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#1072119 - 09/29/21 01:55 AM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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BadSS Offline
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The dished 16cc piston is a bit of a compromise because as Brent said he's considering the possibility of moving to a different set of 64cc heads at a later date. Running this particular piston puts the compression a little on the high side with the small chamber heads and a little on the low side of what I'd consider "optimal" with a set of 64cc heads.  However, the differences between the two compression ratios and “optimal” is minimal when using 93-octane gas.

Yep, Bob, I understand your concern about the oil rings on the long stroke, long rod combos.  I had serious reservations at one time myself and did my best to stay away from support rails when possible until relatively recently.  In fact the 406 engine before last I built for my SS used 5.7 rods due in part to my apprehension on using support rails and to get a thick top and second ring land for the amount of nitrous I planned on using.

I’ve seen comments from well-known engine builders that have said they’ve had no issues regarding support rails – as long as they’re the right ones (you have to check to make sure they put the right rails in the right box). Also, the last few engines I’ve built used rail supports and no one has reported any oil consumption issues.  So, I suspect that if you’re seeing a little higher than normal oil consumption that it probably has little if anything to do with the support rails.

Now, I’ve always been partial to the Napier second rings for their improved oil control over other type second rings (whether using support rails or not). They are a required build criteria for me as opposed to using a different type 2nd ring, so that may be a factor as nothing I've built ever had any oil control issues.

The two ring sets I recommended to Brent both utilize a barrel faced Moly top ring and a Napier second ring.  I’ve used the same series Mahle rings in builds before, but the other set, Hasting rings, look to be better on paper.  I’ve never used Hasting rings, but from what I understand they supply rings to Wiseco and JE for them to put in their boxes. So I asked Brent to consult with his machinist (who is a well known engine builder in his area) to see if he had used and could recommend the Hasting rings over the Mahle.

#1072127 - 09/30/21 04:27 AM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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mmc427ss Offline
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Never gave the 2.00" verses 2.100" crank journal a thought, but a good idea. That would reduced clearancing .050" on each side of the block. There was only one crank available at 4" back in the day, Eagle. at 2.1" Clearancing for that swing is one of the reasons i have a Dart block. Even then one or two of the oil pan bolt holes are very close to the clearancing.
As with any longer stroke SBC rod to cam clearance also becomes a problem. The base circle of the cam usually is effected. With the 2.00" and 3.875 stroke what base circle are you stuck with? The cam from the 305 is that base circle reduced? My 4" stroke requires a toothpick cam, .850" base, good thing it's billet. Had a hard time getting a cam ground that small, only Comp would do it.

So you will be sacrificing quench for compression with the 56 heads?
Do you use this compression calculator or is there one better?
http://www.wallaceracing.com/cr_test2.php

Bob

Last edited by mmc427ss; 09/30/21 04:29 AM.
#1072130 - 10/01/21 01:43 AM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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BadSS Offline
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Below is a chart from Comp Cams that show the differences in the standard/theoretical base circle based on differences in lift and the base circle if you request a small base circle. Now you can get them to cut just about anything on a .900 base circle, but these CompStar rods even in the 2.100 journals have a lot more clearance than most "stroker rods". This is especially so at the block (bottom of the rod bolt pad) - even more when going with the CompStar 2.000 journals.

Cam lobe for the 396 will be in the .360 range - one reason because it will keep the typical "small base circle" at 1.020, which should give ample clearance with the CompStar 2.000 rods
[Linked Image]

Here's the write up that contains the chart.
https://www.cpgnation.com/all-about-that-base-circle/

When I model an engine, the simulation calculates the static compression ratio, which uses the typical entry data. However, if I'm unable to use the simulation, I use the calculator at Diamond Racing. It gives more data and generates a little more precise ratio if you know the top ring land height - if not the Wallace calculator is just as good
https://diamondracing.net/p-10-compression-calculator.html

I know people can get excited over "squish". However, while I typically like to get it in the .040" - 045" range with steel rods and run as thin a gasket as possible, a .051" compressed gasket and anything under .060" will not be problematic - especially when naturally aspirated. Plus, I'd rather have a little too much than not enough - lol.

