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

[Terry in Australia]

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!

Thanks Brent, good to get some "real life experience" input for a change. I remember guys puching holes in their airboxes "back in the day", but I don't remember any real improvement coming out of it, apart from some "water injection" on wet days.

Scott's point is valid too, although as he's pointed out, there's a certain amount of faith required as it's hard to measure any performance gains with anything but your "Seat of the pants-ometer". When I reinstalled my engine in my Cafe Racer yesterday I refitted the stock airbox and noticed that it has two large vents as standard, one in the front, and one in the back, and I really couldn't see any need to enlarge these holes.

My engine is not stock (836cc kit, big cam, Mike Reick stage III head etc) and the stock carb's main jets have been drilled out to 130, and it's running really well. If I've got a spare airbox bottom half I'll punch some holes in the bottom (rather than the back or the front, to avoid water ingress) and see if I can notice an improvement. Cheers, Terry.  

[HondaMan]

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?

I've often wondered that, myself? Although, at the time, Hooker and R/C were marketing right-side 4-1 exits, despite the fact that the extended kickstarter often got in their way (or customized the muffler, when started...). On the other (left) side, some say it was the toe hold for the centerstand that limited the pipe dimension, so maybe that was it?

[Jerry Rxman Griffin aka MuthaF'er]

Taking a look at my 75 750F airbox after seeing that BrandEn's is different. The air enters the airbox from the bottom rear and is routed to the front of the air cleaner only. I cut five 5/8" holes on the rear. 3 on the back and 1 each on the back corners. Besides looking like a cool high performance mod my thinking was to allow air to enter at the rear of the air cleaner also. Does it equalize pressure inside? Does it allow full utilization of the air cleaner vs just the front thus restricting the front as it filters more air crud? Can I justify this? Does it perform better? Does rain affect it? I can NOT answer any of those questions, only theorize. Soooo, at this point, it's just the proven cool factor.    

[Jerry Rxman Griffin aka MuthaF'er]

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?

I've often wondered that, myself? Although, at the time, Hooker and R/C were marketing right-side 4-1 exits, despite the fact that the extended kickstarter often got in their way (or customized the muffler, when started...). On the other (left) side, some say it was the toe hold for the centerstand that limited the pipe dimension, so maybe that was it?

I can't imagine my stock system putting out more than 85db's at the opening of the muffler. Real quiet considering. Sounds MUCH better with the muffler removed!

[HondaMan]

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?

I've often wondered that, myself? Although, at the time, Hooker and R/C were marketing right-side 4-1 exits, despite the fact that the extended kickstarter often got in their way (or customized the muffler, when started...). On the other (left) side, some say it was the toe hold for the centerstand that limited the pipe dimension, so maybe that was it?

I can't imagine my stock system putting out more than 85db's at the opening of the muffler. Real quiet considering. Sounds MUCH better with the muffler removed!

Well, 85dB ain't much noise outside (especially just 3 feet away from something).
My 1967 LTD, stock throughout, won't even pass that test.

[brewsky]

OK.......here are some documented results of tuning WITH the additional holes in a stock K8....for better or worse....

(the carbs had been way over-jetted for previously installed pods and tuned by seat of the pants dyno method by local "expert" mechanic


1. pic of the 4-1" holes on stock airbox
2. dyno of final (to date) adjustments (one run with muffler baffel in, one without)
3. dyno of pre-tuning baseline (note one run at only 1/2 throttle)
4. spreadsheet of trial and error changes and A/F readings from live on board wideband O2 sensor taken while riding on the road

(note....all results are with E10 Regular 87 octane)

Since there was no baseline run BEFORE the addtl holes were drilled, there is no way to tell for sure what effect they by themselves had, or vs a bone stock model.

Next "Dyno day special" that happens at the local shop, I might just tape them up and see what happens....

[Terry in Australia]

Interesting (and disappointing, I guess) results mate, but I'd like to see what the figures are for a set of stock carbs with stock jets, before and after?

Also, if you're gonna spring for another dyno run, it'd be great to see the difference with no airbox bottom on one run, and the airbox but no filter, AKA how Brent used to race them? Thanks for the pics! Cheers, Terry.

