Effect of cam timing on ITB's?

camlob · 06:39 PM

The reason for me posting it here, is because a lot of you guys have installed ITBs. I had installed on a 993. We got different afr readings on both banks. We thought it was the o2 sensors so we ordered new ones, and still the afr's are off by 1.7. We synchronized the itb's and get the same flow at 2k rpm and tried again at 3k.

We also checked for the flow of each injector and they are ok.

We noticed that at the start its ok, then after 5 dyno pulls, the afr's on the banks are off. Some tuners suggest checking out the cam timing again. Which we will do.

What is your opinion?

T77911S · 07-12-2011, 04:16 AM

cam timing would be the logical place to start, but you might consider a compression test and a valve adjustment check. one bad cylinder may be all it takes.

did you test the injectors on the car or off. if you can test them for flow volume on the car it may be more helpfull. i had one bad injector line causing all my problems. the car went through 2 other owners and who knows how many mechanics before i found it. but, i would start with the easiest thing to do first.

914efi · 07-12-2011, 04:54 AM

Where are your temp sensors?

Bill Verburg · Bill Verburg's Garage

i don't know if cam timing is your issue or not but I do know that late 993s have issues w/ cam timing that does not stay fixed.

Up through ~halfway in the '95 MY 993 production run they used the old dowel method of fixing cam timing, some time after that era they switched to a purely friction system which appears to be the source of the issue. When I rebuilt my 3.8(MY97) I back dated the cam timing to the older system.

jpnovak · 07-12-2011, 06:07 AM

I too would verify cam timing.

What ECU are you running? Are your O2 sensors grounded at the exact same place? Voltage drops at the contacts can make a BIG difference on the output reading of the O2 sensor. Are you using a dual wideband or narrowband? What is your AFR that is so different? Narrowband sensors are not very accurate once you drop below 13.5 AFR. The quickly go non-linear in response and any real difference in AFR is magnified by the output curves.

camlob · 07-12-2011, 01:29 PM

Quote:

Originally Posted by T77911S

cam timing would be the logical place to start, but you might consider a compression test and a valve adjustment check. one bad cylinder may be all it takes.

did you test the injectors on the car or off. if you can test them for flow volume on the car it may be more helpfull. i had one bad injector line causing all my problems. the car went through 2 other owners and who knows how many mechanics before i found it. but, i would start with the easiest thing to do first.

They tested with the injectors off. I will also suggest a compression test. Car has had only 65k kms on it so I didnt think of changing the rings. On hindshight, I should have at least changed the rings since they dismantled even the cylinders and not just the heads.

camlob · 07-12-2011, 01:30 PM

Quote:

Originally Posted by 914efi

Where are your temp sensors?

Do you mean to check the egt? I dont know how they did it but I guess with some sort of measuring tool.

camlob · 07-12-2011, 01:32 PM

Quote:

Originally Posted by Bill Verburg

i don't know if cam timing is your issue or not but I do know that late 993s have issues w/ cam timing that does not stay fixed.

Up through ~halfway in the '95 MY 993 production run they used the old dowel method of fixing cam timing, some time after that era they switched to a purely friction system which appears to be the source of the issue. When I rebuilt my 3.8(MY97) I back dated the cam timing to the older system.

I think I have the 964 type of cam gears. But the balance of afr's were already apparent when they started tuning. Too soon I guess to get off-balanced.

camlob · 07-12-2011, 01:37 PM

Quote:

Originally Posted by jpnovak

I too would verify cam timing.

What ECU are you running? Are your O2 sensors grounded at the exact same place? Voltage drops at the contacts can make a BIG difference on the output reading of the O2 sensor. Are you using a dual wideband or narrowband? What is your AFR that is so different? Narrowband sensors are not very accurate once you drop below 13.5 AFR. The quickly go non-linear in response and any real difference in AFR is magnified by the output curves.

We are using an electromotive tec3r. My tuner says not the best out there. My o2 sensores are grounded at the same place, a little after the collector. I know I have a wideband that came with the kit. We bought a PLX o2 sensor and gauge that will show me readings on both banks. Ill double check on the what type of o2 are we using for the afr readings.

We already brought down the engine to check on cam timing. John D also suggests that. Ill also do the compression test.

Thanks everyone for your inputs.

camlob · 07-12-2011, 05:22 PM

We just checked the cam timing and its spot on

It was off by only a hairline. Less than 1mm. Will do the compression test now.

camlob · 07-13-2011, 03:38 AM

We just found out the cause, weak c.r. on some cylinders. Sooo, it looks like I have to do a overhaul. I was thinking of doing a 4l kit for some time, so this might be it. I just have some questions.

I understand that the stock head can support 385 hp, so does that mean I can use my stock valves? No need for RS valves?

The car is primarily a street car with an occasional fun run with the boys. According to Charles at LN Engineering, he says stock rods will do. Of course, Pauter rods will be great, but I am trying to control costs. So maybe stock rods with ARP bolts, with the 4l kit and stock valves.

From my research, the 3.8l kit does not have the bang for buck factor. What do you guys think?

T77911S · 07-13-2011, 05:36 AM

they pulled the cylinders off? is that something you should do and reuse old rings?
rings are cheap, well at the time

camlob · 07-13-2011, 05:11 PM

Quote:

Originally Posted by T77911S

they pulled the cylinders off? is that something you should do and reuse old rings?
rings are cheap, well at the time

On hindsight I should have changed the rings. Well I might go for the 4l kit. Just waiting for the leakdown today.

So any inputs on using aftermarket rods? According to Troy of EBS Racing, stock rods can rev up to 7k rpm easily. Aftermarket rods will make me rev to 8.5k rpm, but would I need that? The longevity of the engine will be compromised at such a high rpm is my fear.

Flieger · Flieger's Garage

lighter rods will be insurance for any missed shifts and will also reduce bearing loads at all rpms.

camlob · 07-13-2011, 06:20 PM

Quote:

Originally Posted by Flieger

lighter rods will be insurance for any missed shifts and will also reduce bearing loads at all rpms.

OK. So the benefit wont be just for high rpms but will be for less stress on the bearings. What will the response be? Will the engine rev faster more noticeably?

Flieger · Flieger's Garage

It will rev faster, but the pistons might become the limiting factor. Whether the rods alone would be noticable, I do not know. But it will make you feel good to know they are there.

Lighter rods can only make the engine rev quicker.

camlob · 07-13-2011, 06:36 PM

Quote:

Originally Posted by Flieger

It will rev faster, but the pistons might become the limiting factor. Whether the rods alone would be noticable, I do not know. But it will make you feel good to know they are there.

Lighter rods can only make the engine rev quicker.

The JE pistons on the 4l kit are much lighter than stock. Hmmm, tempting.

Can the stock 993 crank handle the increased rev limit? Would I need to balance it if I get the rods?

Flieger · Flieger's Garage

The crank would be balanced on its own and you would match the rod weights (which is done when you get a complete set) to keep that balance. Lightest piston + heaviest rod and the reverse. You are only talking minute mass measurements here, though, with good quality parts.

As to the rev limit for the 993 crank, as Steve Weiner of Rennsport Systems or Henry Schmidt of Supertech Performance.