Rarity- Accord V6 6MT Hypermile/Build Thread

[Daschicken]

498.8 miles, 13.884 gallons= 35.93 MPG!

I added the previously mentioned middle tray extensions and rear wheel spats. Also I got one of the OBD II insurance adjuster things, so I won't have my scangauge 2 for the next three months. This also means I have to change my driving style slightly, I accelerate too fast apparently, so I had to slow down. Also, I can't brake hard for fresh red lights anymore, oh well.

Wow, look at that percent over EPA! (65.2% for future readers) I'm number 13 in cars % over EPA!

[Daschicken]

I'm going to be honest, my engine does not make much low end torque. What is my plan to change that? Replace it with a smaller engine that revs higher and makes even less low end torque of course! More on that later. My actual plan to increase low end torque is to install an intake manifold spacer from a 2005-2006 Acura MDX. The manifold spacer shifts the torque curve to lower rpms so I should get more usable torque for my usual low rpm driving. These spacers cost $50 brand new, but since i'm cheap and want this to be able to pay for itself i'll look for one in a junkyard. Unfortunately, the spacer is too tall stock, 1.5", so I will have to mill it down to about 1". Luckily my dad has a mill! People on Driveaccord have reported improved low end power, and in some cases better mpg, both good things. I have high hopes for this mod.

Okay, as for the engine swap plan...

I discovered a while back that I did not actually have the smallest J series engine Honda made, they made an even smaller V6, the J25. That's a whole half liter smaller than my J30! And no, before you suggest it I am not swapping to the 4 cylinder. I like my V6s, and I also like having a tall geared 6 speed manual. The J series transmissions do not fit on the K series 4 cylinders. Back to the J25, it was made from 1998-2003, IN AMERICA! Unfortunately it was only sold in Japan. However, there are plenty of J25s being shipped here with low miles for cheap, because, Japan...I found one such example of a J25 for $399! But it was in New York, so that's that...The J25 shares some features that my J30 has, like a variable intake manifold, and VTEC, yo! However, it also features something different...Direct injection...

Honda has a habit of implementing new technology and not advertising it before they know it works. Case in point: SH-AWD. NO ONE talks about the direct injection in the J25, I really thought that the new earth dreams engines were the first Hondas to have direct injection, guess not. This direct injection makes me a little nervous. If it was in the original car I would be absolutely fine with it, but it won't be. My plan for this engine was to swap my J30 out for the J25, hook up everything like normal, and enjoy better fuel economy. The car wouldn't know the difference. However, with direct injection, I don't think this would be the case. If I plug everything in like normal, assuming the injector connectors fit each other, I think the DI would fire early(comparatively) and cause pre-detonation. The other thing i'm worried about with this engine is the compression ratio, I can't find any info about it, so I have no idea what it is. Does it run on regular gas? I have no idea. What I will probably end up doing is swapping the heads from the J30 to the J25, just to make everything guaranteed to work, and just leave it at that. The car wouldn't know the difference, the engine will just have a shorter stroke.

I don't JUST want to swap to a J25 for better fuel economy, I also want to enjoy the ability to rev to 7200 rpm, instead of 6800 like the J30. However, If I swap the heads over and just plug it into the car I fear it will still be limited to 6800. And I doubt the Honda Inspire ECU is compatible with my accord, so I would probably have to resort to a tuning software to raise the rev limiter. I really want to leave the original J25 heads on, and make everything work, since DI should have a fuel economy benefit. But without hooking up an Inspire ECU I just don't see how I could get it to work. The J25 was made back before Honda integrated the exhaust manifolds into the heads, so that means the possibility of running some nice tuned headers, but again, I don't think I will be able to get the stock heads to work. Another problem with that is emissions compliance, will I get a check engine light? Do the J25's exhaust manifolds have O2 sensors in them? Does it have pre-cats like my J30 does? Do the headers hook up to my stock J-pipe? I have no idea.

