05 2500-hd part out time for its 8.1/allison combo . 101,998 miles on it .
Going in a gmt400 3500-hd project .
Going in a gmt400 3500-hd project .
CK5
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L18 8.1L swap resource thread
- Thread starter Larry
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Got the rusted out at rad oil cooler line bypassed. Fired up and added trans fluid . Lot drives no codes or light for trans .
Yes light is on but its for all the other missing stuff .
Great oil pressure on used black oil it has in it . Fires right up . No noises other than rusty pulleys and belt chirp . . . Guess the fan was moving the air good on 2nd video .
Cant wait to pull it and get it ready for install in her new home .
Yes light is on but its for all the other missing stuff .
Great oil pressure on used black oil it has in it . Fires right up . No noises other than rusty pulleys and belt chirp . . . Guess the fan was moving the air good on 2nd video .
Cant wait to pull it and get it ready for install in her new home .
What is every one doing for intake manifold bolts ? I ordered gaskets and am going to reseal my intake. I have silver bolts in the engine but do not know if they have been replaced according to the Service bulletin. GM say the part number 12561518 is not available. ARP doesn't seen to do them either.
Guys in FB group for 8.1 say ARP 770-1010 2 sets .
Arp catalog sucks, and the web page wouldn't render correctly yesterday.
Thanks Brett
Thanks Brett
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good little video of basic stock for stock on 7.4 to 8.1L ........ and even carb and power adders cant beat the low / mid grunt of the 8.1L
That video is a good demonstration of why I stroke pretty much every engine I build, unless rules limit your cubes and you can't do it, more stroke will pickup everywhere in a typical performance engine. And get the block with the biggest bore you can too.
There is a replacement for displacement, however, if you apply that replacement to the displacement, it's even better.
Big Blocks rule, whether it be Mark IV, Gen 6, or Gen 7.
There is a replacement for displacement, however, if you apply that replacement to the displacement, it's even better.
Big Blocks rule, whether it be Mark IV, Gen 6, or Gen 7.
I see you left the genV off your list. I wonder why? Oh wait, I have one I know why!That video is a good demonstration of why I stroke pretty much every engine I build, unless rules limit your cubes and you can't do it, more stroke will pickup everywhere in a typical performance engine. And get the block with the biggest bore you can too.
There is a replacement for displacement, however, if you apply that replacement to the displacement, it's even better.
Big Blocks rule, whether it be Mark IV, Gen 6, or Gen 7.
There's nothing wrong with the Gen V really other than the tiny heads, it's just more rare and doesn't have a mech fuel pump mount, but can still be built just as strong with a heads, cam ,intake, etc. If you already have a Gen V and want more power, who cares if its a Gen V, its easier to find parts for a Gen V than a Gen 7 actually, you have way more cylinder head and camshaft options.
My father built a Gen V 573 ci BBC (4.5" x 4.5", not the 572 GMPP) over 25 years ago, it's still going strong, made 720 HP on pump gas with iron heads.
My father built a Gen V 573 ci BBC (4.5" x 4.5", not the 572 GMPP) over 25 years ago, it's still going strong, made 720 HP on pump gas with iron heads.
So I have a 2003 avalanche 3/4 ton with 8.1, 267k miles.
Has a cam sensor code and sometimes at idle it tics.
Do you think it's worth doing any upgrades while I have the engine out before I put it in the suburban.
I plan on fixing and updating all the items @Larry posted in the beginning, all the TSBs.
Has a cam sensor code and sometimes at idle it tics.
Do you think it's worth doing any upgrades while I have the engine out before I put it in the suburban.
I plan on fixing and updating all the items @Larry posted in the beginning, all the TSBs.
If you ask me the answer is usually yes. 
What other answer would there be, with a big block?
After reading and ruminating upon manual tranny things, I think the VSS discussion needs to be visited and I'm not totally sure this is the right place for it other than there are a good number of guys running manuals behind 8.1s in swap applications here. After paying attention to some factory manual trans later model trucks (at least as late as you can get with a factory manual...) I'm seeing some VSS related activity that would probably solve some problems on swapped drivetrains and especially with engines running some non stock hardware like bigger cams.
