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#1304024 - 09/15/12 06:43 AM 383/400 stroker - 3.90, 3.91 or 3.92 crank?
YYZ Offline
master

Registered: 01/20/03
Posts: 7091
Loc: Toronto (YYZ) Ontario
For a low deck stroker running 6.700" rods and pistons with a 1.320 CH (Diamond flat tops) - what should the crank be ground to for final stroke?

I've seen both 3.91 and 3.90 used a fair bit, but if I'm doing the math right (and there's a good chance I'm not)

With 3.90 stroke:

1/2 stroke: 1.95"
plus rod: 6.70"
plus piston: 1.32"

Gives 9.97" against a 9.98" deck height, so all things being equal, the pistons will be .010 down in the hole

With 3.91 it goes to .005 down in the hole and 3.92 should be zero deck

So without going into major block work/measurement, which is the best choice to keep both compression up but also a margin of safety for stretch/expansion of the reciprocating assembly?

Heads will be open chamber - worked 346s

Thanks in advance



gives

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#1304025 - 09/15/12 06:49 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
CompWedgeEngines Offline
master

Registered: 11/03/04
Posts: 3286
Loc: Syracuse,NY
You have no intentions of decking or correcting the block deck?
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#1304026 - 09/15/12 07:04 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: CompWedgeEngines]
sr4440 Offline
enthusiast

Registered: 02/06/08
Posts: 231
Loc: Central TEXAS!!!!
Without all the info, how do you expect to make an informed decision? Get the block corrected, then you will have your answer.


Joe
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#1304027 - 09/15/12 07:36 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: sr4440]
gregsdart Offline
master

Registered: 01/21/03
Posts: 6988
Loc: Frostbite falls, Mn ( Home of ...
Quote:

Without all the info, how do you expect to make an informed decision? Get the block corrected, then you will have your answer.


Joe



If you are expecting a good level of performance and anti knock resistance from this build, measuring and doing it right is the only way.
If you are running flt top pistons and will not have any quench, then you can figure compression with a deck of 9.98 and your compression should be close or less than calculated and if you chose you can live with what you get. If it is still a little high you can band aid it with a thicker gasket. There are large bore gaskets in .040 and .051.
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#1304028 - 09/15/12 08:01 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
67_Satellite Offline
super stock

Registered: 01/20/03
Posts: 815
Loc: Andrews,In. U.S.of A.
Since you're asking "what the crank should be ground at", I'll assume you're using a factory crank. When I had mine done,due to the indexing of the factory crank I started with, 3.908 was all the machinist could get without going to a .010" U.S. B.B.C. bearing. Could have gotten more stroke with an undersize bearing,but I thought material left for future machining/freshening was more important than a few thou, extra stroke.Your particular crank may allow more or less depending on what it was originally ground at. Whatever stroke you end up with, machine the block to set the deck height to the desired level after a mock-up and measure session. The factory didn't always hold the decks equal side to side or end to end back in the day.


Edited by 67_Satellite (09/15/12 08:05 AM)

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#1304029 - 09/15/12 08:56 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
MR_P_BODY Online   content
Master

Registered: 06/13/03
Posts: 44415
Loc: Romeo MI
Have a shop check the deck first... that will give
you a idea what you have and the decision will be
alot easier... you would be suprized how bad some
of the decks are
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#1304030 - 09/15/12 10:22 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: MR_P_BODY]
YYZ Offline
master

Registered: 01/20/03
Posts: 7091
Loc: Toronto (YYZ) Ontario
Thanks everyone for the input

The goal was to tread the line between budget stroker vs full-on build and see what's involved

Crank is a new MP 3.75" stroke for B engines

Scenario 1 would be to low-budget - keep the 3.75 stroke, use a reconditioned set of LY rods bushed for the .990 pins and just screw it all together and hope for the best

Scenario 2 is to maximize stroke with the same crank, turn down to 2.200 rod journals and go through the rest with a fine toothed comb. Which appears to be at least a few K more

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#1304031 - 09/15/12 10:45 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
Thumperdart Online   content


