Topic: 77-78 750K engine in '75 K

[68grandprix]

I just bought a good '77 or '78 750K engine to hop up and put in my '75 750K. Will this work? I saw something about the '77s having a wider chain line. Also, are there any other differences between these years of motors that I should know about? Thanks, Jim.

[eldar]

Well the 77/78 had a 630 chain but you can throw on a 530 sprocket no problem. The part I am not sure about is if there is a different offset to worry about. The good part is that the 77/78s are about the best motors to hop up.

[68grandprix]

Why are these motors better to hop up? I've got an 811cc kit, and I was planning on installing an aftermarket cam, and doing some port work. Jim

[eldar]

Well they encompassed all the improvements over the years. They also have the best lube system and the best clutch. The 77/78 seem to be the favorite engines to hop, regardless what some say about the bikes themselves.

[Jerry Rxman Griffin aka MuthaF'er]

I'm with Eldie about encompassing all the improvements. I think it's probably the best stock engine you could use. More compression, better head = more HP. However my thoughts as far as hopping it up compared to other engines is that they would be pretty much identical at that point provided you did the same things. Why not hop up the 75 and save the 77 for a better stocker rather than hop up the better stocker and still have the mildest 750 engine?!   

[eurban]

The late heads are a somewhat better design from an air flow standpoint.  If little or no porting work is going to be done I would think that the late heads would be a better choice.  I am not sure of how much a boon the late head casting is to an experienced "porter" who is making some significant changes but I would guess that it is still a better starting point. . . .

[68grandprix]

I want to hop up the 77-78 so that I can still ride the bike while I'm building the motor, then swap it in in a weekend. Also, the '75 motor need some welding where the chain broke and cracked the case. I'm installing TRW pistons that are rated at 10.5to1 compression. How much more compression does the later motor have than the '75? Thanks, Jim

[eurban]

77/78ks and 75/76F (750s of course) had a CR of 9.2 to 1 vs 9:1 for the earlier.  To the best of my knowledge, the bump in compression is completely attributed to the piston design so if you install different pistons then you should end up with the same CR as if they were installed in an earlier engine.

[Jerry Rxman Griffin aka MuthaF'er]

Eurban,

I used to think the bump in compression was due solely to the pistons until I took my 78 K engine apart. It had what appeared to be earlier style partial dome pistons rather than full dome that was in my 75 F. The combustion chamber design is different in the -392 head used in the 75/75 F and 77/78 K vs the earlier K. Perhaps this is what accounts for the 9.2:1 bump. I'd love to see someone cc the 2 heads for comparison.

[eurban]

Yeah, the "to the best of my knowledge" clause means that I have read and participated a number of discussions on this over the years and the conclusion seemed to be that the pistons were the cause.   The 75/76Fs and the 77/78 Ks are all supposed to have 9.2:1 cr but I guess it is possible that they get the bump in different ways.  I would certainly be curious to know if I am running more than 10:1 cr in my 78K engine with the Wiseco 836 kit.  In fact, I even started a thread on this subject a while back.  If and when I pull her apart again, I guess I will have to check .  Overall, the Wiseco 836 in my 78 performs powerfully well yet does exhibit some negative traits seemingly common to the 836kits:  increased crankcase pressure / tendency to leak oil from the valve cover etc, tendency to rattle / predetonate which is dealt with by installing the d9 plugs, retarding the timing a bit, installing an oil cooler, and using high octane gasoline.  Can't say for sure whether these negative traits would be less in an earlier engine but they certainly are manageable in my k8 engine an the thing really runs like a top!  . . .

[Jerry Rxman Griffin aka MuthaF'er]

Isn't the Wiseco K engine 836 kit either 10.25 or 10.5 to start with? If thats the case with an earlier K and if the -392 head is the cause for the bump in CR then it would stand to reason that it would be more like 10.45 to 1 or 10.7 to 1 when used in a 75/76 F or 77/78 K. SOMEONE CC THESE HEADS !!

[eurban]

Isn't the Wiseco K engine 836 kit either 10.25 or 10.5 to start with? If thats the case with an earlier K and if the -392 head is the cause for the bump in CR then it would stand to reason that it would be more like 10.45 to 1 or 10.7 to 1 when used in a 75/76 F or 77/78 K. SOMEONE CC THESE HEADS !!

10.25 to 1.  (you can get a "race only" 12 to 1 set also)

[rbmgf7]

i installed a 75 engine in my 78 and there was an offset between sprockets. can be corrected by getting the same year sprocket carrier of the engine.

you guys are talking about having wesico kits in your 77/78's; i thought that dropped the compression in these engines? or is it just the F's and not the K's?

[eldar]

Just in the F. It has a larger chamber but that can be shrunk some by domed pistons or decking the head a little.

[Jerry Rxman Griffin aka MuthaF'er]

Depends on which 836 Wiseco kit you use. They apparently USED TO make a 836 for the F2/F3 also. Yes, if you use the 836 for the earlier bikes, it will drop the CR in the F2/F3.