puddles up in spark plug holes i need to do a valve cover gasket
Do another compression test and put a small amount of oil in each cylinder.
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Old 12-30-2015, 11:19 PM   #7
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honestly i don't plan on rebuilding a motor with almost 200,000 anyway i just want to know if it will be a waste of time to do all the work and pay to have it tuned on a dyno if its going to blow up in 1k miles. or should i save the effort and find a lower mileage motor either way i will do the wet test after work tomorrow and report back!
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Old 12-31-2015, 12:41 PM   #8
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Why not just rebuild the existing motor with forged internals?

I guess a stock motor used is less money?
Forged rods alone would give you a lot more boost safety.
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Old 12-31-2015, 12:58 PM   #9
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because if i do the work of rebuilding an engine with forged internals i would rather do it on a low mile motor and not one that is at the end of its life span. i think i am going to just boost it how it sits and see how long i can get away with it. if it does blow up ill buy a lower mileage motor and build it the right way! thanks for all the imput guys!
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Old 12-31-2015, 01:01 PM   #10
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If the oiled comp test shows perfect, then right there you know its rings. If its the same, then it's likely valve seals. Does not look/sound like a HG issue

Bad rings will not result in big metal particulates in oil, not a lot of turbo kill-age potential there. Fried bearings could do that, but worn rings not so much.

An oil analysis would also be a enlightening.

And while that variance is a bit worrysome, bad rings (assuming its that) do not necessarily mean its a time bomb. I have seen engines with 90 psi compression (not kidding) run really well with lots of boost. I don't recommend it, but it did work. You just need God's own PCV setup.

With either rings or valve stuff, you could consider building up a shortblock or longblock while running the current engine. I think I saw a 99 core on here for cheapo depot (like 75 plus shipping) very recently. Nice little upgrade in and of itself. At that point you can also decide if you want to put in rods or rods/pistons if you do a bottom end.

That does require time, willingness to learn and possibly redo stuff, tools, and a good work space though. Known good used long block may be your best bet.
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Old 12-31-2015, 01:04 PM   #11
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Quote:
Originally Posted by Ca11 Me Bubbles View Post
because if i do the work of rebuilding an engine with forged internals i would rather do it on a low mile motor and not one that is at the end of its life span. i think i am going to just boost it how it sits and see how long i can get away with it. if it does blow up ill buy a lower mileage motor and build it the right way! thanks for all the imput guys!
When you rebuild mileage becomes fairly unimportant, as you are tackling the worn parts anyway. You are probably at least rehoning the cylinder walls and certainly doing new rings, so that who deal is good. You are doing new bearings, so that's good. Really, as long there's not actual damage to the block or crank, there no disadvantage to rebuilding a high mileage shortblock vs a low mileage one.

Apparently old F1 teams in the 80s use blocks with a minimum of 100K miles on them for the huge turbo 4 cylinder engines making 1000HP because that large # of heat cycles made them more consistently strong than factory new blocks. Much has changed sine 1982, but then again these engines are more or less 80s tech.
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Old 12-31-2015, 01:09 PM   #12
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wow 90lb!?!?!?!?! that makes me feel alot better! and i will definitely be scavenging for engine parts to build one from the ground up! luckily my dad is a mechanic and i have an entire garage full of tools and car lift at my disposal thanks for the advice! oh and on a side note because im a noob here on the forum how does one acquire cats? LMAO
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Old 12-31-2015, 05:32 PM   #13
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If Dads a mechanic and you have the tools and time, there's no reason not to rebuild something. BP blocks and cranks are quite stout (virtually any iron 4 cylinder is really) and heads are cheap. 99 heads are significant improvements over stock by virtue of just plain flowing better.

Seriously, check the FS section if you have enough posts. I thought I saw some good rebuilders in there lately. Rebuilding is a lot easier if you have a running car in the mean time.

Edit: But it important to figure how your stocker is too - that oiled comp test will tell you a lot.

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Those with many cats are those that have figured out various conundrums related to our cars, or created things that are helpful to MT.net people etc. Or just have excellent cat photography and drinking skills. So people like Joe P who have done a lot with MS (or Brain, or....) have many cats.
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