Idle timing and low compression

[hpcubed]

I just got my 402 running. It is set up for boost and therefore 9:1 comp. I'd like to get some feedback on what people are running for timing at idle and low air flow rates especially if running lower compression. Are we talking 25 - 28*?

Would you see a big delta in AFR when going from say 18* to 28* at idle for low comp motor?

[Redline Motorsports]

Cool! My turbos just showed up last week for our TT 402 going on our C5. Just waiting for the heads to show up from ET Performance.

Typically at idle the timing can vary quite a bit. I have found that some motors with large duration cams can use a bit more timing at idle to stabilize the idle. Because there is no load you can run as much as 40 degrees without a problem. I can only assume that your boosted motor is not using a large duration cam so idle should be fairly easy to calibrate. What are your cam specs?? 115-116 LCA?? What size turbos??

Howard

[hpcubed]

No not running a big cam 226/224 on a 115. Idles like butter. For the last few weeks, I have been tracking down a problem which finally turned out to be electrical but I finally got it.
I have only heat cycled the motor twice. So during the two heat cycles I notice that my commanded AFR is about 14 and my measured is about 12.5 at idle. Background - for initial startup I am running the maf and stock injectors. So this thing should be dead on. I notice that as I give it some gas ( and the timing increases) AFR goes more toward commanded. I will adjust the timing a bit but this may be caused by some issues with the motor which may go away once I break it in. One of the cylinders out of the 4 I compression tested came out quite a bit lower than the rest. So I am hoping that after I break the motor in, that issue goes away.
Any experience with uneven compression and a fresh motor?

[turboberserker]

I found 10* is my antisurging sweet spot. It was quite surprising to me.

Oh, my CR is 8.6:1 and I'm idling at 750rpms

[Redline Motorsports]

No not running a big cam 226/224 on a 115. Idles like butter. For the last few weeks, I have been tracking down a problem which finally turned out to be electrical but I finally got it.
I have only heat cycled the motor twice. So during the two heat cycles I notice that my commanded AFR is about 14 and my measured is about 12.5 at idle. Background - for initial startup I am running the maf and stock injectors. So this thing should be dead on. I notice that as I give it some gas ( and the timing increases) AFR goes more toward commanded. I will adjust the timing a bit but this may be caused by some issues with the motor which may go away once I break it in. One of the cylinders out of the 4 I compression tested came out quite a bit lower than the rest. So I am hoping that after I break the motor in, that issue goes away.
Any experience with uneven compression and a fresh motor?

Yeah that cam should idle real nice! I am waiting for these heads to show up so we can flow them before we get a cam ground. It will most likely be around a 230 duration range.

How low was the cylinder? What is the cranking compression? A couple heat cycles is hardly enough for ring seat. I would get some break in miles before you panic! These motors are real finicky with cylinder wall cross hatch vs. ring tension. Are you running conventional oil or synthetic? Synthetic this soon will make it a much longer break in process.

When it breaks in some more, do a leak down test. Its a much better test to determine ring seal.

[hpcubed]

Cylinder was 125 where the others ranged from 140 to 155. This is the cranking compression.

Conventional oil - 30W

Yeh, I will try not to worry about it before I go break it in. I'll keep you posted. Hopefully everything goes OK. Keep your fingers crossed for me.

[Redline Motorsports]

I think those numbers are low. There should not be too much overlap with that 115 LCA cam you noted. Cylinders also should not vary by more then 5 % MAX!

Even with a static compression of 9:1, it should be closer to 180.

You should do a leakdown test before going to crazy.

[hpcubed]

My 11.2:1 motor was 180. I would expect 9:1 to be lower.

[Redline Motorsports]

I assume the 11.2:1 motor was a NA motor? If so then the camshaft would most likely have more duration and would allow the compression to bleed off. The 115 degree blower cam should keep cranking compression a bit higher.

Take two known motors; same displacement. One motor with a tighter cam and the other with a looser (more duration) and they will have different cranking compressions.

This is where guys with stock displacement motors kill themselves with large camshafts. They blow all there compression out the exhaust valve at low speeds which kills torque.

[hpcubed]

Same duration (224) on a 114 LSA. But i see your point.