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  #41  
Old 12-10-2018, 01:27 AM
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If I let the software optimize both length and diameter for max peak HP, it picks 30" x 0.925" ID.
16.4 HP at 5750 RPM.

You'll notice a dip at 3875 RPM, which corresponds to the negative Overlap %VE (-2.2) at 3875 RPM in the chart. This is due to intake and exhaust tuning.
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30 x 925.jpg   30 x 925 chart.jpg  

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Old 12-10-2018, 01:45 AM
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If I set the diameter to 0.85" and let it optimize length for max HP, it picks 25"...

16.2 HP at 5750 RPM.
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85 x 25.jpg   85 x 25 chart.jpg  

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  #43  
Old 12-10-2018, 01:57 AM
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Going back to 18" x 0.85" ID...
Here is the Analysis Report.



Analysis Report for Mild Street Engine with Desired HP Peak at 5750 RPM

Peak Tq =16. @ 4500 RPM 1.24 Ft Lbs per CuIn
Peak HP =15.7 @ 5750 RPM 1.22 HP per CuIn

Maximum Tq/CuIn is 1.24 Ft Lbs/CuIn.
This is somewhat high, indicating good performance, but will
produce high cylinder pressures and temperatures.


Maximum Exhaust System Backpressure 'Exh Pres' is 0 PSI.
This is low for a street vehicle with a full exhaust
system. This is simulating either an extremely free flowing exhaust
system or open headers or open exhaust manifolds. This may be
illegally loud for street operation.


Maximum Fuel Flow 'Fuel Flow' is 8.3 lbs/hr GAS.
This is equal to 1.4 gallons per hour of fuel flow.
For an injected engine with one injector per cylinder, you will require
at least 8 lbs/hr injectors.


The Maximum Average Piston Speed 'Piston Spd' is 2751 ft/min
at the Performance Calculations Maximum RPM of 7625 RPM.
This is somewhat high (if you want to run this entire speed range),
requiring light, high strength reciprocating components.

A mild street engine should limit PSN SP to a range of 2500-3000 ft/min
with production quality rods. To run at 2750 ft/min or higher, you will
need 'better than production' reciprocating components (connecting rods
& bolts, pistons, etc.).

Maintaining low PSN SP and PSN GS are critical for 'keeping the engine
together'. OVER-REVVING PARTS BEYOND THEIR INTENDED LIMIT IS UNSAFE
FOR THE ENGINE, YOURSELF AND BYSTANDERS.


The Intake Runner Velocity 'Int AvgVel' is 363 ft/sec
at your 'Desired HP Peak RPM' 5750 RPM.
This is somewhat high and indicates you will need a larger Intake
Runner Diameter or larger Intake Port Diameter for less
restriction and stronger intake tuning.

For these engine specs, an 'Int AvgVel' of about 260 should work well.
If the 'Int AvgVel' is approximately 30-80 Ft/Sec higher than this,
you will likely improve torque below this RPM, but lose some HP.


The Inertia tuning of this intake is tuned to 5418 RPM,
which is close to your 'Desired HP Peak RPM' of 5750 RPM.
Since this RPM is about where the HP peak should occur, peak HP
should be good. If you specify longer and/or smaller diameter intake
runners, you will likely gain Peak Torque and lose some Peak HP.


The % Exhaust to Intake Flow Capacity 'Total Exh/Int %' is 90.1 %.
This is Very high, and indicates you could be opening the
exhaust valve too early or you could improve performance by improving
intake valve flow and intake cam profile. The most common 'rule of
thumb' is to design for around 75% EXH/INT flow capability.


Estimated Idle Vacuum 'Est Idle Vac, ''Hg' is 6.9 ''Hg.
This is EXTREMELY low, and will likely NOT idle smoothly or provide
enough vacuum to run vacuum accessories for a mild street engine.