Our experience actually started with trying to tune the 6.0L in our U4 car. It has a full manual valvebody auto but is still a stand alone harness with no VSS. We ran into issues with it dying on decel because the computer just couldn't know that we were jamming on the brakes, steering and letting decel braking drag the engine down. The quick and dirty fix was essentially to raise the idle and run it. Similar situation with our tan convertible K30. Along the way I started paying attention to the rpm on my LB7/ZF6 dually and could see that it held the idle up on decel and even upon stopping for a few seconds before letting it drop to "stationary" idle speed. At one point I could play with letting it neutral roll down a street really slow and there was a speed at which I could see the idle come up, then when I stopped, it would drop again. Similar situation with our '99 K2500 UA truck and my new to me '98 K2500, both with the factory 7.4L/NV4500.
So basically I'm thinking there's a lot more going on with the VSS than just controlling an auto trans. I think they're using the VSS to know when the truck is decelerating so they can keep idle up and support the added drag of the brakes and steering on the engine so it doesn't die when you come to a stop. Why this isn't a more common thing in the tuning world, I don't know. I'm not familiar with tuning software other than very rudimentary stuff so I have no idea how obvious the VSS interaction is. Another problem is that the vast majority of swaps use an auto and probably most often a computer controlled auto so the VSS controlling the idle may just happen as a result of leaving all that activity turned on.
After all this I wonder if at least some of @Larry 's lack of satisfaction with the cam in his 8.1L 'burb swap stems from this problem? I'm guessing any swap with a late model engine will work better with the VSS doing it's job no matter what trans it's paired with.
Our experience actually started with trying to tune the 6.0L in our U4 car. It has a full manual valvebody auto but is still a stand alone harness with no VSS. We ran into issues with it dying on decel because the computer just couldn't know that we were jamming on the brakes, steering and letting decel braking drag the engine down. The quick and dirty fix was essentially to raise the idle and run it. Similar situation with our tan convertible K30. Along the way I started paying attention to the rpm on my LB7/ZF6 dually and could see that it held the idle up on decel and even upon stopping for a few seconds before letting it drop to "stationary" idle speed. At one point I could play with letting it neutral roll down a street really slow and there was a speed at which I could see the idle come up, then when I stopped, it would drop again. Similar situation with our '99 K2500 UA truck and my new to me '98 K2500, both with the factory 7.4L/NV4500.
So basically I'm thinking there's a lot more going on with the VSS than just controlling an auto trans. I think they're using the VSS to know when the truck is decelerating so they can keep idle up and support the added drag of the brakes and steering on the engine so it doesn't die when you come to a stop. Why this isn't a more common thing in the tuning world, I don't know. I'm not familiar with tuning software other than very rudimentary stuff so I have no idea how obvious the VSS interaction is. Another problem is that the vast majority of swaps use an auto and probably most often a computer controlled auto so the VSS controlling the idle may just happen as a result of leaving all that activity turned on.
After all this I wonder if at least some of @Larry 's lack of satisfaction with the cam in his 8.1L 'burb swap stems from this problem? I'm guessing any swap with a late model engine will work better with the VSS doing it's job no matter what trans it's paired with.
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VSS has led to interesting discussions as far back as EFI swaps have been around. Back when TBI swaps were the thing, the topic of VSS came up all the time where it seems nobody really had a true answer if it was needed or not with a manual trans. Some people swear by it and others got by without it just fine.
The way I see it, VSS isn’t needed with a manual trans because sensing load is the job of the Manifold Absolute Sensor. If you’re on the throttle hard then MAP voltage is high then when you let off the throttle the MAP voltage drops instantly. Based upon inputs from the MAP and TPS, there is enough information already going to the ECM/PCM to control injector pulsing and IAC operation to prevent the engine from dying. There is really nothing more VSS can add to control anything, even though adding VSS has been know to fix some swapped engine issues. Makes one wonder if the base tune was a correct manual trans tune to begin with.
From the GM Service Manual: (There is nothing in the VSS component description that says it supplies anything for engine control purposes outside of transmission shift scheduling)
The powertrain control module (PCM) supplies 5 volts to the MAP sensor on the 5-volt reference circuit. The PCM also provides a ground on the low reference circuit. The MAP sensor provides a signal to the PCM on the MAP sensor signal circuit which is relative to the pressure changes in the manifold. The PCM should detect a low signal voltage at a low MAP, such as during an idle or a deceleration. The PCM should detect a high signal voltage at a high MAP, such as the ignition is ON, with the engine OFF, or at a wide open throttle (WOT). The MAP sensor is also used in order to determine the barometric pressure (BARO). This occurs when the ignition switch is turned ON, with the engine OFF. The BARO reading may also be updated whenever the engine is operated at WOT. The PCM monitors the MAP sensor signal for voltage outside of the normal range.
