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Got on the dyno, down on power

5K views 93 replies 30 participants last post by  maverick3429 
#1 ·
Got the car on a dyno after my cam install, and the numbers are pretty low. 250hp/260ft lbs. I should be at least 275hp, and over 290tq from what I've seen.

He showed me that there was a difference in fuel trim between bank 1 and bank 2 of 17%, so it would be pointless to try to get power out of it until it's fixed. It's not throwing any codes though, which is good... but makes finding the problem a little harder.

He got the idle and the driveability locked in, though. Full throttle AFR's are 12.1, it starts right up and idles at 800, runs really smooth and feels like it pulls harder than it did before I put the cams in, but the numbers on the dyno are definitely off. When I'm driving, I don't feel a miss or knocking or any breaking up. It's not blatantly obvious to me that there's an issue.

He thinks it's a spark problem and told me to put new plugs in and front O2 sensors because maybe I fouled them up by running the car untuned for a few days. But I don't know if that would make a 30hp difference. What else could cause a bank-to-bank difference in fuel trim like that? An exhaust leak on one side before the front O2 sensor maybe? I just want to cover all the bases before I go back and try again next week.
 
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#85 ·
I know he said cam timing is not the issue, and it's running great now....but my first thought when he said he installed aftermarket cams and the fuel trim was 17% off between the two sides was that the cam timing was off (or different) between the two cams. It could just be the way the cams are ground? I've heard it's not uncommon for the factory cams to be off several degrees from side to side. I would imagine you would have to use an actual degree wheel to catch it. It seems to me that would certainly effect the combustion between the two banks, but I don't know if that would effect the "fuel trim" as he described.
 
#86 ·
That is always a possibility as well although a skipped tooth would be a more common problem. 17% seems quite significant for it to be a simple problem such as an o2 sensor plus something as simple as an o2 should have been picked up by data logging. It wouldnt hurt to pull the cover and double check before throwing parts at a seemingly unknown issue.
 
#88 ·
The link is on my home computer so I'll post it up when I get home from work

---------- Post added at 03:28 PM ---------- Previous post was at 03:22 PM ----------

Do a leak down test. Fuel trims being off bank to bank after a cam install usually points to one of two things which would be timing is off such as a skipped tooth or bent valve(s). Both of them would also explain it being down on power. With a set of mild cams is not uncommon to skip a tooth and still have no ptv issues and so forth but the fuel trims are off bank to bank. Compared to a large set where ptv would have occurred.
Later in the thread I posted my compression numbers, they all came out good so I'm not losing compression through the valve seals or the piston rings.

I timed the chains to the dots on both cam gears and the timing mark on the crank gears with all the pistons below deck. I rechecked it multiple times after I put the new tensioners on. It runs well now but I'm gonna throw a few parts at it for good measure, get the tune finished up and another dyno run will hopefully show a nice improvement.
 
#90 ·
What amazes me is you went to dyno it with plugs that had ALOT of miles on them lol. That plug is so worn out.
 
#93 ·
I know, all that work and I didn't even think to check them. So dumb.

---------- Post added at 04:21 AM ---------- Previous post was at 03:54 AM ----------

I'm not doubting you installed them properly, I'm just speculating that maybe one or both cams is ground off of spec index by a few degrees (as in a quality control issue)...so the cumulative effect results in a significant difference in cam timing between the LH and RH cams relative to each other. You would never be able to tell unless you used a degree wheel to find the actual lobe centers on each cam and compare them. It could be easily corrected by adjustable cam sprockets. Like I said, Im just speculating and asking if that could possibly explain the difference in fuel trim that you have between the two sides. I don't know if the spark plug issues would cause that much difference in fuel trim, but maybe they did. Glad to hear you got it running better. Hopefully your next dyno pull will produce the numbers you're after!
I didn't go with adjustable gears because I didn't want to mess up the install or have them slip, and I figured a few degrees wasn't a huge deal as long as the valves don't touch. There is no way of knowing now unless I break it all down again. Depending on the outcome of the next try, I might need to do that.
 
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