In 2004 several members of the Pacific Performance Engineering Forum began brainstorming about the possiblity of adapting the true MPFI intake manifold from the Marine version of the Vortec 350 (L31) for use on the street version of this engine. Around the same time TunerCATS OBD-II was released and supported the 96-97 and 98-00 Vortec trucks giving us, for the first time, the ability to calibrate the "black box" PCM for larger injectors ourselves. This manifold has the distinct advantages of supporting all OEM smog equipment and eight individual standard fuel injectors. Because the throttle body is in exactly the same place as the street L31 manifold, most aftermarket supercharger and cold air intake kits can bolt right on the same.
There are now many forum members that have done this swap with the help of the information assembled here. The number continues to grow!
The original text:
Thanks to the resourcefulness of Matt "Pecos" and the collaboration of several others, we have learned a great deal about the Vortec L31 Marine Intake Manifolds we have each had in our possessions for the past few months. We consciously choose to keep a lid on this project because of the limited availability until feasibility for use with our L31 trucks could be proven. Over the past few months over 100 emails have been exchanged in our collaborated effort to identify and overcome the dozens of issues that exist with this piece. With the new PPE forum up and running so well it's time to bring our project into public domain. The editable format the forum now has will provide the means to post updates to the information and pictures as we progress.
Thank You Pecos, NJCKZ71, Speeder, White350, Cannon, RCFast, and Hog for your contributions both past and future.
Here is our information!
1. The Marine Intake Manifold has no provision for a thermostat bypass like the stock manifold. (See PIC)
The manifold casting does have the appropriate internal structure and enough metal to drill through. (See PIC) Some creative use of cold weld or epoxy is required inside to make sure the tapped hole is a sealed passageway through and into the internal structure. (See PIC)
2. The included injectors are Delphi EV6 24lbs/hr. @ 43.5psi. units with USCAR connectors. (See PIC) They work out to about 27lbs/hr. at this fuel pressures of about 53psi. These connectors are used on some Ford SVT models and the LS2 and LS7 engines. They are high impedance (~14ohm) and the same physical dimensions from o-ring to o-ring as standard Bosch type injectors. (See DIAGRAM)
...for naturally-aspirated L31's. Anything blown or having more cubic inches will want more of an upgrade in flow than the OEM Marine injectors can provide. (The whole point of this swap is that be able to put any size injectors in there you want!)
Standard EV1 injectors will also fit the rails and a harness must be made anyway. Connectors for the "oval" USCAR connectors are available.
3. There is no hole for the water temperature sensor.
There is a boss for it in the manifold, and in the stock location. (See PIC) Simply drill and tap it for 3/8" sensor. (See PIC)
4. There is no EGR valve mounting flange like the stock manifold. (See PIC) There is an internal passage for it (See PIC) with an inlet in the stock location matching the stock size. (See PIC)
Using the divorced EGR flange block from a Gen-III engine (See PIC) it is possible to cut and splice into the stock EGR pipe and have EGR. A piece of angle iron drilled to mount between the compressor and its accessory braket can serve as an attachment point for the Gen-III EGR flange. (See PIC) The two stainless steel tubes from the flange need to run behind the compressor in the location taken up by the stock upper radiator hose. It is necessary to reroute the hose forward using a 45-degree thermostat housing and an upper hose from a TBI™ truck. (See PIC) The valve will sit higher than before, but the harness will reach and the hood clears with plenty of room to spare.
5. The Marine Intake Manifold throttle body (See PIC) does not have the EGR diffuser plate (lip) or the hole to diffuse the EGR coming up under the butterfly. (See PIC) This is fine because the pipe inside is directed toward the rear, not up like the stock manifold.
This TB does not have the proper connections on it for throttle and cruise control cables making it unusable anyway. Also, the TPS and IAT are not the same parts as stock even though they appear to be at first glance. (See PIC) The return spring rate is also all wrong. The blade used in the TB would seem to have desirable qualities but the holes in it holding it to the shaft in in different places than the stock unit. The marine TB also has different bearings for the shaft which prevents any interchang eability between the Marine and stock units.
6. Throttle Body sits forward on the intake like the stock unit meaning that aftermarket intakes will work with this manifold. The TB mounting flange of the marine manifold is exactly in the same position and height as the stock manifold. This makes the L31 Marine manifold the ONLY true MPFI intake manifold compatible with bolt-on superchargers and cold air induction kits.