[brewsky]

I don't really know whether to be disappointed or not......the comments from the racing forum guys were those numbers were pretty good for a stocker..... (no engine mods and 40,000+ miles)

I am definately satisfied with the power curve and throttle response, though

I actually did fab a plate and try the bottomless box with the K&N first, but it fell on its face low and mid range without re-jetting, and I didn't like the look, so I gave that up.

Interesting that simply removing the MAC baffle gained about 2HP and 2#TQ max on the dyno but made it almost un-rideable on the street (not to mention the noise!)

The local shop usually does dyno special days once or twice a year for around $20 or so for 2 pulls, so I usually wait for those

[andy750]

I would say 53 hp at 8000rpm is pretty good for a stocker with 40000 miles. Thanks for posting the dyno graphs.

cheers
Andy

[Terry in Australia]

I don't really know whether to be disappointed or not......the comments from the racing forum guys were those numbers were pretty good for a stocker..... (no engine mods and 40,000+ miles)

I am definately satisfied with the power curve and throttle response, though

I actually did fab a plate and try the bottomless box with the K&N first, but it fell on its face low and mid range without re-jetting, and I didn't like the look, so I gave that up.

Interesting that simply removing the MAC baffle gained about 2HP and 2#TQ max on the dyno but made it almost un-rideable on the street (not to mention the noise!)

The local shop usually does dyno special days once or twice a year for around $20 or so for 2 pulls, so I usually wait for those

Sorry mate, I certainly didn't mean any offence with the "disappointed" comment, but it's interesting how things change in these forums. A couple of years ago I dared suggest that most stock CB750's would be lucky to produce any more than around 50 BHP at the back wheel and I was shouted down by the "experts" who wouldn't even consider numbers that low, but there you go, the guys here who're getting upwards of 70 at the rear wheel with 836cc kits, big cams and head mods should take some comfort that it really was money well spent.

Was it you who posted the pics of the "bottomless box" plate mate? So are you saying that the holes in the box gave you a nice linear result, whereas the plate didn't? It's interesting that in a "still air" situation (strapped to a dyno, not out on the road) it would have made such a big difference? I mean, an airbox with normal venting, plus 4 one inch holes punched into it would still be that much more restrictive than a "bottomless" airbox? Interesting. Cheers, Terry.  

[brewsky]

I would say 53 hp at 8000rpm is pretty good for a stocker with 40000 miles. Thanks for posting the dyno graphs.

cheers
Andy

No problem....I wish there were more actual graphs out there for comparison for stock engines also.

My most important reason for getting the runs was to confirm mixture info for tuning. I was happy to be above 50 based on what I had read previously.

[brewsky]

I don't really know whether to be disappointed or not......the comments from the racing forum guys were those numbers were pretty good for a stocker..... (no engine mods and 40,000+ miles)

I am definately satisfied with the power curve and throttle response, though

I actually did fab a plate and try the bottomless box with the K&N first, but it fell on its face low and mid range without re-jetting, and I didn't like the look, so I gave that up.

Interesting that simply removing the MAC baffle gained about 2HP and 2#TQ max on the dyno but made it almost un-rideable on the street (not to mention the noise!)

The local shop usually does dyno special days once or twice a year for around $20 or so for 2 pulls, so I usually wait for those

Sorry mate, I certainly didn't mean any offence with the "disappointed" comment, but it's interesting how things change in these forums. A couple of years ago I dared suggest that most stock CB750's would be lucky to produce any more than around 50 BHP at the back wheel and I was shouted down by the "experts" who wouldn't even consider numbers that low, but there you go, the guys here who're getting upwards of 70 at the rear wheel with 836cc kits, big cams and head mods should take some comfort that it really was money well spent.

Was it you who posted the pics of the "bottomless box" plate mate? So are you saying that the holes in the box gave you a nice linear result, whereas the plate didn't? It's interesting that in a "still air" situation (strapped to a dyno, not out on the road) it would have made such a big difference? I mean, an airbox with normal venting, plus 4 one inch holes punched into it would still be that much more restrictive than a "bottomless" airbox? Interesting. Cheers, Terry.  

No offense taken, Terry, I was just professing my ignorance on what a late model stock should actually show at the rear wheel.

