Educate me on LT1 Weaknesses
03-18-2016, 10:20 AM
#1
Educate me on LT1 Weaknesses Once you start pumping up the power, what's the first holdup? Is it airflow? Valvetrain? Bottom end? I know rod bolts need to be addressed, but how much of a gain in RPM headroom does that buy you? At what RPM are stock bolts a liability? Can they be swapped without yanking pistons out? 2 bolt/4 bolt/4 bolt splayed? When do the mains start to walk? Valvetrain? If I replace pushrods, springs, and go roller rockers how much RPM headroom does that give me? When does stock become a limiting factor and which individual parts are the most vulnerable? Would I be accurate in saying airflow becomes a serious factor around 330whp with an M6? How much lift before valves start dropping? Are certain heads more prone than others? In the LS world, lifts above .600" can be a little hit and miss when used for road racing. Seems if you just leave an LS1/LS6 bone stock internally, they're about nuke proof. Last time I started poking around, it looked like 360whp was pretty doable with some TFS heads and a mild cam. How long would a stock bottom end live on that on a road course? Some of my questions are gee whiz, and some apply to a potential build down the road.
03-18-2016, 11:14 AM
#3
TECH Regular
That is a lot of very general questions, you could get ten different answers for each one. The better question might be "....this is what I want to do and this is how much power I want to make. What do I need to upgrade?"
As for some general info, here goes.
Stock crank is very strong, good to at least 600rwhp in most applications. I've seen folks turn them to 8000rpm without a problem too.
Stock rod bolts can become an issue anywhere over about 6300rpm when the motor sees a lot of time at that rpm. Some folks have used them all the way to 7000 successfully in street cars, but street cars typically don't see lots of time at very high rpm.
In most LT1 cars these days with stock motors, you're talking about 20-year-old engines that often already have over 100k miles on them. Bearing failure is always a possibility when camming these engines and spinning them several hundred rpm past their factory limit of 5800rpm simply because everything has a couple decades and many miles worth of wear on it.
Maximum lift depends on what type of valvetrain components you're running, just like in any other engine. Simple as that. If you want to run lobes with lots of lift, you'll need high-quality valvetrain components.
How long a stock bottom end lasts on a road course depends on what type of top-end setup you're running, how many rpm you're turning it to and what condition it's in to begin with. A completely stock, well-maintained engine that stays within the factory-prescribed rev limit would likely last indefinitely.
For another example, my car. I have a stock, untouched but well-maintained 113k mile bottom end with a fairly aggressive heads/cam setup making a bit over 400rwhp. Peak power is around 6200rpm, the rev limiter is at 6600. On the street, it only occasionally sees over 6000rpm and when it does, it's only for a few seconds. However, if I were to put it on a road course and turn it that high hundreds of times every weekend for miles on end, it'd likely dramatically shorten it's life.
As for some general info, here goes.
Stock crank is very strong, good to at least 600rwhp in most applications. I've seen folks turn them to 8000rpm without a problem too.
Stock rod bolts can become an issue anywhere over about 6300rpm when the motor sees a lot of time at that rpm. Some folks have used them all the way to 7000 successfully in street cars, but street cars typically don't see lots of time at very high rpm.
In most LT1 cars these days with stock motors, you're talking about 20-year-old engines that often already have over 100k miles on them. Bearing failure is always a possibility when camming these engines and spinning them several hundred rpm past their factory limit of 5800rpm simply because everything has a couple decades and many miles worth of wear on it.
Maximum lift depends on what type of valvetrain components you're running, just like in any other engine. Simple as that. If you want to run lobes with lots of lift, you'll need high-quality valvetrain components.
How long a stock bottom end lasts on a road course depends on what type of top-end setup you're running, how many rpm you're turning it to and what condition it's in to begin with. A completely stock, well-maintained engine that stays within the factory-prescribed rev limit would likely last indefinitely.
For another example, my car. I have a stock, untouched but well-maintained 113k mile bottom end with a fairly aggressive heads/cam setup making a bit over 400rwhp. Peak power is around 6200rpm, the rev limiter is at 6600. On the street, it only occasionally sees over 6000rpm and when it does, it's only for a few seconds. However, if I were to put it on a road course and turn it that high hundreds of times every weekend for miles on end, it'd likely dramatically shorten it's life.
Last edited by HellTeeOne; 03-18-2016 at 11:22 AM.