The VSS topic did come up when I was dealing with the Raylar 202 cam in my Suburban a few years ago. Larry at Raylar as well as the local tuner trying to dial it in both stated VSS would do nothing to help my Suburban situation as it was dying from the lack of air. There is nothing VSS is going to do to help get more air into the engine as the engine RPMs were dropping to near idle speed. Being I prefer mechanical operated throttle bodies pared to manual transmissions, the IAC alone wasn’t enough to get the quantity of air into the engine it needed as the MAP voltage and RPMs dropped. What both Larry and the tuner recommended was swapping on an electronic throttle body so they could kick the throttle plate open as the engine slowed down to prevent it from dying. A throttle plate kicker is how the late model LS engines with huge cams idle. That’s not something that can be done with a mechanical throttle body outside of possibly drilling holes in the throttle plate with hopes of not creating too big of air leak.
My preference was to go back to the stock cam and keep the L29 mechanical throttle body because it is a nice combination with an offroad used rig with a manual trans (My bone stock Silverado with a 8.1/ZF 6 speed and electronic throttle is one herky jerky machine as the throttle is as touchy as a slot car, which makes for some less than sterling launches from stops and very unpredictable throttle control off road).
While the 202 cam did add a lot of high range power to the Suburban, the loss of the low end grunt wasn’t something I was happy to loose so the stock cam was welcomed back. The 202 cam is just not going to let a big rig lug around at idle speed offroad without constantly tossing fuel at it. However, the 202 cam did like outrunning Dirtymaxes from stop light to stop light
Back when I first swapped in a TBI 454 into my father in laws 1993 C1500 Sportside (that had a NV3500 5 speed manual at time), it died constantly when coming to a stop and it had VSS. When I contacted Howell Engine Development back they determined they had an automatic tune in the PROM. Once they changed it to a manual trans tune, the dying stopped because the auto tune was looking for the load of the torque converter, which wasn't here so it would die.
Is VSS really needed with a manual? Not sure anyone really knows the definite answer, but I imagine every tuner has their own opinion. Even the old Ram Jet 502's didn't require VSS for non-electronic or manual transmissions and it had a pretty lumpy cam.
Can you tell if your factory 8.1/zf6 truck changes idle speed based on road speed?
I can see the IAC not having enough speed or capacity to handle the cam. At that point the bandaid is likely just what you said and what we've been dealing with: raise the idle till it stays running. I do wonder what is available in the various computer tables that can happen using the VSS signal. On the practical side for us, if we could make our convertible truck idle at the 750-800 rpm that is fine for steady state and not die as you decelerate to that point, it would be a lot more pleasant to be around. It's fine tuning for sure.
I can see the IAC not having enough speed or capacity to handle the cam. At that point the bandaid is likely just what you said and what we've been dealing with: raise the idle till it stays running. I do wonder what is available in the various computer tables that can happen using the VSS signal. On the practical side for us, if we could make our convertible truck idle at the 750-800 rpm that is fine for steady state and not die as you decelerate to that point, it would be a lot more pleasant to be around. It's fine tuning for sure.
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I have never noticed anything out of the ordinary with the Pewter Puff Silverado before that feels any different than the Suburban or K10 with idle speed, but next time I take it out I’ll try to remember to see what the idle does by slipping it out of 6th without stepping on the clutch. It would be interesting to see if the idle stays up or falls. I haven’t driven that truck since last summer, so it needs some exercise anyway.
I’m sure a tuner somewhere could tell us if a VSS would change your life or not. It sure can’t hurt and as we sit here in 2023, there really is no reason not to run VSS as it opens doors to neat instrument clusters, etc. I liked keeping the cable operated speedo for the sake of keeping the instrument panel original although here I am adding a VSS voltage generator to the cable drive so I can run GMT400 Electromotor cruise control. Adding Electromotor cruise control to the Suburban was one of the winter projects that didn’t happen, as I cruise module I had in stock had the wrong throttle cable end. I need a L29 454 Electromotor cruise control box w/cable from a truck, SUV or van, which seem to be getting hard to find. A Pre-2003 LS truck with Electromotor cruise may fit the L29 throttle body as well.
Are you running a factory ECU?
With the Holley ECU you can tune the IAC hold position, ramp down RPM, decay rates, etc. This allows you to open the IAC more under load and ramp it down slower.