Registered: 07/12/04
Posts: 14143
Loc: behind my drums in ft. of you
I must be the only guy running a 3.900 stroke w/a 413 crank and 6.385 rods. Can`t tell ya any more as far as pin height etc. but almost 13 years later and all`s good.
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#1304032 - 09/15/12 11:19 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: MR_P_BODY]
jafr Offline
member

Registered: 12/31/09
Posts: 83
Loc: Southern IL.
More than lkely you will end up with a deck less than 9.98 I have seen blocks that need .020 or more cut to square things up. You need to square deck the block. The deck heights from the factory are all over the place. Decking it will get rid of alot of the chamfer on top of the deck, give you a nice flat surface for the gasket to seal and equalize the height of all your pistons for quench. I have a 470 on the stand for a backup and it runs very well for for it's size.
My opinion is to run the quench as tight as you safely can, this will make more power and reduce the chance of detonation. On a big bore like a 4.375 I think .040 is safe and on a small block I think you can tighten it up to .035 without seeing witness marks on the pistons or heads.

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#1304033 - 09/15/12 11:28 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: CompWedgeEngines]
JohnRR Offline
I Win

Registered: 01/20/03
Posts: 71061
Loc: U.S.S.A.
Quote:

You have no intentions of decking or correcting the block deck?




DING DING DING ... WINNER .

Vlad, my crank is 3.895 stroke with that rod and piston CH combo, but that was for other reasons .

Get the block squared first , find out what the deck is and cut the crank to put the piston somewhere between 0 and .005 in the hole .

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#1304034 - 09/15/12 10:42 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
Cab_Burge Offline
Too Many Posts

Registered: 08/23/03
Posts: 24421
Loc: Bend,OR USA
With iron heads and pump gas I would shoot for 9.5 to 1 or less compression ratio With aluminum heads I'll settle for 10.5 or less on pump gas with 10 to 15 % Ethanol The 3.90, 3.910 strokers like to rev, in a B or RB motor, the last one I built had a set of B1-BS heads with CNC porting to M.W. sizes, it made 720 HP at 7000 RPM and was still climbing in HP at 7000 the ciustomer siad to stop the pulls at 7000 RPM because he is a bracket racer and wanted to keep the revs. under 7000 RPM Check your cylinder heads CC and go from there I have another stock Mopar 440 crank offset ground to 3.90 stroke for another 440 mild pump gas street build with a set of large valve ported 452 heads, I'm shooting for 9.5 to 1 on it I'm going to use a 7.100 long BB Chevy type rod with Icon # IC-838 pistons, looking for 9.5 to 1 comp. ratio with decent quench
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#1304035 - 09/15/12 10:52 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
Belvedere1 Offline
super stock

Registered: 01/20/03
Posts: 840
Loc: Phoenix, AZ
Be prepared to spend some extra $$ on getting that crank to balance and cutting the counterweights for clearance. I hope yours works out better than mine did.
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#1304036 - 09/16/12 09:30 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: Belvedere1]
440Jim Offline
I Live Here

Registered: 01/19/03
Posts: 15563
Loc: Southern Maryland
Quote:

Be prepared to spend some extra $$ on getting that crank to balance and cutting the counterweights for clearance. I hope yours works out better than mine did.


To get clearance of the counterweights in the low deck block, there are several ways.
1) lathe turn (round) the counterweights to about 7.25" dia, keep bobweight ~2350g
2) put large bevels on the edges of the counterweights
3) grind the clearance in the block and leave the crank about 7.50" dia

The larger the bevels, the larger dia you can leave the counterweights. The front and rear (large) counterweights will fit at a larger dia than the inner (smaller) ones.


Attachments
7381275-Counterweight_bevel800.jpg (41 downloads)


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#1304037 - 09/16/12 09:57 AM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: 440Jim]
YYZ Offline
master

Registered: 01/20/03
Posts: 7091
Loc: Toronto (YYZ) Ontario
At this stage it's a bit of a 'what if' exercise but the point on the block is well taken.

The crank in question should not require further machining to fit the B-block as it was made with smaller counterweights and B-sized main journals for exactly that purpose. Plus an 8-bolt flange.

