Topic: Air Box Mod Per HondaMan - But for an "F" CB750

[RSchaefer]

OK so I have done all of the "Cheap Top-End Improvements" per HM's book.  He talks about "tall baffles" in the "K" air box and drilling holes in the front and/or rear vents and shows this picture:


Here are pics of the "F" box, were do you think the holes should be added, on the bottom at the rear vent?  Apparently the "F" only has the one rear vent.

[brewsky]

4 one inch holes in the bottom rear

see pics in reply #14 of this thread
 http://forums.sohc4.net/index.php?topic=112182.msg1267017#msg1267017

[Terry in Australia]

I really don't think punching holes in your airbox is good for anything more than letting water and crap into your airbox. Install a K&N airfilter and it'll breathe better with no additional holes necessary, the K&N filter is much less restrictive than the OEM filter.

I ran the stock airbox on my bike with my hotrod 836cc engine when I removed the troublesome CR's, and I used a K&N filter and 130 main jets and it breathed fine. The stock carbs will never flow as well as CR's at higher RPM's obviously, but for legal riding, they're fine. Cheers, Terry.

[RSchaefer]

4 one inch holes in the bottom rear

see pics in reply #14 of this thread
 http://forums.sohc4.net/index.php?topic=112182.msg1267017#msg1267017

How is the noise with these large holes?  You think it makes any difference to use smaller holes?

[RSchaefer]

I really don't think punching holes in your airbox is good for anything more than letting water and crap into your airbox. Install a K&N airfilter and it'll breathe better with no additional holes necessary, the K&N filter is much less restrictive than the OEM filter.

I ran the stock airbox on my bike with my hotrod 836cc engine when I removed the troublesome CR's, and I used a K&N filter and 130 main jets and it breathed fine. The stock carbs will never flow as well as CR's at higher RPM's obviously, but for legal riding, they're fine. Cheers, Terry.

I'm using the K&N as recommended by Mark, but he seems to suggest the next step of increasing the intake in addition to the K&N.

[Terry in Australia]

Up to you mate, it's your airbox, but I'd like to see some Dyno numbers before I started punching holes in a perfectly good airbox. Cheers, Terry.

[RSchaefer]

Yea thats cool!  I'm sure in some cases I'm going overboard, I do tend to take HM as gospel and was just wondering what others have done.  I did drill out my '69, CB750 airbox back-in-the-day and my '75 GL1000.  I remember hearing a little more of a roar during hard acceleration after drilling and I have pretty much upgraded every ride I have had with the K&N.  I have the K&N in my GL1800.

[brewsky]

I also used a k&n...the additional holes were to take advantage of it's flow capability.
The k7 and k8's have much more restricted stock airbox inlets than earlier models.
Noise.....I haven't noticed any difference
a/f spreadsheet is attached later in the same referenced thread and dyno posted in dyno thread

[RSchaefer]

I copied your pics and the excel screen shot from your referenced thread, thanks!!

[brandEn]

I really don't think punching holes in your airbox is good for anything more than letting water and crap into your airbox. Install a K&N airfilter and it'll breathe better with no additional holes necessary, the K&N filter is much less restrictive than the OEM filter.

I ran the stock airbox on my bike with my hotrod 836cc engine when I removed the troublesome CR's, and I used a K&N filter and 130 main jets and it breathed fine. The stock carbs will never flow as well as CR's at higher RPM's obviously, but for legal riding, they're fine. Cheers, Terry.

What clip did you run the needles at?

[BobbyR]

I like Terry wonder if just drilling holes is effective. I think it my disrupt the airflow inside the box. Honda wanted a linear flow of air coming from a quiet spot. When air is confused it tends to stay that way. That is how the Blackhawk went down during the Bin Laden mission.

What I do think if the 77-78 or others are more restrictive, I would think opening up the existing opening may be a better choice and reduce the possibility of turbulence in the box.

[RSchaefer]

Well again I was going with HM.  The inlet and the addition of holes is prior to the air filter and thus outside of the other side of the air filter and the stock velocity stacks so I'm not sure why this would mess with the stock air flow or lets say dirty the air, it would just allow the stock air box to take a deeper breath if you will.

