questions about LT1 SR motor and power
08-23-2010, 03:48 PM
#1
questions about LT1 SR motor and power A buddy of mine built a solid roller 383 LT1. It has fully ported LT4 heads, a fully ported LT4 intake. The SR camshaft specs are: 248/256 .609/.609 with a 20 gap on a 110LSA. Which lift was at .589 after you account for valve lash. He still has stud mounted 1.6 RR. Everything along the valve train is within spec to meet the big camshafts needs. He's running a brand new MSD optispark along with TR6 plugs.
We got the SR motor done a month or two ago and sent it to the tuner. Last time it was presenting a similar breakup as mine did when it was tuned. It was jumping a lot around 6300-6500. So he replaced the opti and now here we are. Also, we both have relocated the coil to the back side of our intakes.
Now here's the problem, the car is STILL lacking A LOT of power. In my opinion on a 383 SR motor he should be in the 450+ range (470+ being my initial guess).
The questions:
Would relocating the coil affect anything? It's bolted to the back side of the intake and throughout the RPM range acts fine until jerking and bucking at 6300+.
Is this SR camshaft too large for a 383 LT1? Are the heads and intake just not efficient enough to work with it?
He is running 3/8" stud mounted 1.6 RR. Do you think the studs could be distorting and not opening the valves enough? Do you think they just aren't strong enough?
We're all blow out of the water in curiosity on why his car isn't putting down the numbers it should.
Opinions and suggestions welcomed.
We got the SR motor done a month or two ago and sent it to the tuner. Last time it was presenting a similar breakup as mine did when it was tuned. It was jumping a lot around 6300-6500. So he replaced the opti and now here we are. Also, we both have relocated the coil to the back side of our intakes.
Now here's the problem, the car is STILL lacking A LOT of power. In my opinion on a 383 SR motor he should be in the 450+ range (470+ being my initial guess).
The questions:
Would relocating the coil affect anything? It's bolted to the back side of the intake and throughout the RPM range acts fine until jerking and bucking at 6300+.
Is this SR camshaft too large for a 383 LT1? Are the heads and intake just not efficient enough to work with it?
He is running 3/8" stud mounted 1.6 RR. Do you think the studs could be distorting and not opening the valves enough? Do you think they just aren't strong enough?
We're all blow out of the water in curiosity on why his car isn't putting down the numbers it should.
Opinions and suggestions welcomed.
Last edited by tbird31; 08-23-2010 at 05:54 PM.
08-23-2010, 04:00 PM
#2
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3/8" studs are far too weak to be handling a SR camshaft. That cam is pretty close to what I am running and I have springs that are 250 on the seat and around 500 on the nose. I would never trust a 3/8" rocker to handle that kind of load. He needs to step up and get 7/16" studs due to his spring pressure needing to be around that range. Probably a real good idea to take the valvetrain apart and examine all components closely for damage just to be on the safe side. What kind of fuel pump is on the car? I wouldn't suggest putting the coil on the back side of the engine. There is a lot of heat in that area and could cause the coil to prematurely fail. This car has been dynoed? What does the curve look like? That can tell a lot.
08-23-2010, 04:29 PM
#5
That's what I assumed with the rocker studs. The car only has about 100 miles on it since the build. It has been dynoed but I do not have the chart unfortunately. He is running a Walbro 255 in-tank pump through stock lines. The valve train was completely gone through before putting it on the motor, do you think it's necessary to pull it all back apart? Or should be just upgrade to 7/16 studs and roll on our day. He does not want to rev the car over 7k, just wants a nice cruise around/show car. He MIGHT drive it 1000 miles a year...
Also, the stock optispark has been gone for years. The optispark unit he had to replace was an MSD one that MIGHT not have been bad in the first place, but since MSD offered to replace it free, we didn't pass it up and replaced it.
Also, the stock optispark has been gone for years. The optispark unit he had to replace was an MSD one that MIGHT not have been bad in the first place, but since MSD offered to replace it free, we didn't pass it up and replaced it.
