Square Bore 454 TBI Adapter

[dave w]

I've been thinking about making a 1" thick aluminum 454 TBI to Spreadbore adapter plate. Here are some CAD screen shots.

All thoughts are appreciated.

dave w

[gregs78cam]

looks good Dave. This is obviously a CNC job, so while you're at it put a radius on those 4 outside corners.

[EagleMark]

Looks funny? Guess that is why all the available spread bore adapters are rear set TBI instead of center.

GM has the best design, would be expensive to CNC but would be the best choice... 3/4 to 1 inch plate would do the trick...

[dave w]

A couple pics of my 0.500" thick aluminum spread bore design. CNC not required.

dave w

[dave w]

looks good Dave. This is obviously a CNC job, so while you're at it put a radius on those 4 outside corners.

Maybe this design, which does not divide the two planes, could be done on a manual mill. The throttle bore holes are 2" diameter at 22.5 degrees.

dave w

[gregs78cam]

Maybe this design, which does not divide the two planes, could be done on a manual mill. The throttle bore holes are 2" diameter at 22.5 degrees.

dave w

That is definitely do-able on a manual mill.

[1project2many]

Big block TBI engines were more likely to suffer fuel / air separation than with the old Quadrajet. GM switched to using peanut ports to increase velocity and reduce the chance for fuel to drop out of suspension. Additionally, they used a heated adapter with big transition in the early years, and a heated intake on later engines. They also switched to using smaller injectors at higher pressure, presumably to decrease droplet size. This really suggests your spacer should be more than just a piece of aluminum which connects the TB and manifold.

At part throttle fuel tends to flow along the throttle bore while at larger airflow fuel will be drawn away from the injector faster causing any potential contact to be at the base or even below the TB. A spacer that's slightly larger than the TB bores, which transitions from two holes to one with an increasing diameter will create a low pressure area below the TB to boost airflow and help vaporize fuel as it drops below the TB walls. The increasing diameter will allow the fuel to slow before it needs to to turn the corner at the plenum entrance to head toward the intake runners. With too much velocity the air will turn while the fuel continues it's path toward the floor of the plenum and once the fuel's out of suspension, it's wasted.

Some of the made-for-racing two barrel to four barrel carb adapters are worth looking at. Wilson racing, for example, makes a nice one:




This is a tapered spacer. You can clearly see what the bottom looks like along with the transition from two hole to one single hole. The slotted holes allow the spacer to be positioned properly fore and aft, while the two small diameter holes are for alignment pins, installed after the spacer position is finalized.

[EagleMark]

Looks like the tapering inward is wrong idea, looking at GM BB adapter they knew that...

[dave w]

Looks like the tapering inward is wrong idea, looking at GM BB adapter they knew that...

The 454 TBI is about 4.6" from outside to outside of the throttle blades. The Square Bore is about 3.60" from outside to outside of the throttle blade opening. An angle of 22.5 degrees with an adapter thickness of 1" is required to make the transition. I liked the idea of keeping the two planes separate. I could do a few CAD calculations to "volume match" the transition equal to a 2" diameter x 1" long cylinder, which in theory keeps for flow velocity unchanged. I'm thinking even with the "volume match" the fuel air mixture will wash on the dividing wall that separates the planes. CNC machine time is a huge factor at this point, and the very nature of using an adapter typically means there is a budget concern. It's likely to be less expensive to get an intake manifold that would accept a 454 Throttle Body, than build a "flow designed" adapter. My thinking is an engine making less than 1 HP per cubic inch, there isn't a performance need, or a budget need to use a "flow designed" adapter.

dave w

[1project2many]

The 454 TBI is about 4.6" from outside to outside of the throttle blades. The Square Bore is about 3.60" from outside to outside of the throttle blade opening.

Does a TB have to be inline with the engine to work? You've got 5.09" from corner to corner.

My thinking is an engine making less than 1 HP per cubic inch, there isn't a performance need, or a budget need to use a "flow designed" adapter.

It's not just about power. Fuel that drops out of suspension gets blown out the tailpipe to be burned in the converter.

FWIW, I have a homemade adapter here like your second design. I have thought about using it on my Smoke Ram manifold on the 302 in the '57. But every time I set it on there, it's just not the direction I want to go. Chances are I'll use a stock GM spreadbore adapter and carve up the old intake manifold to match.