Cam lobe to crank throw clearance & cam lobe to rod

Discussion in 'Small Block Tech' started by Mart, Oct 5, 2018.

  1. Jim Blackwood

    Jim Blackwood Well-Known Member

    Mart, if the base circle you are referring to is the root of the cam lobe (and it'd pretty well have to be at under 1-3/16" now wouldn't it?) then any increase in lift has to be ground out of the base circle so yeah, ANY aftermarket cam is going to have a reduced base circle. You can't really make the lobes stick up farther than the bearing journals. That's why they are so big to begin with. Which means anything except a reground cam will have lobes that are the same height at the tip. And any aftermarket cam would be a reduced base circle cam. But that doesn't help with lobe to rod clearance.

    If "Base Circle" is used as a reference, then it can only mean the circle described by the tip of the cam lobe. In that case a reduced base circle cam would have lobes that don't stick up as far and again lift would determine how deeply the lobe is ground into the root. In this case a reduced base circle cam would increase lobe to rod clearance at the expense of camshaft rigidity. I think this has to be the definition of a reduced base circle cam as used in building stroker motors, otherwise it makes no sense. It's not the size at the shaft, it's the size of the invisible circle the tip travels through.

    And does anybody make those for the SBB? Not that I have seen. Whoever is using them, please provide your source.

  2. gsjohnny1

    gsjohnny1 Well-Known Member

    you guys are just too funny....:D:D:D
  3. Jim Blackwood

    Jim Blackwood Well-Known Member

    OK Johnny, you explain it. How's it even remotely possible to have a consistent lobe root diameter (what we usually understand to be the base circle) from one cam to another without the lobe peaks being all over the place? And I'm not seeing that. You look at a stock cam and the lobe peaks are out there at about the bearing diameter, easy to notice as the cam is going in. If it stuck out more I think it'd scrape up the bearings, don't you? So how can a cam with 600 or 700 lift have the same diameter at the lobe root?

    And then if they all have different root diameters what the heck is a reduced base circle cam anyway? Reduced from what? It just doesn't make any sense.

  4. gsjohnny1

    gsjohnny1 Well-Known Member

    I'm not an advocate of making a diesel engine. I prefer the safer side of engine building. but the closest I ever got was doing a mopar 273 cam which the base circle has to be smaller for any lift and to fit in the cam bearings. maybe this winter I will sit down and look at the engine.
    but somebody has to be a guinea pig in doing this and mart is JUST THE GUY. lol
    Mark Demko likes this.
  5. Mart

    Mart Gold level member

    Jim, I think what your trying to understand is the base changes on cam depending on lobe height. Your right, it wouldn't fit threw the bearings if the base wasn't proportionate to the lobe height.
    In my case, the base of the roller is 1.045, plus the .365 lobe height, so "the reduced base" would have to be smaller than 1.045 but still with the .365 lobe height, to offer up a little more clearance to the rods. How much is up to your added clearance requirements and if the cam grinder can grind the base to that smaller size, and still have good lifter function with that size.

    John is going to suprise us all after he hugs that block this winter and it starts talking ideas to him......:D
  6. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    In post #24 of this thread you will find a simple answer to your "base circle" question.

    I'll try a different way here to explain it as well, the base circle can be measured 90* from the lobe of the cam on the centerline of the camshaft.

    The if you measure the peak of the lobe to the "base circle" that is 180* from the peak of the lobe and subtract the measurement of the base circle(90* from the peak of the lobe) the answer will be the height of the lobe.(lobe lift)

    Now, if the base circle is smaller or larger, it doesn't matter, the height of the lobe off of the base circle dictates the lift.

    So you can have the same lift with a larger or smaller base circle if the measured difference is the same, see post #24 of this thread for more.

    The sbb roller cams have to be custom ground, so you can order them with a reduced base circle if that's what you need or want.

    So, this thread was always referring to a roller cam from the beginning, not sure if the flat tappet junk is available in a reduced base circle or not if that's what people are confused about? Maybe as a custom grind?(not sure)

    The only other way to get more lift would be with larger rocker ratios. Used nascar shaft rockers can be found cheap on eBay once and a while with ratios as high as 2.2:1! Would need to make custom rocker stands to make them work though.