I have read many times how the early engines developed more power, and were progressively neutered as time went on......with more restrictive mufflers, more restrictive air boxes, milder cams leaner carbs, etc.

I did try the bottomless air box after I saw someone post about it, but concluded it would take some major jetting changes to work, didn't like the look anyway, so I decided to tune for the additional hole option instead.

The additional hole decision was an attempt to "regain" some of the "lost" performance from the early, more open airboxes.

I think the lesson learned is more the importance of carb tuning to match intake and exhaust conditions than anything else.

The reason I added the onboard A/F monitor was to see the real effect of jetting, airbox, and exhaust changes during actual riding conditions at speed.

I certainly didn't mean to imply the extra holes resulted in the linear curve.

It just implies to me that you can get that result, with additional holes, if the carbs are properly jetted with them in place.

[BobbyR]

I think the idea that the late models were nurtured is not true. The heads are improved on the K7-8 models and are used for improving older motors.

See how she runs when you are on the road. The mod u made can be reversed if u find sidewinds giving you problems.

[Duke McDukiedook]

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!

You don't have to sweat cleaning the damn thing after a rainstorm though!
Pesky cleaning chores.

[RSchaefer]

I do think the engines were detuned over time (neutered), HM speaks to that at length in his book in each of those areas, specifically the cam, timing (advancer), muffler, and air box.  A Lot of it appears to have to do with what was going on in the U.S. at the time relative to the oil embargo in the mid 70's more stringent EPA requirements imposed on importers, Honda in particular.

HM describes in his book those things you can do to the later model CB750's that would allow you to regain some of the original K0 HP, or to tune your mount based on your desire, in-town driving verse highway, two-up with a faring, etc.

The dyno info is interesting but some would say not important relative to what I seemed to have gleaned from this thread, which is the need to draw still air and not to modify the air box in a way that disturbs the air.  Should one want to get a little more out of the girl on the top end then eliminating some of the breathing restrictions in the air box would help, but not necessarily by drilling holes in the outside of the air box as this may contribute to making the air dirty within the air box at other than full open throttle.

Also it seems that dyno tuning may not take into consideration this whole issue of still air, in fact it might overstate HP, power range, etc. on a vehicle that IS effected by the quality of the air around the air box at speed.

[seanbarney41]

I ran an air box with no filter for years, but kept some nylon panty hose stretched over the intake to keep out debris...k1 cb750 motor and carbs, 120 mains, stock needle position and idle jets, mac 4-1 header with short exhaust pipe and glass pack insert, worked pretty good

[andy750]

I would say 53 hp at 8000rpm is pretty good for a stocker with 40000 miles. Thanks for posting the dyno graphs.

cheers
Andy

No problem....I wish there were more actual graphs out there for comparison for stock engines also.

My most important reason for getting the runs was to confirm mixture info for tuning. I was happy to be above 50 based on what I had read previously.

I will get my stock K4 dynoed next time I need a new rear tire. I have wanted a good stock reference for my 810cc K2 which gets 72 hp (with Mike Reick head).

thanks again
Andy

[Terry in Australia]

I'd love to have access to a Dyno, but the only time I've ever put a bike on one (my Triumph Rocket III) it cost me $300 for 2 runs. Consquently, I have to rely heavily on my "Seat Of The Pants Meter" which seriously needs calibrating........

[brewsky]

If I had the space, I'd love to build one.........

Say ......maybe an old car torque converter, couple sprockets and some chain, old conveyor belt head, and a spring scale?

[bwaller]

brewsky I commend you for your work. I can tell you back in the early to mid seventies I couldn't spell dyno let alone know where to possibly find one. I worked in a roadrace shop and a lot of the go-fast stuff we tried were trial & error and often likely never worked. Today we have dyno's, transponders & Mylaps to see results instantly.

You've done the right thing to help your bike run well. be happy with 53 at the rear, that's probably what a new one would have made.

As an aside I test rode a lot of 750's in the day working in a Honda shop. The fastest one belonged to a friend of mine, a K6. It was way faster than my K3 & another buddies K0. I know it was stock because I rebuilt it at some point and I could see no reason why it was so good. Maybe it was a freak, I'm sure not saying K6's were the fastest, only saying most often it's the guy twisting the throttle.

