Thread: Gems Injection

I believe GEMS refers to the distributorless system for the early P38 4.0 and 4.6 engines whose plenum is distinctly different in appearance to the 14CUX in the 3.5/3.9 D1/RRC system using a mechanical/vacuum advance distributor for engine timing.

Garry, I assume you mean the 4.0 and the 4.6 GEMS. (Yes, I know the 4.0 is really 3.9).

Ok - now I am confused - the V8 fuel injection system with the big box on top. 3.9 vs 4.6. I thought 4.0s were all Thor plenums along with later 4.6s.

Likewise I thought there were only 3 injection systems used on V8s - starting with the old flapper on the 3.5 (maybe early 3.9s), Gems on 3.9 and early 4.6 and then Thor on 4.0 and later 4.6s.

Happy to be educated.

Cheers

Garry

Originally Posted by garrycol

Ok - now I am confused - the V8 fuel injection system with the big box on top. 3.9 vs 4.6. I thought 4.0s were all Thor plenums along with later 4.6s.

Likewise I thought there were only 3 injection systems used on V8s - starting with the old flapper on the 3.5 (maybe early 3.9s), Gems on 3.9 and early 4.6 and then Thor on 4.0 and later 4.6s.

Happy to be educated.

Cheers

Garry

G`day , depends how involved but if only RR , Disco and not car etc .

Pretty much as they came .

Flapper 3.5 RR , Hotwire 3.9 RR , Hotwire 3.5 D1 , Hotwire 3.9 D1 , Gems 4.0 and 4.6 RR , Thor 4.0 and 4.6 RR plus Thor 4.0 D2 .

Can`t help with the plenum volumes but pretty sure the trumpets are the same and do know the head ports have a greater volume in the later 3.9/4.0/4.6 10 bolt heads than the earlier 3.5/3.9 14 bolt heads .

GEMS 4.0's are different to 3.9's in that they use the same cross bolted block and bigger main bearings of the 4.6. They used sequential injection and coil packs whereas the 3.9 ran a dissy and group firing injection. Any resemblance is purely superficial. Both LR versions of the bread box plenum were superseded by the THOR system in 1999 in the P38 and the D2 when D1 production stopped.

Thanks - we are talking "bread box" injection systems.

Are all the boxes the same size ie the one with no number on it, the one with 3.9 on it, the one with 4.0 and the one with 4.6 on it - the 4.6 looks a little higher but may be optical.

What are the advantages of the sequential injection system that has the breadbox plenum compared to the group firing injection system with the breadbox plenum.

Thanks

Garry

GEMS (4.0 and 4.6) plenums are a little taller and narrower compared to the unlabelled (3.5) and 3.9 ones, but I suspect the difference in volume is not much.



Sequential injection should give better fuel economy, especially with the later GEMS running oxygen sensors. Also having full electronic timing control with knock sensors should in theory be an improvement (but you can't easily adjust it). And the coil packs are more resistant to getting drowned as they are sealed components with all terminals water resistant. However if the ECU fails you don't get to run on LPG as you can with the simpler system. The dissy in a D1/RRC is stand-alone and doesn't require anything other than one 12V wire to run.

Thanks for that - I thought the plenum on these was higher - maybe a bit too high to fit under the engine cover of my 101. I have the injection system off a 97 3.9 which makes it pre sequential. I will not be running a dissy and going coil packs etc.

Might be easier to get a Thor system and a mega squirt ECU and be done with it.

Cheers

Garry