I just did a compression test on my 3.5L motor. The front bank ranged from 140-155 and the rear ranged from 180-190.

I guess this would be a sign of the cam timming being off on the front bank. If so that would deff cause the symptoms i am seeing.

Here is a link to the thread describing how the car is running
http://forums.maxima.org/showpost.ph...09&postcount=1

Any thoughts?

 

Horrible front numbers. Time to pull cams

 

just pull the timming cover and adjust the timming chain. I was pi$$ed at first but i figure it is better this than the timming being right and the cams themselves not being able to work.

we already have the SC off and are in the process of taking off the timming cover.

Its kinda bitter sweet

 

Wait what? So the timing chain was off?

 

Taken from the other thread.

 

Dont know yet. I am just assuming the front bank must be off since that whole bank has way lower compression than the rear bank.

Is this assumption correct?

 

I'm not saying we are going to purposely set the timming chain to retard the cams. I am saying that we think the front bank must have jumped a tooth or two when assembling the timming equip on the 3.5. The reason we think this may be so is because of the compresion test results

 

From reading everything i think you have a compound problem. Excessive overlap due to what we talked about and the front bank cam jumping a tooth are so. Excessive overlap won't cause low compression numbers.

 

but the cam jumping a tooth would cause low compression numbers correct


Although i may have extra overlap, i just don't see it being a crippling problem like what i am experiencing right now.

 

Yes it would.


If you were n/a, the extra overlap wouldn't concern me as much. But seeing that you are boosted, overlap is your worst enemy. It might not be causing the idling issues (then again it might play a part in it) but it's gonna cost you some power.

 

That is true but we are talking about a centrifugal SC which doesn't build any boost at idle. And builds really no boost until around 3k rpm. In which case it wouldn't affect the idle and wouldn't really affect the low end that much

Say you have a 350z SC'd w/ a vortech and nismo cams. Would they be running the same amount of overlap above 5k rpm as me or more? Say i do in fact have 10 degrees of overlap.

 

I wouldn't be able so say since i don't know what the VTC maps are like. All i know is for boosted applications the ideal cam grind is one that adds lift and duration without increasing overlap.

Although i would consider your amount of overlap excessive, the fact that the r-tune cams have considerably more duration than stock gives the cylinder more time to be filled with air after the exhaust valve closes, which is a plus.

 

The cams and overlap will be fine, I would not worry at all. Your timing chain is off since the compression readings are different on each side of the engine. That would cause a bad idle, and revving slowly/ heavy flywheel feeling.

 

You don't think 4 or more degrees more valve overlap than stock sans VTC with boost is an issue? Like i said, excessive overlap won't cause low compression numbers. Fixing the timing chain will improve the idle and restore compression, but the overlap will reduce his low end and cause boost loss through the exhaust valves on top-end.

 

Well i may be wrong but it seems that in order to run run more aggressive cams you will have will have to sacrifice overlap. There are other cars out there that are running far more aggressive cams than mine on boosted cars and they see substantial power increases. I know these cams are not ideal for my setup, but they are installed and i think they will preform better than the stockers.

 

It will not be ideal to have excessive overlap on a boosted engine, you are correct. These are N/A cams, not FI cams. But, that being said, they will still make more horsepower because of the duration and extra lift. I would stick with them.

 

I checked the cam timming this afternoon and it was dead on. I am about to give up on this F'ing car and sell it as is.

 

Drop vq30 cams in it and call it a day.

 

What about the possibility someone raised that you may have swapped the Right and Left bank cams? They said they were nearly identical to the eye and had similar symptoms until they swapped them, IIRC.

 

.................

 

Did you check the exhaust cam chains too? The symptoms pointed to the main chain, but it never hurts to check it all. Are you sure they didn't get the cams mixed up, left and right is easy to get mixed up? Take a picture of the cams left and right, and also the chain setup if you can. I took several pictures when I did mine so I could compare.

 

i thought they were both the same

 

Those couldn't have possibly been mixed up since the front bank cams are longer than the rears since they have to reach out to the 6 cylinder.

 

^^ exactly.

 

sorry to thread jack but i did mine also today and they were all around at 120-130.
where the compression needs to be?

 

according to the FSM that is below the spec range.

 

So did you check if cams were backwards?

 

even with cams? i don't know if the compression changes or stays the same

 

I am going to do a leak down test on monday to see what is causing the low compression. Any guesses as to what the cause may be? I was thinking maybe a blown headgasket, but wouldn't the readings be a lot lower on the front bank than what i am seeing if that were the case?


Btw i looked up the specs on the Nismo cams and i remember someone saying that they increased overlap by 3 degrees from stock, not 4 degrees. Being that the JWT cams have 2 degrees overlap from stock i am still not convinced that the cams themselves are the problem (at least not yet). The only possibility could be that Stephenmax drilled them wrong, in which case we would have to have a little chat about that. I am fairly careful about thinking this though becasue he is a very experienced and meticulous person.

 

It is impossible for them to be backwards from what i understand. That post that icey linked was talking about a guy who got the vtc mixed up. It wasn't the cams that were backwards.

 

compression will stay the same even w/ upgraded cams

 

Yeah. You need to get this bad boy running asap.

 

Seeing that you had the issue with aftermarket cams, I contacted Tilley about my cam spacers/adapters to see if there would be any issues since I'm going to be using the Debuloz (JWT knockoff cams) and he said they should be none and the adapters will be made as if we were using stock 3.5 cams. I believe the cams are not the issue. I really think you should get the motor running without the super charger and go from there.

 

How would the SC cause a low compression reading? It has nothing to do w/ the SC. I will leave it off for the time being though so i don't have to keep taking it off

 

I hear what you are saying. But what I meant is just so you can isolate the problems. Once you figure out the compression issue, there may be another issue. With the supercharger off, you can get everything figured out and get this bad boy moving. Can't have you saying sillt things like "going to sell the car as is" due to a little frustration. Remember, you are the first to take the 3.5 swap to these measures (3.5L with SC) so the pioneer will have the most issues. Good luck!!!

 

Bad motor? Maybe you got a 35 that consumed oil. (precat pieces broke and got sucked into head..)

 

Actually, I don't think that's 100% correct. The static compression ratio will be the same, however the change in overlap will cause the dynamic compression ratio to change IIRC.

Sorry about the misinfo on the 350Z thread, I need to read better.

Also, I agree with you that it isn't the cam grind/advance angle causing this issue, it has to be something else causing that much of a compression difference. My feeling is that something to do with the install, clearances, etc..

Keep up the fight man...it will be a BEAST once you get it figured out and worth it I'm sure.

 

It is pretty impossible to get the cams switched. The lobes are in different places since the valves are different lengths away from the timing chain. The rear would also be out of spec.

More agressive cams will lower dynamic compression, but for such a mild cam it would not do it this much. Again look at the rear bank.

Any news Chris? This has me stumped. Did you check the exhaust cam timing on the front bank?

 

no update yet guys. The shop is closed for memorial day weekend. First step is to do a leak down test to see where exactly the pressure drop is coming from then we will go from there.

BTW alex, the clearances were checked when intalling the cams and they were dead on.

 

Did you change the buckets?

If not, I thought all but the S-tune required ordering new buckets to put the valve lash? at the proper spec?