So since I have little confidence in the DI system working on my accord ECU (honestly I wouldn't even want to test start it, cause it might blow itself up), I have three potential solutions:

1. Most realistic solution: Swap the J30 heads onto the J25, the car WILL NOT know the difference.

2. The solution I want to work: Keep the stock J25 heads on, leave the direct injectors in place and swap all the J30 hardware onto the J25, ie: Intake runners, manifold, fuel rail(assuming the fuel injectors plug into the runners, and not the head), injectors.

3. The solution that won't work but I wish it could: Sprinkle some unicorn dust onto the fuel injectors and injector connector plugs, cross fingers and hope it doesn't explode

My current questions regarding the J25:

What fuel economy benefit does it offer? It seems to get 10% better fuel economy than the J32 according to one site, still looking for user feedback.

Fuel system questions:

Can my fuel pump supply the correct pressure for the DI system? No Idea.

Will my fuel rail bolt up to the DI system? Probably not.

Will my accord's ECU work with the DI in the J25? Probably not.

Will the ECU limit the engine to 6800 rpm instead of 7200? Probably.

Can the Inspire ECU work on the accord? Probably not.

Will the engine run on regular gas? No idea.

Will it be emissions compliant with stock heads? No idea.

Mechanical questions:

Are the mounting holes the same? Apparently all J series blocks are the same so they should be.

What type of oil to use? No idea.

Will my J30 flywheel fit on the J25? Probably.

Will the engine mounts be in the same places? Probably.

Will the heads have to be swapped ? Most likely.

Will the accord J pipe fit the manifold on the J25? Doubt it.

Is the throttle body compatible with the J25? It will be if the manifold from the J30 is swapped over!

Is the intake manifold the same on the J25? From the pictures, it doesn't look the same, however it still has a variable intake function.

Also on my engine mods to try list: I kind of want to try adjusting the valve clearances on one intake valve each to give an intake swirl effect like in the i-VTEC engines. I found the valve lift to be about 10mm on a performance K24, its probably similar on a J30, and it looks like when in low lift mode a i-VTEC engine lifts one of the intake valves about 40-50% of the height of the normal lobe. I fear it may be bad for the valves if I do this, and I am sure it would be very noisy. What I am not sure of is any benefit offered by this. Also, my engine would be unable to revert back to normal lift for both valves when I need to step on it, so I don't know what that would do to the engine.

[Daschicken]

I researched what cars came with the intake manifold spacer, and found two different part numbers of spacers.

17108-PGK-A00
2003-2004 Honda Pilot
2002-2004 Honda Odyssey
2001-2002 Acura MDX

17108-RDJ-A00
2005 Honda Pilot
2006-2008 Honda Pilot 4WD
2006-2014 Honda Ridgeline
2003-2006 Acura MDX

Aside from the price, I couldn't find out any difference in the parts. The PGK costs $44, but the RDJ is only $23. After looking back here, I think I found the difference. The PGK has knubs that stick out, and have threaded holes. The RDJ, of which is pictured above does not have these knubs. However, the car I took mine from (2003 Honda Odyssey) did not have anything attached to them. After milling down the spacer, the knubs interfered with the fuel rail, so I sawed them off.

Here is how it looks before the spacer:

After:

Here is where the spacer sits, right on top of the intake runners. Before:

After:

Those pictures were taken with the standard sized studs still in place. I was smart enough to grab the studs from the Odyssey the first time I went to the junkyard. Here is a comparison.

What a difference!

Unfortunately, I didn't think I needed the bolts from the Odyssey because I thought they screwed into something else I guess. So I needed to take another trip to the junkyard to grab these:


The spacer was too tall stock, so it had to be milled down, I already knew this from the start. I test fit the manifold with spacer before milling it, and was surprised to see the hood actually close without bulging. Clever placement of an iphone in video mode showed that the hood was resting on the manifold when closed. It needed to be milled. Milling this thing was a huge PITA until I made a somewhat obvious yet brilliant revelation. Screw the friggin' clamp that required a ridiculous amount of leveling to get to an acceptable level. Just take the damn thing off and set it on the mill's table directly. Voilà! No leveling needed! The spacer I got was about 1.55-1.6" stock, and we milled it down to about 1.3". On driveaccord I heard most people were using .75-1" spacers, so more torque for me!

