Author Topic: Turbocharging the Digger  (Read 4199 times)

0 Members and 1 Guest are viewing this topic.

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Turbocharging the Digger
« on: August 21, 2014, 02:50:34 PM »
Hey Performance Gurus!

I recently completed my 78F Digger bike. Then I came across an ATP Turbo Kit on eBay that I couldn't pass up. So it's going on the Digger.





Cheap, Fast, and Reliable; you can only pick two. Well I don't plan on running this bike down the track, so for now I'm choosing Cheap and Reliable.

I know the 78F pistons produce too much compression, so I plan on picking up the $120 836 kit from eBay to lower compression and to increase displacement. I also plan on swapping the F cam out for a 75K cam that I have to reduce duration.  Eventually I plan on going to a Mikuni carb, but not immediately. Are there any other mods that I SHOULD do that fall under cheap and reliable?

Offline Coyote13

  • Hot Shot
  • ***
  • Posts: 534
Re: Turbocharging the Digger
« Reply #1 on: August 21, 2014, 02:54:49 PM »
Yesssssssss, I'm IN!

Would a thicker head gasket help reduce compression enough?  I know some of the Harley guys use that approach, not sure how effective it is.
'78 CB750K.  Throttle ripper.
'71 CB100.  Grocery getter.
'01 XL883.  Panty dropper. Gone but not forgotten.

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #2 on: August 21, 2014, 02:57:53 PM »
I'm not sure that it would reduce it enough. But $120 for the 836 kit plus about $200 to bore the cylinders is money I'm willing to spend. Should bring compression down to about 8:1 from what I understand.

Offline Retro Rocket

  • Eggs are hard due too a
  • Really Old Timer ...
  • *******
  • Posts: 19,279
  • ROCK & ROLL
Re: Turbocharging the Digger
« Reply #3 on: August 21, 2014, 03:04:08 PM »
If you were going to use a spacer to lower compression it would be better to use a thicker copper base gasket, i wouldn't ever use thicker head gaskets, thats just asking for trouble IMO... ;)  I'm in Digger.. ;)
« Last Edit: August 21, 2014, 03:09:27 PM by Retro Rocket »
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Coyote13

  • Hot Shot
  • ***
  • Posts: 534
Re: Turbocharging the Digger
« Reply #4 on: August 21, 2014, 03:04:46 PM »
Really? I thought I had read the eBay pistons were 10.25:1

Edit:  yes, I meant base gasket. Brain fart.
'78 CB750K.  Throttle ripper.
'71 CB100.  Grocery getter.
'01 XL883.  Panty dropper. Gone but not forgotten.

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #5 on: August 21, 2014, 03:09:59 PM »
The eBay pistons are 10.25:1 with a stock K head. I will be keeping the F head which has a larger dome. Sorry, should have mentioned that.

Offline Retro Rocket

  • Eggs are hard due too a
  • Really Old Timer ...
  • *******
  • Posts: 19,279
  • ROCK & ROLL
Re: Turbocharging the Digger
« Reply #6 on: August 21, 2014, 03:11:52 PM »
Really? I thought I had read the eBay pistons were 10.25:1

Edit:  yes, I meant base gasket. Brain fart.

Weisco are 10.25 :1, cheap ebay pistons are stock comp from what i've read here...  There's some very good turbo builds on this forum, don't be shy to ask questions... ;D ;)
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline Coyote13

  • Hot Shot
  • ***
  • Posts: 534
Re: Turbocharging the Digger
« Reply #7 on: August 21, 2014, 03:13:19 PM »
I was about to mention the F head in my previous post, makes sense
'78 CB750K.  Throttle ripper.
'71 CB100.  Grocery getter.
'01 XL883.  Panty dropper. Gone but not forgotten.

Offline TurboD

  • Hot Shot
  • ***
  • Posts: 680
Re: Turbocharging the Digger
« Reply #8 on: August 21, 2014, 03:18:32 PM »
The Feebay pistons are basically a copy of the early K pistons, they just have a bigger bore. I bought a set for my turbo build with a F3 head. I did the math at the time, but did not write the numbers down. If my memory is correct, I believe they were in the 9.5 range. 

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #9 on: August 21, 2014, 03:20:40 PM »
There's some very good turbo builds on this forum, don't be shy to ask questions... ;D ;)

I won't be! I've been reading a lot of the turbo builds. A lot of guys are taking it further than I can right now, but I am definitely getting some good info.