For turning it down if/when the time comes, being friends with a crank shop does occasionally have its benefits....

Thanks all for the great advice!

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#1304038 - 09/16/12 02:45 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
Evil Spirit Offline
master

Registered: 01/02/05
Posts: 3380
Loc: Newport, Mi
I'm sure this will rattle a few cages - oh well. Setting the block deck height to set your quench/combustion chamber/piston clearance is important, as eliminating mechanical interference and checking compression is again, important. Decking a block solely to square it up? More important to help make the intake seal than anything else - don't waste your time or money if you think there's "magic" power there. There probably isn't 5 HP gained by decking the block just to square it up, since most blocks aren't that far off, usually in the .010 range, and cutting only .010 off a block won't change compression enough for a big power gain. You will get the same power increase by a thinner head gasket. So if you are cutting the block to set piston to deck height - whack away. Otherwise there isn't the gain there by trueing the deck to the mains that lot of people seem to believe. I've heard of a lot of people decking blocks, but not equalizing combustion chamber volumes - HUH - whats the difference? Ones just promoted better I guess.
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#1304039 - 09/16/12 03:04 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: Evil Spirit]
ou812 Offline


Registered: 01/31/03
Posts: 5124
Loc: SoCal
Quote:

I'm sure this will rattle a few cages - oh well. Setting the block deck height to set your quench/combustion chamber/piston clearance is important, as eliminating mechanical interference and checking compression is again, important. Decking a block solely to square it up? More important to help make the intake seal than anything else - don't waste your time or money if you think there's "magic" power there. There probably isn't 5 HP gained by decking the block just to square it up, since most blocks aren't that far off, usually in the .010 range, and cutting only .010 off a block won't change compression enough for a big power gain. You will get the same power increase by a thinner head gasket. So if you are cutting the block to set piston to deck height - whack away. Otherwise there isn't the gain there by trueing the deck to the mains that lot of people seem to believe. I've heard of a lot of people decking blocks, but not equalizing combustion chamber volumes - HUH - whats the difference? Ones just promoted better I guess.




Piston to cylinder head quench is main reason to zero deck... .010 tolerance is unacceptable.
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#1304040 - 09/16/12 03:21 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: YYZ]
JohnRR Offline
I Win

Registered: 01/20/03
Posts: 71061
Loc: U.S.S.A.
Quote:


The crank in question should not require further machining to fit the B-block as it was made with smaller counterweights and B-sized main journals for exactly that purpose. Plus an 8-bolt flange.






Is the crank in question a crank sold by Mopar performance in/around 2003 ? If it is have you actually tried putting it in the block ?

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#1304041 - 09/16/12 03:42 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: JohnRR]
RUNCHARGER Offline
I Live Here

Registered: 01/20/03
Posts: 14657
Loc: Kelowna B.C. Canada
Maybe I reverse engineer stuff. I would set it up with as much stroke as you can get with that particular crank. I would then mock it up before decking and see where it was at. Then I would decide how much to deck it.

Sheldon

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#1304042 - 09/16/12 03:49 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: RUNCHARGER]
23T Hemmee Offline
mopar

Registered: 01/06/10
Posts: 519
Loc: Franklin, TN
Quote:

Maybe I reverse engineer stuff. I would set it up with as much stroke as you can get with that particular crank. I would then mock it up before decking and see where it was at. Then I would decide how much to deck it.

Sheldon


BINGO!!! We have a winner.....
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#1304043 - 09/16/12 04:28 PM Re: 383/400 stroker - 3.90, 3.91 or 3.92 crank? [Re: RUNCHARGER]
BSB67 Offline
master

Registered: 02/19/05
Posts: 2727
Loc: Prospect, PA
Quote:

Maybe I reverse engineer stuff. I would set it up with as much stroke as you can get with that particular crank. I would then mock it up before decking and see where it was at. Then I would decide how much to deck it.

Sheldon




Me too. You'll be able to figure out if the shelving piston height will work and either move the CD around, cut the deck, or gasket it to what you want.
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