Maximun hp for a cast Scat 383 crank-bearing clearance
11-26-2002, 02:34 PM
#1
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Maximun hp for a cast Scat 383 crank-bearing clearance
I’m curious and wondering if anyone has any objective information regarding power/rpm levels where a forged crank is absolutely necessary. I’ve known of a couple 500rwhp cars that are using a good ol $199.00 cast Scat 3.75 inch stroke crank with no problems.
Has anyone of the list actually broken one of these things?
Also what bearing clearance is optimal for a street driven car? My main bearings now are .028 giving me a little lower oil pressure than I’d like to see. Any suggestions?
Has anyone of the list actually broken one of these things?
Also what bearing clearance is optimal for a street driven car? My main bearings now are .028 giving me a little lower oil pressure than I’d like to see. Any suggestions?
11-26-2002, 06:10 PM
#2
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".028".. yeah, I can see the problem.
I know, you meant .0028 which is a little loose. I'd shoot for .0022-.0025.
As for the crank, I've run quite a few to 7500 rpm. Usually about 10 times a night, twice a week.
The crank needs to be prepped correctly... it needs to be straight (check it) and it needs to be nitride hardened. That'll help it live. I did notice a bit less bearing wear when I switched to a forged Crower crank here a few years back in one of my engines (had been running a cast Summit crank) but the engine never gave me a problem otherwise. Even ran it on a progressive shot of Nawss a few times.
Just make sure you use a lightweight piston, pin and rod. That'll go a long ways in insuring the life of your crank. Get plenty of oil to it too... goes without saying.
-Mindgame
I know, you meant .0028 which is a little loose. I'd shoot for .0022-.0025.
As for the crank, I've run quite a few to 7500 rpm. Usually about 10 times a night, twice a week.
The crank needs to be prepped correctly... it needs to be straight (check it) and it needs to be nitride hardened. That'll help it live. I did notice a bit less bearing wear when I switched to a forged Crower crank here a few years back in one of my engines (had been running a cast Summit crank) but the engine never gave me a problem otherwise. Even ran it on a progressive shot of Nawss a few times.
Just make sure you use a lightweight piston, pin and rod. That'll go a long ways in insuring the life of your crank. Get plenty of oil to it too... goes without saying.
-Mindgame
11-26-2002, 06:33 PM
#3
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Thanks!
Guess the jest is I traded for a complete engine. I got a cast crank, good forged rods and pistons, but just slightly off on balance and real loose clearances out of the deal. Besides that……. runs fine with no other issues. For sure the engine is going to be torn down in the next month, rebalanced, refreshed and was just wondering if I should put in a forged crank being I gave up on the 409ci project.
The reason I bring it up is I’ve never heard of anyone having problems with one and doubted if it really was worth spending the extra money for an intended 6800 rpm engine. Sounds like those cheap cranks (actually $299.00) are better than most people realize.
I was looking to set it up according to the LPE book of .020-.025, but I respect the experience of the people on this list and wanted second opinions.
Thanks again.
Guess the jest is I traded for a complete engine. I got a cast crank, good forged rods and pistons, but just slightly off on balance and real loose clearances out of the deal. Besides that……. runs fine with no other issues. For sure the engine is going to be torn down in the next month, rebalanced, refreshed and was just wondering if I should put in a forged crank being I gave up on the 409ci project.
The reason I bring it up is I’ve never heard of anyone having problems with one and doubted if it really was worth spending the extra money for an intended 6800 rpm engine. Sounds like those cheap cranks (actually $299.00) are better than most people realize.
I was looking to set it up according to the LPE book of .020-.025, but I respect the experience of the people on this list and wanted second opinions.
Thanks again.
11-26-2002, 07:19 PM
#4
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denny, .0028 shouldn't cause abnormally low oil pressure.
I try to set my mains around .0025, and rods around .0022.
Just spring for a cheap forged crank.. you can get eagle units for <$600, if you wanna rev/make power.
if you are gonna go the 450rw NA route, no nitrous... the scat unit will be more than enough however.
I try to set my mains around .0025, and rods around .0022.
Just spring for a cheap forged crank.. you can get eagle units for <$600, if you wanna rev/make power.
if you are gonna go the 450rw NA route, no nitrous... the scat unit will be more than enough however.
11-26-2002, 09:01 PM
#5
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The oil pressure won't go past 50 psi with 15-50 Mobile 1. Ain't bad......ain't good.
The build sheet says .0028 but it may have loosened up some (may also not be correct) as the engine has 10k miles on it. New bearings certainly won’t hurt and it needs to be rebalanced anyway. If it wasn’t for the balance issue, wouldn’t even mess with it.
Actually all I'm shooting for is around 450rwhp or so N/A staying below 7000 rpm. I just can’t see buying all new crank, rods, and pistons for just 13ci more so I’m just going to stay with the 383 stuff I’ve got. Ah…… Still open for a new forged crank just because it sounds better even though I think I’m spinning my wheels.
Believe you did a supercharged 383 with the same Scat crank if I remember right. In fact, that’s what really facilitated the idea that another crank may not be necessary.
Heads…………. That’s another story. The best that I can find……. Course finding killer heads is easier said than done. Somehow the spec sheets I’ve received never match the heads I’ve paid for.
The build sheet says .0028 but it may have loosened up some (may also not be correct) as the engine has 10k miles on it. New bearings certainly won’t hurt and it needs to be rebalanced anyway. If it wasn’t for the balance issue, wouldn’t even mess with it.
