What compression should the cylinders be in a low compression FI motor?
02-24-2007, 12:51 PM
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What compression should the cylinders be in a low compression FI motor? I have had some constant misfiring issues since I dynoed a few weeks ago...
One of the plugs was completely melted, and almost all of them when I pulled them, cracked and broke since it seemed to be stuck in there pretty good.
Another thing is the last furthest plug on the drivers side seems to be hitting something...
Ive changed plugs twice in a week on that one cylinder, and when I pull it the gap is completely closed and i see little tiny chunks of metal of the end of the plug scraped up...
Pulled the valve cover and that looks fine. My friend thinks something might have fucked up in the heads or block and thats what keeps messing up the plugs...
We did a compression test and pulled 90 on that one, and 100-105 on the ones right next to it.
Motor is 8.5 CR in the 408.
Is that 90 bad? What things can I check for before having to pull the head and get down and dirty?
One of the plugs was completely melted, and almost all of them when I pulled them, cracked and broke since it seemed to be stuck in there pretty good.
Another thing is the last furthest plug on the drivers side seems to be hitting something...
Ive changed plugs twice in a week on that one cylinder, and when I pull it the gap is completely closed and i see little tiny chunks of metal of the end of the plug scraped up...
Pulled the valve cover and that looks fine. My friend thinks something might have fucked up in the heads or block and thats what keeps messing up the plugs...
We did a compression test and pulled 90 on that one, and 100-105 on the ones right next to it.
Motor is 8.5 CR in the 408.
Is that 90 bad? What things can I check for before having to pull the head and get down and dirty?
02-24-2007, 01:45 PM
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we only did it on a couple of them on the passenger side, and they were right at 100 or so...
****, this is not good. What options do I have then? What do you think happened to **** it up?
****, this is not good. What options do I have then? What do you think happened to **** it up?
02-24-2007, 05:01 PM
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Pull those heads!!!!!!!! Did you try to rotate motor by hand before you started it up to see if anything was getting hung up. Is it a new build?
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02-24-2007, 05:34 PM
#8
i dunno my motor was 103-107 across all cylinders and runs fine.if theyre supposed to all be within 10% of eachother that 90psi cylinder might be toast.did you do a leakdown test or just a compression test?
edit....trans zam you have more car problems then me and i didnt think that was possible.just try to keep going cause evertime you go WOT it makes it worth it.
edit....trans zam you have more car problems then me and i didnt think that was possible.just try to keep going cause evertime you go WOT it makes it worth it.
02-24-2007, 08:51 PM
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Don't start or run that thing until you pull the head. That spark plug damage might be serious. I hope its nothing bad. Your car is flat out amazing. Good luck!
02-24-2007, 09:31 PM
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Originally Posted by engineermike
I blew up 4 motors in a year before deciding to back off the boost level.
02-24-2007, 10:08 PM
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Originally Posted by Got Me SOM
being a fellow engineer i would have figured after 2-3 times you would have come to conclusion then lol.
02-25-2007, 02:45 AM
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You need to start running better gas at the power levels your pushing. Damn near 900rwhp on 91 octane is just asking for it. Run at least some 100 for those power levels just to be safe. 104 or 110 when you're beating on it wouldn't be a bad idea either...but anything is better than 91!
02-25-2007, 12:41 PM
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Originally Posted by TS6
Did you do the test with the motor warm?
Ditto on the having problems thing, it sure seems like that car breaks alot.
Ditto on the having problems thing, it sure seems like that car breaks alot.
When we did it, all plugs on the passenger side manifold were plugged in.
all the other plugs except for 1 were out on the drivers side... So we had for example the compressor kit plugged in the back cylinder, then open, then open, then the first cylinder had the plug in it.
We havent done a compression check on the passenger side yet. I hoping something just got caught up in the head and fucked something up and maybe I will just need some machine work... But with my luck the block is scratched up.
If it is something like having to have the block punched out, would it be most cost effective to just bore it out to a 427 and throw it back in there?
Then just sell my 408 rotating assembly and try to recoop some money?