03-18-2016, 03:42 PM
#4
I'll just address one of the many: rod bolts
If you price out the cost of replacing rod bolts you're basically spending the same amount of money as buying new rods. Used to be common practice until quality affordable aftermarket rods became available.
I had one of my rods stretch the bolts around 130k miles with tons of racing and nitrous, later in it's life an aftermarket cam. They are a well addressed weakness but then again I put that motor through hell and back so it's not like it happened the moment I started racing it but years later.
There really is no magic safe number of horsepower or rpm since every setup, tune, and use is different from the next. Road racing will be a huge stress on any motor due to constant rpm fluctuations, oil starvation under heavy braking/turning, extensive run time, etc.
The fact that it is a 20+/- year old motor with an unknown past I wouldn't be surprised if it didn't last a long time, I also wouldn't be surprised if it went for years with no issues. A quality refreshing of the valvetrain and bottom end would guarantee your many years of service though.
If you price out the cost of replacing rod bolts you're basically spending the same amount of money as buying new rods. Used to be common practice until quality affordable aftermarket rods became available.
I had one of my rods stretch the bolts around 130k miles with tons of racing and nitrous, later in it's life an aftermarket cam. They are a well addressed weakness but then again I put that motor through hell and back so it's not like it happened the moment I started racing it but years later.
There really is no magic safe number of horsepower or rpm since every setup, tune, and use is different from the next. Road racing will be a huge stress on any motor due to constant rpm fluctuations, oil starvation under heavy braking/turning, extensive run time, etc.
The fact that it is a 20+/- year old motor with an unknown past I wouldn't be surprised if it didn't last a long time, I also wouldn't be surprised if it went for years with no issues. A quality refreshing of the valvetrain and bottom end would guarantee your many years of service though.
Last edited by myltwon; 03-18-2016 at 03:50 PM.
03-18-2016, 04:25 PM
#5
I know there's no magic number as some motors outlast others just like some make more power than others. I should have been more specific though, this would never be with a 100k junkyard motor. The motor I have now is a rehone with all virgin internals (actually found a guy who parts out stock motors), polished crank, and new crank/rod bearings. At the time of its demise, it was an otherwise clean low mileage (60k) engine. Were I to cam this one up, my thinking was rod bolts (which, with some recent googling, shows resizing is required 
) and cam bearings would be addressed as well as pushrods, springs, and roller rockers. Kind of wondering what the expected life span of that would be keeping it under 6500. HellTeeOne mentioned the crank being a nice, stout piece which is great. I'll never build an LT1 to make the kind of power it'd take to break a crank. I am quite curious about main cap walk though. When does that become an issue and is a stock 4 bolt the least bit better than a 2 bolt when it comes to that?
) and cam bearings would be addressed as well as pushrods, springs, and roller rockers. Kind of wondering what the expected life span of that would be keeping it under 6500. HellTeeOne mentioned the crank being a nice, stout piece which is great. I'll never build an LT1 to make the kind of power it'd take to break a crank. I am quite curious about main cap walk though. When does that become an issue and is a stock 4 bolt the least bit better than a 2 bolt when it comes to that?
03-18-2016, 04:44 PM
#6
Here's some data for you... ...take it for what it's worth.
Limit revs to 6500 instead of 7100 as I did, and it would give you a long service life.
https://ls1tech.com/forums/lt1-lt4-m...alvetrain.html
Limit revs to 6500 instead of 7100 as I did, and it would give you a long service life.
https://ls1tech.com/forums/lt1-lt4-m...alvetrain.html
03-18-2016, 05:21 PM
#7
I know there's no magic number as some motors outlast others just like some make more power than others. I should have been more specific though, this would never be with a 100k junkyard motor. The motor I have now is a rehone with all virgin internals (actually found a guy who parts out stock motors), polished crank, and new crank/rod bearings. At the time of its demise, it was an otherwise clean low mileage (60k) engine. Were I to cam this one up, my thinking was rod bolts (which, with some recent googling, shows resizing is required ) and cam bearings would be addressed as well as pushrods, springs, and roller rockers. Kind of wondering what the expected life span of that would be keeping it under 6500. HellTeeOne mentioned the crank being a nice, stout piece which is great. I'll never build an LT1 to make the kind of power it'd take to break a crank. I am quite curious about main cap walk though. When does that become an issue and is a stock 4 bolt the least bit better than a 2 bolt when it comes to that?
