BUILD THREAD 86 GP 2+2~Blown 6.0

motorheadmike

Geezer
Nov 18, 2009
7,108
113
Ottawa, Ontario Canada
The 4.8s live longer lives because they can typically take more RPM, which if the shift points are done correctly, will keep the engine above MBT. Below MBT there are huge problems with cylinder pressure effecting metallurgy.
 
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Northernregal

Sloppy McRodbender
Supporting Member
Oct 24, 2017
2,126
113
Red Deer AB
I don't know why folks are on about bent rods? If you sneak up on the limit, the knock sensors can save your @$$ with a stock PCM. Even if not, I'd see detonating and blowing ring lands/pistons first.
Respectfully disagree...

Bent a gen 4, 10psi on pump 91. It's easier than you think.
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,943
113
Upstate NY
The 4.8s live longer lives because they can typically take more RPM, which if the shift points are done correctly, will keep the engine above MBT. Below MBT there are huge problems with cylinder pressure effecting metallurgy.

Never seen that abbreviation, but from the comment it has something to do peak tq rpm number. And I agree fully. As mentioned before, the majority of the damage to rods, bearings and ring lands is done at or before peak torque rpm in a boosted application. Matt H has documented this very well over the course of the last 3-5 years. It's pretty easy with E or race gas to make 500 or so wheel hp before peak torque rpm by adding gobs of boost before then, and then making all of the power (HP) 500ish or so rpm's above that. You can see it in some of his tune files where there is more timing above 5500 than below. The idea of more timing with more boost and rpm is contrary to old school thought process of cutting timing by boost number, i.e. take out 1 degree per each psi of boost from a base WOT timing number when NA. In a nutshell, he was minimizing cylinder pressure by setting the timing to a stupid safe number below peak torque rpm. He's definitely leaving some power on the table below 5000 rpm's, but the idea is to not even get close to the maximum attainable HP in that rpm range so as to keep the motor in tact.

Another reason the 4.8's are 'easy' on the rods and bearings is because when they hit the peak torque rpm at a given boost level, the peak TQ number is a bunch lower than that of a 5.3 and 6.0. Which really lightens the burden on the rod and bearing as well. I would estimate 75-150 ft/lbs less with a 4.8 than 5.3/60 combo depending on the cam if using stock heads.

I've applied this thought process to my own stuff as well, and from looking at a datalog from a strip run, you can see how little bit of time is spent in the 4000's ideally. In my previous post I mentioned to Jake to not add any timing based on the rpm range he's running despite the advice he's received indicating that he could add another 8 degrees+. Now seeing where his rpm range is, I'd reckon ihis 12 degrees of timing is the only reason his rods are still straight.

I don't know why folks are on about bent rods? If you sneak up on the limit, the knock sensors can save your @$$ with a stock PCM. Even if not, I'd see detonating and blowing ring lands/pistons first.
You are correct about sneaking up on the tune, but you are living life on the edge tuning against a knock sensor below peak torque - that respectfully will not work. It won't work with a Haltech or Holley, let alone a stock PSM. I have to admit though, used to think the same until I busted a ring land. I had an aftermarket knock sensor, Knocksense was the brand, that worked fabulously for tuning for mileage in the 40-80 kpa range. When I broke the ring land, I seen the light come on at approx 10psi, by the time I lifted the damage was done - that a SBC with a set of KB hypers. And I was on the program of dropping 1 degree per each psi. That had worked on the dyno very well, but a dyno doesn't tune boost at 3000 rm's very well - as I learned when I broke the ring land. The reason I broke a ring land rather bending the rod was because the rods were not bendy rds - Crower H-beams that were way overkill for that motor. I wadded/destroyed the bearings as well with that tune.

I 110% agree with Jake's plan as to how to get his car cranked up. The benefit of the LS over conventional a SBC, BBC, etc. is it's ability to spin a lot of rpm's on stock components - hence the need to try to not make much power until they see a lot of rpm's. Trying to spin a BBC on a stock rod to 7K is going to be a short lived program obviously. I don't see any reason why the 2+2 is not able to be seeing consistent sub 10.5 runs once you get this stuff taken care of. And perhaps much faster.


p.s. anyone notice my use of 'reckon' - I am very proud of that ;)
 

Supercharged111

Royal Smart Person
Oct 25, 2019
2,177
113
Colorado Springs, CO
Respectfully disagree...

Bent a gen 4, 10psi on pump 91. It's easier than you think.

Weird. What I've experienced, and the people I know, has been more losing pistons to cylinder pressure and rods to hydrolock. The GenIV is supposed to have some stout rods too.

Never seen that abbreviation, but from the comment it has something to do peak tq rpm number. And I agree fully. As mentioned before, the majority of the damage to rods, bearings and ring lands is done at or before peak torque rpm in a boosted application. Matt H has documented this very well over the course of the last 3-5 years. It's pretty easy with E or race gas to make 500 or so wheel hp before peak torque rpm by adding gobs of boost before then, and then making all of the power (HP) 500ish or so rpm's above that. You can see it in some of his tune files where there is more timing above 5500 than below. The idea of more timing with more boost and rpm is contrary to old school thought process of cutting timing by boost number, i.e. take out 1 degree per each psi of boost from a base WOT timing number when NA. In a nutshell, he was minimizing cylinder pressure by setting the timing to a stupid safe number below peak torque rpm. He's definitely leaving some power on the table below 5000 rpm's, but the idea is to not even get close to the maximum attainable HP in that rpm range so as to keep the motor in tact.

