7.4L Mercruiser balancer timing marks

[Michigan Blackhawk]

I built a 7.4L - 0.030, 10:1, full roller valvetrain with an angry camshaft. I need to set 20 base timing and the marker wont go that far.....I need to hit 20 BTDC....can I measure 10 degrees on the balancer indicator and add to the 0 TDC, mark it and set the timing with the 10 deg (on the original marker) and the new balancer mark I made?

 

[Scott Danforth]

welcome aboard

you cant run that motor in a boat. what cam do you have? define angry cam. as in post your cam card

I can tell you from personal experience more than 10 degrees of initial timing on a big block will cause reversion. and any cam bigger than 270 degrees duration will cause reversion.

 

[Scott Danforth]

now if this engine is in a car or a jet boat with dry exhaust, you can set as much base timing as you want. then use timing tape

 

[Michigan Blackhawk]

Thank you for the concern....this is a comp cam marine cam...running 240ish duration...the cam is a 296HR. I've heard the tape falls off, so thought I'd paint stripe a 10 deg increase, then time to "10" on the indicator for a total of 20 deg on the base

 

[Scott Danforth]

the comp XM296HR has 296 degrees of duration intake and 302 degrees duration exhaust. that is a jet-boat only cam, or a dry-exhaust only cam.

I have never had timing tape come off.....ever in 40 years of using it. it was on my motor originally from volvo penta and lasted 30 years until I pulled the motor to go bit.

 

[Michigan Blackhawk]

Good to know the tape holds......Regarding the cam, what countermeasures can be done to minimize the chance of reversion? .thank you...much appreciated

 

[tpenfield]

:welcome: to iBoats.

Is this cam/engine going in a sterndrive (I/O) with wet exhaust? Sounds like an angry cam that will result in an unhappy engine.

BTW - the stock Mercruiser cam has almost NO overlap, and for good reason. Reversion and hydrolock.

 

[Michigan Blackhawk]

To minimize reversion, does adding a GIL marine exhaust riser make sense?

 

[bruceb58]

Use an advance type timing light. Then you won't need tape.

 

[alldodge]

To minimize reversion, does adding a GIL marine exhaust riser make sense?

You should know this answer prior to building the motor. I'm pulling 530HP (509) and my base timing is set at 10 BTDC. Have +3 inch exhaust elbows straight out the transom with turn downs.

 

[Scott Danforth]

To minimize reversion, does adding a GIL marine exhaust riser make sense?

No.

you cant use a wet exhaust with that cam..... period. even with a 6" riser, i still had reversion with the much milder cam that I am running. I had to make a custom cross-over and drop my base timing to 6BTDC and change my advance curve.

if you want to run that cam, you will need to have a dry jacketed exhaust all the way thru the transom and have the water mix with the exhaust at the tip. about 32" or more from the manifold collector.

 

[Michigan Blackhawk]

Thanks for the info....is the concern only at idle or includes closed throttle maneuvers? If only at idle, if I turned off exhaust water prior to shutoff, would that help?..... or am I back to replacing the cam? Appreciate your patience...... as to asking the question before building....I made a bad choice

 

[Scott Danforth]

Its idle to about 1500 RPM. The cam overlap is sucking in water every intake stroke where there is overlap with the exhaust. Not until the exhaust stream gets enough velocity that the reversion wave cant make it back up the exhaust

After about 5 minutes of running at idle, you will have sucked in enough water in the cylinders that you may hydrolock or you will have enough water in the oil you will get foam out your breathers unless you run dry exhaust.

You will be able to see each exhaust pulse at idle, and you can see the water getting sucked back in the exhaust with every cylinder event

Your options are to change the cam, or run dry exhaust.

 

[Michigan Blackhawk]

Are there any good dry exhaust systems you are aware of?

if I regrind the cam (likely solution based on your feedback), how much duration do you think I could keep ....250? You mentioned 270 above, but you also mentioned 10 degrees.....