However, a larger camshaft does indeed need to idle a little higher because of the overlap, are you positive this is not a case where it just needed a higher idle to run properly in the first place? Also, are you running hydro power brakes or vacuum?
I don't recall anywhere in the regular software where it changes IAC position based on vehicle speed, but I don't tune the factory ECUs, just speaking of aftermarket like Holley, Edelbrock, FAST, BS3, etc. Holley does offer "advanced tuning tables" where you can offset the IAC position based on other factors like gear or speed, this could be used to do what you speak of.
What about your torque converter? If you have a performance cam but have a tight torque converter for engine braking or rock crawling it may cause other issues that are just a side affect of what you need. When you increased the idle, how far, what RPM was it? Was it enough it was hard to hold the vehicle stopped at idle, or not nearly that much?
I am not saying any of these are your case Stephen, just throwing out possibilities and potential solutions.
Very interesting conversation between you guys. What about manually adjusting the set screw and relearning the IAC counts and TPS for increase air flow and let the computer do idle speed? When I converted to Holley Terminator X, I had to take the VSS out of the tcase and add it into the adapter housing to get the tail shaft speed for the trans to electronically shift in low range. For those that may remember in Moab I was "slow cause my truck never shifted unless I flogged it hard and with a big group it was hard to get the real estate to do this.
Here is a interesting read on HP tuners and cams for this exact purpose.
https://forum.silveradoss.com/topic/49683-idle-tuning-using-hptuners/
Here is a interesting read on HP tuners and cams for this exact purpose.
https://forum.silveradoss.com/topic/49683-idle-tuning-using-hptuners/
For those of us using gm pcm’s we don’t have a way to relearn the idle speed without using hp-tuners or similar software. Without making the adjustment the pcm would force the iac to close to try and slow the idle to its target speed. It would make the idle inconsistent all the time not just on decel.
Mine is really consistent with the L29 cable throttle body and I’ve not had any issue with mine stumbling or stalling when slowing down. It’s probably rooted in the fact I’m running the stock cam.
I did have a problem with an iac that was getting stuck in different positions. It acted like someone adjusted the throttle opening manually sometimes. I would watch the iac command trying to slow the idle speed down and it would still be humming at 1800-2000 rpm. It wouldn’t always set a code but it did cause the engine speed to flare when you pushed in the clutch to shift gears. It gets back to the earlier idea to manually crack the throttle blade to let more air in. Without taking the time to adjust the pcm for what the target idle speed is you’ve effectively taken away the pcm’s ability to automatically adjust the idle speed as it normally does for warm up and through the normal drive cycle. Which defeats the purpose of having the computer control it in the first place.
I found with the iac hanging at higher engine speeds taking off from a stop on the street wasn’t an issue but if I was trying to do some slow and more controlled stuff off road it would be too much in first gear or low range. Considering all the torque on an 8.1 down low off idle too much engine speed makes it jumpy in low range or low gear or both.
I think the MAP on a stock setup works really good to monitor load and adjusting accordingly. I have a vss in mine since it’s a ‘91 but it is not tied into the pcm at this point.
Mine is really consistent with the L29 cable throttle body and I’ve not had any issue with mine stumbling or stalling when slowing down. It’s probably rooted in the fact I’m running the stock cam.
I did have a problem with an iac that was getting stuck in different positions. It acted like someone adjusted the throttle opening manually sometimes. I would watch the iac command trying to slow the idle speed down and it would still be humming at 1800-2000 rpm. It wouldn’t always set a code but it did cause the engine speed to flare when you pushed in the clutch to shift gears. It gets back to the earlier idea to manually crack the throttle blade to let more air in. Without taking the time to adjust the pcm for what the target idle speed is you’ve effectively taken away the pcm’s ability to automatically adjust the idle speed as it normally does for warm up and through the normal drive cycle. Which defeats the purpose of having the computer control it in the first place.
I found with the iac hanging at higher engine speeds taking off from a stop on the street wasn’t an issue but if I was trying to do some slow and more controlled stuff off road it would be too much in first gear or low range. Considering all the torque on an 8.1 down low off idle too much engine speed makes it jumpy in low range or low gear or both.
I think the MAP on a stock setup works really good to monitor load and adjusting accordingly. I have a vss in mine since it’s a ‘91 but it is not tied into the pcm at this point.
I'm glad you guys are smarter then me.
