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Cam timing question

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aquaholic72

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« on: June 30, 2016, 02:51:41 AM »
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 O0
« Last Edit: June 30, 2016, 03:05:36 AM by aquaholic72 »
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« Reply #1 on: June 30, 2016, 07:11:45 AM »
Valves are 2.19/1.88?  Intake and exhaust rocker ratio is 1.7?
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

aquaholic72

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« Reply #2 on: June 30, 2016, 01:28:16 PM »
Yes x2
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Racesbc4life

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« Reply #3 on: June 30, 2016, 11:09:00 PM »
Can't wait to hear the responses !!

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« Reply #4 on: June 30, 2016, 11:37:25 PM »
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


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« Reply #5 on: June 30, 2016, 11:46:53 PM »
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


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Well here's the thing.

The cam will be ground 4° advanced,  so "strait up" isn't really.

With forced induction,  even at a very low boost rate,  you could gain a little bit of upper rpm torque with some more retarded valve timing, the issue is piston to valve clearance and peak compression issues.

So I think it becomes a risk versus reward issue.

I personally would run it at a true 4° advance after checking valve clearances,  but it depends on valve diameter,  rocker ratio,  deck height,  gasket thickness,  spring type, valve relief size, rpm range.


Just too many variables to say "go for it"...

You know?

GT
« Last Edit: July 01, 2016, 12:05:19 AM by GT Jets »
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If i get some free time tonight at work, ill play with it and post it for everyone to see.

Time to man up and yank it John!  :banghead:
Ray

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« Reply #6 on: July 04, 2016, 08:13:34 AM »
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
« Last Edit: July 04, 2016, 09:02:17 AM by Flusher »
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

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« Reply #7 on: July 04, 2016, 08:19:20 AM »
Performance Trends Engine Analyzer

Here are some numbers for you to ponder.  I have been playing with Engine Analyzer by Performance Trends, it I a good way to consume an incredible amount of time.  I input the above 402 to run some basic simulations to compare advance and retard utilizing the CompCams XM278H cam.  The XM278H already has 2-degrees of advance ground into the cam, so this 2-degrees advance will be referred to as the “straight up” condition.  The results of the baseline test are in the first and second pictures.

The second and third tests are based on; most timing sets with two additional cam timing positions typically offer “straight up,” 4-degrees advanced, and 4-degrees retarded.  Keep in mind that “straight up” on the timing set is actually 2-degrees advanced on the cam, so unless extra measures are taken, the actual cam timing possibilities are 2-degrees advanced, 6-degrees advanced, and 2-degrees retarded.

4-degrees advanced (timing set), 6-degrees advanced (at the cam) – In this test, both torque and horsepower were up everywhere below 5500 RPM with a slight drop in both at 5500 RPM.

4-degrees retarded (timing set), 2-degrees retarded (at the cam) – in this test, both torque and horsepower were down all across the board.

Engine Analyzer has a feature which runs many different permutations to offer suggestions on optimizing a combination.  The first test was to optimize Maximum Average Horsepower.  Compared to the as ground 2-degree advance in the cam, Engine Analyzer recommends 5-degrees advance.  This would place the intake LCA at 107-degrees.

The next optimization test run was to optimize Maximum Peak Horsepower.  Engine Analyzer recommends 1-degree of advance of the cam which would be 1-degree retard in the timing set.  It is interesting to note that peak torque and average torque are down slightly while peak horsepower remains the same.

The Real World

A 1-degree change in cam timing is most likely going to be unnoticeable.  As the timing chain breaks in, the cam will end up about 1 to 1.5-degrees retarded.  If you have a timing set that provides easy cam timing changes, I would try both 110 intake LCA and 107 intake LCA.  If not, I think I would go with 110-degree LCA or “straight up” on the timing set, for maximum peak horsepower.  After running these numbers, I think that is where it would most likely end up.  It is also possible that the slight reduction in cylinder pressure might make the engine a little more user-friendly regarding octane requirement.

Hope this helps.

Cheers,

Joe
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

aquaholic72

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« Reply #8 on: July 04, 2016, 10:21:24 AM »
Holy chit!!!! That was a f#ckin awesome piece of information right there... Thanks so much for your time Joe.
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« Reply #9 on: July 04, 2016, 10:53:55 AM »
You are always welcome brotherman
"I want to roll with my brother Joe" - Joe Bateman - January 29, 1950 ~ November 27, 2013

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« Reply #10 on: July 06, 2016, 09:27:13 PM »
Joe that was awesome!


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« Reply #11 on: July 07, 2016, 07:54:18 PM »
Joe that was awesome!


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My brain hurts.

I will re read this.

Again.

and again

Thanks Mr. Joe.

Daniel
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« Reply #12 on: July 07, 2016, 08:08:45 PM »

My brain hurts.

I will re read this.

Again.

and again

Thanks Mr. Joe.

Daniel


Joe that was awesome!


Thank you
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« Reply #13 on: February 12, 2020, 06:47:10 AM »
I know this is an older thread, but since it's such good information, I had a quick question, Joe.  I am running a solid roller with 242 248 at .050, and .646 .653 on a 110 lsa set up straight up with no cam timing.  I believe I can confidently say my static compression ratio is 8:1.  This a blower setup (6-71).  It used to be a 1:1 pulley setup at about 6-7 lbs of boost and 30 degrees timing

.Because I can never leave anything alone, I had to go one tooth lower on the upper pulley making it 2.5% over driven and now 8-9 lbs of boost.  31 degrees timing.  On 93 octane, I didn't hear any detonation on the dyno, but being a blower motor and making a lot of heat because of the smaller charger, would I be safer to drop the timing to 29-30 degrees and putting the smaller 1:1 pulley back on top and lose about 50 hp? (isn't a big deal if that happens)

I just want a super safe reliable setup that will last long cruising runs.  Am I too aggressive to stay on pump gas? Or do you think I am still at a good spot and just leave everything alone? 

Thanks a lot for your help guys!!

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« Reply #14 on: February 13, 2020, 05:56:23 AM »
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.
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« Reply #15 on: February 13, 2020, 05:38:24 PM »
Joe.  I appreciate your help once again.  The cam was degreed at 106 intake centerline.  It has 25 degrees of overlap.  When at 28 degrees timing, I was almost identical as the 31 degrees timing except for slightly higher egt's (and lower hp, tq, boost, etc).  It's a little rich, but I wanted that to help keeping it cooler and also with detonation.  Detonation is what I am most worried about.  On the dyno, there was no account of detonation and that was at 31 degrees timing and 8 lbs boost on 93 octane.  I do have a knock sensor, so maybe that will be helpful in figuring out if I need to lower the timing and boost.  I just don't want to lower the timing too much because it will really get things hotter faster.  I guess I am just wondering if I am mainly running at 4000 rpm, with my setup, do I need to be worried, or would you consider me safe, even under load for long periods?  I do plan on at least running some 100 octane av gas mixed with the 93 octane 90% of the time.  Should I be running that all of the time?  (our summers over here in PA never get hotter than 80 degrees for the most part btw).   Thanks again for all your help!!!!
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