TH 400 bolting to 350 LT1 Chevy Vette Engine - Team Camaro Tech
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Old Feb 21st, 16, 09:59 PM   #1 (permalink)
 
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TH 400 bolting to 350 LT1 Chevy Vette Engine

Does anybody know if you have to change anything up or make any modifications when bolting up a 350 LT Chevy Block Vette engine to an existing mounted TH400 transmission. 1981 Z28 project. I have confusing information about maybe having to change the "Slip Yoke" due to size difference. Thanks for any advise and guidance. The engine attached before was a Big Block.
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Old Feb 22nd, 16, 06:04 AM   #2 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

The TH400 uses a larger diameter out put shaft because it was designed to have a much higher torque rating than the TH350 that was never installed behind anything hat made more than 300 factory rated horsepower. That out put shaft requires a 32 spline front yoke. The newer 3L80 made after the TH400 was renamed (as it looks the same on the outside so it's hard to tell them apart) uses the same smaller 27 spline output shaft that was used on the Muncie four speed and the TH350 so the only thing is to shorten the drive shaft to fit the longer length. You have to shorten it with either tranny as they are both larger than the smaller TH350 (taller, and longer as well as heavier).

The TH400 also requires a round tube cross-member that fits in the Firebird because a Camaro TH400 cross-member is offset to fit a BBC in the Camaro. With your SBC you will need the Firebird cross-member that centers the motor in the car. You need the round tube version over the two piece flat stamped metal cross-member because you won't be able to drop the transmission's oil pan with the TH350 version of the cross-member. The stamped steel one is two wide to get to the bolts or drop the pan.

The flex plate ids also different as the TH400 has a larger flex plate that matches the size of the 11 inch clutch fly wheel, and the Th350 uses a smaller 10-1/2 inch clutch fly wheel starter ring. So that means the starter motor also has to be changed out to match the 168 tooth flex plate.

Big Dave
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Old Feb 22nd, 16, 07:01 AM   #3 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

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Originally Posted by Larger Dave View Post
The TH400 uses a larger diameter out put shaft because it was designed to have a much higher torque rating than the TH350 that was never installed behind anything hat made more than 300 factory rated horsepower. That out put shaft requires a 32 spline front yoke. The newer 3L80 made after the TH400 was renamed (as it looks the same on the outside so it's hard to tell them apart) uses the same smaller 27 spline output shaft that was used on the Muncie four speed and the TH350 so the only thing is to shorten the drive shaft to fit the longer length. You have to shorten it with either tranny as they are both larger than the smaller TH350 (taller, and longer as well as heavier).

The TH400 also requires a round tube cross-member that fits in the Firebird because a Camaro TH400 cross-member is offset to fit a BBC in the Camaro. With your SBC you will need the Firebird cross-member that centers the motor in the car. You need the round tube version over the two piece flat stamped metal cross-member because you won't be able to drop the transmission's oil pan with the TH350 version of the cross-member. The stamped steel one is two wide to get to the bolts or drop the pan.

The flex plate ids also different as the TH400 has a larger flex plate that matches the size of the 11 inch clutch fly wheel, and the Th350 uses a smaller 10-1/2 inch clutch fly wheel starter ring. So that means the starter motor also has to be changed out to match the 168 tooth flex plate.

Big Dave
The info I've put in bold above is for a first gen Camaro not a second. If you already have the 400 in your 81 and it was a drivable car the driveshaft issues (slip yoke) have already been addressed. I think the flywheel on the Vette engine may work, some of them had a dual bolt pattern to fit the larger torque converter. I'm not sure how to confirm this without bolting them together and trying to install the bolts.

