What happens if one cam is 180* out?

billjam · billjam's Garage

Do you get two cylinders at compression at same time? That is, three compressions per two crank turns instead of six compressions?
EDIT: Make that 360 crank degrees out!

I have been having starting problems from day one with my 3.6 and I think I have finally figured out why. It seems like I managed to screw up the cam timing so that I have two cylinders firing at once! Is this possible?

Funny thing is that once it starts, it runs perfectly, however starting is a real PITA. I suspect that the wild fluctuation in the cranking speed, due to the double compression, is causing the MS3X to lose sync and spit its dummy.
When the weather is hot or after the engine has run once, it usually starts OK and I am putting this down to faster cranking which is just enough to avoid sync loss.

I have not been running wasted spark, so one thing I don't understand is how the out of sequence cylinders are getting their spark.

What's the easiest way to check if there are pairs of cylinders at compression? Rocker position? Which pairs of cylinders would likely be aligned?

tobluforu · tobluforu's Garage

I have no idea what will happen on your motor, but I installed one cam 180 like a goofball. I went to start car and realized something is wrong. Anyhoo, nothing at all happened, but I had ground T cams and 8.5 pistons so luckily nothing hit.

Flat6pac · 10-01-2017, 04:20 AM

I did a 2.7 in the early 80s, went to lunch while timing cams.
Installed the motor and started it.
Bank 1-3 was hot on the exhaust. 4-6 was cold
Spun the distributor 180 degrees and 4-6 exhaust was hot, 1-3 was cold.
Lesson, don't go to lunch timing cams..
Bruce

hcoles · 10-01-2017, 08:28 AM

I was a dummy and did this during my top end rebuild. Engine ran rough and didn't have much power. Since then (10?) years ago I've been worried that the rings on that bank may not have seated correctly. I got 204HP on a chassis dyno run, so maybe all is fine.

Catorce · 10-01-2017, 08:41 AM

The side that installed wrong won't "fire" - the firing is a function of ignition timing not cam timing. However, the side that is installed wrong will not make proper compression - valves will be open when they are supposed to be closed, fuel will leak out of the cylinders into the exhaust spark will go off in the cylinders with valves open.....all of the above equals no start.

hcoles · 10-01-2017, 09:10 AM

Well, my engine started and ran rough as stated. I don't remember how I figured it out. I do remember getting under the car and feeling one bank was cold after running. In any case, it should not have happened, something for people to double check by looking at the cam/rocker action and at the same time watching the distributor. If you see the exhaust and intake valves overlap when the spark should fire then you have the cam in wrong.

76FJ55 · 10-01-2017, 02:47 PM

As stated earlier it will run on 3 cylinders. If you should happen to be working on an engine with wasted spark ignition (coil packs, no distributor) it would run on all cylinders. You would just have 2 firing at the same time.

Plays with cars · Plays with cars's Garage

Quote:

Originally Posted by billjam

What's the easiest way to check if there are pairs of cylinders at compression? Rocker position? Which pairs of cylinders would likely be aligned?

Take out all but one spark plug. Leave the one plug in one of the cylinders you suspect is timed wrong. Remove the distributor cap but note where the post is that goes to the suspect cylinder. Turn over the engine by hand (or gently tap it over with the starter, but I prefer by hand). If timed right, you will feel it building compression as the rotor is coming around to the point you noted where the post is that feeds to the suspect cylinder. If its building compression as the rotor is pointing 180 degrees opposite of the right spot, well... its not good news.

rsscotty · 10-01-2017, 04:51 PM

If I recall correctly this engine does not have a distributor....it is COP.

jpnovak · 10-01-2017, 05:01 PM

Bill,

This is easy to check. Pull the valve covers and put the engine crank pulley at #1 TDC. Then check the rocker lash. The #1 cylinder will be loose and the #4 cylinder should be tight as it will be on overlap. (or the converse will be true). if they are both loose or tight then then one cam is out of phase by 360 crank degrees.

Since you have COP and wasted spark (and twin plug) you could ignite the mixture with #1 and #4 at the same time. Both exhaust would be hot. I think the engine would run like it was on 3 cylinders but make much more power since it is firing 2 cylinders simultaneously on every other crank rotation.

Hard to start could be EFI settings. There are different solutions depending on what symptoms you have being hard to start. Another topic entirely.

billjam · billjam's Garage

Jamie,
I figured out that checking the rockers was the easiest way to confirm cam timing, just hadn't sussed out exactly what to look for. You saved my brain cells a little bit of work there.

The engine has been firing on all six and it has been running like this for four years! I have COPs and they have been set to fire sequentially, so I don't understand how come both cylinders on compression have been firing ... don't really care actually ... just so happy to have finally figured out its starting problem.
When it is running, the engine runs and pulls surprisingly smoothly from idle to the red line. I can't wait to get it sorted and see what difference it makes.

A side benefit might also be a better exhaust note. I have tried a couple of different exhaust configurations to quieten it down, so maybe this will solve that problem too.