[HondaMan]

Those rearward-facing airboxes of the post-1975 "F" bikes have several advantages over the earlier ones:

1. They face backward. While not obvious, this does improve the still-air feed supply to the carbs, and mimics all later bikes where this feed was taken from under the seat. When still(er) air is available, it is at higher pressure than moving air. If one of these airboxes would fit the earlier "K" bikes, it would make a nice improvement over the early versions. There are some fairly rare late 1975 and 1976 K5/6 airboxes out there that are exactly this: they fit the same carbs the "K" uses with the smaller velocity stacks (the holes in the front of the box are smaller, to fit), and they have the rear-facing vent. I think the early 1975 "F" bikes also have this same airbox with the smaller velocity stacks.

2. The vents in the bottom of the lower half can be notched forward about 4mm to obtain the same increased opening of the earlier mods, with similar results. This will prevent weakening of the web between the two vents: don't let it get too thin there, or it will warp later.

The rear-facing vents are much quieter than the bottom-opening versions. One of the advantages of using a full fairing like a Vetter with the earlier bikes is: it creates a large still-air zone behind the carbs, which enhances intake performance at hiway speeds (disadvantage: it brings the engine noise right to your face...). I think Craig (Vetter) did this on purpose because he learned from the Phantom fairing (the original one-piece full Vetter fairing for the 750) that the open rear side of that fairing caused a sharp power loss at speeds over 90 MPH. It took a while to realize that it was disturbing the still air behind the engine because the bow wave created a slight vacuum around the rider's knees. I noticed right away the improved throttle response at 80+ MPH when I installed the Lowers after the first year without them: the Lowers didn't come out in enough quantity to actually GET a set until the summer of 1972 (I bought my K2 in early Spring that year, with the Vetter).

Ah, the early days... 

[Terry in Australia]

I really don't think punching holes in your airbox is good for anything more than letting water and crap into your airbox. Install a K&N airfilter and it'll breathe better with no additional holes necessary, the K&N filter is much less restrictive than the OEM filter.

I ran the stock airbox on my bike with my hotrod 836cc engine when I removed the troublesome CR's, and I used a K&N filter and 130 main jets and it breathed fine. The stock carbs will never flow as well as CR's at higher RPM's obviously, but for legal riding, they're fine. Cheers, Terry.

What clip did you run the needles at?

I didn't move the needles mate (too much like hard work on K carbs) so they stayed in the middle position. Cheers, Terry.

[brewsky]

I hesitated drilling the addtionall holes for a while too.......

But I measured the box inlet to be smaller than the carb openings, and figured why add a less restrictive air filter, and keep a choked down inlet to it?

Then I found these pics of the early boxes which showed a large additional rectangle shaped opening at the rear, and that did it.

Unfortunately, I didn't get a before dyno run for just the airbox mod.

[RSchaefer]

Cool Beans Mark!

So I always get confused about location

Are you talking about notching on the bottom



or on the inside



In looking at things it kinda looks like the size of the opening at the bottom rear is similar to the opening into the box on the inside?

[BobbyR]

Those rearward-facing airboxes of the post-1975 "F" bikes have several advantages over the earlier ones:

1. They face backward. While not obvious, this does improve the still-air feed supply to the carbs, and mimics all later bikes where this feed was taken from under the seat. When still(er) air is available, it is at higher pressure than moving air.

This is what I was alluding to put more eloquently. When you are creating a vacuum you want still air. 

[brandEn]

The timing of this topic is about three days late. I just modified the airbox for my K4 build. Cut 3 holes on the backside of the lower box for my build.
Wiseco 836, Stage 1 head, Megacycle 125-65 cam, K&N filter.
Hopefully it will run OK.

[Terry in Australia]

I think it looks lovely mate, although it'd flow better if you just left the bottom off completely and fabbed a plate to seal the air filter and hold it in. Make sure you punch some more holes in the bottom to let the water drain out when you ride it on a rainy day, or wash it, etc. Cheers, Terry.

[brandEn]

I think it looks lovely mate, although it'd flow better if you just left the bottom off completely and fabbed a plate to seal the air filter and hold it in. Make sure you punch some more holes in the bottom to let the water drain out when you ride it on a rainy day, or wash it, etc. Cheers, Terry.