Offline Retro Rocket

  • Eggs are hard due too a
  • Really Old Timer ...
  • *******
  • Posts: 19,279
  • ROCK & ROLL
Re: Turbocharging the Digger
« Reply #10 on: August 21, 2014, 03:22:40 PM »
The Feebay pistons are basically a copy of the early K pistons, they just have a bigger bore. I bought a set for my turbo build with a F3 head. I did the math at the time, but did not write the numbers down. If my memory is correct, I believe they were in the 9.5 range.

Yes, you only have to compare the piston tops on the cruzin image pistons to the weisco to see that they are lower compression than the 10.25:1 weisco pistons.
750 K2 1000cc
750 F1 970cc
750 Bitsa 900cc
If You can't fix it with a hammer, You've got an electrical problem.

Offline TurboD

  • Hot Shot
  • ***
  • Posts: 680
Re: Turbocharging the Digger
« Reply #11 on: August 21, 2014, 03:26:49 PM »
Yes that is true. As a general rule with all things being equal, whenever a bore size is increased the compression ratio increases.

Offline Jim F

  • Need a bigger house for more toys
  • Master
  • *****
  • Posts: 1,887
  • go baby go
Re: Turbocharging the Digger
« Reply #12 on: August 21, 2014, 03:32:00 PM »
I have an extra set of MTC turbo pistons
In an 836 configuration if you interested
I sent these down to MTC to get checked
Out and they are good
Compression is 7.1
Jim
2002 RC51 1000 (SP2)
1983 GS1100EC Suzuki
2002 998 Dukati (Customers Bike)
1992 KTM500 2 stroke
1975 CB750/836 Honda
1978 GS750/840 Suzuki

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #13 on: August 21, 2014, 03:37:45 PM »
I have an extra set of MTC turbo pistons
In an 836 configuration if you interested
I sent these down to MTC to get checked
Out and they are good
Compression is 7.1
Jim

Is that with an early K head? I'm ultimately looking for something fun but still very streetable. Would 7:1 be too low for that?

Offline TurboD

  • Hot Shot
  • ***
  • Posts: 680
Re: Turbocharging the Digger
« Reply #14 on: August 21, 2014, 03:43:24 PM »
I would (and did) keep the ratio around 9 to 9.5. Current thinking is to keep a little compression and tune for it. Too low of a ratio can and will take away from throttle response, off boost power and increase lag time.

Offline KayOne

  • Enthusiast
  • **
  • Posts: 190
Re: Turbocharging the Digger
« Reply #15 on: August 21, 2014, 04:48:49 PM »
Methanol/water injection is very effective and will cost less than engine mods. Turbo D is spot on with his reference to lowering the compression too much.
CB750 K1 restored
1979 CB750Fz - original except for exhaust, 14000km
1984 VF45F Interceptor - all original, 12000 km
1968 S90 - all original, 2100 miles
1973 H2a, Restored
1973 H1D, next project
CB750K1 (sold)
1976 KZ900 (sold)
1981 CB900F (under restoration)
2015 Yamaha FJ09, my appliance rider

Offline Jim F

  • Need a bigger house for more toys
  • Master
  • *****
  • Posts: 1,887
  • go baby go
Re: Turbocharging the Digger
« Reply #16 on: August 22, 2014, 06:39:34 AM »
Now that I think about it
Those turbo pistons are 8:1
Sorry about that
It's been another Monday
2002 RC51 1000 (SP2)
1983 GS1100EC Suzuki
2002 998 Dukati (Customers Bike)
1992 KTM500 2 stroke
1975 CB750/836 Honda
1978 GS750/840 Suzuki

Offline 76kcafe

  • Full Member
  • *
  • Posts: 32
Re: Turbocharging the Digger
« Reply #17 on: August 23, 2014, 01:47:04 PM »
You bought that turbo kit from me off ebay. You should have it shortly.
76 CB750 Cafe
74 CB750 dragbike in progress

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #18 on: August 23, 2014, 01:48:37 PM »
You bought that turbo kit from me off ebay. You should have it shortly.

It's like waiting for Christmas!!!

Offline Bill/BentON Racing

  • Really Old Timer ...
  • *******
  • Posts: 6,725
  • Ex Honda Service Manager, Cert. Honda Tech - Racer
    • BentON Racing
Re: Turbocharging the Digger
« Reply #19 on: August 23, 2014, 02:32:15 PM »
Seen 17lbs of boost racing! It will make you smile while it's stretching your arms! Hang on and have fun!  ;D Bill
BentON Racing Website
OEM Parts | Service | Custom Builds
BentON Racing Facebook
Over 35 years of experience working on vintage motorcycles, with a speciality in Honda SOHC/4 with a focus on the CB750 and other models as well from 1966 - 1985.
______________________________________
1993 HRC RS125 | 1984 NS400R | 1974 Honda CB750/836cc (Calendar Girl) | 1972 CB 500/550 Yoshi Kitted 590cc | 1965 Honda CB450 Black Bomber | 1972 Suzuki T350 | 1973 88cc | Z50/Falcons Pit Bike | 1967 CA100| 1974 CB350 (400F motor)...and more.
______________________________________
See our latest build 'Captain Marvel' CLICK HERE

