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I am working on my 1981 Avanti II. When built the car was powered by a Corvette 305 CID and a 350 transmission. Prior to my ownership, the car received a 350 CID engine possibly from a 1996 Chevrolet pickup truck. I am just about finished with the body and paint and have started to look into the engine bay. The car runs, but not real strong. This is where the Corvette forum can hopefully help. I have purchased a Corvette factory manual to help with wiring harness and connections. Does anyone know if the PCM needs to be changed. Is it going to be possible to get everything working like it was when the 305 CID was originally in the car. Another thought that I had was to eliminate the PCM and all peripherals. Your recommendations would be appreciated.
The 305 was a CA only Corvette engine in 1980. The 81s were all 350s. I know that the 81 computer control system worked just fine on larger bore and stroke engines so I’d imagine the same would be true for an 80 305 to 350 swap. If your current 350 lacks power, heads and cam are where I’d look first. Also, what exhaust system are you running?
It’s not the lack of power. I have a check engine light on. It seems to run very rich. I’m not sure if the ECM is connected to everything as required. The exhaust is dual Flowmaster. The seller said that he believed the engine to be from a 1996 Vortec pulled from a Chevy pickup.
Just a suggestion, as a place to start, what does the Avanti II normally have for an OE exhaust system? Was it a dual or single system, and does it have, or should have, an O2 sensor(s) in place? Also, is the carburetor a "regular" 4 barrel or an electronic feedback 4 barrel, and if it's of the feedback variety, is the wiring in place and functioning?
It’s not the lack of power. I have a check engine light on. It seems to run very rich. I’m not sure if the ECM is connected to everything as required. The exhaust is dual Flowmaster. The seller said that he believed the engine to be from a 1996 Vortec pulled from a Chevy pickup.
Pics of the engine and everything you know about the setup will help. A 96' engine will have Vortec heads.......it uses a very different intake manifold.
Rich running CCC usually points to the Idle Mixture Control solenoid in the carb. When it fails it goes full rich.
Personally.....having worked on these CCC engines, delete the system. All that is required is a new distributor and carb.
Lol! Yeah, I explained my reason for joining and posting here. It's a factory fiberglass corvette power train classic, so I knew the members here would be able to help.
I have tried to locate the casting numbers. Easier said than done. The Avanti was a strange company when it comes to parts. As I was looking for casting numbers I fund that Avanti installed Dana Cruise control at the factory. I can see from the intake that it is a 1981 Corvette. I am posting pictures of the engine in this post and I will try again tomorrow with a mirror to find the casting numbers. I will try to take my 64 Studebaker off the lift and put this on to locate O2 sensors and look at the transmission for a speed sensor.
I have tried to locate the casting numbers. Easier said than done. The Avanti was a strange company when it comes to parts. As I was looking for casting numbers I fund that Avanti installed Dana Cruise control at the factory. I can see from the intake that it is a 1981 Corvette. I am posting pictures of the engine in this post and I will try again tomorrow with a mirror to find the casting numbers. I will try to take my 64 Studebaker off the lift and put this on to locate O2 sensors and look at the transmission for a speed sensor.
The carb is an Edelbrock with an adapter. I doubt any of the computer equipment works anymore.
Don’t waste time looking for O2.....you need to get that carb off there and either change the intake or get a spreadbore unit that matches the intake. You could
also swap the intake for a 2101 or 2701 Performer and keep the carb you have. You will never get this engine properly tuned with that adapter. The following picture will show you why:
The mismatch on the primaries is horrible and the throttle plate leans into the mismatch.
These adapters should be banned for purchasing.
The pictures are from a 72’ Vette I have here that I put one of my Q-Jets on as the owner complained about poor idle, bad fuel mileage and hesitation.
The casting number for the block is on the drivers bellhousing pad....six or seven digit number....post your findings.
The cylinder heads have 1987 or newer centerbolt valve covers.....but the center two intake bolts indicate a 1986 or earlier intake manifold....I bet these are World S/R heads or something equivalent.
The EGR is blocked. Somebody has done a lot of fooling around here. Pull a valve cover and look for head casting numbers.
Lol! Yeah, I explained my reason for joining and posting here. It's a factory fiberglass corvette power train classic, so I knew the members here would be able to help.
I get it and no problem. Jus bustin' your stones. Haven't seen one of those cars in like forever.
Is that an Edelbrock carb? The fuel line (which should be metal for safety, but not performance), has a big kink in it. Your distributor has an aftermarket adjustable vacuum advance can connected to a vacuum port on the carb (check that this is manifold, not ported, vacuum).
It looks like someone has already taken @Jebbysan's advice, and deleted all of the CCC-dependent components.
The engine stamp pad should be in front of the passenger side head on top of the block (below the AC compressor and hard to see of course).
