: 4.9 throttle body interchange data



ehall
03-08-11, 06:11 PM
I am once again looking to replace my 4.1 with a 4.9 setup and am researching throttle body replacements. Specifically I want to do a 4.9 TBI setup so I need an interchange TBI throttle body and will then try to find injectors with longer pulse rates to trick the ECM (rather than messing with an adjustable FPR or MAP).

My assumption is that the Chevy 305 TBI setup bolted onto the 4.9 manifold will probably end up working the best (the closest displacement), but there are a bunch of factors there that I need to work through.

First question is if anybody knows the bore size for the 4.9 throttle body and/or intake manifold?

drewsdeville
03-08-11, 06:21 PM
I am once again looking to replace my 4.1 with a 4.9 setup and am researching throttle body replacements. Specifically I want to do a 4.9 TBI setup so I need an interchange TBI throttle body and will then try to find injectors with longer pulse rates to trick the ECM (rather than messing with an adjustable FPR or MAP).

My assumption is that the Chevy 305 TBI setup bolted onto the 4.9 manifold will probably end up working the best (the closest displacement), but there are a bunch of factors there that I need to work through.

First question is if anybody knows the bore size for the 4.9 throttle body and/or intake manifold?

TBI or TPI? If TBI, a '88-'89 4.5L TBI setup should physically bolt onto a 4.9 with no trouble.

Injector pulse length is calculated and controlled by the PCM and is not inherent per injector. Larger injectors (by flow rate) will simply force the PCM to adjust the integrator and block learn (PFI only) to reduce pulse length. Conversely, smaller injectors will force the PCM to lengthen pulse length.

Sorry, don't know the bore sizes.

ehall
03-08-11, 07:36 PM
Sorry I meant to say higher lb flow but was rushing out to dinner. You are correct that pulse duration is controlled by the ECU so a higher flow rate is needed to get more fuel per pulse (needed to offset the larger quantity of air). There are numerous TBI injectors and they all appear to be interchangeable (much like the throttle bodies) so I should be able to get close on this.

At this point my main concerns are with the throttle body (finding one with a compatible ISC setup, etc), and particular matching the bore size.

A throttle body from a 4.5 may turn out to be a better option however a 4.9 is 299 CI so 305-299=6 CI difference while a 4.5 is 275 CI or 299-275=24 CI difference... 305 is much closer and the slightly larger size would allow for better breathing

ehall
03-09-11, 01:25 PM
Little more information just to keep it all in the same place.

The 305/350 and other SBC seem to share the same throttle body (and bore size), while the 454 and other big blocks have another bore size. Depending on the size of the 4.9 manifold opening, I may be able to use either of them.

All of the GM TBI setups use the same basic fuel system back-pressure (9-13 PSI), and just vary the injector flow. http://diy-efi.org/gmecm/component_info/tbi_injectors.html gives the differences on the different engine sizes. Injectors can be swapped between throttle bodies. The FSM for my DeVille says 9-12 PSI, so it seems line pressure is compatible.

Also looked at some interchange on ISC data and rockauto says the 81-87 Pontiac Trans Am uses the same ISC part number (with mounting bracket).

I bet if I can find a throttle body for a mid-80s 305/350 TBI Trans Am and 305 injectors I will be golden.

Ghost C
03-09-11, 02:15 PM
I read the stock throttle body openings on the 4.9 TB are 44mm. My caliper measures 45mm on both holes at the back of the TB, and when it stops raining, I'll get a measurement on the manifold itself for you but as far as I remember it's an exact match to the TB.

ehall
03-09-11, 03:25 PM
Thanks for the data. Supposedly the stock 305/350 TBI is 1-11/16 inches or 42-43mm so it should be a good fit. Maybe I can get one bored out to 44 and replace the butterflies... I found some people selling 46mm so might explore that too. Thanks again for the data

carnut
03-10-11, 11:23 AM
On my 84 Eldo, I replaced the throttle body with one from an 81 Cadillac 368 cu in engine. The infamous 8 6 4. You must use the thick base gasket because the throttle blades are much larger. Was necc to swap over the TPS switch as well only because the 368 one hit the manifold. I tried using the 368 injectors as well, but they flowed too much fuel and super heated the cat. converter. I swapped the 4.1 injectors onto it. I drove this set up for a year before I sold it. The larger throttle bores improved throttle response dramatically! Felt as if I'd added 20 HP. This change did not affect fuel mileage or its ability to pass a smog test. The system on your 85 is a speed density system meaning that larger injectors will effectively flood the engine.

ehall
03-10-11, 02:02 PM
Thanks for the info carnut. Based on the info in wikipedia the 8-6-4 was a blg-block with a smaller bore, so it probably had the 2" throttle body plates same as the 454. The small-block throttle bodies should be alright on fitment but obviously won't have the airflow that you found, hopefully it will be enough.

Also, to reiterate, I'll need larger injectors to compensate for the larger displacement of the 4.9.

ehall
03-11-11, 03:02 PM
I exchanged emails with a throttle body place and it was suggested that since all the parts were interchangeable the easiest thing to do for compatibility with my 4.1 plumbing would be to bore a 4.1 throttle body to 305/350 specs. So that is my new current plan.

ehall
05-13-11, 04:13 PM
For the record... I measure the bottom of the 4.9 throttle body at 44-45mm, the 4.5 throttle body at 42mm, and the 4.1 throttle body at 36-38mm (could not measure exactly).

This is rather interesting because (1) these are different from the SBC throttle bodies, and (2) it's not even the same within the engine series while the 305/350 use the same.

The 4.5 throttle body uses a bore size that can safely be considered the same size as an SBC TBI throttle body. The extra boring on the 4.9 probably opens up some higher top-end performance. This is also interesting because the 350 SBC is ~17% larger displacement than the 4.9. Now I am kind of curious what the SBC PFI throttle bodies measure at.

Sevillian273
05-13-11, 05:28 PM
I reckon illumina used a TB spacer for a chevy 4.3L on his 4.9. As far as how well it matched up, I dont know. He gave it a 'D' in the performance improvement department. :yawn:

ehall
05-18-11, 04:04 PM
I wonder how well any of the Holley performance TBI throttle bodies could be adapted

http://www.holley.com/types/Direct%20Replacement.asp