08-23-2010, 05:11 PM
#6
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"I have a 96 lt4 vette with a 383, 10.5 comp, heads, cam, long tubes, etc. made 414 rwhp. I had a digital 6 box, on it and it was running great until my stock opti took a crap. I put an MSD opti on it and now it has a high rpm hesitation misfire around 6000-6300rpm. It just falls on it face like it has too much timig in it. I played with the advance screw and it breakups the least with about 3 1/2 turns out. I have tuned the car and played with the timing / fuel maps but it is still breaking up on the big end. Plugs, wires, coil are new and working fine. Any help is appreciated."
Looks like its in the same RPM range on this setup as well. If you know anyone with a stock unit, check it out to eliminate suspects in your setup. Also at this point I would consider testing sensors in your signal chain, codes?
Looks like its in the same RPM range on this setup as well. If you know anyone with a stock unit, check it out to eliminate suspects in your setup. Also at this point I would consider testing sensors in your signal chain, codes?
08-23-2010, 05:54 PM
#7
yea...I guess it is an option to throw a stock opti back on there and see if that's the issue. That guy with the 96 LT4 Vette though has interesting numbers. My 355ci motor with a somewhat large camshaft and 11.5:1 CR made 408rwhp on a 100* day and a trashed MAF. I cleaned my MAF and I'll throw new plugs in it and I think I'll see 415-420rwhp just on a cooler day. lol.
The SR camshaft specs are: 248/256 .609/.609 with a 20 gap on a 110LSA. Which lift was at .589 after you account for valve lash.
I'm curious as well do you think the plug choice is wrong? Should we go back to a TR55 from the TR6 we're running now?
The SR camshaft specs are: 248/256 .609/.609 with a 20 gap on a 110LSA. Which lift was at .589 after you account for valve lash.
I'm curious as well do you think the plug choice is wrong? Should we go back to a TR55 from the TR6 we're running now?
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08-23-2010, 07:34 PM
#9
It's at a .248/.256 duration...the springs I have no idea, but they are capable of holding the lift #'s because he was pissed he had to replace his old springs. Dyno graph I don't have, I do know that it dropped 100hp at 4500ish rpm then recovered.
I don't believe the missfire is caused by the lack of strength in the studs either but I do believe it's a contributing factor to the lack of power it made.
I don't believe the missfire is caused by the lack of strength in the studs either but I do believe it's a contributing factor to the lack of power it made.
08-23-2010, 09:46 PM
#10
It's at a .248/.256 duration...the springs I have no idea, but they are capable of holding the lift #'s because he was pissed he had to replace his old springs. Dyno graph I don't have, I do know that it dropped 100hp at 4500ish rpm then recovered.
I don't believe the missfire is caused by the lack of strength in the studs either but I do believe it's a contributing factor to the lack of power it made.
I don't believe the missfire is caused by the lack of strength in the studs either but I do believe it's a contributing factor to the lack of power it made.
08-23-2010, 10:16 PM
#11
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He's going to be even more pissed to find out he'll have to replaced the rockers to accommodate 7/16" studs. He'll be even more pissed if he just leaves it alone, starts having geometry issues and drops a valve.
Was this dynoed on a Mustang or Dynojet or etc...?
Was this dynoed on a Mustang or Dynojet or etc...?
08-24-2010, 08:01 AM
#13
With a cam that big he'd want to run a girdle as well to make it sturdier but not sure how possible it is with an LT4 head. You'd need to fab up a way to get the valve covers on. LT4 heads don't have provisions for perimeter bolts. A girdle is cheap maybe $150-$200 and adds a lot of stability. Ideally shaft rockers are always the best choice but they can be pricey.
I seriously doubt that the coil is causing the issue. It would need a LOT of heat soak to get hot enough to fire erratically, and you are just doing a few dyno pulls with the hood up. Usually when a coil fails it simply doesn't work at all, meaning the car simply shuts off. Try to run a fuel pressure check at WOT and see what happens. When my opti went my spark was scattering at anything above 6000rpm. Anything below that and it was clean as a whistle. I put in the MSD opti and it cured it 100%.
