Alright gentlemen, after totally tearing my engine down there are no signs of where my mystery metal came from! So it had to have been something that fell in during an intake swap! Sucks!! So I'm gonna clean everything up and put it back together.. But my question is I had the cam in straight up dot to dot, I have a nice made in the USA billet double roller chain in the engine with a lot of adjustment, how do you guys feel about advancing or retarding cam timing? My boats a heavy flat bottom v-drive, 402 BBC, 9to1 compression, large oval port heads ,cam is a comp xm278h , if I remember correctly .564-567 lift 278-292 advertised duration on a 112 lc, but its gonna have a b&m 250 supercharger on it this time around, so any thoughts on advancing or retarding cam  timing and why would be greatly appreciated... Thanks

Yes x2

Seems like there is tons of information in the heads of the members here would it be different if he had a jet boat? Or is this something that no one knows? Or is just everyone just worried about having a fun weekend while this boat is broken


Sent by a Sleekcraft SST owner on the SoCal Jet boat app

Piston to Valve Clearance

The intake valve is substantially closer to the piston at TDC as it is (or should be) aggressively opening at that point.  This means that the piston and the intake valve are quickly approaching each other.  Along with a smaller diameter, the exhaust valve is located much higher in the chamber relative to the intake valve.  To put this into perspective, while placing an unsurfaced 188 BBC head on a flat surface, combustion chamber side down, a 2.19 intake valve will hit that surface opening with approximately .070” lift and a 1.88 exhaust valve will hit that surface opening approximately .520” lift.  Using the numbers of the CompCams XM278H cam and a true flat top piston with .055 of quench, you could have full exhaust valve lift and still have a few thou clearance at TDC.  The intake, on the other hand, with the cam installed on a 110-degree Intake Lobe Centerline, has a theoretical .124” of valve lift, about .001” of interference without a valve relief notch in the piston.  Advancing the cam decreases piston to intake valve clearance but increases the clearance on the exhaust.  Retarding cam timing has the opposite effect of increasing piston to intake valve clearance while decreasing exhaust clearance.

Affects of Advancing Cam Timing

Cylinder pressure increases.  Dynamic compression ratio is why this happens.  Dynamic compression ratio theorizes that the air/fuel mixture cannot be compressed until the intake valve is sealed closed.  Then, and only then, can the piston’s motion towards TDC compress the air/fuel mixture.  To calculate dynamic compression ratio, the position of the piston ABDC where the intake valve closes and the remaining distance that the piston will travel from that point to TDC is substituted as the stroke length in the static compression ratio calculation.

Why is this number beneficial to us, it helps to estimate octane requirements.  It is also an indication of performance.  The power output of an engine is directly proportional to the expansion ratio.  The combusting air/fuel mixture expands 3.5 to 4 times that of the initial compression pressure.  A higher compression ratio translates to higher cylinder pressure, during combustion, and therefore greater output.

To see how advancing or retarding cam timing will affect cylinder pressure, using the OP’s 9:1 402 BBC with a CompCams XM278H cam, I have made some assumptions with the engine specs to arrive at a 9:1 static compression ratio.  These assumptions are 17cc piston domes, 105cc combustion chamber, and a 9.7cc head gasket.  All other numbers are assumed to be to original BBC 402 blueprint dimensions.  The CompCams XM278H cam is ground with 2-degrees of advance built in and that will be used as baseline as if installed by simply lining up the dots on the timing chain set.

One important thing to note here is that this is a hydraulic cam and calculating or even measuring the actual closing point of the intake valve has proven to be frustratingly impossible.  Movement of the hydraulic plunger can drive you insane while trying to measure.  Substitution of a solid lifter for measuring purposes is not entirely a viable solution because the opening and closing ramps of cam lobes have special approach and departure ramps so that, combined with the action of the hydraulic plunger, gently take up the lash as the valve is opened and softly returning the valve to the seat during closing.  The ramps of a hydraulic cam versus a mechanical cam are completely different.  Estimating by subtracting duration doesn’t really work either because of hydraulic lifter bleed down.  Bleed down is a function of oil flow over time and there is more oil has bled down by the time the tappet moves off of the closing ramp.  This essentially advances the cam timing slightly.  The final HUGE assumption is using the advertised cam timing events (.006” Tappet Lift) as the actual opening and closing of the valve.  Also note that the .006” tappet lift is only for CompCams hydraulic flat tappet cams.  Mechanical cams and roller tappet cams as well as other brands use different numbers.  I am not writing this to give actual factual numbers, as I feel that that is impossible to do, but to present trends that can help evaluate performance and influence build decisions.

