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-   -   400 tbi problems (http://www.hotrodders.com/forum/400-tbi-problems-231247.html)

Matty8 03-28-2013 05:34 AM

400 tbi problems
 
I have put the top end from a 350 tbi onto a 400 sbc. After some wiring issues I have the thing running now. But it runs better with the wire connector for the map sensor unplugged. I'm running an inline fuel pump rated at 18 psi. With the map sensor plugged in the engine lopes and won't idle or stay at one consistent speed. However it runs nearly perfect without it plugged in. When the map sensor is unplugged the computer makes the mixture rich right? Is it possible that since it is a 400 instead of 350 that it's not getting enough fuel? I have tried multiple map sensors and triple checked the wiring. The sensors are working fine and the engine is making 20 inches of vacuum.

joe_padavano 03-28-2013 12:22 PM

Quote:

Originally Posted by Matty8 (Post 1661071)
Is it possible that since it is a 400 instead of 350 that it's not getting enough fuel?

Well, GM used different injectors and different PROMs on the 305 and 350 motors, so what do you think?

Matty8 03-28-2013 01:25 PM

Will injectors from a 454 do it?

joe_padavano 03-28-2013 01:53 PM

You've increased displacement by about 15%. Assuming you have the stock 55 lb/hr injectors (orange/black color code, P/N 5235206), you need to upgrade to the "cop car" injectors for the 350 that are 65 lb/hr (yellow/brown color code, P/N 17084327 or 17112412). That's just over a 15% increase in flow rate, so all else being the same (including the PROM), those should get you close.

Matty8 03-28-2013 02:25 PM

So if I can use those injectors without changing the ECM? What about tweaking the fuel pressure regulator?

joe_padavano 03-29-2013 05:43 PM

Quote:

Originally Posted by Matty8 (Post 1661177)
So if I can use those injectors without changing the ECM?

They'd probably get you close enough.

Quote:

What about tweaking the fuel pressure regulator?
Actually, if you can get enough flow through the stock injectors by raising the pressure, yes that also works. The whole issue is to get enough fuel into the engine to match the larger displacement. There are three ways to do that: Increase the amount of time the injector is open (that's a PROM change), increase the flow rate of the injector, or increase the fuel pressure. I don't know what the flow rates of TBI injectors are at higher than stock fuel pressure, but I'm sure that info is available. There's also a limit to how high you can to with those injectors. I think its around 16 psi or so.


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