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200psi compression help??

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Discussion starter · #21 ·
sweet yeah i was able to talk to one of my mechanics today that helped me with a couple of things and he said my compression will be up a little cause we have the 248 intake cam advanced some so that will cause my compression to be high, i do appreciate all the help though guys
 
#22 ·
interesting.... a camshaft increasing compression pressure????? lol@urmechanic
 
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Discussion starter · #24 ·
well its because i have it advanced above the normal 4 teeth, cause other then the random shaking it does at idle it runs great, normal partial drivability is freakin awesome
 
#25 ·
You guys don't know your engine theory very well if it surprises you that the way a cam is adjusted can affect cranking pressure. Ever heard of cam overlap? How the more overlap you have the more compression that is lost at low rpm. If you advance your intake cam, you decrease overlap, more low rpm torque and HP, less at high RPM.
 
#26 ·
cam overlap doesn't matter whatsoever when you're compression testing... the piston is never (unless you're running an anti-lag system) pushing air out of the exhaust port under compression...so there is always a time when the maximum compression is able to be read on a gauge

and ch183958312989080123xz3 is right about it only needed to be cranked 4 times... the piston only needs to go through intake compression power and exhaust (that's four things) once to get a reading of compression
 
#27 ·
Not so much overlap but cam timing affects cylinder pressure. Most cams leave the intake valve open after bottom dead center and stays open during the begining of the compression stroke,it relies on the high velocity of the intake to continue filling the cylinder, even though the piston has started compressing the mixture, this is the main reason radical race cams run so bad or not at all at low rpm, because at low rpm where the intake air velocity is low they will actually blow mixture back thru the intake. I've seen this myself on some radically camed dirt track engines, their static compression might be 13 to 1, but their cranking pressure might be only 130-140 lbs. if you don't put their idle up to about 1500 or higher, you can actually see the backflow thru the carb just before it stumbles and dies. What you're seeing is lost compression from cam timing. If you can lose compression thru radical cam timing, you could certainly gain it by going more conservative with a cam.
 
#28 ·
japside4life said:
you have 205 205 205 205 which is great all your numbers are even across the board. Yes they are higher than factory spec. That could be due to carbon build up. Is it a problem? Yes and no, its a problem if your worried about boosting really high, but if you have a tune and good fuel to compensate for it you will be great.
Click to expand...
bingo....
 
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#30 ·
haha at this thread.

cept for dark hawk and tyrodtom.


did anyone mention anything about pulling the ecu or fuel pump fuse? if not you are still dumping fuel into the cylinders and that will throw your reading off.
 
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#31 ·
since when does fuel lower your compression? you cant run a motor without fuel. is fuel not in your cylinder during the compression stroke? why take away the fuel during a compression test if its there when its running. the only thing you take away is the ignition so you dont fire the mixture. ive never read that in any of the articles ive read or seen and not one tech has mentioned it when i talked to them about it, and they sure as hell didnt do it when nissan did a compression test on my brothers old ka..

few articles to look at.

http://www.aa1car.com/library/compression.htm
http://www.georgiazclub.com/tips/ztech/comptest.htm (this ones really good)
http://www.motorage.com/motorage/article/articleDetail.jsp?id=16170

none of them say shit about fuel...
 
#32 ·
did i say it would lower the compression? nooooooooooo

it will actually make it seem higher, as its going to displace air, as carbon, etc will. it starts pooling on the pistons, since its not being fired.

you should stop reading so much crap online and go get some actual experience.


and all these techs you talk to are either questionable, or you arent listening good.
 
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#34 ·
turbo510 said:
haha at this thread.

cept for dark hawk and tyrodtom.


did anyone mention anything about pulling the ecu or fuel pump fuse? if not you are still dumping fuel into the cylinders and that will throw your reading off.
Click to expand...

well since he already has the god of all mechanics telling him how a compression test should be done, the mechanic already told him to use a remote start switch, and you don't have to worry about spark and gas,,,or running back and forth.

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=35448

Image




"you should turn it over until the gauge stops rising, but no more than 12 seconds"

about anywhere you look for PRINTED (or reliable internet DIY's) instructions, ^that's what you'll see.
 
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