Another reason the 4.8's are 'easy' on the rods and bearings is because when they hit the peak torque rpm at a given boost level, the peak TQ number is a bunch lower than that of a 5.3 and 6.0. Which really lightens the burden on the rod and bearing as well. I would estimate 75-150 ft/lbs less with a 4.8 than 5.3/60 combo depending on the cam if using stock heads.

I've applied this thought process to my own stuff as well, and from looking at a datalog from a strip run, you can see how little bit of time is spent in the 4000's ideally. In my previous post I mentioned to Jake to not add any timing based on the rpm range he's running despite the advice he's received indicating that he could add another 8 degrees+. Now seeing where his rpm range is, I'd reckon ihis 12 degrees of timing is the only reason his rods are still straight.


You are correct about sneaking up on the tune, but you are living life on the edge tuning against a knock sensor below peak torque - that respectfully will not work. It won't work with a Haltech or Holley, let alone a stock PSM. I have to admit though, used to think the same until I busted a ring land. I had an aftermarket knock sensor, Knocksense was the brand, that worked fabulously for tuning for mileage in the 40-80 kpa range. When I broke the ring land, I seen the light come on at approx 10psi, by the time I lifted the damage was done - that a SBC with a set of KB hypers. And I was on the program of dropping 1 degree per each psi. That had worked on the dyno very well, but a dyno doesn't tune boost at 3000 rm's very well - as I learned when I broke the ring land. The reason I broke a ring land rather bending the rod was because the rods were not bendy rds - Crower H-beams that were way overkill for that motor. I wadded/destroyed the bearings as well with that tune.

I 110% agree with Jake's plan as to how to get his car cranked up. The benefit of the LS over conventional a SBC, BBC, etc. is it's ability to spin a lot of rpm's on stock components - hence the need to try to not make much power until they see a lot of rpm's. Trying to spin a BBC on a stock rod to 7K is going to be a short lived program obviously. I don't see any reason why the 2+2 is not able to be seeing consistent sub 10.5 runs once you get this stuff taken care of. And perhaps much faster.


p.s. anyone notice my use of 'reckon' - I am very proud of that ;)

What's the deal with under MBT being worse than over? I always figured near MBT was the big danger zone.
 

81cutlass

Comic Book Super Hero
Thread starter
Feb 16, 2009
3,298
113
Western MN
I washed the car off Saturday AM and headed to the cruise one of the guys from the local LS groups was hosting.

3 cars showed up. One was the photo guy who had a burning oil VW jetta and the other was the host who had a 4th gen Camaro.

The cruise dates back 10-15 years when the local LS forum used to exist and they had a 20+ car turnout. The problem is all the people who used to go to the cruise turned their street/strip cars into 7 and 8 second track oriented cars, people got out of the game and as a result nobody goes anymore. 2 of the dudes in the photo car just wanted to sit on the patio and drink so we stopped for lunch and the wife and I decided to just go back home.

Anyways put on like 150 miles and it didn't overheat, handles good and there isn't anything fishy.

The 5401 rear springs I put in are too soft however. It bottoms out hard on bumps. The ride height is awesome but I think the car has too much weight with the big rear window for those springs. I need to do some digging and find the next catalog up or two spring rate. They are probably good on a stripped interior malibu but not for my tank.

Also my fuel table is all jacked now. Now that the base fuel pressure is at 60 and it's been tuned on a decreasing pressure ramp through boost for all these years it goes pig rich. I need to spend a few hours dialing it back in.

And no word on shocks yet. Turnout at the track Saturday was great with killer weather so it was a bummer my shocks didn't turn up and I've got another month before I can get the thing to the track.

So I am at the weird point in the build were I can't do much until I find out how it does at the track.

So the honey-do list happened and my garage got infested with wood.
1620068160325.png


Oh and I stumbled on this today. Halp. I am getting distracted.
1620067342916.png
 

64nailhead

Goat Herder
Supporting Member
Dec 1, 2014
2,943
113
Upstate NY
Weird. What I've experienced, and the people I know, has been more losing pistons to cylinder pressure and rods to hydrolock. The GenIV is supposed to have some stout rods too.



What's the deal with under MBT being worse than over? I always figured near MBT was the big danger zone.

Cylinder pressure is greatest at peak torque with maximum timing. So it's really easy to break pistons just before and at peak torque because peak torque is where maximum cylinder pressure occurs if yo have timing maximized for HP. The way to cheat this is to get past the peak torque rpm and minimize cylinder pressure around peak torque by dropping the timing 5-8 degrees from optimal. Once you get past that rpm level, then start feeding the timing back in to the peak HP rpm. I want to be clear, this is not the way to make the biggest HP curve throughout the entire operating range, but it is a way to keep the ring lands in tact and the rods in the block. And the idea is that when you're at the strip that you are over the peak torque rpm all of the time. This is why I think 81cutlass should move his rpm window up so as to stay over 5K when he shifts.

Oh and I stumbled on this today. Halp. I am getting distracted.
View attachment 174507

OK, I see that Craigslist is like p*rn to you - can't stop once you start (or so I've been told.) But I do believe you need another project or two, I'm sure your wife agrees.
 

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