How does the power look at 5500rpm? A motor like that should still be well above 400hp at that point.
I seriously doubt that the coil is causing the issue. It would need a LOT of heat soak to get hot enough to fire erratically, and you are just doing a few dyno pulls with the hood up. Usually when a coil fails it simply doesn't work at all, meaning the car simply shuts off. Try to run a fuel pressure check at WOT and see what happens. When my opti went my spark was scattering at anything above 6000rpm. Anything below that and it was clean as a whistle. I put in the MSD opti and it cured it 100%.
How does the power look at 5500rpm? A motor like that should still be well above 400hp at that point.
08-24-2010, 09:10 AM
#14
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A stud girdle creates more problems than they solve. They are junk. To install them takes the most intricate of adjustments to keep the lash true. It's not worth the trouble. Properly built, the valvetrain will be able to do just fine on its own with no outside help. If a girdle is to be considered then just forget it and go with shaft mounts.
08-24-2010, 09:32 AM
#15
ok....ive got some decent experience with the SR stuff heres what im thinking.......
1. Those 3/8" studs are never going to make it on that motor loose them for some 7/16" at minimum....but at that point you have to buy studs plates and rockers.....might as well spend the money and put a shaft setup on it and call it a day I think jegs has the Jessel Sportsmans for like 890. This is a durability issue but its not causing the breakup in the motor.
2. The opti could be subject as well as the ICM on the cylinder head. Im saying its not the coil becuase it would be doing it a a lower rpm under load as well. If the opti is infact good (which ive seen two bad ones in a row before) then I'd start looking into fuel. Is the pressure remaining steady through the rpm range? Also what is in the car for injectors?
3. What is the spring pressure at the seat and over the nose on the valve springs? (could be starting to float)
4. How did you adjust the lash on the motor? (ie. what procedure did you use for lashing the valves??)
5. What is the duration at .004 and what is the lift at TDC on the cam and the LSA?
6. Is the cam installed straight up? Retarded or advanced?
7. How bout the tune who did it and what does it look like? Timing/AFR?
Answer those and we may be able to get closer on this.....hope i can help
and just to add this......I've got a stock opti on mine with an msd cap and rotor.....it worked fine untill 7200 and i started misfiring....i switched to a msd crank trigger for the signal now but still retain the opti to send the spark......i take my car to 8000+ and it runs fine......so the opti will last if it is built good and correctly....
1. Those 3/8" studs are never going to make it on that motor loose them for some 7/16" at minimum....but at that point you have to buy studs plates and rockers.....might as well spend the money and put a shaft setup on it and call it a day I think jegs has the Jessel Sportsmans for like 890. This is a durability issue but its not causing the breakup in the motor.
2. The opti could be subject as well as the ICM on the cylinder head. Im saying its not the coil becuase it would be doing it a a lower rpm under load as well. If the opti is infact good (which ive seen two bad ones in a row before) then I'd start looking into fuel. Is the pressure remaining steady through the rpm range? Also what is in the car for injectors?
3. What is the spring pressure at the seat and over the nose on the valve springs? (could be starting to float)
4. How did you adjust the lash on the motor? (ie. what procedure did you use for lashing the valves??)
5. What is the duration at .004 and what is the lift at TDC on the cam and the LSA?
6. Is the cam installed straight up? Retarded or advanced?
7. How bout the tune who did it and what does it look like? Timing/AFR?
Answer those and we may be able to get closer on this.....hope i can help
and just to add this......I've got a stock opti on mine with an msd cap and rotor.....it worked fine untill 7200 and i started misfiring....i switched to a msd crank trigger for the signal now but still retain the opti to send the spark......i take my car to 8000+ and it runs fine......so the opti will last if it is built good and correctly....