Supercharger Specifics – Supercharged engines are subtly different than Naturally Aspirated (NA) engines.  With greater intake manifold pressures, the short comings of the intake ports are minimized and it isn’t so difficult for and engine to ingest the air and fuel when it is being force fed.  Problems arise when it comes time to exhaust the additional waste gasses.  It is generally accepted in the performance industry that the exhaust port should flow about 75% of the intake for an NA application.  The factory heads fall a bit short of this as it is.  The idea is that, when the exhaust valve opens, the blow-down phase of the exhaust cycle should reduce the pressure in the cylinder to less than 35 PSI.  Since this particular application utilizes a small blower producing low boost, the volume of waste gas present in the combustion chamber needing to be exhausted is greater than a NA application, but far less than all-out race application.  Performance will benefit from an earlier exhaust valve opening event.  Reducing the pressure, and therefore the volume of waste gas remaining in the combustion chamber, pumping losses will be reduced because the engine won’t have to work as hard to exhaust the additional waste gasses.

Dynamic Compression Ratio Calculations

With the cam installed straight up, 110-degree intake Lobe Centerline Angle (LCA), the advertised intake valve closing point is 69-degrees ABDC and the dynamic compression ratio stroke is 2.810 instead of 3.76 which yields a dynamic compression ratio of 7.029:1.

With the cam installed 4 degrees advanced, 106-degree intake LCA, the intake valve closes at 65-degrees ABDC and the dynamic compression ratio stroke is 2.916 instead of 3.76, yielding a dynamic compression ratio of 7.256:1.

With the cam installed 4-degrees retarded, 114-degree intake LCA, the intake valve closes at 73-degrees ABDC and the dynamic compression ratio stroke is 2.699 instead of 3.76, yielding a dynamic compression ratio of 6.790:1.

CompCams XM278H Specs

Lobe Separation:112-degrees

Intake Lobe Number:5446

Duration in Degrees:
278 @ .006” Tappet Lift
234 @ .050
147 @ .200

.332 Lobe Lift

Tappet Lift at TDC:
.087” @ 106-degree Intake CL
.073” @ 110-degre Intake CL

.564 Theoretical Valve Lift at “0” Lash 1.7:1 Rocker Ratio

@ 110-degree Intake Lobe Centerline:
Valve Timing @ .006” Tappet Lift
Intake Opens 29-degrees BTDC
Intake Closes 69-degrees ABDC

Exhaust Lobe Number:5214

Duration in Degrees:
292 @ .006” Tappet Lift
244 @ .050
154 @ .200

.334 Lobe Lift

Tappet Lift at TDC:
.100” @ 106-degree Intake CL
.087” @ 110-degre Intake CL

.568 Theoretical Valve Lift at “0” Lash 1.7:1 Rocker Ratio

@ 110-degree Intake Lobe Centerline:
Valve Timing @ .006” Tappet Lift
Exhaust Opens 80-degrees BBDC
Exhaust Closes 32-degrees ATDC

http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=405&sb=0

Holy chit!!!! That was a f#ckin awesome piece of information right there... Thanks so much for your time Joe.

Joe that was awesome!


Sent from my iPad using SoCal Jet Boats

My brain hurts.

I will re read this.

Again.

and again

Thanks Mr. Joe.

Daniel

Joe that was awesome!

Thank you

I'd really need to know more about your cam.  Sorry, I really haven't had the time to research and try to discover which cam you have. 

Your cam could very well have advance ground in.  That means that if installed "straight up" on the timing set, it would already be advanced.  Even then, you are still at the mercy of manufacturing tolerances of the block and timing set.  The only way to know for sure is to degree it at all four corners and set it at a best average.

Detonation should show up in the torque curve.  The dyno probably had knock sensors.  I wouldn't be able to hear knock unless it was excessive, but then it would be too late.

The dyno testing that I have been involved in, the tuner typically tunes for best power.  The operating conditions will never be as ideal as in the dyno cell, compared to a hot afternoon on the water.

I think it's not the best idea to cruise around with it always set on kill.  Fuel loses octane sitting around.  I would pull 2 degrees of timing, maybe 4 degrees.  Do you have dyno sheets from previous tests while arriving at your current total timing?

Learn how to read plugs, especially for signs of detonation.  While you are learning your new iteration, continue monitoring.  You will learn where it likes to run and you will be able to feel when it's not happy.  It will talk to you, you just have to listen.

If you really want to know what your IAT (Intake Air Temperature) is doing, install a gauge or some type of data logging system.

BTW, That's a pretty impressive increase in boost for just one tooth.  I wouldn't go back.  You will probably find yourself wanting to overdrive it harder.

Don't let the doomsayers knock you off square.  Just because some new hot damn part has "higher efficiency," doesn't mean that your older technology won't work.  Keep your focus on your program.  You have a beautiful boat, enjoy it.