305 to 350 swap

[1EVHFAN]

I have a 1987 R10 with the factory 305 TBI and 700R4. I'd like to do an easy swap to a L05 or L31. Do I need a new ECU from a 350 vehicle? Also Holley makes an aftermarket TBI rated at 670 CFM that seems like a worthwhile upgrade. I plan to get a donor block, have it machined, and add a cam and heads. Tips tricks and suggestions appreciated. TIA.

 

[Bextreme04]

I have a 1987 R10 with the factory 305 TBI and 700R4. I'd like to do an easy swap to a L05 or L31. Do I need a new ECU from a 350 vehicle? Also Holley makes an aftermarket TBI rated at 670 CFM that seems like a worthwhile upgrade. I plan to get a donor block, have it machined, and add a cam and heads. Tips tricks and suggestions appreciated. TIA.

Just the engine swap would probably require a different chip, but with all the additional stuff, you will definitely need either a custom chip or an all new EFI setup. The Holley terminator TBI kit is like $2500... which would get you a MPFI Edelbrock Pro-flow system that would work better with the Vortec heads anyways. @Vbb199 has a custom chip in his TBI for his 350 crate motor and knows way more about that stuff than I do.

 

[1EVHFAN]

https://www.holley.com/products/fue...hrottle_bodies/gm_throttle_bodies/parts/502-6

$731.95 at Summit. This is what I have in mind.

 

[Bextreme04]

https://www.holley.com/products/fue...hrottle_bodies/gm_throttle_bodies/parts/502-6

$731.95 at Summit. This is what I have in mind.

That's just the throttle body, you'll still need to tune the ECU for the motor changes, and definitely for the bigger throttle body if you get it.

 

[mcarlo86]

Check out https://harristuning.com/Tbi/

I had them burn a custom chip for me and am pleased with the results. They also have a lot of recommendations on engine modifications that work best with GM TBI.

 

[Rusty Nail]

https://www.holley.com/products/fue...hrottle_bodies/gm_throttle_bodies/parts/502-6

$731.95 at Summit. This is what I have in mind.

Imo that is an absolutely ridiculous amount of money to spend for something that costs half that much.
I have plans to buy a TBI from these dudes in Texas.

https://ebay.us/brpB1L

$700..... my ass!

Bro! I was gonna spend $250 for new throttle bushings and a milled airhorn.

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Dude... It's even bored out!

 

[1EVHFAN]

Imo that is an absolutely ridiculous amount of money to spend for something that costs half that much.
I have plans to buy a TBI from these dudes in Texas.

https://ebay.us/brpB1L

$700..... my ass!

Bro! I was gonna spend $250 for new throttle bushings and a milled airhorn.

You must be registered for see images attach

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Dude... It's even bored out!

That IS a significant discount. Thanks for the tip. I did look at the e-bay listing and noticed it's rated for 590 cfm. The Holley unit is rated at 670 cfm. Apples and Oranges??? IDK.

 

[Rusty Nail]

I'd send em a message.


670 is overkill for a stock 350 tbi engine anyways. Not too interested in the conversation about required cfm but there is one. There's a formula.
Axe the diggy diggy doc y'all!


Lemme play some music and drink some coffee -
You could install a freaking 1050 DOMINATOR if you wanted - the engine will only take as much as it needs.
I put a 670 on my 12 second hotrod to replace a 795.
Bigger is not necessarily better (in this case).
...
More research is necessary, agreed?

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The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456. Any ordinary stock engine will have a volumetric efficiency of about 80%.

Hope this helps!

 

[Bextreme04]

That IS a significant discount. Thanks for the tip. I did look at the e-bay listing and noticed it's rated for 590 cfm. The Holley unit is rated at 670 cfm. Apples and Oranges??? IDK.

That Harris Tuning site that Vince linked above has everything you could possibly need on it, including recommendations for crate engine setups and a link to the guys that he knows does a good job with modifications of the throttle body. It looks like the TBI system even with the 650cfm throttle body will support about 400 net HP, which should be plenty. Depending on your budget and level of experience, you will likely be time and money ahead to just buy one of their recommended crate engine combinations and then get the modified TBI unit and intake adapter from SPR Performance. https://www.ebay.com/str/sprperformance

The sportiest engine that they recommended would be that ATK aluminum headed 383 stroker motor. The long Block comes in at about $4000 shipped. You'd be another $1000-1500 into the throttle body, intake manifold, manifold adapter, fuel pump, and chip to support the new motor.