Here is the spacer milled and de-knubbed:


After getting the correct bolts, the spacer was ready to go on. Or was it...? That little extra height added by the spacer caused three major problems, and one minor one. First, the connector for the variable intake motor was not long enough. Thankfully after some not-so-careful tugging, it gave the extra room needed to reach the plug.

Here is the plug in question, and a good picture of how the spacer fits in there.


Second, there was some sort of sensor attached to the back of the intake manifold that no longer cleared the strut tower bar. Again I shall use the words not-so-careful to describe the solution to the problem. Bang on the bracket with a hammer till it was bent enough to clear the bar with some room to spare. I need to remember to not back up too aggressively or else the engine may pitch forward and take out that sensor.

The third problem was not so major, but it did prevent my car from going back together for another day and necessitated two trips to the auto parts store. This problem was a coolant line connected to the throttle body. With the added height of the spacer, the line was too short and would not allow the throttle body to reach the height of the manifold. After a new hose from the parts store, my dad discovered the old hose clamps were not sufficient to hold on the new hose. Cue the second trip to the parts store for proper hose clamps. I removed this exact hose on my 4 cylinder accord and it resulted in air getting into the cooling system and overheating the engine, so I remembered to bleed the cooling system this time.

The last, and thankfully, minor problem was that due to the added height of the intake manifold, the intake hose leading to the throttle body would be more likely to tear because of the awkward angle it is stretched at. I simply unbolted the airbox and pulled it slightly further away from the downstream intake piping to give it more room. I later reattached one bolt because the airbox was rattling.

I got the car back together after two days of downtime, bled the cooling system and took it for a ride. First impressions are that there is more torque at low rpm. Can't say anything about higher rpms, since I never got above 3000 rpm. It could just be that I was driving a miata around for two days, but I think it did improve low end torque. On one of the usual hills I go up at 45 mph in 6th gear at 1500 rpm, I usually have to really put my foot down to maintain speed, but with the spacer installed I didn't have to. After I got back from my test drive I discovered that the bolts holding on the throttle body were only finger tight. I tightened them up and haven't driven it since.

[serialk11r]

I don't think that your spacer is doing you good as far as fuel economy is concerned, since you didn't tune it and the engine probably runs slightly rich at high load. If that wasn't the goal then ignore that comment lol.

[gizzardgulpe]

This has been a fun read. I just bought a 2007 Accord V6 with the automatic transmission. My tank averages have been between 26 and 30 mpg so far with no modding, so I'm really pleased to see that the car responds well to little more than careful driving.

Still a bit jealous about that ~36 mpg tank. Awesome for a V6.

[Daschicken]

Originally Posted by gizzardgulpe

This has been a fun read. I just bought a 2007 Accord V6 with the automatic transmission. My tank averages have been between 26 and 30 mpg so far with no modding, so I'm really pleased to see that the car responds well to little more than careful driving.

Still a bit jealous about that ~36 mpg tank. Awesome for a V6.

Good to see i'm attracting other accordians!

I have not driven any honda automatics at speeds higher than 15 mph, so I can't offer advice there, but try to keep the torque converter locked up, and don't be afraid to coast in neutral as long as the engine is still on. As of now, I do ZERO engine off coasting, and it will continue to stay that way until I can wire up a kill switch. Those are some pretty decent tank numbers, especially for an automatic, I just checked the EPA ratings and it looks like you have a 1 mpg highway penalty. However, EPA ratings for manuals are so much easier to beat.

For the last 2 months I have been without my scangauge, still have one month to go until I get this stupid insurance thing off my car. I'm up to a 22% discount . After that one month, I can resume hard braking for fresh red lights and turn off my engine for lights again. Along with those techniques restored, I will get my scangauge back so I can properly drive with load again, maybe block some of the upper grill for short trips, and I will be getting 2 new tires, likely Goodyear Assurance Fuel Maxes. I will finally get to see how my intake spacer affected mileage, yay!