End of Analysis Report



So Does a Mod2 give that mean sounding Rumpity Rump?
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Old 12-10-2018, 06:26 PM
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Quote:
Originally Posted by bob58o View Post
So Does a Mod2 give that mean sounding Rumpity Rump?
I wouldn't think so, it only has about 29-30° of overlap and is meant to pass a lift check at 0.225".
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Old 12-10-2018, 07:31 PM
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I wouldn't think so, it only has about 29-30° of overlap and is meant to pass a lift check at 0.225".
Well without ever using it, I’m not a fan of the Mod2. Stock lift and only a bit extra duration. There’s a 272 mongoose grind that must be pretty new. Sounds appealing for $50. Same duration as Mod2 but 0.272” lift instead of 0.231”ish.
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Old 12-10-2018, 10:02 PM
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on to the big blocks that have not been dynoed yet
*****Big Block Midas Mild Build*****
Bore? 88mm
Stroke? 64mm
Rod Length? 4.410"
Head Type? wedge
Flat Head, Wedge, or Hemi wedge
Compression Ratio? 10.2:1

Valve Size? 35mm in/31mm ex
Port Dimensions? unported/mild port job (as seen in the video in this thread iirc)
Carb cfm rating?stock gx390 carb

Exhaust Length and Diameter? 1" ID, 24"

Camshaft Intake lobe duration and lift? NR 280 0211 grind
Intake lobe center line?
Exhaust lobe duration and lift?
Lobe separation Angle?
Rocker Arm Ratio? stock rockers

premium pump gas 92 octane
26 DBTDC

*****Big Block Stroker Mild Build*****
Bore? 90mm
Stroke? 70.4mm measured
Rod Length? 4.360"
Head Type? wedge
Flat Head, Wedge, or Hemi
Compression Ratio? 10.2:1 (not milled stock head mild port job)

Valve Size? 35/31
Port Dimensions? stock/mild port job
Carb cfm rating? Mikuni TM32 Flatslide

Exhaust Length and Diameter? .990x .995 x 1.070 x 1.220 x 13"

Camshaft Intake lobe duration and lift? Dynocams 275
Intake lobe center line?
Exhaust lobe duration and lift?
Lobe separation Angle?
Rocker Arm Ratio? 1.2 ratio rockers (test against 1.3 just to see)

*****Big Block Stroker Block Buster Build*****
Bore? 90mm
Stroke? 70.4mm measured
Rod Length?4.360"
Head Type? wedge fully ported for MAX HP at some loss of torque, 3 angle valve job
Flat Head, Wedge, or Hemi
Compression Ratio? 13.5:1

Valve Size? 36mm in/31mm ex
Port Dimensions? no idea, max hp port job by NR
Carb cfm rating? 33mm Mikuni Pumper

Exhaust Length and Diameter? 1.2" ID 16" length for now. id starts at 1.2" and will go 3 stage with larger steps.

Camshaft Intake lobe duration and lift? Dynocams 380
Intake lobe center line?
Exhaust lobe duration and lift?
Lobe separation Angle?
Rocker Arm Ratio? 1.2:1

Carb is set up for methanol but i have an unopened 5 gallon pail of VP racing M5 fuel instead of M1 methanol because it was only $5 more.


*****Please let me know if you need me to find out about any of the predator hemi/non hemi engines. IIRC you needed the cam profile of a predator 212? i have a degree wheel and a dial gage and im sure i could find the time to do this. I have a sneaky suspicion that there are really large gains to be had just by swapping to a cheater cam and advancing timing by 6-8 degrees. More gains than you would see back in the day with the briggs 5 hp flathead because the stock cam BLOWS and timing BLOWS. Really great for 3600 rpm and low end torque*****
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  #47  
Old 12-11-2018, 12:28 AM
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I'm three or four bottles of merlot deep into this mess and need a day to recoop. I work in the AM.

I'll catch you when I forget about the girls who forgot about me.
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Old 12-11-2018, 05:21 AM
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Old 12-11-2018, 01:48 PM
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Several years ago I plotted a stock GX340 intake lobe. With stock 1.1 rockers and zero lash it had 0.269" max lift and duration of about 205° @ 0.050".

Seat-to-seat duration is absurdly high at nearly 330°, but the ramps are a mile long at about 60-65° on each side of the lobe so the effective duration is quite conservative.

I would suspect that the smallblock cams have similar events just with less lift.

Quote:
Originally Posted by bob58o View Post
1.) Well without ever using it, I’m not a fan of the Mod2. Stock lift and only a bit extra duration.