#1072145 - 10/02/21 07:44 PM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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MAP Offline
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The only thing I wonder about regarding a small base circle stems from the fact that the cam is basically a cylindrical (+/-) torsional spring. Think of it as a lumpy sway bar that's driven at one end. The smaller the base circle diameter, the more compliant the spring, and the lower its fundamental torsional resonant frequency. Should this frequency enter the cam's rotational speed at or near top engine rpm, it could wreak havoc with valve events. Below this frequency, high torsional compliance would tend to manifest as a slight shortening of duration, especially for lobes near the back end of the block, farthest the cam gear. Ditto all this for the distributor, btw - in fact, timing jitter at high rpm may be a good sign that the cam is misbehaving.

One can always assume that Comp Cams wouldn't sell a cam with this kind of behavior, but from past experience with many hotrodding vendors, the vast majority of whom tend to be avid technicians rather than engineers, I wouldn't make that assumption. - HTH.

Last edited by MAP; 10/02/21 07:49 PM.
#1072151 - 10/04/21 01:04 AM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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mmc427ss Offline
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Unfortunately 15 years ago rods with offset big end, cranks with small rod journal and a 4" stroke into an engine design for a 3" stroke were not readily available. To get .050" rod to cam clearance that toothpick of a cam was required. An option would have been to use the tall deck Dart block but that need a lot of very specific parts to put it together. The price and complication of a tall deck 427 SBC made my second choice for a build an LS7 at that time. The crate price of a dry sump LS7 back then forced the building of an old school SBC 427" truck engine. Tons of low end torque and lower rpm limits. And you are right, the VERY small cam base circle was the scary part. Even a 383 with a large cam normally needs a very small base cam.

Fortunately due to a large cubic inch, 427, a mild cam could be used, only .570" and 240 at .050", E and I are different but close enough to those numbers. That cam falls on it's face at just shy of 6K and the engine never really need to buzz that high. Good for it, the cam.

Valve springs and retainer were changed out a few years to beehive and tool steel which reduced weight drastically. Springs are PSI LS1511 which are a good match for the cam and AFR heads.
Sure would be cool to put that engine's cam/valvetrain on a Smokey's "Smoketron", but that will never happen.

Believe me if, when, that cam comes out will be curious to see how it held up. Just over 25k on the build of not easy miles.
Bob

#1072153 - 10/04/21 07:44 AM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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BadSS Offline
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I ran a solid roller .940 base circle with 212lb seat pressure (565lb over the nose) in my last 406 for about 5 years of daily driver use and then an addition 5 years or so of weekend use. Ran 1.6 rockers with 0.640 net lift and I had over 200 grudge races with it shifting at 6800 rpm - many more test and tunes and lots of street play. I would go months before checking the lash and can't remember a single time when I HAD to make an adjustment. Built a number of engines with .900 cams just to be sure there would be no clearance problems and no one has had issues.

The late, great Joe Sherman said in one of his posts about .900 base circle cams - " I run 285 on the seat, and 810 open (at 875 lift on the intake) In five years, the springs have not lost any pressure- lash never changes , and we rev it 8500 twice on every run- NO PROBLEMS at all."

Now, I don't think anyone would argue best practice is to go with the largest base circle that will clear (that's what I did with my last and current build). However, if drag racers running big spring pressures and running high RPMs aren't having issues and based on my own experiences, I wouldn't sweat running something as small as Bob is when using hydraulic roller spring pressures.

#1072198 - 10/07/21 08:31 PM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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MAP Offline
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Hi BadSS,

Great news! I didn't say a small base-circle cam would necessarily cause problems, but simply that it was more prone to do so than its larger-base-circle cousin.

All's well that ends well.