Offline PeWe

  • Really Old Timer ...
  • *******
  • Posts: 15,529
  • Bike almost back to the 70's 2015
Re: Turbocharging the Digger
« Reply #20 on: August 24, 2014, 12:45:35 PM »
Thicker base gasket... copper...
How much thicker can be used/ how high can the cylinder be raised without getting problem with cam chain getting too short?
CB750 K6-76  970cc (Earlier 1005cc JMR Billet block on the shelf waiting for a comeback)
CB750 K2-75 Parts assembled to a stock K2

Updates of the CB750 K6 -1976
http://forums.sohc4.net/index.php/topic,180468.msg2092136.html#msg2092136
The billet block build thread
http://forums.sohc4.net/index.php/topic,49438.msg1863571.html#msg1863571
CB750 K2 -1975  build thread
http://forums.sohc4.net/index.php/topic,168243.msg1948381.html#msg1948381
K2 engine build thread. For a complete CB750 -75
http://forums.sohc4.net/index.php/topic,180088.msg2088008.html#msg2088008
Carb jetting, a long story Mikuni TMR32
http://forums.sohc4.net/index.php/topic,179479.msg2104967.html#msg2104967

Offline SOHC Digger

  • I'm not an
  • Expert
  • ****
  • Posts: 1,171
Re: Turbocharging the Digger
« Reply #21 on: August 24, 2014, 01:12:46 PM »
I'm not splitting the case, so the stock rods are going to be used. Keeping that I'm mind, if I use the 836 kit, what would you guys recommend for max boost?

Offline Greggo

  • Somebody's
  • Old Timer
  • ******
  • Posts: 4,164
  • Helmets Save Lives. Period.
Re: Turbocharging the Digger
« Reply #22 on: August 24, 2014, 01:14:00 PM »
Can't wait to see how this one comes out! 

Offline NitroHunter

  • Radical Street / Strip Turbo
  • Hot Shot
  • ***
  • Posts: 711
  • A man's motorcycle is a reflection of the man
Re: Turbocharging the Digger
« Reply #23 on: August 25, 2014, 07:54:37 PM »
Cool build.
Here's my .02 for what it's worth.
Use the F cam with it's 108* lobe centers, it has less overlap.
I've seen stock rods live for short periods with stupid amounts of boost - IF YOU DON'T RATTLE THE MOTOR... but they are NEVER a good idea with much any boost. However when used with decent gas (110 is good) <8:1 cc'd static compression and 15lbs boost, stock timing, I had one go for a long time with no issues, so it can be done.
Just remember that cheap gas will get REALLY expensive in an instant.  :o  It has a way of spitting your newly "S" shaped rods out through the case.
 Big bore low compression has more torque than small bore but make no mistake, low compression sucked for street riding until that little gauge points to it's happy place.
If you shim up the cylinder base much you get into oil supply issues, like having to make special O rings and support spacers. Do-able but not advised. I used 5 base gaskets and the cam chain still fit. Barely.
 Make sure you have enough fuel flow through your petcock (or better yet fuel pump) if you intend to see much boost for more than a few seconds, it's surprising how quickly the float bowl empties and black death appears on the cylinder walls.  ::)
Robbie the NitroHunter                      Fuel Coupe Hired Gun                  NHRA T/F 640

DRAGBIKE USA XH/MB Recordholder: http://forums.sohc4.net/index.php?action=dlattach;topic=127179.0;attach=332735
DRAGBIKE USA XH/SS Recordholder: http://forums.sohc4.net/index.php?action=dlattach;topic=63940.0;attach=103300
Young mans glory days in the lanes: http://forums.sohc4.net/index.php?action=dlattach;topic=45685.0;attach=66341

Offline BLUE71TURBO

  • Master
  • *****
  • Posts: 1,849
Re: Turbocharging the Digger
« Reply #24 on: August 25, 2014, 08:59:26 PM »
 If yer gonna run any high boost levels, you need low compression turbo pistons. All other pistons are not designed for high boost turbocharging.
 The piston domes on turbo pistons are thicker. You will need a copper head gasket along with better cylinder studs. Also heed the advice that NitroHunter gave you. Use the " SEARCH "  above for all kinds of info on turbocharging.  Good luck !  ;)    Cause yer gonna need it.    :o
Remember; Before you can be old and wise, you must first be young and stupid !