Possible back pressure problem??

[bigLSHO]

I am still getting used to this car. Its my 3rd SHO so I have an idea on how it should perform.
After ressurecting it from someone elses neglect, I have put a total of 75 miles on it since tuesday. Just cruising around trying to get the clutch worn in as well as get a feel for how this car performs and what needs done outside of a planned 60K and rod berrings in the near future.

The issue I am noticing, besides a few minor things non mechanical, is something that may be hard to describe:

To get it road worthy I needed an exhaust. I went with the full SHO Source catback with equal length Y and cats.

I noticed when finally getting it on the road that the exhaust was quite loud. I thought it was just the Borla mufflers. But I had also realized that the car was pulling very weakly. I was even having trouble with hill climbs in 2nd gear.

Yesterday there was a change in tone, it was breif but I didnt pay it much mind. I was almost home and parked it. Started it this morning and it was loud on my way to work.

Took my extended trip back home after work today. Right near the end of the journey I noticed the change in tone to a quiet, more even note. Skipped the turn home and I tested it. Hmmm...there is the power I was missing, a strong pull lots of low end and an even top end feel. I didnt push it too hard but I know the difference. Extremely noticeable difference.

It seems upon reflecting on it, that the loud noise comes from underneath me sitting in the drivers seat.
Could this be a gasket issue between the y and the cat back?
Would this be enough to cause a back pressure issue?
Are there any possible other culprits?
I was already planning to take it to an exhaust shop and not my mechanic to make sure everything was tight and sturdy this weekend.

Thanks in advance

 

[bigLSHO]

Let me add one thing to this. I also noticed a difference in exhaust tone when coming down hill. Coasting was a queit tone, even at a reasonably high rpm. Once the gas was hit the loud beast awoke again. Could it be a bad motor mount causing a shift in the position of the exhaust? Just a side thought. Although, I'm relatively aware of what that's felt like in my other cars. This one seems sturdy.

 

[jayro]

Checking the motor mounts is easy enough. Just pop the hood and put the parking brake on. Easy forward and then reverse. Watch how much the engine moves. As far as the exhaust. If the gasket between the y pipe and catback is messed up it will be really loud, but I don't think it would effect the power to the extreme you are talking. I wouls check the codes and run the cylinder balance test. Also check for oils in the plug wells ect. Doing your 60k would be the best place to start.

 

[bigLSHO]

Its just strange to me that the difference in sound directly effects its overall performance. Because both of these are drastic in their change. I assumed it would be something with the new exhaust system.
Ill check codes tomorrow
It does need 60K'd(hopefully soon)...it sat for a looong time.
Thanks for the reply.

 

[mrecoolgar]

I agree with jayro.
Check engine mounts as well as the subframe mounts.

 

[lowc]

any chance that the new cats are defective clogging up on you. try hitting them (while cool) with the palm of your hand and see if you can hear anything flopping around in there.

 

[sho pleaser]

check the all the ****** gaskets .. y pipe to the manifolds most likley the back one is m.i.a just my 2cent

 

[SHOtime2511]

Could be the cats coming apart,I've heard a few bad reviews of those magnaflow cats.

 

[DirtySho]

yeah i had magnaflow cats on and i had nuthin trouble since the where in stalled. there really bad.

 

[bigLSHO]

check the all the ****** gaskets .. y pipe to the manifolds most likley the back one is m.i.a just my 2cent

This is a possibility. They had problems with the old Ypipe and had to retap the studs. He lost a bit in the process. Maybe frustration caused him to forget to place the gasket on.
I'm going to have the system looked at today.

As far as the cats, ill have them looked at as well. I just can't see them being clogged. Wouldn't it run like crap all the time instead of intermittently?

 

[bigLSHO]

Another symptom that I wanted to make certain of, I was just able to confirm. RPM-MPH difference.

I hope that this doesnt turn out to be a major issue. When performance is down/loud exhaust output, the RPM range per MPH is low ie. in second gear 60MPH at 4K RPM. I know that this is unnatural as my other car would be close to redline at that speed. This isn't the exact MPH to RPM ratio but its close.

I actually noticed a jump in the tone while cruising in first gear from loud to quiet and smooth as well as a jump in the RPM from low to a higher more regular pace.

Also, while parking...two times I have noticed the oil light. It sometimes idles low and does when I am parking but that oil light bothers me a lot. I know that usually means big trouble.
Ill check codes shortly and report any findings