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3400/3500 Hybrid Parts

The LX9 3500 gives 3400 owners an excellent upgrade path, but also introduces more confusion.  Here is a simple breakdown to how it all works.  Sorry, 3500 heads should not be used on a 3100 due to the bore/combustion chamber overlap.

Using 3500 heads on a 3400 lowers compression to about 9:1 on a stock bottom end 3400.  Pushrods, valve springs, retainers, keepers, and valves are all compatible.  8mm rocker arms are the only option for the 3500 heads, so for those with 96-02 3400, you will need the 8mm rockers to do the swap.

The 3500 lower intake must be used with the 3500 heads.  The coolant temp sensor is no longer located in the lower intake, but rather the rear head instead.  Drilling and tapping for the stock location is 1 option, along with plugging the rear head like the front head is plugged.  The other is to extend your coolant temp lines and use a coolant temp sensor in the rear head.  This will require a new sensor as the threads are not the same as the 3400 style.

Porting for the upper intake is where the confusion really sets in.  There are 2 gasket sizes available, the 3400 sized paper gasket, and the 3500 sized metal (or felpro paper of the same size found in the metal lower gasket set).  The 3400 upper intake should not be ported for this larger size, as it is much larger than can be properly ported.  The larger size increases flow, but requires the 3500 upper to be ported to this larger size as well.

The 3500 upper requires grinding to clear the fuel pressure regulator, and the 3500 swap FPR bracket helps with this as well (otherwise it is a fine line between clearing, and grinding a hole through the upper).  The cannister purge will no longer bolt up to the 3500 upper, the DIS mounting bracket won't line up, and the stock 3500 purge location needs to be tapped for a vacuum line.  For these reasons, the 3400 upper intake is the recommended route to take.  With the 3400 upper, everything else bolts up like stock.

The advantage of the 3500 upper is a broader powerband and not necessarily an increase in peak power.  Due to the tube that runs into the middle of the upper, cylinders 5 and 6 see less flow than the rest, while cylinders 3 and 4 see the best flow.  A 2000+ 3400 upper flows almost exactly the same across all cylinders.

The parts below are all designed for using the 3400 size gasket, even the 3500 upper intake.  If you want to use the larger port size, please check the 3500 section of the site.


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