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More 670 questions for the knowledgeable Doo guys

Are H.O. and non-H.O. cylinders the same? What about timing? I know that the heads and pistons are different but was wondering about ports, port timing, etc? and it seems like I can remember reading somewhere that the ignition timing is also different. Someone school me on these things, please.
 
Differences

The cylinders are different. Different sizes, timing, and additional machining to go with the tunnel porting of the case. The HO also has a different initial timing spec as well as a different timing curve in the CDI box. The HO was/is a very refined 670, most out ran the standard 670s, however, there were a few "spot on" standard 670s that would run with and/or beat an HO. The HO had more potential and in fine tune was a better engine.
 
The cylinders are different. Different sizes, timing, and additional machining to go with the tunnel porting of the case. The HO also has a different initial timing spec as well as a different timing curve in the CDI box. The HO was/is a very refined 670, most out ran the standard 670s, however, there were a few "spot on" standard 670s that would run with and/or beat an HO. The HO had more potential and in fine tune was a better engine.

Wasn't aware that the cylinders are different. Bore and stroke are the same on both motors (78x70) and most of the other components are the same. Head design was different and the HO pistons were flatter for more compression. The ignition timing was changed to match the increased compression (hence the new timing spec). I think the compression ratio was like 12:1 vs the standard 670 that was about 11.4:1 or something like that. Still ran on pump gas. I wouldn't say that it was "very refined", it was basically a 670 with increased compression to make a little more power. The rotary valve specs and carbs sizing etc, were all the same as the standard 670 if I remember correctly. They both had the 502 rotary valve as I remember, correct me if I'm wrong though, its been a while since I looked at them side by side.
 
The non-HO/non-X motors had 40mm carbs. The HO and X motors had 44mm carbs. RV timing was the same among the models at 164* stock.

The port timings are essentially the same among the models. However the HO and X models had much better cases from the factory- in that the cases did not have as much mismatch requiring massive tunnel porting.
 
HO engines

Other improvements were in the y-pipe, pipe and muffler. The cylinders are different and require different sleeves when needed. Not trying to argue here, but I have been building and racing these things since 1993 and have had a lot of them apart!! Sad to say I just have one left now, my 1999 MXZ HO that I bought new. Gonna keep this one.
 
Ok few have hit the high points but heres the skinny...

670HO VS 670

same bore/stroke
flat top piston/acorn head (91 pump gas, even summit X)
44 mm carbs and DPM
500 RV
CNC ported cases = cut to match cylinder and RV intake area opened up
crank is different (timing key/cageless bearings = bigger rod ends)
cylinders have cut in intake side (base of sleeve)..port timing is the same enough (a gasket change is all they did) other-wise the same (interchangable**)
timing spec is different to match DPM specs (@3500 rpm DPM doesnt change timing no matter the elevation)
new pipe/can

Low elevation hop up as follows- bore .010" over and new pistons, port match sleaves to ports (pretty ugly stock), 8 hole base gasket, cut head* to .055" squish, 501 RV at 78 deg closing and BPR9ES plugs @ .018" makes a solid 130+ hp with stock exhaust.

For high elevation hop up- do same as above PLUS cut the head for .050" squish and CC for 13-1 (usually a .040"/.030" cut) and keep stock RV...BJ

*head cut includes squish area to TRUE domes to piston, as in a .020"/.010"...stock cast domes are pretty bad

**only the 95/96 Mach 1 670 twin pipe had goofy cylinders...could still use them BUT needed to play with base gaskets and heads to achieve safe compression ratio/squish
 
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