Cam Timing Advance/Retard

[bs009]

So one of the things I recently discovered is that the L36 and L67 have different cam timings. Doing some googling I found that most people will say the L67 has about 2 degrees advance over the L36.

I had to do some research to figure out what the benefits of that are, but if that were true then the L67 would have had a peak power about 100 rpm sooner than the L36. Not a big deal but I wanted to see it for myself.
I found from cross-referencing parts on rockauto that the difference in timing comes from the cam sprocket since the crank sprocket is the same part number between the two.

Checking my own sets of timing chains from my motors in my garage I found though that the L67 having 2 degrees of advance is pretty incorrect.

Here is the L67 sprocket laid on top of the L36 sprocket with the keyways lined up.

You can see that the cam sensor is a bit further counter-clockwise on the L67 cam proving that the L67 cam is actually retarded compared to the L36.
Here are the timing marks between the two.

So the L67 cam is half a tooth retarded, and on a 48 tooth gear this would roughly 4 degrees of difference there. So the L67 should in theory have about a 200 rpm later power peak according to what would be expected from retarding the cam.
Based on what I've been able to find online and on an episode of engine masters, it seems like retarded cam timing will even make more power across most of the rpm range as well, but I doubt anyone has tested this too much on a 3800.

Not a big deal if you're already running a rollmaster since you can adjust the cam timing, but I still thought this was pretty cool information.

Click to read more...

 

[10sec]

Ron Vogel was going to build the ultimate stock cam (blower) car and probably knows incredible amounts about little stuff like this.

For me, just throw a turbo at any 3800 and it makes more power than any transmission will handle anyways so none of this matters.

 

[1quick]

I think Ron messed with the cam timing on his stock cam car but I could be wrong

 

[v6buicks]

Interesting observation.

 

[bs009]

Yeah I'll need to do some digging but I'm thinking of running the L36 sprocket but with a tooth behind where it should be. Should take out 7-8 degrees of cam advance and shift the power band up a couple hundred rpms.
It would probably test the limits of what the heads can flow though without rockers though. I'd love to be able to break the stock cam turbo record - I think I've gotta be pretty close

I'll still need to do some more research though to see if it's worth it though

 

[1quick]

The heads were never really a choking point, a lot of people ran pretty high rpm cams on stock heads

 

[v6buicks]

I'll be really interested to hear what you and Ron have to say about this. I agree that the Rollmaster stuff kinda makes it a moot point though. I guess if you convert from L36 to L67 and have both options laying around, it's kind of a free mod.

 

[Ron Vogel]

The L36 cam is 2 degrees advanced so it doesn't feel like a turd at lower RPM's. You want to be reterded for a FI application. If you were doing a turbo build an addition 2 degrees retarded with a proper stall would be a nice quick spool set-up that wouldn't fall on it's face on the top end. I'd check valve to pistion clearance though if you were doing a bigger cam.

 

[bs009]

Went and got a picture of both on the same camshaft just to get a better idea of how much they are. 2* didn't seem like enough based on those first pictures I showed.

Looks like it's almost exactly half a tooth ahead.

With 48 teeth this would be 3.75* advanced compared to the l67

 

[bs009]

I was thinking that had to be part if the reason why as well. Definitely going to try retarding it now to see how she does!

The L36 cam is 2 degrees advanced so it doesn't feel like a turd at lower RPM's. You want to be reterded for a FI application. If you were doing a turbo build an addition 2 degrees retarded with a proper stall would be a nice quick spool set-up that wouldn't fall on it's face on the top end. I'd check valve to pistion clearance though if you were doing a bigger cam.

 

[GTPpower]

Are both cams the same part number?

 

[bs009]

Yup, at least according to rockauto

 

[GTPpower]

That's what I found as well.

 

[v6buicks]

Hold up... you're saying L36 and L67 have the same cam just different timing? I thought for sure they would be different. Nice!

 

[Ron Vogel]

Not same cam, at least not OEM.

Even different years had different cams, ‘98 being the “hottest”

I used to have a spreadsheet with duration/ lift for the cams. I doubt it’s still around though, I didn’t save a lot of the stuff after I got rid of my car

 

[bs009]

Welp, thought I'd update this thread and report on my very disappointing results.
L36 timing set a full tooth retarded for whatever reason failed pretty hard on my setup.

Engine fell on its face after 4200 rpm in the first few pulls where it should be peaking at 5200rpm with stock timing, I had hoped for a power peak around 5,500 rpm instead.