[brewsky]

I haven't checked lately, but there may even be a cell phone dyno app by now!

There is a pretty good free android version for 0-60 and 1/4 mile time called "Speedview"

[Terry in Australia]

As an aside I test rode a lot of 750's in the day working in a Honda shop. The fastest one belonged to a friend of mine, a K6. It was way faster than my K3 & another buddies K0. I know it was stock because I rebuilt it at some point and I could see no reason why it was so good. Maybe it was a freak, I'm sure not saying K6's were the fastest, only saying most often it's the guy twisting the throttle.

Yep, there were definitely "Freaks" out there Brent, I bought a pair of rough old K2's from a guy back in the late 1990's and one of them was an absolute joy to ride. The clutch and gearbox was the best I'd used, the engine was smooth and powerful, but when I stripped it down (to replace the head and cylinder block that had some fins knocked off, it was all stock standard, and I don't think it'd ever been touched.

I restored it and sold it to an English "Back Packer" who was going to ride it around Oz. He got as far as Sydney and had it serviced, and it started to blow smoke, so he contacted me to ask if I could get him new pistons and rings, but I suggested that he dump the crappy thin Pennzoil out and put some Penrite HPR30 back in, and magically, it stopped smoking, and I never heard from him again. Cheers, Terry.

[nccb]

As an aside I test rode a lot of 750's in the day working in a Honda shop. The fastest one belonged to a friend of mine, a K6. It was way faster than my K3 & another buddies K0. I know it was stock because I rebuilt it at some point and I could see no reason why it was so good. Maybe it was a freak, I'm sure not saying K6's were the fastest, only saying most often it's the guy twisting the throttle.

Yep, there were definitely "Freaks" out there Brent, I bought a pair of rough old K2's from a guy back in the late 1990's and one of them was an absolute joy to ride. The clutch and gearbox was the best I'd used, the engine was smooth and powerful, but when I stripped it down (to replace the head and cylinder block that had some fins knocked off, it was all stock standard, and I don't think it'd ever been touched.

I restored it and sold it to an English "Back Packer" who was going to ride it around Oz. He got as far as Sydney and had it serviced, and it started to blow smoke, so he contacted me to ask if I could get him new pistons and rings, but I suggested that he dump the crappy thin Pennzoil out and put some Penrite HPR30 back in, and magically, it stopped smoking, and I never heard from him again. Cheers, Terry.

He died after his engine blew and he lost control over a cliff into the pacific. Good going Terry

[HondaMan]

If I had the space, I'd love to build one.........

Say ......maybe an old car torque converter, couple sprockets and some chain, old conveyor belt head, and a spring scale?

We made one once for a Weedwacker engine, from an old automotive generator (from a Ford, I think?). Just install enough big resistors to eat all the power the generator can make and keep switching them in until either the motor stalls out at RPM or the generator burns up, and you get pretty close on how many HP it was!

It only ran once...

[Terry in Australia]

As an aside I test rode a lot of 750's in the day working in a Honda shop. The fastest one belonged to a friend of mine, a K6. It was way faster than my K3 & another buddies K0. I know it was stock because I rebuilt it at some point and I could see no reason why it was so good. Maybe it was a freak, I'm sure not saying K6's were the fastest, only saying most often it's the guy twisting the throttle.

Yep, there were definitely "Freaks" out there Brent, I bought a pair of rough old K2's from a guy back in the late 1990's and one of them was an absolute joy to ride. The clutch and gearbox was the best I'd used, the engine was smooth and powerful, but when I stripped it down (to replace the head and cylinder block that had some fins knocked off, it was all stock standard, and I don't think it'd ever been touched.

I restored it and sold it to an English "Back Packer" who was going to ride it around Oz. He got as far as Sydney and had it serviced, and it started to blow smoke, so he contacted me to ask if I could get him new pistons and rings, but I suggested that he dump the crappy thin Pennzoil out and put some Penrite HPR30 back in, and magically, it stopped smoking, and I never heard from him again. Cheers, Terry.

He died after his engine blew and he lost control over a cliff into the pacific. Good going Terry

Damn, I didn't think anyone else knew. OK, send me your Paypal details and I'll send you some "hush money".............