Topic: help raising comression on cb650

[nuzzi]

i recently picked up a 81 cb650 so im new to this engine, im looking to raise the compression, can the head be shaved to bump it up? if so how much. thanks.

[jaguar]

why do you want to start with compression?

is EVERYTHING else up to 100% on the bike?
i dont know alot about the 650s but i would think they are similar to other sohc4 hondas and the weak link is the head.
if you shave the head you need to start thinking of new ways and parts needed to time the motor.

[Frankenkit]

Correct me if I'm wrong (and I often am) but doesn't the 650 already have a hemi'd cylinder head?  From what I understand, it's basically a 550 bored out to 650, so structurally you're pushing it a bit to bore it further (not that that ever stops anyone).

I'd +1 Jaguar on making sure everything else is at 100% performance-wise first.

[Pinhead]

No, the 650 is NOT an overbored 550. The ports, chambers, and pistons are miles ahead of the 550 in terms of design.

Simply removing the base cylinder gasket will raise the CR noticeably.

I'm running '77 CB750 pistons with a final CR of 11:1 on my '79. Unfortunately, the machinist that made the pistons fit hasn't been online for a while (look up soos for the details and relevant threads).

GM HEI ignition is a worthy upgrade as well (esecially if you run higher CR).

[nuzzi]

thanks for the replies but i dont need the bs run around about why i want more power. i may be new to the board but am by no means new to the performance world so i deserve a little respect. obviously i would upgrade other components to create a well balanced machine.

that said, ive heard of the coil upgrade so ill be looking  into that thanks, i also read about the gasket trick, im just curious how the gasket being removed affects the seal, or is it not important? do you just use gasket maker or silicone? any idea what the cr is by doing this? i wouldnt take more than 10 thou off the head anyway, ill be doing the headgasket so i figured while i was in there i might as well. thanks again for the good info.

[bollingball]

No one deserves respect you earn it. Yes use the silicone.

[jaguar]

Im not giving you a run around about why you want power.
i was just asking a basic question about why start with compression.

I wasnt going to assume anything more then what you placed in the post.  so we have no info as to other upgrades that you want to do that goes along with compression.
myself im interested to hear what some of the experienced SOHC guys say as i havent heard of many people building HC street motors. 

so why dont we start with the "package" that you are looking for in your build
what are the goals for the motor?
street?
race?
pump gas?
reliability?


on a kind of side note, what are the basic specs on the "gentleman's cafe"?  seem to remember it was a 500/550 with 650 parts.
are there inherent benefits to a 550/500 over a 650 in terms of performance?
if the head really does flow that much better then a 500/550 then i would assume that gains could be made with the bottom end.

[Jon]

Depending on how far you want to go an increase in displacement is not too hard.

Check out Paulage's 550/650 Big Bore, it covers it well.

Respect is earned not asked for.

[Frankenkit]

on a kind of side note, what are the basic specs on the "gentleman's cafe"?  seem to remember it was a 500/550 with 650 parts.
are there inherent benefits to a 550/500 over a 650 in terms of performance?
if the head really does flow that much better then a 500/550 then i would assume that gains could be made with the bottom end.

One of the inherent benefits of using a 550 bottom end with a 650 head is that you still get to have a kickstart and (I believe) the 500-550's PM charging system instead of the often faulty 650 electromagnetic system, wherein after about 30 or so years the rotor likes to short internally and cease to provide a charge.   Not that the 650 charging/starting system is that awful, but there's an undeniable bit of cool factor derived from kickstarting your cafe instead of hitting the start button. 

[jaguar]

on a kind of side note, what are the basic specs on the "gentleman's cafe"?  seem to remember it was a 500/550 with 650 parts.
are there inherent benefits to a 550/500 over a 650 in terms of performance?
if the head really does flow that much better then a 500/550 then i would assume that gains could be made with the bottom end.

One of the inherent benefits of using a 550 bottom end with a 650 head is that you still get to have a kickstart and (I believe) the 500-550's PM charging system instead of the often faulty 650 electromagnetic system, wherein after about 30 or so years the rotor likes to short internally and cease to provide a charge.   Not that the 650 charging/starting system is that awful, but there's an undeniable bit of cool factor derived from kickstarting your cafe instead of hitting the start button. 

see you learn something new every day....even on the end of days....lol

[nuzzi]

i apologize for coming off strong. ill be using 93 octane, street driven, not a racer, im not looking for top speed and will never take it past 100, ill be going up a few teeth on the rear sprocket, stg 3 dynojet kit, k&n oval pod filters, cut stock exhaust (4-4) upgraded ignition, most likely will throw a larger tire on the rear too so ill probably go with a 45t rear sprocket. not looking for a rocket just want good low end and more pep, with more growl. im attempting to keep it low cost so the gasket trick will surely be used. when removing the gasket, is there enough adjustment in the valve train to compensate or are other mods needed? say if i removed the gasket and took .010" off the head, where would i be and is that still within the range of the stock adjusters? thanks.
oh and a kick start would be bad ass.

[Pinhead]

I'm not sure about the clearances when removing the base gasket; I told soos that I was planning on running with no gasket and he machined the clearances to fit.

The 650 bottom end has more stroke so unless you want to do a complete overhaul (Paulages 550/650 Hybrid) than I'd stay with the complete 650 motor.

The 650's primary drive ratio puts the clutch in a class above the 550's, as well.

Really, the only technical advantage that the 550 has, is the kick starter. EVERYTHING else in the motor is superior.

The main weakness is the alternator rotor but if you do have to replace any parts, they'll be good for another 30 years.

[nuzzi]

good to know, well i think my best option at this point will be just to shave the head enough to be flat, and look into the base gasket removal and leave it at that. so anthother question is will i need to do a hone job since the pistons will be traveling higher in the bore and going past the wear mark?