Topic: Big Bore Kits

[Cpcracing]

I have a K5 and want to build a reliable street performance engine to run on 91 octane. I am planning on major head work, camshaft, crank, rods and larger carbs.  What does it take to break 100 HP? What size and Brand of Big Bore kits do fellow forum members recommend? What components work well together ( like displacement via carb size and camshaft recommendations)?

[Retro Rocket]

Its not an easy task chasing 100hp on a streetable 750. Ape engineering have 970 and 1000cc forged piston sets that are very good, 10:25 to 1 comp I believe, cycle x also has a nice 915cc kit as well as 970 and 1000cc kits, they may be the same as the APE kits. Forum member MRieck has gorgeous billet aluminium blocks, much better than over boring stock blocks to these sizes, they are not cheap, but they are state of the art, mine has nikasil bores, no sleeves, better for heat dissipation and weight...ALso loo at the kibblewhite 5 mm oversize valve conversion for these bikes...  I'll post a couple of links...

APE
http://cbrzone.com/sohc.html

CycleX
http://www.cyclexchange.net/Engine%20Parts%20Page%20%28Non-Cafe%29.htm

[Jerry Rxman Griffin aka MuthaF'er]

What does it take to break 100hp? Seven, eight grand $$, the right people, the right parts. Reliable, street..... I'd say this depends on how well it is built and how you want to ride it.

It IS possible. The 915 even with a Mike Rieck Stage 4 head will be questionable to achieve above 90hp with 10.5:1 compression pistons. I'd say you're going to have to go 1000 or larger.

[cbr954]

Last I talked to APE they are out of the 970 and 1000.  Cycle X only had 970 in stock but they may have the others now.  Dynoman Performance has a good selection of pistons in stock and they will custom build any piston you want.

[754]

80-90 hp can be achieved for about half of what 100. Will run you..

[HondaMan]

The only way I know of to make 100 HP with these as streetable and reliable (for more than 5000 miles of life) is with a turbo.
80 HP is no problem, even 90 HP if done carefully (and completely). The crankcases will need some assistance above about 85 HP (a la R/C Engineering mods of old) lest it end up a leaky mess.

[SOHC4 Cafe Racer Fan]

100 bhp is a tall order for a streetable CB750 running 91 octane pump gas.  I think Mick is right that you have to consider 1000cc -- best form would be a Mike Rieck billet block and a Mike Rieck Stage IV head.  Even then, with Carrillo rods and other go fast, light parts, you will be challenged to break 100 bhp. 

You can get a torquey 80-ish bhp 836cc with a Stage IV head with less money expended. 

[PeWe]

100 hp on the real wheel or crank hp?
I do not know the power difference,crank vs rear wheel power. Car threads talk about -15%, some bike threads about -10%
If you search this forum you'll find several bikes with 80-85hp on rear wheel. I guess reaching 100hp will cost a lot more.

836cc (only bore your std cylinder), ported head (w larger in valves), cam, carbs will take it there. Maybe with std carbs to save money.
I regret I did not invest in stronger rods when I enjoy reaching +9000rpm with my 836 that measured +83hp on the dyno last week. This year better carbs, but I do not see better top speed.

I have used my bike as a reliable touring bike back in the 80's, mostly with std carbs. It ran well with a cam not that hot I had last week. I'll see next weekend what the old cam will do on the dyno.

[Cpcracing]

Thank you guys for your response. Maybe 100 hp is too high of a goal to achieve for street use. To achieve 85 hp with reliability, what displacement and brand are fellow forum members using? What size of carbs are interfaced with your big bore?

[MRieck]

Thank you guys for your response. Maybe 100 hp is too high of a goal to achieve for street use. To achieve 85 hp with reliability, what displacement and brand are fellow forum members using? What size of carbs are interfaced with your big bore?

I got your PM....I'll give you a call tomorrow.
Mike

[cbr954]

I am running a cylce x 970 on the street.  Dont know the hp.  Using cycle x super rods  and lightened crank and alt.  CR31 carbs.  It runs strong and terrorizes the Harley guys last couple of years in Sturgis.  Makes them look:)  But even a lot of them compliment the old "750"

[SOHC4 Cafe Racer Fan]

Mike will steer you in the right direction.