Best,
MAP

#1072216 - 10/09/21 04:05 AM Re: 88ssBrent's Build! [Re: MAP]  
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BadSS Offline
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Originally Posted by MAP
Hi BadSS,

Great news! I didn't say a small base-circle cam would necessarily cause problems, but simply that it was more prone to do so than its larger-base-circle cousin.

All's well that ends well.

Best,
MAP


No, you brought up a valid point about the small base circle cam and I certainly do not disagree with any of the points you made. In this case, bigger is better - or at least the biggest you can run the better - lol

#1072236 - 10/12/21 08:42 PM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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MAP Offline
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Hi BadSS - Just thinking that a roller cam may behave very differently from flat-tappet cam. A roller side-loads the cam lobe (= exerts a disruptive torque) as the roller rides up and down the side ramps. A flat tappet only loads the cam as a function of changing friction. Somewhere I'm sure I've got a book (Blevins?) that I can look up to quickly calculate the torsional resonance... 8,500 rpm = 4,250 cam rpm, and with 16 lobes (17 counting the fuel pump lobe,) with 34 total side ramps, we are exciting the cam at roughly 1.3kHz. I need to see how close that is to torsional resonance.

BUT anyway before I go off the deep end - Brent, given BadSS's good experience, I see no cause for alarm, thankfully.

Last edited by MAP; 10/13/21 06:33 AM.
#1072250 - 10/14/21 01:45 PM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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Travis Jones Offline
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Originally Posted by MAP
A roller side-loads the cam lobe (= exerts a disruptive torque) as the roller rides up and down the side ramps. A flat tappet only loads the cam as a function of changing friction.


I'm sorry I don't agree with your assertion here. First a flat tappet rides up and down side ramps just like a roller does, if it doesn't it will trash the lifter. The difference is that the roller lifter uses only the roller to spin, where as the body of the flat tappet lifter rotates because of the offset convex surface ground into the lifter and angle of the lobe on the camshaft.

The force load on the cam is directly proportional to the weight of the valvetrain components and spring pressures and RPM... F=MA, but with a flat tappet amplifying frictional losses when spun by the crankshaft. Both have to act as lever and fulcrum is in the same spot. For a cam with the same ramp profile and valvetrain component weights, the forces exerted by the valvetrain are the same, but the torque to turn would be higher because of the higher friction flat tappet actuation resulting in higher torsional stress within the camshaft.


86 SS 6.2l LS3, Ilmor intake, Summit Stage 4 Cam, Stainless long tube headers, Stainless 3in exhaust, Tremec T-56 Magnum 6 speed, Eaton Truetrac 8.8 LSD, UMI Cornermax Front Suspension, 3-link Rear suspension w/ UMI Control Arms, UMI Front & Rear Braces, Brembo Brakes
#1072251 - 10/14/21 07:58 PM Re: 88ssBrent's Build! [Re: 88ssBrent]  
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MAP Offline
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MAP  Offline
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Hi Travis,

The physics you cite is incomplete and partially wrong. Please see my forum profile for my background.

The very short description I gave two posts back was even more incomplete - sorry about that, but I hit just the two very top points for the sake of brevity.

Pictures would help immensely here, and even better if the pictures could evolve in real-time along with a verbal explanation, but I don't do Youtube.

Bottom line is that some forces, and therefore some moments, are not the same. Do a FBD for the lobe with the roller and the flat tappet and compare. Don't forget to apply appropriate boundary conditions. Also, don't forget that for the same valve lift profile, the cam lobes for a roller and a flat tappet are completely different. The roller has steeper ramps and a rounder, broader nose; these alone would indicate different forcing functions acting on the cam, but the differences between the two cam types continue.
________________________

Brent, I take full responsibility for the rabbit trail here - I apologize - feel safe to ignore it and just go by BadSS's posts.

Best,
MAP


Last edited by MAP; 10/18/21 01:20 AM.
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