Actually all I'm shooting for is around 450rwhp or so N/A staying below 7000 rpm. I just can’t see buying all new crank, rods, and pistons for just 13ci more so I’m just going to stay with the 383 stuff I’ve got. Ah…… Still open for a new forged crank just because it sounds better even though I think I’m spinning my wheels.
Believe you did a supercharged 383 with the same Scat crank if I remember right. In fact, that’s what really facilitated the idea that another crank may not be necessary.
Heads…………. That’s another story. The best that I can find……. Course finding killer heads is easier said than done. Somehow the spec sheets I’ve received never match the heads I’ve paid for.
11-26-2002, 09:38 PM
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The main bearing clearance that you will need depends upon what block you are using. A stock GM 400 block (any of them) is not very strong and the bottom ends tend to be pulled out of shape in 500+ horsepower applications. The mains will become misaligned under the loads that equate to that much power and the bearings will contact the crank. The recourse is to open the main bearing clearances up to the 0.0025+ range. The Dart, Motown or Rocket blocks have much stiffer bottom ends that stay put much better. Unless you plan to run a blower or a serious load of nitrous, stock (like .0010-.0015) main clearances will be adequate. The more limber the block the more clearance is needed.
11-26-2002, 10:19 PM
#7
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Denny, it sounds like thats just what the pressure relief spring on your oil pump is set at.. not a bearing clearance issue.
Yes, you are correct.. I have used a scat crank in a couple blower build ups.. its a stout piece.
as far as heads 'n stuff.. drop me a private email and ill give you my thoughts on it..
my primary email is down, so hit me up @ jordonmusser@hotmail.com
Yes, you are correct.. I have used a scat crank in a couple blower build ups.. its a stout piece.
as far as heads 'n stuff.. drop me a private email and ill give you my thoughts on it..
my primary email is down, so hit me up @ jordonmusser@hotmail.com
11-27-2002, 05:20 PM
#8
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It’s just a plain ol Jane two bolt LT1 block that I'm going to have splayed mains installed while I’m at it.
Not sure how much strength splayed mains actually adds, but I’d really would like to keep the clearances a bit on the tight side for a hot rod. It’s my turn to use the thin slippery oil that gives ya a bit more power.
Not sure how much strength splayed mains actually adds, but I’d really would like to keep the clearances a bit on the tight side for a hot rod. It’s my turn to use the thin slippery oil that gives ya a bit more power.
11-28-2002, 08:26 PM
#10
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odd. I bet its more than .0028.. cause it should make more presure than that.
I am gonna run about .0022 or so I think(gotta do final assembly next week) and run a standard volume pump.
I am gonna run about .0022 or so I think(gotta do final assembly next week) and run a standard volume pump.
12-01-2002, 03:16 AM
#13
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I can't remember my clearances. I'll have to check the file but it was under .025 for the crank.
I was experimenting where to shift and I've revved it to 7000rpm a few times with no problems.
Over 450hp here.
I was experimenting where to shift and I've revved it to 7000rpm a few times with no problems.
Over 450hp here.
12-01-2002, 10:10 PM
#14
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"Just make sure you use a lightweight piston, pin and rod. That'll go a long ways in insuring the life of your crank. "
mindgame, i'm a litte confused by your comment. the reason i DIDNT use JE pistons on my motor is because although it's light, it's not long lasting. That's why i used a heavier speedpro 11.9:1 pistons instead, because i wanted my motor to last a good while without a re-build (80k).
would you mind explaining why a light piston would make the crank live longer?
mindgame, i'm a litte confused by your comment. the reason i DIDNT use JE pistons on my motor is because although it's light, it's not long lasting. That's why i used a heavier speedpro 11.9:1 pistons instead, because i wanted my motor to last a good while without a re-build (80k).
would you mind explaining why a light piston would make the crank live longer?
12-02-2002, 11:53 AM
#15
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The lighter the piston, pin and rod combination is the less mass that the crank has to "reciprocate"(make switch directions). The higher the RPMs the more this causes high loads the crank which continually has to accelerate, decelerate, stop, change directions, accelerate, etc, etc, over and over again.
The higher you spin the more these forces build- and not in a stright line- it's the sqaure of RPM (or maybe it's the cube of RPM- I'm not much of a theory guy) but it gets to be a damned large amount of force if you try to spin a heavy reciprocating assembly to a high RPM. Hence, lighter is better for crank life at high RPM.
Weight and strength are constantly at odds with eachother in issues like this. Gotta be strong engough to live but it's gotta be light enough to live, too!
Regarding bearing clearances: If you're building say, 500 HP in a high RPM NA/motor generally going a little on the loose side helps. If you're going to make the same HP but at a lower RPM by building torque (nitrous, supercharger) then it's better to keep clearances tighter, in my expereince. They're different situations even though both are the same HP.
The higher you spin the more these forces build- and not in a stright line- it's the sqaure of RPM (or maybe it's the cube of RPM- I'm not much of a theory guy) but it gets to be a damned large amount of force if you try to spin a heavy reciprocating assembly to a high RPM. Hence, lighter is better for crank life at high RPM.
Weight and strength are constantly at odds with eachother in issues like this. Gotta be strong engough to live but it's gotta be light enough to live, too!
Regarding bearing clearances: If you're building say, 500 HP in a high RPM NA/motor generally going a little on the loose side helps. If you're going to make the same HP but at a lower RPM by building torque (nitrous, supercharger) then it's better to keep clearances tighter, in my expereince. They're different situations even though both are the same HP.
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