Improving the Nissan Xterra Throttle Body

OK, so I didn’t “need” a new, bored out throttle body for my ’00 Nissan Xterra but I sure did want one.  Always a fan of cheap and easy mods, this one had always intrigued me.  A long proven upgrade, common to practically anything with an engine and wheels, boring out a throttle body works on the same basic principal that a high flow air filter or aftermarket exhaust does as the more air you can get into and out of an engine, the more power you can make.

I had already done something with the stock air filter, having replaced it and its restrictive box with the Jim Wolf Technology (www.jimwolftechnology.com) “pop charger”, but had done little else in the “food chain” to improve performance.  At one time, Spencer Low Racing (SLR) offered a throttle boring service for Nissan (and specifically the 1st generation Xterra VG33E engine) vehicles.  That option and its reported 6 HP increase ceased to exist in late ’07 when SLR closed its doors.

Having bounced around the internet enough to know that no other aftermarket Nissan company offered such a service/product, I took it upon myself to find another viable option and prevent this potentially inexpensive and effective modification from being lost in the dustbin of history.

After a mostly fruitless search for people offering a throttle boring service of one kind or another, I settled on MaxBore.com (www.maxbore.com).  They (well, more like he) has specialized in boring out Honda throttle bodies for some time but can realistically work on any brand given that the basic principals remain the same.  MaxBore not only bores out the throttle body but also replaces the butterfly valve with a new one, made to fit the now larger opening and then cleans/refurbishes the entire unit, returning it to “as new” condition.  At a cost of only $150 (less than that of many air intake systems) and with literally scores of pages of positive feedback on the www.honda-tech.com website alone, I felt I was taking a fairly minimal risk in sending out my spare throttle body (purchased on Ebay for $19) to be worked on.

Our new throttle body (top) arrived from Maxbore looking much better than the one we had pulled off our Xterra (bottom).

About ten days after sending the throttle body out, I received and email from MaxBore stating that it was already on its way back to me.  Three days after that it was sitting on my doorstep ready to be installed.

Putting aside a full Saturday afternoon for the install, I made sure to leave myself plenty of time as even the simplest of procedures can be a chore when working on the rat’s nest of an engine that is the Nissan VG33E.

To be honest, the removal of the old throttle body and the installation of the new bored out one was much simpler than I anticipated.  It is basically a plug-and-play operation, involving little more than disconnecting the rubber air intake tube which runs to the airbox (on in my case the JWT pop-charger), two coolant hoses, the throttle and cruise control cables, the throttle sensors and the four bolts that hold the throttle body to the intake manifold.
What I was not anticipating was what the interior of the intake manifold would look like.  The interior surface of the manifold was coated in a sticky, black, oil based substance.  My first thought was that this must be due to the oil from my air intake (an oil and cotton gauze style) getting into the intake.  In looking at the old throttle body, however, there was no residue on the intake side of the butterfly valve but plenty of residue on the manifold side, leading me to believe it was an internal issue. 

Now given that the truck has 130,000 miles on it, I have never had any problems with burning or leaking oil and I have never previously removed the throttle body to clean the manifold, I chalked the residue up to a lack of proper, routine maintenance on my part vs. any serious problem.  This feeling was later corroborated by a number of other Nissan Xterra owners who have observed the same accumulation in their higher mileage trucks.  A quick search on the internet also revealed that this is a normal build up seen in almost all vehicles and not something in particular to be concerned about unless there are other symptoms of engine problems.  A good hose down with some intake manifold cleaner (who knew they had a special product just for this?!?!) and out ran all the nasty carbon buildup.

The new bore on the intake side of the refurbished throttle body measures out slighty over 1 millimeter bigger.	Originally our stock throttle body measured just over 63 mm making the new bore two percent larger.

Outside of the obvious cleaning and refurbishing that the “new” throttle body had undergone, comparing the original throttle body with the modified one revealed the differences. The original throttle body opening (intake side) measured 63.27 mm in diameter with its exit (manifold side) at 60.08 mm in diameter.  The bored out throttle body opening (intake side) now measured 64.46 mm in diameter with its exit (manifold side) at 61.78.  I made sure to do my best to measure each with the caliper at identical depths and position to get as accurate a reading as possible though a small margin of error is still likely.  Even factoring in that margin for error, it is clear that there has been an enlargement of the throttle body on both the intake and outflow side.  The differences in diameter detailed above equate to the bored out throttle body now having a 2% larger intake capacity and a 3% larger outflow capacity. Not exactly earth shattering numbers but when you are talking about a truck that started out at a whopping 170 HP, I’ll take all the help I can get!!

On the manifold side the original intake measured out to 60 millimeters.	After boring, the manifold side measures almost 62 millimeters making for a three percent more outflow.

So with a nice clean intake manifold and a freshly refurbished and bored out throttle body, my intake system may be in the best shape it’s been since the truck rolled off the assembly line.  After all the hoses, sensors and throttle cables were reattached, new throttle body gasket ($3-$4 at any Nissan dealership) in place and the four throttle body bolts screwed back in, it was time to turn her over and see what (if any) changes there would be.  Starting right up and idling smoothly, the new sensors (my Ebay obtained throttle body came with its original throttle sensors) and butterfly plate seemed to be working fine.  Revving the engine resulted in nothing unexpected, cycling the engine smoothly up and down the RPM range.  The actual sound of the engine had changed slightly. Not at idle nor with the engine under a heavy load but only during initial acceleration is there a deeper tone. A more pronounced rumble if you will.

Lastly and most importantly came the road test, which would involve taking the truck out for a decent tour (1/2 hour and about 15 miles) through a range of demands.  Now admittedly in this case there has been no pre or post modification dyno test to quantify if there have been any demonstrable performance enhancements, so all I can give in my general observations and feelings.

Installation is a fairly simple process of disconnecting the throttle/cruise control cables, a couple coolant hoses and the throttle body.	Once in place the new throttle body did not produce a noticeable difference in seat-of-the-pants testing but does help maximize our air intake setup.

I would not have expected any major power gains from this mod (if it obtained the 6 HP increase noted by the now defunct SLR it would equate to roughly a 3% power gain) and I felt none, though this doesn’t necessarily mean there isn’t one. Its just hard to feel small HP gains on a truck that has gone from stock 30 inch tires to its current 35 inch shoes. What was noticeable was the smoothness and responsiveness in the acceleration of the truck.  Now whether this is actually due to the bored out throttle body or more the result of the intake manifold being cleaned and the new sensors installed (the more likely of the two) is up for debate. Regardless, I was pleased with the result.

So if for short money ($150-$200) you want to make sure that you are maximizing the benefits of your intake (be it a drop in filter, cold air intake or snorkel) and high flow exhaust AND have an excuse to perform what I now believe to be necessary maintenance of cleaning out the intake manifold, then this reborn modification for your Nissan is well worth the minimal time, effort and money necessary.