Engine and Transmission Tech

While researching the cam position sensor for the GM "411" controller it has come to my attention that Buick and Chevy engines appear to be timed differently. Not just the new firing order for the LS but apparently going all the way back to initial introduction of the SBC.

On the Buick (so also all BOPR) when you install the timing chain, with the dots towards each other as always, as I now understand it the #1 cylinder is on the compression/power stroke whereas in the Chevy it is on the intake/exhaust stroke. Makes a big difference to the cam sensor.

Anyone else care to confirm this?

Jim

Both valves should be closed. At that point you should continue turning the crank and check to see where the exhaust opens - should match the cam card.

Yeah I know. I cranked the 300 on the engine stand over with the new timing set and watched the lifters for #1. They are both down with the marks together (cam dot at 6 o'clock) and then go through the exhaust/intake cycle at 12 o'clock. Apparently the SBC is on compression with the cam at 12 o'clock. That's what I'd like to confirm, the different mark locations.

It makes no difference when timing the engine, just the location of a mark. Only I never noticed it before now. Actually it makes no difference when installing the stub oil pump drive/ cam sensor unit either. I just never suspected there would be this difference.

Jim

That is a big help on chevy when rebuilding engine. Less turning of crank to adjust valves then just drop in dist. In Durham i messed up Tim's timing by doing it the chevy way. I am sure he and carl remember that. I thought they would be the same.

Not sure I understand your question. On a Chevy when the dots are together (cam down and crank up), #1 cylinder is at TDC and in between the compression and power strokes and has just fired. When both dots are up #6 is in the same position and has just fired. SBC Chevys and the LT1 like mine have a 1-8-4-3-6-5-7-2 firing order, LS engines swap cylinders 7&4 and 3&2 in their firing order but #6 is still firing when the cam is halfway through its rotation.

Better double check that Jim. I always thought so to but what I'm seeing on the chevy and LS forums is that TDC on the compression/power stroke is with the dot at 12 o'clock. Some pretty heavy hitters on those forums say so. I'm not claiming to know, myself.

Jim

With the two timing marks pointing to each other #6 is TDC firing postion. One more rotation of crank puts the cam mark at 12 o'clock & #1 is TDC firing position. Drop in the distributor.

#1 & #6 are TDC together, but not both on firing TDC.



Edited 1 time(s). Last edit at 05/13/2019 10:31AM by MGBV8.

Yeah that's what they said. But on the Buick it's the opposite.

Jim

I've never had an LS apart so I can't say for sure about them. I can say for sure on SBC's and the Gen 2 LT1, #1 cylinder is firing when the dots are pointing at each other. #6 is firing when both dots are pointing up. On my 215 it was that same way as well.

Well crap. Looks like I'll have to find an intact SBC to visually inspect then. Not that I'm doubting any of you guys but it can't be both ways, it's got to be one or the other. Wonder if my neighbor Gary still has his spare SBC sitting in the shop (and if he'd let me pull the timing cover). I did have one up until recently, let it go for a song 'cause I needed the stand for another Buick.

Maybe I needn't bother. Maybe I shouldn't. After all time is getting short and I need to get on with rewiring the engine. I can get pressure on #1, go to TDC and install the distributor stub, same as always. The problem is, I look at photos of the LS cam gear and see the orientation of the raised arc for the cam position sensor, and the location of the dots, and I really do need to know if that's on the overlap or the firing stroke. LS guys say it's on the overlap. The guys from chevy engine building shops say the old SBC is the same. Carl says they are the same. Jim says no. Well, I respect all of you guys but I have no easy way to independently verify one or the other. I'm sure you can see how that puts me in a quandary. Not that it really matters to me how the older SBC was set up since I'm working off of LS parts, but it does seem odd that Chevy would change that after all these years and not mention it. Especially if the older engines were consistent with the other GM V8's. I find it very puzzling.

Jim

Maybe I am not sure what you are asking. I believe SBC, BBC, & Buick have the same firing order. The LS engines do not. The 4 & 3 are switched with the 7 2. The timing gears install the same, though.

What Jim S. is stating is correct for Buick. It's the opposite for Chevy. Would be nice to have a list of engines that install distributors 180 degrees out from the timing gear marks. I think some MOPARs are the same as Chevy on this.

From the Edelbrock timing chain installation instructions:

NOTE: Alignment of the timing marks as shown below does not
always indicate Top Dead Center (TDC) of the number 1 cylinder on
the compression stroke. Many engine families (Chevrolet, Pontiac,
etc.) will be at TDC of number 1 on the exhaust stroke and the
crankshaft will have to be rotated one full turn before the distributor
can be installed and timed.

That's what Steve DeGroat said too, and he and his brother had a business building SBC engines.

Yes the firing order is different on the LS but that's not the issue. It's really the question of whether the LS cam sensor picks up the 180 degree bump from the intake stroke to the sparkplug firing, or whether it does it during the power and exhaust strokes. The old SBC is only relevant to this question inasmuch as it is either consistent with or opposite to the LS.

But so far everything points to the LS cam being installed at TDC 180 out from ignition, and the cam sensor picking up the bump from there until the plug fires.

Jim