400 external balance crank question

Discussion in 'Rotating Assemblies' started by SLO-C10, Jun 18, 2017.

  1. SLO-C10

    SLO-C10 New Member

    long time lurker, first time poster with a question. I have a 406 Chevy with external balance crank. Motor came with a PPI 80001 dampener, 6-3/4" with bolt on external weight. Motor did not come with flexplate so I bought an SFI B&M unit for external balance crank.
    This is my first foray into external balance world, I've Always had 350s before.
    For my own edification, I am noticing that the bolt on weight on the dampener is clocked at about 7:00 with the timing mark at 10 before TDC while the weight on the flexplate is clocked at about 2:00ish give or take a little based on the center of the weight, and installed using the index pin on the crank. By clocking I mean looking at both from the front of the engine. The weights are basically opposite each other on a clock dial.

    Is this about where they should be in relation to each other? It seems like they would sort of cancel each other out taken as static balance.

    I work in the tire industry and I understand that to dynamically balance a wheel/tire assembly there are usually weights on both the inside and outside of a wheel and rarely are they clocked the same or even the same amount.

    I just want to make sure before this engine goes in my truck that all is as it should be.

    Thanks in advance. IMG_4466.JPG IMG_4480.JPG
  2. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    Nice looking engine and truck you have there.
    Now for the bad news - you have to know how this engine was balanced to know what damper
    and flexplate or flywheel to use. It might need a neutral balanced damper with a counter-weighted
    flexplate. Or the opposite. Obviously, neutral/neutral gives the least confusion.
    If you know what company built this engine, give them a call and hopefully they can tell you exactly
    what you need. Or if you can find any numbers or stampings (other than block casting numbers),
    you can post them here and somebody might be able to help.
    BTW, if you get it wrong, you will immediately know it. Good luck.
  3. SLO-C10

    SLO-C10 New Member

    Thank you for your reply, the motor was built locally and I actually saw it in its previous vehicle and was definitely external balanced. We also ran it on a dyno before I actually took it home also. My question has more to do with my unfamiliarity with what the relationship between the positioning of the weight on the dampener with the weight on the flexplate. The dampener is the same one that was on the motor before I bought it, the owner kept the flexplate because he needed it for another project.

    Since the positioning (clocking) of either one is not negotiable, the crank keyway determines the dampener position and the index pin determines the flexplate position, I'm just curious about the relationship between the two.

    This being an aftermarket dampener is not what the typical external ones look like, instead of weight removed from an area on the backside it has weight added to an area to do its share of the external balance job.
  4. SLO-C10

    SLO-C10 New Member

    Thinking about this, an easier way I could have posed the question would have been to ask what the positioning of the flexplate weights should be in relation to the crank keyway. That would eliminate the differences between the PPI balancer I have from a more standard external dampener with weight removed.

    In my case the keyway at the nose and the index pin at the rear are clocked pretty close to the same if you were looking front to back straight through the engine.
  5. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    the real question, here ... is...
    did the engine run smoothly when you had it dyno tested?
    if the weights were not in the correct location the engine would vibrate very noticeably by the time you exceeded about 3500 rpm.
    notice the weight here
    most better and experienced balance shops will have the precision balancing equipment that tells the guy balancing the rotating assembly where the weight needs to be located and how much it needs to weigh, the shop will bolt on or weld on a slightly heavier weight in the lighter side of the damper or flywheel/flex-plate,in most cases, or otherwise add weight with heavy MALLORY METAL< where its needed to crank counter weights and make fine weight adjustments by drilling holes or grinding off a bit of weight.


    Last edited: Jun 18, 2017
  6. SLO-C10

    SLO-C10 New Member

    I answered incorrectly at first, yes the motor ran smoothly on the dyno and in the truck it was removed from. Dyno sheet attached. I chose to stop at 6000rpm because that's as far as I'll push it in my C10 but the motor originally dynoed at 575ish at 7500rpm.
  7. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    THEN OBVIOUSLY, per your experience,
    for that particular engines rotating assembly,
    the balance weights are correctly indexed, by 6000 rpm,
    it would be very noticeable if it was out of balance

    BTW nice dyno , CONGRATS!
    having the torque peak at 4400 rpm frequently indicate's the ports are slightly restrictive or the cam could use a bit more duration.

    the formula for horse power is

    torque x rpm /5252= hp

    575 hp= 7500 rpm x 402 ft lbs
    Last edited: Jun 18, 2017
  8. SLO-C10

    SLO-C10 New Member

    Thanks Grumpy, I think not having the flexplate from the motor at the time it was in the previous truck had me a bit worried, and I honestly don't know what goes on the rear of an engine when it is on a dyno, so the B&M unit is the unknown compared to my prior knowledge that will be part of the rotating assembly when it goes into the truck. Which will hopefully be the next few weeks.
    Thanks for the reply.
  9. Loves302Chevy

    Loves302Chevy "One test is worth a thousand expert opinions."

