removing intake manifold divider plenum
06-15-2005, 11:05 AM
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removing intake manifold divider plenum
Anyone have any thoughts on the effects of grinding down the plenum separating the two planes of the intake manifold ...allowing both sides of the engine to draw fuel from both injectors ...kind of like what CFM tech does with their "open plenum" style TBI spacers? I know that it would allow a carbed motor to breath a little better at higher rpms and rob some bottom end torque, but what would the effect be on a TBI motor. I believe CFM tech says it produces a smoother idle?
06-15-2005, 11:20 AM
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Car: 93 GM300 platforms
Engine: LO3, LO5
Transmission: MD8 x2
A neighbor of mine did it to his factory intake on his LO3 engine in his 91 Caprice. He only removed the upper bridge, smoothed the edges, and left the lower knife edge alone. (note: the factory intake has a small window that connects the upper and lower plena; removing the bridge allows more air to flow between the plena).
Removing the bridge does expose two coolant reserviors, front and aft of the TBI holes --- so those would need to be welded closed. The coolant reservoirs are probably to help thermally stabilize and cool the intake from the EGR feed.... as well as to possibly help prevent ice formation on the throttle blade at part throttle, high humidity, and near freezing conditions.
He also did some mild porting on the rest of the manifold. I can't say how much it helped by itself because he did several mods all at once: dual exhaust, copcar injectors (and a revised injector constant in the EPROM), the aforementioned manifold mods, plus the Caddy bonnet + sewerpipe + cone filter dropped his 0-60 mph time from the mid to high nines down to under 8 sec (7.6 was his best IIRC). He never got it to the track (it was a daily driver for the winter) but it probably would have run high 15s, which is much better than the low 17s sec the car would run factory stock. It was quite a good/cheap gain for minimal mods on the 4200 lb with LO3 and he got 26+ mph highway. It also didn't change his hood clearance, because it's the spacer mod without using the spacer.
I have done the same mod but I need to get it welded up before I install it. HTH.
Removing the bridge does expose two coolant reserviors, front and aft of the TBI holes --- so those would need to be welded closed. The coolant reservoirs are probably to help thermally stabilize and cool the intake from the EGR feed.... as well as to possibly help prevent ice formation on the throttle blade at part throttle, high humidity, and near freezing conditions.
He also did some mild porting on the rest of the manifold. I can't say how much it helped by itself because he did several mods all at once: dual exhaust, copcar injectors (and a revised injector constant in the EPROM), the aforementioned manifold mods, plus the Caddy bonnet + sewerpipe + cone filter dropped his 0-60 mph time from the mid to high nines down to under 8 sec (7.6 was his best IIRC). He never got it to the track (it was a daily driver for the winter) but it probably would have run high 15s, which is much better than the low 17s sec the car would run factory stock. It was quite a good/cheap gain for minimal mods on the 4200 lb with LO3 and he got 26+ mph highway. It also didn't change his hood clearance, because it's the spacer mod without using the spacer.
I have done the same mod but I need to get it welded up before I install it. HTH.
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