Boost is all in at 4200 rpm ish to 16-15 psi in all of the runs, I tried to turn it up for the 2nd run but was running into issues with the controller.
In the third run I bumped timing up by 3* across the board and that gained a little bit up to 4,500 rpm, but still nowhere near where I expected power to peak at. I was limited on dyno time so not much else I had time to try.

run 1 log data:

Spoiler: run1 table

run 2 log data:

Spoiler: run 2 chart and table

run 3 log data:

Spoiler: run 3 chart and table

It's possible the tight bend I have after the turbo is causing some sort of restriction, but I'm a bit doubtful that there's something else physical preventing it from making peak power up there.

Do you think I've given this a fair shake? or do you think I have something way out of whack preventing this from making power up top?

I was having some clutch problems, but didn't notice any slippage on the dyno. I've also ran the turbo up to 20 psi and it felt pretty good there too so I don't know that the turbo or exhaust is a restriction but I'm planning on addressing that soon - parts are on the way right now. The turbo is a holset hx35 from a 12v cummins and supposedly it can do ~400 hp, but it's possible that I'm hitting the limit of it. IATs on the charts are post-intercooler

Right now I have a new L67 timing set that I'm planning on putting back in the car now unless it's really possible there's more left here in the L36-1tooth timing setup

 

[GTPpower]

Which hx35 do you have? 7 blade? They made like 6 different hx35's.

 

[bs009]

Which hx35 do you have? 7 blade? They made like 6 different hx35's.

The one I have is one of the better ones IIRC
it's a 3592767
7 blade compressor, 12 cm twin scroll exhaust housing, internal wastegate. Looks like it came from a manual 99 cummins

edit: found some more specs:
Compressor:
54 mm inducer
78 mm exducer

turbine:
70 mm inducer
60 mm exducer

I'm about to replace this one with a neat GT35 I found online that seems like it could be a pretty good match for the stock cam and the way I have the exhaust routed right now

 

[GTPpower]

Yup, I believe that's one of the better ones. I know people claim 400hp on them, and maybe on Dsm's they can, but on a 3800 it seems 350hp is pretty tapped on them.

 

[Turbocharged400sbc]

1 tooth is like 8 or 10 degrees retarded....may be too much

 

[bs009]

True if the reference is the L67 cam timing, I'm using the L36 timing set though which seems to be off by a half a tooth. So it should be ~8 degrees retarded from the L36 timing, but only ~4 degrees retarded from the L67 timing

 

[TurboTPI]

The one I have is one of the better ones IIRC
it's a 3592767
7 blade compressor, 12 cm twin scroll exhaust housing, internal wastegate. Looks like it came from a manual 99 cummins

edit: found some more specs:
Compressor:
54 mm inducer
78 mm exducer

turbine:
70 mm inducer
60 mm exducer

I'm about to replace this one with a neat GT35 I found online that seems like it could be a pretty good match for the stock cam and the way I have the exhaust routed right now

This turbo is VERY similar to mine spec wise. It also has a 12cm T4 twin scroll exhaust housing. I love this thing for a daily driver/street car. not sure what 800cfm is capable as far as HP, but i feel like this turbo should make 500whp easily. How is spool on the HX-35?

Turbo Model	TE06H-24V-16.0, TE06H-24V
Engine	6H, LG4, 6HK1XS, 6HK1XSL
Engine Manufacturer	Isuzu
Displacement	7.3L, 7300 ccm, 6 Cylinders
Fuel	Diesel
Bearing Housing	(Oil-Cooled)
Turbine Wheel	(Ind. 58. mm, Exd. 121. mm, 11 Blades)
Compressor Wheel	(Ind. 57.7 mm, Exd. 78. mm, 7+7 Blades, Super back)
Flow Rate	800 CMF @ 1.0 Bar

 

[GTPpower]

800 cfm is probably about 600whp. But I'm questioning that size of turbo flowing 800 cfm at 1 bar.

 

[TurboTPI]

Kinda what I thought as well, but I will say that @ 15-16lbs of boost (non intercooled), it's making some pretty good power as far as I can tell with the cars I've raced. Either way, I like how it responds and it definitely will make more power than I need, or should I say my trans needs haha.

 

[sktchy]

So my take from this is get an l67 cam gear for my l36? Keep up the good work of figuring out all the hard stuff tho I always look forward to seeing a new thread from you lol