I am now more convinced than ever that all my starting problems are directly related to the starter not being able to turn the engine smoothly enough to maintain sync.
In situations where the starter spins the engine faster (warm engine or no compression), sync loss doesn't occur, and it fires right up, so I don't think there are any tuning problems.

For reference, this tooth log during cranking is what really highlights the problem. The tall single spikes are from the missing teeth and the humps are where the engine is slowing down due to cranking against double compression. There should be twice as many humps and they should be flatter.

Tippy · 10-01-2017, 07:10 PM

Bill, the official name of what you did would be a "big bang" motor. Motorcycle racing found that more power could be used if an engine hit really hard with multiple cylinders than sequential, with a gap between powerstrokes letting the tire regain traction. The Dodge Viper motor I believe is this. That's why it doesn't sound good. It's literally a 5 cylinder.

76FJ55 · 10-02-2017, 05:40 AM

Not intending to hijack this thread, but the viper does not fire 2 cylinders at the same time. It is a 90 deg V10, with shared journals, so it finds up with alternating 90 and 54 degree crank rotation between firing cylinders.

dap930 · dap930's Garage

Jamie,

I have been trying to help billjam on the Megasquirt forum. The engine was struggling to turn over to the point it was losing sync. He installed a high torque starter which helped, but the engine still slowed down a lot when coming up on compression. His posted tooth log is really odd. Only one compression spike on one engine revolution and two spikes on the next revolution. I had him post a composite log and it showed the same thing, however, the cam signal was missing. I checked his MSQ and did not fine any reason for the cam signal not being there. He is using a Clewett cam sensor. He will have to troubleshoot this later once the engine able to run correctly. He had previously conducted a compression check of each cylinder, they were all good, 155 - 160 PSI. I suggested he turn the engine over manually to verify compression every 120*. What he found was the same as the tooth logs. So, there is some sort of cam timing problem or maybe both cams are the same??? I can not figure how his engine will run (when it will start) when set for sequential spark unless Megasquirt defaults to wasted spark when the cam signal is not present.

76FJ55 · 10-02-2017, 12:02 PM

Looking at your cranking pulses it definitely looks like it is running with the cams synced to cause the power strokes on both banks to coincide. With the 2 banks phased together you will essentially have a large displacement triple which will give you compression/firing pulses every 240 degrees, just as the log above shows.
For it to be generating good power I can only assume that the MS is running wasted spark or the other possibility is that with the dual plug heads one set of coils is timed 180 out from the other effectively giving you single plug wasted spark ignition.

jpnovak · 02:55 PM

Bill,

I agree that the tooth log shows some missing cylinders based on rpm. If you pull the plugs one bank at a time you should see the same trace. If you pull all the plugs I expect it will be fairly flat. What puzzles me is that you should see a single crank revolution with 3 compression pulses and then the next crank rotation should have no compression pulses. This would be the alternating events between the missing tooth. I am trying to imagine if the L and R cams were reversed.

Going back to your wiring diagram. The connections show the sequential spark With Channels A-F. According to the diagram you have correctly connected the top and bottom coils in your bypass jumper. You should ohm out the signal wires for all 4 cylinder 1 and cylinder 4 sparks and see if they are cross-wired. Same for 2/3 and 3/6. This would confirm that you have a wasted spark configuration and running two cylinders at the same time during the potential dual compression stroke.

76FJ55 · 10-03-2017, 06:39 AM

Quote:

Originally Posted by jpnovak

Bill,

I agree that the tooth log shows some missing cylinders based on rpm. If you pull the plugs one bank at a time you should see the same trace. If you pull all the plugs I expect it will be fairly flat. What puzzles me is that you should see a single crank revolution with 3 compression pulses and then the next crank rotation should have no compression pulses. This would be the alternating events between the missing tooth. I am trying to imagine if the L and R cams were reversed.

The assumption that all your pulses on a single bank will occur in 1 revolution is incorrect. If you look at the original firing order, 1-6-2-4-3-5, you will see that each bank is evenly distributed evenly about the 360 degrees of distributor, or 720 degrees of crank rotation.
It is true that each cylinder on either bank will pass TDC each revolution, but they will not all be on the compression stroke. Look in at bank 1,2,3 assuming 1 approaching TDC on compression you will have first revolution: 1 compression, 3 exhaust, 2 compression and second revolution: 1 exhaust, 3 compression, 2 exhaust.

jpnovak · 10-03-2017, 06:42 AM

Yes. Clear this morning after the margarita has worn off...

dap930 · dap930's Garage

Bill,

Have you made any progress finding out what the cam timing issue is?

billjam · billjam's Garage

Sorry, no progress to report. I have worked every day for the past two weeks so haven't really felt inclined to crawl around my engine bay when I get home.
The only possible explanation is incorrect cam timing, so I am working towards an engine drop, however I will check the timing before I do that.

I'm not too upset about having to drop the engine. I have a cam housing leak that I need to track down and I need to change the colour of my engine bay.
Last time I had the engine out, I repainted it in readiness for a set of turbo flares and a complete respray (to Carmine Red) but I have since gone off red and will probably go white.