Is that an Edelbrock carb? The fuel line (which should be metal for safety, but not performance), has a big kink in it. Your distributor has an aftermarket adjustable vacuum advance can connected to a vacuum port on the carb (check that this is manifold, not ported, vacuum).
It looks like someone has already taken @Jebbysan's advice, and deleted all of the CCC-dependent components.
The engine stamp pad should be in front of the passenger side head on top of the block (below the AC compressor and hard to see of course).
Further review shows a Pertronix Flame Thrower distributor and a complete serpentine belt system that was not available until 1987.....
I think someone has transplanted a whole engine out of a newer 87-88 Monte Carlo SS or a C/K pickup.
Further review shows a Pertronix Flame Thrower distributor and a complete serpentine belt system that was not available until 1987.....
I think someone has transplanted a whole engine out of a newer 87-88 Monte Carlo SS or a C/K pickup.
Jebby
I’m pretty sure you are right! It appears to be from a Chevy Pickup. No luck on the block casting number. They are buried way deep in the mess. Is the adapter used because of the Chevy intake manifolds mounting bolt pattern? I have a brand new in the box Holley 0-80555C Carburetor that I’ve saved for future need. Would this be a good match for this intake? Would there be any reason to leave the ECM in the car if I eliminate all of the peripherals? It would be nice to completely remove everything to clean up the engine bay and under the dash.
I’m pretty sure you are right! It appears to be from a Chevy Pickup. No luck on the block casting number. They are buried way deep in the mess. Is the adapter used because of the Chevy intake manifolds mounting bolt pattern? I have a brand new in the box Holley 0-80555C Carburetor that I’ve saved for future need. Would this be a good match for this intake? Would there be any reason to leave the ECM in the car if I eliminate all of the peripherals? It would be nice to completely remove everything to clean up the engine bay and under the dash.
That carb is perfect. Install that. Look again at the spacer pics I posted.....it is not the bolt pattern, it’s the size and location of the throttle blades....
When you get the Edelbrock carb off and install that Holley.....you will understand.....that Holley is a direct Q-Jet replacement unit.
You have a lot of tidy to do......
A couple of guys here run that carb.....ask them for a pic....pics are your friend.
I had to do a double take when I saw the pics...looks like one big pic to the bleary morning eye.
"Holy Toledo, that cat's got two engines in there! I gotta see how he did that!!!"
Casting number on the block will be on the back ledge, where it bolts to the transmission, driver's side. Your smartphone camera can be your friend in tight spots like this.
You're in the right spot, we'll help you get it lined out.
I don't think you need the ECM at this point, but please confirm that the distributor is a 4-Pin HEI (I'd think it must be, with the vacuum advance, but who knows). If that is the case, the ECM is not doing anything, and won't be controlling that Holley carb either.
What transmission do you have? There is a tiny chance you have a TH350C, which could benefit from an ECM signal, but I'm certainly no expert on that, and I think there are ways around this.
One other observation: Your heater core feed line comes off of the waterneck. C3s do not do this (there is a plugged hole on your intake where a hose barb would connect). I s'pose there's nothing wrong with this, but it means you won't get heat until the engine is fully warmed up and the thermostat is open.
I don't think you need the ECM at this point, but please confirm that the distributor is a 4-Pin HEI (I'd think it must be, with the vacuum advance, but who knows). If that is the case, the ECM is not doing anything, and won't be controlling that Holley carb either.
What transmission do you have? There is a tiny chance you have a TH350C, which could benefit from an ECM signal, but I'm certainly no expert on that, and I think there are ways around this.
One other observation: Your heater core feed line comes off of the waterneck. C3s do not do this (there is a plugged hole on your intake where a hose barb would connect). I s'pose there's nothing wrong with this, but it means you won't get heat until the engine is fully warmed up and the thermostat is open.
It has a TH350 transmission. I will put it up on the lift in the next few days to verify if there is a harness connected to it. I installed a 700R4 in my 53 Chevy Suburban and my transmission guy installed (if my memory serves me correctly) a vacuum TCC and it works perfect. I don't have a switch or any wire going to the transmission. I'm a little foggy when it comes to the TH350. It's not an overdrive transmission, so the work around may be different.
You guys (sorry I didn't mention you all by name) on this forum are some of the best and have been super helpful. I'm glad to have joined. I will get the car on the lift and verify the items you have asked me to post. I really appreciate the help getting this sorted out, looking and running right.
Due to my many projects it’s been a while since I followed up on your recommendations. I have removed the Edelbrock and adapter plate and installed the Holley. I am working on bending and installing steel fuel lines and an in-line fuel filter. I will be searching for pictures of a tidy engined compartment to help with cleaning mine up. I have finished with most of the paint and interior finish (picture included).
11-10-2020, 06:06 PM
305 CID to 350 CID Swap and PCM