My recommendations to you as for getting good fuel efficiency with this car, well, there is a big list of things.

Driving style:
The city rating for this V6 is pretty abysmal, at 18 mpg. I can confirm that it is an accurate rating. The secret to getting good city mileage is to avoid having to start from a stop. These are heavy cars, and this engine drinks fuel when accelerating, so try to coast as much as possible.

This engine also drinks fuel at idle. In mine, the scangauge indicated .35 gph at warm idle, my 4 cylinder accord idled at .21 gph. If you are going to be waiting longer than 30 seconds or so, shut off the engine. The starter is very easy to replace in this car, so don't worry about that. I always shift to neutral when I come to a stop in an automatic, it is usually good for a 20-25% reduction in fuel consumption at idle.

The highway rating is what is really impressive for these cars. They like steady driving. Stock aerodynamics are decent, but the underside leaves something to be desired, so try to keep the speed down.

Maintenance and mods

1. Air pressure. The door on my car specifies 32 psi front, and 29 psi rear. I laughed at the sticker and instead set the front to 48 psi and the rear to 44.

2. Maintenance. Cleaning out the intake in my V6 was good for a 10% improvement in fuel economy, if my back to back trips were accurate. Not only did it improve economy, but also power. Cleaning the intake in these engines is deceivingly easy, just unbolt the intake manifold cover, and below it are the bolts holding the manifold to the runners. You will have to unplug some stuff and disconnect a few hoses, but it is fairly simple.

After removing the manifold, you will see the 6 intake runners, and they will likely be filled with carbon. Mine were. Grab at least 2 cans of intake/throttle body cleaner, spray it onto a rag, and then wipe all that carbon off of the runners. The carbon inside the manifold is much harder to clean off since you can't reach all of it, but try and reach what you can. Clean the throttle body too, just make sure to unplug the electrical connector if you are going to move the throttle plate. I have taken that area of my engine apart probably 4 times now, and have yet to replace any of the gaskets, so don't worry about them.

One big thing to be concerned about with the accord v6 engines of this time period is loose spark plugs. The number 5 spark plug is notorious for coming loose and popping out. Cylinder number 5 is the front middle cylinder, closest to the bumper, as opposed to the firewall. My number 5 plug was actually fine, but my number 2 plug was HAND TIGHT! Check yours!

3. Mods
As far as aero mods go, I would suggest that you make an engine undertray first. I noticed an improvement in coasting when I installed mine. I would not suggest trying to add onto the existing front undertray thing, instead I suggest starting at the bumper and making your way back. That is the best way to make sure your undertray stays ON your car.

After that, I would suggest making a lower radiator panel like the one I made in post 48. It will reduce lift and improve the cooling system efficiency, therefore making you more confident in a grill block. I know the 4 cylinder automatics have a transmission cooler built into the engine radiator, but i'm not sure about the V6s. So don't get too carried away with the lower grill block.

After I get some sleep and get back from work tomorrow (LATE!) I will post some pictures of the underside of a V6 accord of our generation. Mine looks the same underneath. I will also post some comparison pictures of what mine looks like now with the undertray on it.

Good luck with the modding!

[gizzardgulpe]

Those are some pretty decent tank numbers, especially for an automatic, I just checked the EPA ratings and it looks like you have a 1 mpg highway penalty. However, EPA ratings for manuals are so much easier to beat.

Thanks. The automatic transmission is one of the very few things I dislike about the car. The other is the inability to turn off the AC during defrost. I'm willing to turn up the heat to reduce relative humidity, but Honda apparently thinks I'm too dumb to make that decision for myself.

The city rating for this V6 is pretty abysmal, at 18 mpg. I can confirm that it is an accurate rating. The secret to getting good city mileage is to avoid having to start from a stop. These are heavy cars, and this engine drinks fuel when accelerating, so try to coast as much as possible.