[Cpcracing]

Thanks for those of you who have offered advice regarding the size and brand of big bore kits. I am leaning towards purchasing a 915 cycle X brand kit. Have any of you had good luck with this kit? Also which style and brand of exhaust will go well with the 915cc Big Bore kit ie a 4 into 2 or a 4 into 1 style exhaust? Which style and brand is the quietest?

[MCRider]

I'm posting mostly to belong to this thread. I hope to get mine on a Dyno in the next several weeks. Mike did the topend work incl valve and spring setup.  APE provided the bottom end.  APE Racing crank w/ Crower rods.  APE fitted rods to crank. Me and buddy assembled the rest of it.  At my choice its 888cc Arias pistons 12:1. Medium+ head porting (I'm not sure what Mike would call it)
with 1.5mm OS intake valves, Webcam 41 cam, Mikuni VM29 Smoothbores, and a Marshall Deeptone exhaust. The exhaust is probably the weak link for power but I wanted a streetable sound. It is a quiet 4-1.  I'd be happy with 80hp.

I had an 888 before it was very reliable, 2 up touring etc. Same carbs, Yosh head, stock cam and valves, racing pipe. Took on all comers in its day, but HP ??   

[Cpcracing]

What Brand and style of exhaust have forum members been using on their Big Bore SOHC motors? Are 4 into 1 or 4 into 2 exhaust preferred? Which systems are the quietest?

[Retro Rocket]

What Brand and style of exhaust have forum members been using on their Big Bore SOHC motors? Are 4 into 1 or 4 into 2 exhaust preferred? Which systems are the quietest?

Most would be running 4 into 1 pipes as they are usually a performance orientated pipe, its hard to equate performance with "quiet", its kind of an oxymoron, quiet pipes restrict performance...

[SOHC4 Cafe Racer Fan]

I use 4-1 pipes on my bikes, but I've seen some ridiculously awesome 4-4 race megaphones on Doctor D's CB750.  None of these performance pipes are quiet -- and the race 4-4 pipes are colossally loud.

[ivanhoew]

maybe it might be worth considering a 4 into 1 or 4-2-1 header , and then a modern bike silencer , that flows enough for 100 bhp , but has to reach oe spec silencing levels ...best of both worlds ? anything form a 600cc up would do ...lots of different styles  out there at the breakers too.


robert.

[Cpcracing]

Ivanhowe, That is a great idea! I wonder if anyone else has tried that?

[bwaller]

It would be a stretch to find something that fits. So expect to make a mid pipe or modify an existing header. This is 4-1 header I picked up cheap and used some thin wall SS swimming pool ladder side rail to make a mid pipe. Lots of nice radius bends that I just cut & welded to tuck in close. The can is GSXR750. This is not a race pipe obviously.

[PeWe]

I have heard that 4-2-1 has better midrange power.
I have had thoughts about a 4-2-1 header like this: http://www.ebay.com/itm/Cycle-X-Honda-CB750-4-2-1-Race-Exhaust-Chrome-Header-Only-Chopper-Bobber-Cafe-/151526109837?rmvSB=true

With a chromed megaphone like Russ Collins had or Vance & Hines with an ear to fit in the rear foot peg holder. The versions of megaphones CycleX has is either too short or too long with not that nice shape as the mentioned RC/V&H had.

Vance & Hines is still produced but I have not seen any megaphone only. (with baffle)
This seems to have correct angle for CB750 K0-K6.
http://www.ebay.ie/itm/VANCE-HINES-DRAG-RACE-PIPE-HONDA-750F-900F-CB110F-/291458717811?hash=item43dc4b1873

K7 had angled upwards as my RC 4-1 for K7 had on my K6 (photo). I had to use brackets for rear foot pegs.
Maybe megaphone is close: http://www.ebay.ie/itm/Vance-Hines-4-1-Exhaust-System-Megaphone-Chrome-Fits-Honda-CB750F-1979-1982-/331546675903?hash=item4d31b8cabf