7. The upper intake manifold does not have an integrated gasket for the throttle body like the stock manifold does. Instead the Marine setup has this seal integrated into a channel in the throttle body. Since the Marine TB cannot be used, a gasket is needed between the stock TB and the upper manifold. (See PIC)
The stock L29 Vortec 454 uses a TB gasket between its TB and upper intake. Part number 12570168. (See PIC)
8. The fuel rail has 3/8" and 5/16" quick-connectors like a Gen-III manifold. (See PIC)
9. Entire fuel rail is stainless steel and brazed together as a single unit with no possibility of leaking. Fuel pressure regulator has a lower pressure than stock at about 50-55psi instead of 60-63. (See PIC)
10. There is no port for a vacuum brake booster.
There is a boss in the upper (aluminum) portion of the intake that can be drilled and tapped out for it.
11. PCV routing is a little different but will work. (See PIC) I heated up the stock plastic tube to straighten it out. The tube connects to a 90-degree checkvalve that PCV needs with forced induction.
12. There are no ports for all the EVAP stuff. An existing nipple can be tee'd and external divorced adapters can be fitted to the various EVAP parts and plumbed in.
13. The lower manifold is cast iron. (Yes, really, and very very heavy.)
14. There are mounting points miscellaneous stuff like the coil, but they are in a different place. The coild can be mounted directly to the intake manifold between the two pair of injectors. The drilled holes in the manifold are about 1/16" closer together than the holes on the coil - the coil holes can be enlarged to fit. The forward ear of the coil assembly needs to be cut down to clear the #4 fuel injector if wide-body injectors are used. Also, the module and heat sink need to be unscrewed from the coil bracket and flipped over to put the harness connector toward the rear. The wiring harness can not reach it otherwise.
15. The stock wire harness bridge will not fit.
16. There is no drain hole in the bottom of lower manifold because it's flat with no cavities. This is a good thing.
17. Stock heater-hose quick-connector will thread right in.
18. Stock distributor and hold-down will work, however it may be necessary to clock it one tooth off counter-clockwise so that the driver-side plug wires clear the fuel line depending on how you choose to run them..
19. Stock Vortec-style thermostat will work. (See PIC)
20. Stock Thermostat Housing will work if you do not intend to run EGR. The lower manifold also has holes for clocking the housing. (See PIC)
21. MAP sensor is different. (See PIC) Marine unit is 5-pin with integrated IAT.
The stock unit can be adapted with a custom hold-down bracket. (See PIC) The sensor needs an o-ring over it to seal to the hole in the upper manifold. (See PIC)
22. Stock GM Vortec intake manifold gaskets work. GM Part# 89017465 (See PIC)
If a conventional paper-type gasket is desired, Flepro 1255 can be used. (See PIC)
23. Fuel lines with Quick-Disconnects are needed. (See PIC)
There are a few companies making QD to AN to O-Ring adapters and lines. (See PIC) The line used for OEM installations of the Marine Intake are about 34" long with o-ring and QD ends. (See PIC) It is possible to cut off the o-ring ends and clamp onto the stock hardline under the footwell where the stock hardlines make the transition to flexible line.
Pure Choice makes quick-disconnect to -6AN Male Flare adapters. Part# 5800 (See PIC)
For the lines on the firewall two female o-ring to -6AN male flare adapters are needed from Pure Choice, one 14mm 1.5 thread and 16mm 1.5 thread. These are part numbers 5860 and 5870.
Two -6AN fuel lines with female flare on both ends are needed, one 6" long the other 7" long.
The optimal solution is to use 45-degree -6AN fittings at one end of each fuel line. This solution ends up needing one line 2" long between fittings and the other 1.5" long. (See PIC)
24. The proprietary connector for the CPI injection assembly is most practical to simply cut off and replace with a spliced in custom built harness. The new harness can be loomed and zip-tied to the fuel rails. (See PIC)
25. Throttle-Body height. The cruise control peg on the throttle body hits the passenger side fuel rail.
Use a throttle body spacer or any type to obtain the needed clearance to the fuel rail.
Use Stan Weiss's Injector Calculator to figure out what your injectors will flow at with the 50psi fuel pressure the Marine regulator puts out. (See PIC) TunerCAT can convert lbs/hr to gm/sec with its built-in calculator. The "Injector Flow Rate" in the "Fuel System Constants" section of the tuning needs to be updated with the new value. The Marine Intake Manifold flows a great deal better in the upper RPMs than the stock piece. Over time the Long-Term Fuel Trims will reflect this greater volumetric efficiency. The VE table can be updated youself through trial and error to get those trims dealed in perfectly. The alternative is dyno-tuning. If you opt to go this route, make sure the tuner is not just changing the IFR number to get the trims to zero out. The difference in flow characteristics are similar to installing a cam with a higher-rpm band but not losing any low-end torque - it needs VE tuning.
This is a PROVEN solution to the Vortec L31 fuel problem! Congratulations to RCFast for being the first one up and running on this research!
Watch the Quicktime Video of John's truck running!
Here's a Video of my truck at idle, revving, and accellerating with the Marine Intake.
Last edited by James B. on Sun Aug 28, 2011 9:08 am; edited 11 times in total