Hi,

after a rod failiure I build me a new 305 TPI engine. It's stock except for a better oil pump and a Comp Cams XR270HR-10 cam. I know it's a big cam, but as the old cam was broken, I decided to use this one as a starting point. Next winter a new exhaust including Hooker Super Competition Headers will follow.

The problem is that the engine will run only if i set the ignition to about 30-40 Degrees in advance. If i want to go back to about 20 -25 degrees, it runs very poor and at 10 degrees it needs full throttle to hold the engine at a extreme poor idle. The TPS is adjusted and everything is mounted except for the smog pump.

The engine is getting hot at the moment which I think is caused by the extreme advance, but I have no idea why I can' set the ignition to a normal timing. I have bridged Pins A and B on the ALDL connector while checking the ignition.

I changed the "Fan on temperature" in the eprom from 225 to 205, but the engine is running up to 220° F.

The car is driving well, except for stalling out sometimes when stopping, but not on every stop. A diagnostic drive with WinALDL shows nothing special except for a little lean mixture, average BLM is 132, which is in my opinion caused by the ignition too.

The cam specs are:
Intake Exhaust
Gross Valve Lift 0.495 0.502
Duration At 0.006 Tappet Lift 270 276

Duration At 0.05 218 224
Lobe Lift 0.33 0.335
Lobe Separation 110

The cam is degreed to 4 Degree by default, and I havn't changed this.

Did i made a mistake or overlooked sonething? I know that I have to work on my chip, but I want to adjust the ignition first.

I know some of you guys have the same cam. Please help me.

Andreas

 

I think that it stalls when coming to a stop because it's lean. My car was also running lean, and doing that too after my cam change. I had also changed to 22lb. fuel injectors, but had not yet changed the injector size constant in the PROM.

Maybe you should raise the fuel pressure about 3 psi to get to a more rich fuel mixture.

 

is it a computer compatable cam? how much vacuum does the engine have?

 

The cam isn't computer compatible.

It's this one:
Competition Cams Xtreme Energy Camshafts: CCA-08-422-8 - summitracing.com

I measured a vakuum of about 10" Hg @750 RPM with no load. But the connection to the gauge wasn't that good. So it might be slighty higher.

Andreas

 

when i rebuilt my 86 iroc 305 tpi, i was told i could only use a computer compatable cam. not sure of how high the lift, (mine is .444 on both intake and exhaust). i think you should have appx. 20" in. of vacuum at idle. i don't know if this is your problem, but i think you have too big of a cam if you went back with the tpi. i would call summit help line and see if they have any idea. let us know. best of luck.

 

The term "computer compatible" is completey loaded. What the cam companies mean is that it will work with a stock tune. You can run a big ole 240/250 deg dur cam if you want, but it will require lots of tuning to work well. I ran your identical cam in a 350 and it wouldn't even idle when I fired it up, this was with a SD TBI setup. The fact that your car is MAF is probably the only reason it runs.

Your timing problem is throwing a huge red flag to me. Are you setting the timing with the timing connector unhooked? If so you should set it to whatever you have the base timing set to in the prom. I would set this to around 10deg in the chip and at the dissy with the connector unhooked. With the connector hooked back up you'll have much more timing, if you need more just change it in the chip instead of getting your hands dirty (gotta love prom tuning ). If you set your timing correctly I think your damper may have slipped and your timing indicator is no longer accurate. Best way to check for this is to get a top dead center locator that fits in your spark plug hole and verify your timing indicator is correct.

 

The cam you chose has a 110 degree lsa. This is great for a carbed motor with some gears. A TPI motor should have at least 112 degrees of lsa if not more to run smooth . My two cents.

 

Vincode8 is right. 110 LSA will work but you should program it to stay in open loop when under 1200 RPMs. That should fix the low speed problems. When you have a tight LSA, theres a lot of reversion at idle and that screws up the MAF reading and the high scavenging screws up the gases that are sampled by the O2 sensor. Like he said, 112-116 will work well with stock tuning.

 

Not necessary. I have the same cam and I'm no where close to having to force open loop. And I'm running SD which is *supposedly* harder to tune than MAF. Some would probably consider this a mild cam.

Have you adjusted the throttle stop so the blades are farther open at idle? What are the IAC counts, at idle, with the engine warm (you might shoot for 15 or so)?

 

@BMMonteSS
You are right. I measured it with a top dead center locator, and the timing mark was wrong. So I ordered a new damper, but I had to wait a week, because I couldn't get one in Germany.

I installed it, and checked the ignition again. It was nearly right, so I set it to 10 degrees. The engine was running pretty well.

Then I started to adjust the valves again, because I've adjusted it with the slipped damper last time. Unfortunetly al least one pushrod was bended, so I have to buy new ones that will arrive in two or three days.

Hopefully I can test drive on the weekend.

Andreas

 

Glad you found out what was wrong, remember KISS Keep It Simple Stupid. I always start with the most simple things and then work my way out to something more complicated.


Sheez guys, and you thought it was that 2 degrees worth of extra overlap that wasn't "computer friendly". Nothing like having your timing off by 10+ degrees and bending a few pushrods to mess up your idle.

 

Go ahead and pat yourself on the back .

 

It's called sarcasm......I couldn't find a smiley for that.

 

Your cam is fine. It just needs to be tuned.

You will need to adjust the first maf table for the lean idle and add some timing to the timing table where you idle. Keep adding timing in the chip until the vacuum goes up and stays steady and driveability gets better. It will take a few chips to get you there. Also drop your fan on temps lower than 205, to 180. That cam probably doesn't want to idle at the stock idle speed either, so you could raise the idle speed to at least 750 rpm.

 

I too am having problems with my new cam 1 step below your cam in 305 sd. I am always at about 30-40 advance and comp is pulling timing and throwing a 43 code. Just curious how do you know if you have a bent push rod.

 

Yup. FWIW, my spark @ idle is 28* - idle is set at 800rpm.

 

I wanted to adjust my valves and noticed then, that one of the rods can't be twisted. So I removed it ans saw, that it was bent.

@89gta383
I raised the idle to 900. Just to keep the engine running. When all other is fine, I will try with a lower idle.

At the Moment I made the following changes.
Replaced stock temp sensor for secondary fan with 200 degree sensor
Replaced stock thermostat with 180 degree
Set temp for primary fan to 185 degree
Set base ignition timing to 10 degree
Done the same in chip
Raised idle from 750 to 900 rpm in chip
New adjusted minimum idle air

Now I have a stable idle and no temp problems, but the engine is still running to lean, the car is bucking under accelleration. I wanted now to change the parameter for the injectors to raise the fuel amount. The blms are about 140 - 147. I thing they are getting better, if i make this change.

But I will also try your suggestion concerning the MAF and timing table.

Andreas