Well not sure if your messin with me or being serious Terry, but if I have any tuning problems I will just pick up a new airbox. I don't think I will be leaving the bottom off and fabbing a plate. Small drain holes may not be a bad idea though.

[Terry in Australia]

I'm trying to remember who it was here who did that, he posted pics of the mod and said it worked well, but like lots of these "performance mods", without dyno proof, there's more "hot air" than still air...........

[HondaMan]

Cool Beans Mark!

So I always get confused about location

Are you talking about notching on the bottom



or on the inside



In looking at things it kinda looks like the size of the opening at the bottom rear is similar to the opening into the box on the inside?

Do it like your second picture, inside the box. The outer vent is plenty big for these carbs, and you want a quiet air zone inside the enclosure as much as possible: the stiller this air, the more pressure it will push down the carb throats (sounds backward, but Bernoulli had it right...). The one place where a little trouble erupts is when the air has to turn 90 degrees from the bottom to the back: at higher RPM, this causes turbulence in those inner slots, which makes a little bit of a choke. So, opening them up a little will reduce that tumbling effect.

Honda was struggling with the DOT noise regulations with these bikes under the Carter administration, who had directed the DOT and EPA to wage war on import cars and bikes in those days. Only Honda was targeted of the Japanese bikes, but they were hit hard with both emissions rules and noise regulations (no more than 85 dB at full throttle, measured from the left side of the bike as it passed a test stand 3 feet away). This resulted in the restrictive pipes and airboxes of the later 750 bikes, although this particular box enjoyed better breathing (once modified) than the earlier ones because of the bottom vent (even if it is the Devil's own to install on the bike...). The emissions were satisfied by delaying the cams to open at 0 BTC intake because the [dumb] EPA directed the bikes be tested at idle and 2500 RPM...

[Bailgang]

I'm trying to remember who it was here who did that, he posted pics of the mod and said it worked well, but like lots of these "performance mods", without dyno proof, there's more "hot air" than still air...........

+1 Good point except the problem is that it would take a pretty sophisticated dyno to prove it. The reason I say that is because it seems these mods are taking into account the bike is in motion and taking advantage of the high pressure areas developed because of it and that's not going to happen if the dyno is stationary. I'd think you would have to have both the bike and dyno in a wind tunnel or have some REALLY big fans directed at the bike while it's on the dyno to match the wind speeds the bike would see at in what ever gear and rpm. Maybe back to back 1/4 mile runs would be a more practical alternative.

A friend of my brother has a 71 Pontiac GTO with a fully functional factory "ram air" hood and factory air cleaner that mates to that hood when it closes. After making a wheel dyno run the dyno operator was going over the air/fuel ratios the exhaust had been showing during the pull and was saying how his air filter setup was too restrictive and blah blah blah. The first words that came out of my mouth was did anybody take into account that the car was motionless during that dyno pull therefore knocking out of the equation any benefits the ram air hood may have at speed? I had everybody scratching their heads on that one.

[bwaller]

We used to "relieve" the bottom airbox, with no filter, when we production raced these in the early days. Otherwise it's an entry point to soak your filter when you ride in the rain!

[RSchaefer]

So I learned a couple of interesting things on this thread - I wonder if the EPA testing for the 85 db's as the bike passed on the left side is what caused the standard for the location of the muffler on the right side?

Also I wanted to include a little more detail from Mark that he included in a personal message as I thought it would be good for the general community.

"Many times, folks pursue the [general] idea of making a bigger hole to feed the box itself: this is not needed, as the area of the vent is already larger than 1.5 carbs. Since only 1.25 carbs are actually breathing in at any given second, that's far more than enough."

Since only 1.25 carbs are breathing in at any given second - I was obviously thinking about the intake dimension of all 4 carbs added together and not realizing this concept of only 1.25 carbs.

So the issue of increasing some internal dimension within the box, assuming a stock configuration, is to allow more air and hopefully "clean" air to enter the box at the highest RPM.

Now I'm starting to get it.  To me one of the key things that Mark brings to the table is the real detail, the physics or engineering behind some of this stuff.  I always thought I understood an internal combustion engine, seems like pretty simple stuff, right!

Interesting how such a minor change/improvement (adding holes to the air box) could have a negative effect if the concept is not well understood.