Jeff
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Old Feb 22nd, 16, 08:39 AM   #4 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

What you are saying is that you have a 1981 Camaro with a big block engine with a th400 transmission. Now you want to swap in a LT 350 in place of the big block. What year engine is the LT1 350, it makes a difference because the 1970 LT1 350 has a 2 pc rear main seal and the 1992 has a 1 pc rear main seal. The flexplate for the 1992 LT1 is weighted. So if you have the 1992 you can use the flexplate from the big block engine. If not you will have to buy a 168 teeth non-weighted flexplate for a 350 engine. If your starter has the offset bolt pattern you use a 168 teeth flexplate for the 11' bolt pattern torque converter. Remember this: a 10 3/4 bolt pattern torque converter uses 153 teeth flexplate and straight bolt pattern starter. You also use the same crossmember.
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Old Feb 22nd, 16, 09:00 AM   #5 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

What you are saying is that you have a 1981 Camaro with a big block engine with a th400 transmission. Now you want to swap in a LT 350 in place of the big block. What year engine is the LT1 350, it makes a difference because the 1970 LT1 350 has a 2 pc rear main seal and the 1992 has a 1 pc rear main seal. The flexplate for the 1992 LT1 is weighted. So if you have the 1992 you can use the flexplate from the big block engine. If not you will have to buy a 168 teeth non-weighted flexplate for a 350 engine. If your starter has the offset bolt pattern you use a 168 teeth flexplate for the 11' bolt pattern torque converter. Remember this: a 10 3/4 bolt pattern torque converter uses 153 teeth flexplate and straight bolt pattern starter. Since a big block engine didn't come in 1981 Camaros you will use the same crossmember. Someone probably used mounts from a 72 big block camaro. So you will have to make sure of the correct mounts for the 350 engine. Here is some good info on engine mounts. http://www.nastyz28.com/2gcog/motmount.html
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Old Feb 22nd, 16, 09:27 AM   #6 (permalink)
 
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

Thank you Gentlemen, that is all excellent information. Yes I have a new starter motor with 168 teeth. I plan to buy a flywheel to match if I need one. The current converter on it is a 10 inch. The Engine is a 1970 LT1 Vette 350 V8, so I believe it is a 2 piece main rear seal. I still have to pull the engine apart, clean it all up and rebuild. I will know better when I get to it. Still a good several weeks away until I do so. Once I dig into it, I will post some pics and sure to have many questions for all to share tech ideas. Thanks Kel
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Old Feb 22nd, 16, 09:35 AM   #7 (permalink)
 
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

Just to be clear, the TH 400 trans is already in place, the big block that was in it, is out of it now. So I just making sure of anything I may have to do special when dropping in and bolting up the 350 SBC engine. I am hoping it's a simple plug and play understanding having a 168 tooth flywheel and starter to match.

Yes I am bolting in new small block engine mounts. I can go full steel or rubber flex ones, I think I will go with soft mounts as the all steel mounts make it a rougher ride, vibration and what not. It's Not going to be a full blown Race car, but Street machine and able to throw down at the local drag track. That said I am hoping to make this thing go low 12's or mid 11's all day long and still be able to cruise in reasonable comfort.
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Old Feb 22nd, 16, 10:53 AM   #8 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

The 1970 LT-1 engine (370 horse factory rated) used 11.0:1 pop up pistons with cast iron heads. It ran on premium pump gas back in 1970 that had an octane rating of 98 to 104 depending upon the brand. On pump gas today you will need to drop the static compression or run a lot more aggressive a cam to kill your overall dynamic compression.

Big Dave
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Old Feb 22nd, 16, 01:48 PM   #9 (permalink)
 
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

Well noted. I have plans to make this little puppy push out over 400 HP. new Cam, intake, port and polish, the right carb, msd and some other little goodies and tricks to AMP it up. We will see when it's done and get's on a Dyno.
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Old Feb 22nd, 16, 03:55 PM   #10 (permalink)
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

Here is the flexplate you will need for the 1970 LT1 350 engine. You should be able to use the starter you take off the 454 engine. http://www.summitracing.com/parts/su...make/chevrolet Just make sure the starter"s bolt pattern is staggered. Measure the bolt pattern on your 10" torque converter. It will be either 10.750 or 11.500.
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Old Feb 27th, 16, 08:10 PM   #11 (permalink)
 
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Re: TH 400 bolting to 350 LT1 Chevy Vette Engine

Thanks, that's great. I have a brand new in box 168 tooth Monster Starter for this puppy, all good. But I will be getting a new "Flexplate" for sure...
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