    If TA figures out the sbb 350 heads and gets some good flow numbers, maybe they'll look into higher ratio rockers so the cam lobe doesn't have to be so big to get big lift? Or with a big lobe will give huge lift!
  7. gsjohnny1

    gsjohnny1 Well-Known Member

    mine is a flat tappet by Schneider. never thought to look at how it was made. important thing with them was min clearance to piston at zero setting. like I said before the 273 sbm requires a smaller base circle for lift and install.
    now the nascar world is way ahead of us. our blocks are nowhere near theirs. just look at the camshafts and rockers. the valves use tips that can be replaced. that's all 'mad scientist' work.
    all talk of all this theory.....who has the money to go and build a nascar style engine. (secretly, I want to do a nitro 350)
    300sbb_overkill likes this.
  8. Mart

    Mart Gold level member

    Ok, we're done with the base circle, too much crap to read. As John says, Sheesh....
  9. Jim Blackwood

    Jim Blackwood Well-Known Member

    I just thought we should know what we were talking about before just broadly saying that was the remedy to the rod clearance issue. All of that was the reason I clearanced my new Scat rods instead of trying to talk my cam grinder into it. He didn't want to, and it took long enough just to get the custom grind he was working up for me. If I had insisted I might still be waiting.

    I didn't like having to chamfer the rod bolts all the way to the center but that's what it took.

  10. Mart

    Mart Gold level member

    Yep, just finished these up. Stretch and torque values all measured in spec. .0055 - .006 stretch @ 58 lbs.
    300sbb_overkill likes this.
  11. Extended Power

    Extended Power Well-Known Member

    The rods, and bolts are made of much stronger material...having to remove some of it to gain the benefits of having the added performance of a stroker engine, was worth every bit of it.
  12. Jim Blackwood

    Jim Blackwood Well-Known Member

    Ordinarily I'd just agree with you Ivan. But in my case it wasn't to add stroke but to use aftermarket rods with a long rod length (7") in my 340. I expect I could have the same issue with Carillo rods in the 300 with a 340 crank, even using the Toyota pin size. Or maybe not. The real question for that engine is, would it make sense to offset grind the rod journals since they need to go smaller anyway. It may or may not make the difference in clearancing the rods, and if not may mean going deeper. I don't really have any way to answer that question without mocking it up, and that could mean a number of trips back and forth to the machine shop.

  13. Mart

    Mart Gold level member

    Jim & Ivan,
    I think my rod bolts didn't have to be ground quite to center. In fact about only 1/3 of the dia. was removed. Of course I am leaving my clearance @ .050 as TA thinks .040 is adaquate.

    On the rod bolts , they are extremely tough. I did i.d. / o.d. grinding a great portion of my working days and like titanium, there is virtually no spark when grinding bolts compared to the rods. They are one tough composition!

    All this is really hot rodding at its best!:D
    300sbb_overkill likes this.
  14. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    Jim, when Bob Mackley and son built their 370 sbb 350 supercharged stroker they used 6.300" nascar take out rods and "Cam is a custom competition components pieces ground for us as a blower motor cam". They didn't mention any cam to rod clearance issues with the block @ .040" over and the stroke @ 3.990" = 369.67 CID, rounded up = 370.(round up if the number passed the decimal is higher than .5, round down if its under .5)

    Here is the thread;

    I also asked Sean 76 because he has the details for the build for his book(which should be out shortly after the sbb 350 heads are out)and he said that there was no mention of rod clearancing or a reduced base circle.

    So Jim, if you use 6.200" rods you may have even more clearance even with the added stroke(same cam to crank distance with the 215 all the way to the 350). There is another twin turbo sbb 350 stroker running around in Canada(has 455 in his screen name) with nascar take out rods with no mention of rod to cam clearance issues with a flat tappet cam but he is only an extra .100" more stroke with the Honda 1.889" journal size.(2.015" or 2.008" housing bore rods will have the 1.889" rod journal size bearings and the 2.008"/1.976" housing bore rods have the 1.850" rod journal size bearings available for them)

    With a 3.800" bore and a 3,950 stroke would be 358.38 CID vs 3.49.307 CID with a 3.800" bore and the factory 3.850" stroke, so another 9 CID with the extra .100" stroke.

