79 DOHC 750 Turbo project

[turbocity tom]

Hi Everyone,   first post here and would like to share my project.

begin with:



and 14 months later




all parts are custom fabricated by myself.



let me know if anyone would like additional details on the build.

Thanks     Tom

[TOOLS1]

"let me know if anyone would like additional details on the build. "

Well yea!!!!!!!!
TOOLS

[turbocity tom]

CB750 Modification list

Instrument panel – refurbished- needles replaced- mileage reset to 0-  all new bulbs
Mount Lowered 1 ¾” and angle reversed- boost gauge mounted in place of the ignition switch


Headlight- sealed beam replaced with  halogen  from F model- new trim ring
Mount Lowered 2”


Front fender-  10” sectioned from the center to retain the original rolled edge- mounting bracket shortened 1 ¼” to bring the fender closer to the tire.



Front wheel- 78 CB400T “Comstar” using the original CB750K front disc (drilled) custom wheel spacers- 90/90-19 tire, re-built master cyl and caliper cyl- new performance brake pads



Modified (1 ½’ longer each side) “drag “ bars – fuss holder removed from triple clamp area- factory ignition switch removed- Choke cable removed- custom throttle cables, New tach cable, New speedometer cable, New clutch cable, original front brake hose- original Honda CB750C (shorter) rear view mirrors re-bent for better look on the drag bars. CB750SC clutch/ mirror/ choke mount added to handle bar. Custom choke cable.

Custom wiring harness- keyless ignition - Headlight on delay


Custom solo seat



Rear fender- sectioned original “Boat-tail” fender – re mounted


CB750C steel chain guard- modified

Custom tail light/ license plate mount-  mid 60’s original Honda tail light housing

Mid 60’s original Honda turn indicators – front/ rear- custom  mounted

Rear wheel- 78 CB400T “Comstar” with 530 chain conversion  17/41 gearing- custom sprocket spacer- custom wheel spacers- custom brake arm. 110/90-18 tire, new performance brake shoes



Suspension-  Progressive suspension front and rear springs- lowered 1 ¼” both ends


and of course the custom turbo system








Body and paint



Thanks for looking and let me know if you have any questions.

Thanks  Tom

[Maz]

Absolutely stunning!
That's some REAL quality work you've done, there. Paintwork kind of reminds me of a Suzuki GSX 1100 EFE. I s'pose the obvious questions HAVE to be......How does it go and what does it sound like??

Maz

[turbocity tom]

Here is  a quick little video- nothing crazy or wild- just to show how the turbo works.

http://www.youtube.com/watch?v=aqzPuDY5s0k

I really like how it works- the bike was built to ride and not a racer.

Turbo 101

For those of you that are not familiar with how a turbo works:

The point at where a turbo “makes” boost is dependent on engine RPM, throttle position, and to a lesser extent, engine load.

There are certain design points around how the system operates also (size of wheels- A/R ratios of the housings) – size of the exhaust- size of the carburetor.

There is a lower RPM boost threshold. It is the point at which the engine does not produce enough exhaust volume to drive the compressor to produce more air than the engine is consuming.  (My system is approx 3700 rpm)

The position of the throttle (carburetor opening) will also limit the amount of air drawn into the turbo- allowing modulation of the boost at any RPM over the lower boost threshold.

The system will run both non boosted and under boost. The boost gauge normally swings to both sides (in a draw thru system).  When it is on the left side of the gauge, the engine is operating as a naturally aspirated engine- power levels will be very close to stock levels. As the gauge nears the 0 point (center) the turbo has begun to supply extra air to the engine. The power level is increased to around the max potential of the naturally aspirated configuration. When the gauge is to the right, which is where the turbo really begins to function. Typically, the power increase is approx 7% per pound of boost (may system is waste gate limited to 7 psi or approx 50% more power than stock.

At low rpm/ full throttle, the gauge will be at 0 (centered) and as the rpm increases over the lower rpm boost threshold, the boost will increase up to the controlled limit. (7 psi)

Under cruising conditions, above the lower boost limit threshold, the system will still operate under vacuum (left side of gauge). Now the boost level becomes a function of throttle position more than RPM. In the video, you can see the tach (rpm) change very little, but the boost will swing up dependent on throttle position. This is “power on demand” and is why turbos work so well for driven type applications.


Thanks for looking    Tom

[masiste]

Very nice job. Congratulations.

[shinyribs]

Woooow....I totally missed this bike! Very nice job! On everything!

Loved the video,too. Thanks for sharing!

[LukeM]

Great video!  The bike is very "street"able, and it looks like a smooth transition from non-boost to boost. You did a great job with this project. Are you planning to make this a kit for other owners?

I would ask if you ran pods with this project, but I'd just be asking for trouble...

Luke M

[turbocity tom]

Thanks for all the comments.

the turbo does work very smooth and is a lot of fun to drive- like adding a 400cc kit to the 750.

the carb tuning is just about done- will clean up the idle just a little more and call it good.....

I do NOT plan to build any more of these as kits- the kits would cost 2-3 times what the bikes are worth. don't think I would sell many.  I did it because I wanted to, and not many have tried to turbo the DOHC Honda's. There may be weak spots in the engine, but so far there has been no indication of any issues.

It does have a "pod" filter but uses a flat slide standard carb-  so it works fine.

Thanks    Tom

[shinyribs]

Since you are running draw-through you were able to regain gravity fed on the fuel, or are you using a pump? Draw-through turbo with a carb is famous for tuning headaches. Props for sticking with it.

How did you go about sizing the turbo and the carb?

[turbocity tom]

I have a slight advantage - I have worked in turbos for 35 years and have built many systems over the years.

The turbo is a custom compilation of parts- largest T25 compressor wheel/ smallest turbine wheel and smallest A/R exhaust housing. including a positive seal to work in the draw thru set up. Carb is sized for a 1000-1200cc engine- that is equal to the 750cc plus the boost.

I have a lot of tuning practice on Weber carbs and have the tools to make custom sized jets. Tuning has really not been that difficult. the biggest issues have been the Mikuni HRS42 is really designed for the V-twin Harley engine and the needle profile doesn't fit the 4 cyl turbo. really should have a custom needle made. Needs to be stepped, not a constant taper.  tried to make one on my cheap little lathe - BIG fail.....
The fuel curve leans just slightly as the engine passes in to boost. Above or below that point it runs perfect.

At this time I do not have a fuel pump on it- I have built a relay into the harness just in case. You really can not hold it in boost for very long (on the street)- it will go over the ton easily/ quickly.

Thanks     Tom

[LukeM]

Tom-
There is a long-standing controversy about the DOHC motors with stock carbs, and people wanting to run pod filters on them. It is generally preferred not to run pods on those carbs, unless you're prepared to really dig into jets and needles and mixture adjustments and all sorts of headaches. The joy of CV carbs, I guess.

I see where the turbo shop is up for sale. I wish you well in finding someone to take over the business.

Luke M