08-24-2010, 09:53 AM
#16
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With a cam that big he'd want to run a girdle as well to make it sturdier but not sure how possible it is with an LT4 head. You'd need to fab up a way to get the valve covers on. LT4 heads don't have provisions for perimeter bolts. A girdle is cheap maybe $150-$200 and adds a lot of stability. Ideally shaft rockers are always the best choice but they can be pricey.
http://www.summitracing.com/parts/MOR-67045/
08-24-2010, 10:41 AM
#17
ok....ive got some decent experience with the SR stuff heres what im thinking.......
1. Those 3/8" studs are never going to make it on that motor loose them for some 7/16" at minimum....but at that point you have to buy studs plates and rockers.....might as well spend the money and put a shaft setup on it and call it a day I think jegs has the Jessel Sportsmans for like 890. This is a durability issue but its not causing the breakup in the motor.
1. Those 3/8" studs are never going to make it on that motor loose them for some 7/16" at minimum....but at that point you have to buy studs plates and rockers.....might as well spend the money and put a shaft setup on it and call it a day I think jegs has the Jessel Sportsmans for like 890. This is a durability issue but its not causing the breakup in the motor.
2. The opti could be subject as well as the ICM on the cylinder head. Im saying its not the coil becuase it would be doing it a a lower rpm under load as well. If the opti is infact good (which ive seen two bad ones in a row before) then I'd start looking into fuel. Is the pressure remaining steady through the rpm range? Also what is in the car for injectors?
To my knowledge the cam was installed straight up with NO retard or advance.
08-25-2010, 07:37 AM
#19
I will find out...
The car has a Hot Wire kit on it, and he's got 36lb injectors
I will find out...
The builder gave us a tool to use to stick between the rocker and pushrod to adjust lash.
I will find out...
To my knowledge the cam was installed straight up with NO retard or advance.
The tune was done by a well known tuner, Alvin at PCM 4 Less. He's done all the cars we have built, as well as my car and has always done a top knotch job.
The car has a Hot Wire kit on it, and he's got 36lb injectors
I will find out...
The builder gave us a tool to use to stick between the rocker and pushrod to adjust lash.
I will find out...
To my knowledge the cam was installed straight up with NO retard or advance.
The tune was done by a well known tuner, Alvin at PCM 4 Less. He's done all the cars we have built, as well as my car and has always done a top knotch job.
It should've been done like this.....
Rotate the motor till the exhaust valve begins to open....at this point adust the intake valve lash.
Then rotate the motor till the intake valve starts to return from full lift....at this point adjust the exhaust valve lash.
Doing it this way will ensure that you are on the bottom of the lobe of the cam. Adjusting them both while both valves are closed does not ensure that you are on the bottom of the lobe. (Just an FYI)
08-25-2010, 02:35 PM
#20
Alot of good advice here. You may want to check out the knock sensor settings in the PCM as well. I've had to completely turn off all KS input on my SR motors. I'd be ditching those 3/8" studs/rockers asap. Make your life alot easier and just buy a set of shaftmounts. Stud girdles help, but as mentioned, they are TOUGH to get everything dialed in. I messed with two different girdle combinations on past motors, and I could never get them just right (oil leaks). It's a pain with centerbolt heads. Perimeter style would probably be easier.
A local friend and I spent months trouble shooting a similar problem (power dip at 4500rpms) on a HR car this past winter. After looking at absolutely everything, the problem was completely solved by going to the EFI connection LS1 24x PCM setup. We tested, analyzed, and replaced every electrical and mechanical variable in the motor/ignition followed by dyno testing each time. Some unknown qwirk in the LT1 PCM/harness. It was the most odd problem we have ever encountered.
A local friend and I spent months trouble shooting a similar problem (power dip at 4500rpms) on a HR car this past winter. After looking at absolutely everything, the problem was completely solved by going to the EFI connection LS1 24x PCM setup. We tested, analyzed, and replaced every electrical and mechanical variable in the motor/ignition followed by dyno testing each time. Some unknown qwirk in the LT1 PCM/harness. It was the most odd problem we have ever encountered.