You aren't going to be much cheaper than that if you do the work yourself if you count your time as money in it. You are going to burn at least a week or two in disassembly and delivery of your donor L31 to a machine shop, picking it up and cleaning/prepping the motor and parts for assembly plus the time of checking and adjusting clearances as you assemble it. Then you are going to be at least $1500 into parts and machining just for a decent short block and $1000 for the cheapest decent aluminum heads you can find. Plus no warranty on a motor you built.

 

[Catbox]

get the modified TBI unit and intake adapter from SPR Performance. https://www.ebay.com/str/sprperformance

I may have to go this route on my 1995 Suburban that won't pass DEQ.
@Maxwellvis was just going to do the new seals in the rebuild kit he picked up, but this might be the bees knees on the well used throttle body we are running.

 

[Bextreme04]

I may have to go this route on my 1995 Suburban that won't pass DEQ.
@Maxwellvis was just going to do the new seals in the rebuild kit he picked up, but this might be the bees knees on the well used throttle body we are running.

Between the throttle body, a decent cam, and that tuning chip, you'd probably pick up a good amount of power and drivability while still passing DEQ with flying colors. That guy that vince linked to even has good cam recommendations linked with HP numbers, which is always nice. Your 95 is probably a roller motor already... although it might still be flat tappet.

 

[Bextreme04]

That IS a significant discount. Thanks for the tip. I did look at the e-bay listing and noticed it's rated for 590 cfm. The Holley unit is rated at 670 cfm. Apples and Oranges??? IDK.

I'd send em a message.


670 is overkill for a stock 350 tbi engine anyways. Not too interested in the conversation about required cfm but there is one. There's a formula.
Axe the diggy diggy doc y'all!


Lemme play some music and drink some coffee -
You could install a freaking 1050 DOMINATOR if you wanted - the engine will only take as much as it needs.
I put a 670 on my 12 second hotrod to replace a 795.
Bigger is not necessarily better (in this case).
...
More research is necessary, agreed?

xc_hide_links_from_guests_guests_error_hide_media

The formula for calculating how much CFM (cubic feet per minute) your engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456. Any ordinary stock engine will have a volumetric efficiency of about 80%.

Hope this helps!

Just adding to this with a link to some decent reading. https://performanceimprovements.com/blogs/news/carb-size-matters-what-size-carburetor-do-i-need

Basically engine masters did this specifically on a dyno and found that it doesn't really matter, the engine will only take what it needs no matter what size you have on there as long as it is big enough. They even tested almost exactly the highest power situation you are thinking about doing. A high power 383 making ~470 HP and ~480 lb-ft of torque only took 600cfm of airflow at around 6,000rpm. For a normal lower power truck 350, that 590 cfm TBI should have no problem keeping up.

 

[Rusty Nail]

The price has dropped, i'm very excited to finally pick one up thanks to this dude makin me remember about em - but I'm going to the Dentist first.

 

[Rusty Nail]

K I messaged dude . Gave the specs of my Blazer and said I wanted the whole 9. Injector spacer and fuel pressure spring - all that jazz.
Dude said NP. We can install it all here at time of purchase!

Ok. Cool. Hook it upp!

https://www.ebay.com/itm/192189774061

Here. I'm only at half of your $700 , but it doesn't say Holley.
The price didn't drop, we were only looking at half a TBI unit.

 

[RecklessWOT]

305 and 350 technically use a different ECU, but it doesn't matter much in the scheme of things. Maybe 6 months after I got my '87 V10 burb (it has a 350) the ECU took a ****, and a guy at work gave me one from his '87 R10 (his truck originally had a 305, he swapped it out for a carbed crate motor and didn't need it anymore). I was just gonna run it for a while to get me by, but it's still in there to this day. The truck ran fine so I kind of just forgot about it over the years, though I suppose I would like to get the correct one eventually. I don't drive the truck much anymore, but I have put many thousands of miles on it with the 305 ECU, towed with it, etc, and it never skipped a beat.


EDIT: oh but I read farther and see you're going with an aftermarket TBI. You'll probably need something entirely different anyway