) and cam bearings would be addressed as well as pushrods, springs, and roller rockers. Kind of wondering what the expected life span of that would be keeping it under 6500. HellTeeOne mentioned the crank being a nice, stout piece which is great. I'll never build an LT1 to make the kind of power it'd take to break a crank. I am quite curious about main cap walk though. When does that become an issue and is a stock 4 bolt the least bit better than a 2 bolt when it comes to that?Trending Topics
03-19-2016, 11:44 AM
#8
Long technical post comparing apples to oranges. Reasons you can't spin a stock LT1 like you can spin a stock Gen 3:
Stock LT1 pistons are are a lot heavier than stock LS1 and a lot of this has to do with larger compression height. This paired with the shorter deck height of the LT1 means the rods must be shorter-- shorter rods translates to more cylinder side loading (a result of increased rod angularity).
Increased stroke is harder on an engine because of more piston acceleration, rod angularity, side loading, etc. Even though the LT1 has less stroke than a stock LS1/LS6 (3.48 vs 3.622), the added deck height, longer rod length, and lighter pistons of the Gen 3's more than makes up for it.
Gen 3's also have cracked cap rods which makes the big ends a little more stable. Can't resize them though. Our rod bolts are weaker for a high RPM NA application. If you made the piston lighter (less compression height) and used longer rods in a high RPM NA app I don't think it would be as much of a problem.
Not only is head flow better on the Gen 3's because of 15* valve angles and straighter ports, they can also handle more aggressive cam profiles. This is mainly because the base circle of the cam is a lot bigger and greatly reduces the pressure angle on the lifter. They're also billet steel and gun drilled. The valves and springs the cam has to control are also a lot lighter generally speaking which is less hard on everything.
Other little things:
-The ring pack is also thicker on the LT1... more drag.
-The stock LS1 crank is also gun drilled which decreases rotational weight.
-6-bolt mains... The deep skirt block is a nice reinforcement but I think this aspect is overhyped. The stock straight 4-bolt caps on the Vette LT motors can take a lot of punishment. Beyond that, a good billet steel cap with splayed mains wouldn't be a limitation.
-Firing order of the LS motors is also improved. Less crank torsion and deflection.
The few things the LTx has to brag about:
-Reverse cooling. Only reason GM dropped this is because they stole it from Evan's Cooling Systems and a huge lawsuit ensued. LT1/4 can handle more compression.
-5 bolts per cylinder. Deck stability becomes a problem with the stock LS blocks when you get deeper into 4 digit power numbers. This is why things like the ERL superdecks and aftermarket 6 bolt blocks exist.
Stock LT1 pistons are are a lot heavier than stock LS1 and a lot of this has to do with larger compression height. This paired with the shorter deck height of the LT1 means the rods must be shorter-- shorter rods translates to more cylinder side loading (a result of increased rod angularity).
Increased stroke is harder on an engine because of more piston acceleration, rod angularity, side loading, etc. Even though the LT1 has less stroke than a stock LS1/LS6 (3.48 vs 3.622), the added deck height, longer rod length, and lighter pistons of the Gen 3's more than makes up for it.
Gen 3's also have cracked cap rods which makes the big ends a little more stable. Can't resize them though. Our rod bolts are weaker for a high RPM NA application. If you made the piston lighter (less compression height) and used longer rods in a high RPM NA app I don't think it would be as much of a problem.
Not only is head flow better on the Gen 3's because of 15* valve angles and straighter ports, they can also handle more aggressive cam profiles. This is mainly because the base circle of the cam is a lot bigger and greatly reduces the pressure angle on the lifter. They're also billet steel and gun drilled. The valves and springs the cam has to control are also a lot lighter generally speaking which is less hard on everything.
Other little things:
-The ring pack is also thicker on the LT1... more drag.
-The stock LS1 crank is also gun drilled which decreases rotational weight.
-6-bolt mains... The deep skirt block is a nice reinforcement but I think this aspect is overhyped. The stock straight 4-bolt caps on the Vette LT motors can take a lot of punishment. Beyond that, a good billet steel cap with splayed mains wouldn't be a limitation.
-Firing order of the LS motors is also improved. Less crank torsion and deflection.
The few things the LTx has to brag about:
-Reverse cooling. Only reason GM dropped this is because they stole it from Evan's Cooling Systems and a huge lawsuit ensued. LT1/4 can handle more compression.
-5 bolts per cylinder. Deck stability becomes a problem with the stock LS blocks when you get deeper into 4 digit power numbers. This is why things like the ERL superdecks and aftermarket 6 bolt blocks exist.
Last edited by Catmaigne; 03-19-2016 at 02:46 PM.