    There are under size bearings available with the Honda 1.889" journal size bearings available also, so you could get an extra .010", .020" and .030" under available for .110", 120" or .130" extra stroke for 359.287, 360.194 or 361.101 CID.(probably in this case better off using the standard size to save for regrinds, especially gambling that a flat tappet cam won't get wi0ped out and send metal throughout the engine ruining your freshly ground crank!:eek:)

    Of coarse the more bore you can add, the more cubes you'll end up with if your block has the cylinder wall thicknesses to go more than .050" over? A 4.00" bore with a 3.990" stroke is 401 CID! Although a 4.00" bore is highly unlikely with a factory block, an aftermarket sbb 350 block would be WAY cool with a 4.00" bore like the aftermarket v6 block can go to!
  15. Jim Blackwood

    Jim Blackwood Well-Known Member

    Yeah, I think you're dreaming a little bit there Derek but then again, why not? Bench racing at it's finest. I'll have to speak with Dustin and see what thinks about the offset grinding. If he thinks it isn't worth the trouble for 10ci then I'll have to go along with that. Odds are he'll put it in terms of dollars if I ask him to, I'm not sure how much extra I'd pay. A hundred bucks, well maybe, more than that, unlikely. I'm just not desperate for cubic inches. As for the overbore, I can't see going under 3/32 on the off side cylinder wall and on the thrust side I'd really like at least 1/8", with 3/16" being just about right. Not that easy to get even with standard overbore sizes. What I'm NOT going to do is build a dixie cup. Seen far too many of those. So in the end I may still be below 350cid, but that's fine. It'll have a blower on it and that makes up for a lot of other shortcomings.

    Now if the Carillo rods clear with the stock stroke that'd be a wonderful thing. Honda bearing size I'm sure, and I'll take a .040" clearance and like it. Maybe they'd also clear with the offset grind. The Scat rods didn't but they could just be bulkier at the shoulder. And the bolt spacing on the Carillos could be closer. Lot of variables. I'm not sure the rod length makes that much difference, it's not that big of a change in the rod angle when it approaches the cam lobe.

  16. Mart

    Mart Gold level member

    I would think that a stroked sbb with even a flat tappet cam would need after market rod corners ground. You need to talk to Bobb to see what he actually did on that build instead of assuming.

    Do we have a new block out now with a 4.000 bore????? Wow! Then we could bore that .060 over.:rolleyes:
    Last edited: Nov 9, 2018
  17. Mark Demko

    Mark Demko Well-Known Member

    Im thinking the same thing, doesn't a longer stroke take away from an engines ability to spin at high rpm with reliability?
    Diesel engines make massive amounts of torque at low rpm, is this due to the long stroke of a diesel, the 20 to 1 plus compression, the fuel itself, or a combo of all three?
    Marts build is interesting for sure, Im curious whats going to increase more, torque output (low rpm) or horsepower (high rpm), or both, or is the longer stroke going to limit rpm ability?
    I think too much:rolleyes:
  18. gsjohnny1

    gsjohnny1 Well-Known Member

    ok, i'll bite. lets get 2 alum v6's and weld them together and make it v12 at 800cid??
    I'm going to do twin v6's in the dragster, but can't afford welded 2 alum blocks together
  19. Jim Blackwood

    Jim Blackwood Well-Known Member

    Great idea! While you're at it can you weld together 2/3 of two V6's and get a V8? I'd love to have one.
    Don't know about long term durability at max rpm, but I can tell you guys that with light rods, wrist pins, pistons and flywheel, engine response is pretty phenomenal even with the 3.8" stroke. Can't see another 1/10" making any difference at all. Turn it into a diesel? Funny. That never occurred to me.

    Also, rod clearance entirely depends on what rods are used. By and large aftermarket rods have larger rod bolts and that makes probably more difference than any other factor. So unless somebody else has built one with the Carillo NASCAR rods I can't see where that build could tell me anything about clearance. OTOH, at the smaller journal size the Carillo rods should be a little narrower across the big end and that should help. But how much? My GUESS is enough to clear at the standard stroke. Maybe. Regardless, unless it's really cheap to do the offset grind I can't see it being worth it.

  20. Mart

    Mart Gold level member

    Yep Jim,you are right again. The actual corner of the rod isn't the tight spot, its up the rod some on my offset/roller combo. I'll post pics of where that is later.
    The narrower rod(bolt to bolt) doesn't really help the issue at all in my case. It'll all fit by making it fit.

    Offset grind is an add'l $100-150 over the standard charge.

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