2.) There’s a 272 mongoose grind that must be pretty new. Sounds appealing for $50. Same duration as Mod2 but 0.272” lift instead of 0.231”ish.
1.) I had the same thought when I saw the MOD2 specs. To be fair, it's designed around a specific controlled racing class and not really aimed at minibikes or fun karts.

2.) The NR Mongoose looks like a decent profile for the hemi head. 0.275" is about all the stock geometry can handle and it's practically stock-class price. Probably the knarliest limited-lift cam would be an F275 grind, although I'd prefer to see options in the 280°+ range. Isky/SEC is probably the main option for custom profiles like that though and the cost is commensurate.
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Old 12-11-2018, 02:09 PM
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here are tidbits of flow data about gx390 stolen from bobs4 cycle (thanks inernet!)

"On my flowbench a stock 390 head flows about 89cfm @ 28" at .350 lift, but it is nearly topped out from .250 lift where it flows about 86cfm. My basic portwork that was just a cleanup around the bowl and guide got that up to 106cfm without trying too hard and gained flow at all lift points. "
source: post #4 https://karting.4cycle.com/showthrea...t=dyno+numbers
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Old 12-12-2018, 11:44 AM
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Quote:
Originally Posted by bob58o View Post
Well without ever using it, I’m not a fan of the Mod2. Stock lift and only a bit extra duration. There’s a 272 mongoose grind that must be pretty new. Sounds appealing for $50. Same duration as Mod2 but 0.272” lift instead of 0.231”ish.
so in my very limited experience with cams. It seems like there are the cams that are meant for cheating in karting classes (e.g. run this cam if you have a lift rule but no duration tech). Or run this cam if you have a duration and lift check, that ramps faster before or after x inches of lift and it will pass tech. I'm sure cam manufacturers have jacked with lift, duration, lobe separation, ramp rate, and cam timing so much that they are able to find advantages in stock and other type of regulated classes. Thats cool!

I think these cams are really really cool! because they provide a substantial advantage over a stock cam in a predator or clone engine, and are relatively inexpensive.

But when it comes to power, i like the cams that just perform well where you don't have to worry about rules. I currently am a fan of the higher LSA that makes more cylinder pressure and has a wide power band, or an OVERALL/good performance cam.

In racing mini bikes where we have no rules on that stuff in outlaw classes, i cant help but wonder if a mild cam or overall cam has any advantage other than the fact that you will get a better start off the line, and that it may be a disadvantage to not be able to rev to 9krpm with a wild cam. It seems to me that the more compression you have to have with a wild cam, the more torque and hp you will have down low anyway to make up for any 'losses' you would have had if you kept compression the same with a more mild cam.

So there is that. The more and more i learn, the more complicated I realize things are in the real world as they truly are.

For example, there was a 'header of the day' loop pipe that is advertized to make more power at certain rpms. On a dyno it does! But the dyno also shows that even with jetting adjustments the loop pipe destroys engine hp down low and up high.

When you race a kart with that loop header on it, you may have a 0.1 hp advantage over the guy next to you on the corner, but as soon as you accelerate in a straight like, the guy with the straight header is able to pass you at the end of the stretch because his header makes more hp from 5500 rpm to 8500 rpm.

Things to think about.
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Old 12-12-2018, 06:50 PM
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Some cheater cams for flatheads would float the valves in a way that improved power while still passing tech. That's risky in its own right, but riskier with a pushrod engine.

Loop headers touch on resonant tuning. The longer pipe afforded by a loop design has a lower resonant frequency so it scavenges better at a lower RPM. Predictably, short pipes scavenge better at higher RPM. Usually its a fairly narrow window the length is effective across, maybe 2000rpm wide.

Quote:
It seems to me that the more compression you have to have with a wild cam, the more torque and hp you will have down low anyway to make up for any 'losses' you would have had if you kept compression the same with a more mild cam.
If only! Part of the reason cam cards say "X compression required" is because the profile usually reduces dynamic compression below the power range and the static ratio has to be increased to get cylinder pressure back. That's why putting a big cam in a mild engine can not just make it drive much worse, but actually slow down. The cam bleeds off low-end and the combo can't breathe enough to get up into the power. Often those cams also have poor intake vacuum that plays heck with the carb tune so the result is a fast sound and a slow ride.
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