Preaching to the choir. I think my two low MPG tanks came from a 20:80 city:highway driving mix, instead of my usual 5:95 ratio. Absolutely kills the efficiency to wait at a light and take off to avoid causing traffic problems.

My biggest concern with neutral coasting is how hard the transmission shifts. I only have 40k miles on it, but it probably has the old Dextron III or IV fluid, which I guess Honda changed to DW-1. I'm planning to change it in the next couple of weeks and I hope it smooths out the shifting a bit so I don't feel so queasy about pulse and glide and neutral coasting.

To be clear, it doesn't shift harshly enough to worry me, but I kinda figured a 2007 transmission would be about as perfect as auto trans technology gets. My wife's 2001 shifts a hair smoother though, so I dunno.

One big thing to be concerned about with the accord v6 engines of this time period is loose spark plugs. The number 5 spark plug is notorious for coming loose and popping out. Cylinder number 5 is the front middle cylinder, closest to the bumper, as opposed to the firewall. My number 5 plug was actually fine, but my number 2 plug was HAND TIGHT! Check yours!

Good looking out. I'll check mine later today. I pulled one out when I bought the car last month to see if they needed to be replaced or cleaned, and it was tight, but I didn't check them all.

As far as aero mods go, I would suggest that you make an engine undertray first. I noticed an improvement in coasting when I installed mine. I would not suggest trying to add onto the existing front undertray thing, instead I suggest starting at the bumper and making your way back. That is the best way to make sure your undertray stays ON your car.

I made a wind dam of the front of the bumper with some aluminum sheet metal and some L brackets. It looked great and had a nice shape. Until I hit a burst of wind and it got deformed, then I hit a floodway from a parking lot and the whole thing got mangled to hell.

I haven't yet decided if I'm going to improve the wind dam and reattach it or shape the aluminum into other smaller mods.

I would sure like to get a hold of the hybrid diffuser though. Maybe drop that .29 drag coefficient to .28

[Daschicken]

Here are some comparison pictures of the underside of our V6 accords:

Stock rear bumper area


Modified with hybrid diffuser plus extensions


Stock under engine


Engine undertray


Stock Further up


Modified Further up

[Daschicken]

Originally Posted by gizzardgulpe

I would sure like to get a hold of the hybrid diffuser though. Maybe drop that .29 drag coefficient to .28

.29? Do you have a coupe? Sedans are .30 (hybrid is .29) and coupes are .29

Here is the part number for the rear diffuser: 74550-SFY-000 It is called "COVER, RR. (LOWER)" and is part number 22 in this diagram: Honda Automotive Parts

You would not believe how long it took me to figure out which part it was. The name does not suggest rear diffuser at all, and the diagram shows the underside of the diffuser, so it doesn't really look like that's what it is. Don't bother buying the bolts, they won't work. What you need is two long bolts/screws and two nuts to go onto them. Installation will require you to drill two holes into the spare tire area of the trunk. The sheet metal is way too thin to tap threads in, so you need to use nuts.

Do not get this diffuser because you expect it to pay for itself. (I noticed no improvement in coasting nor stability. However, I have no doubt it vastly reduced bumper parachuting and reduced rear end lift.) Get it because it looks cool, it certainly won't make things worse, and it is relatively cheap ($45).

[serialk11r]

@gizzardgulpe

I've driven my parents' 2005 Honda Pilot quite a bit, which has a similar engine and 5AT. I think if you want to P&G, you should try "rev matching" it. For example with the torque converter unlocked in 5th gear on the freeway at 60mph, the engine is at 2500rpm. So before shifting from neutral to D, I give the gas a jab to bring it to 2500rpm ish, and it is very very smooth.

Similarly, you can "rev match downshift" if you are coming to a stop or descending a hill. Coming to a stop this might be able to save a few drops of fuel by avoiding the engine from idling, which it seems to do as you slow down past 40mph. Drop it to "2" while giving it a jab of throttle to bring the rpm up, and it goes into gear very nicely if you do it right.