    Please explain more, Grumpy. How can you tell that? Would I be correct in assuming that his torque
    peak should be higher, more like 5000+ rpm given that HP peaks at 7500?
  10. SLO-C10

    SLO-C10 New Member

    I never knew that formula, but being the cynic that I am I had to plug a few numbers from my Dyno sheet in and try, and to nobody's surprise of course the math works.
    As for the peak torque being at a lower rpm than ideal, it's still plenty for my needs. I will get to NE Dragway once or maybe twice per year, the rest of the time it will be car show and cruise night basically.
    The part of the combo I'm missing so far is the cam specs, heads are Brodix Track 1, vintage 1994!! Performer RPM intake, 1.6 full roller rockers, Comp beehive springs, cam is full roller but I'm really hoping the builder can locate the card.
    I could pretty easily measure lobe or valve lift at this point. Carb was a 850 proform but I may go FITech. Also, not sure if torque curve might change with mufflers? We ran open headers.
  11. Grumpy

    Grumpy The Grumpy Grease Monkey Staff Member

    an engines Torque peak is almost always very closely related to the point in the rpm curve where the most effective/efficient cylinder fill, cylinder fill is related to both intake port cross sectional area and exhaust scavenging,efficiency, and is limited by port stall, and cam duration in relation to displacement, compression and valve train stability, ...all factors are easily calculated
    links below


    I see guys have long discussions about things like the difference in port cross sectional area or the best connecting rod length, to use, no one factor is going to make your engine totally dominate the competition, its a combo of small almost insignificant individual component choices being made and a good deal of time and effort taken during the assembly and clearancing work, that stack up to give you or prevent you from maximizing the engines performance.
    you may not even think about factors like polishing crank journals, or valve train geometry or intake runner cross sectioinal area or length ,or intake runner port matching or surface finish, but the combined effects of your choices and components selected do mater!
    look guys I think a good deal of this discussion is missing the point here, Ive built well over 150 engines in the last 45 years, (I lost cound decades ago)
    but I can assure you that longer rods and the easily verifyable slight increase in dwell time, the longer rods produce will be totally meaningless UNLESS, you design the engine for and select components too take full advantage of the minor increase, by carefully calculating the REQUIRED compression ratio,fuel octane required,all the factors related to the cam timing,(duration,lift, LCA) you calculate and build and install, and tune the engine for , a matched exhaust header scavenging (header primairy length and diameter plus collector design) and the intake runner length and cross sectional area, to maximize the cylinder scavenging effects, plus you match the fuel/air ratio, and ignition advance curve, to maximize that longer dwell times potential advantage.
    the valve seat throat area as a percentage of the intake valve diameter effects the port flow rate, yes the port cross section and angle also effect flow rates and larger valves and larger throat areas with less restriction obviously have some potentially lower flow restriction.
    SO HOW do you MEASURE THEN??
    lets do a bit of math, and keep in mind that a correctly designed header and exhaust system, if matched to the correct cam timing can significantly increase the engines potential power/rpm band
    port cross sectional area can be measured and the stall speed , accurately calculated, as can the required matching header configuration, and cam timing, yeah! it takes some reading but the infos readily available


    just a bit of info on intake gaskets sizes to match port cross sectional areas

    Calculating the valve curtain area
    The following equation mathematically defines the available flow area for any given valve diameter and lift value:
    Area = valve diameter x 0.98 x 3.14 x valve lift
    Where 3.14 = pi (π)
    For a typical 2.02-inch intake valve at .500-inch lift, it calculates as follows:
    Area = 2.02 x 0.98 x 3.14 x 0.500 = 3.107 square inches
    SO lets do a bit of math
    a cylinder head with a 2.02' intake valve and a cam with a .450 lift at the valve with a 1.5:1 rocker will in theory produce a valve curtain area of 2.79 sq inches, swapping to a 1.6:1 ratio increases the lift to .480 lift 2.98 sq inches, increasing the available port flow potential at least in theory by about 6%, but keep in mind the port can only flow at full valve lift for the limited time the valve remains at full lift and if the narrowest section of the port cross sectional areas less that the valve curtain area that not the valve restricts flow

    if you were to look at a performance big block chevy cylinder head your largest standard intake valve size is either a 2.19" or in a few cases the larger 2.3" valves
    a bit of math shows that you won,t reach the max potential flow until valve lift reaches or slightly exceeds about .575-.600 inches of lift with a big block chevy
    and a bit more math suggests a minimum of 4.2 square inches of port cross sectional area would be about ideal to match that potential flow,
    if you built a 496 BBC stroker that 4.2 SQ inch port would max out at about 6000 rpm and would be best matched with a single plane intake and a cam with a tight 105-106 LSA


    useful RELATED INFO you might want to read










    Last edited: Jun 19, 2017

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