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Thicker Base Gasket ?

18K views 32 replies 6 participants last post by  DaveyG 
#1 ·
I know the taller exhaust port helped out the regular 670, but I wondered what your experiences were on the X or HO. Anyone with dyno info on this? With squish and compression left at stock level do you think there are any gains with say a .8 or 1mm gasket and a head cut to match.

All opinions would be appreciated.
 
#2 ·
Hello H.O. rider. The port timing on the h.o. is the same as the standard 670 which is 200 degress ex. 130 degress transfer's. The port timing on the 670 has been the same since 1993, again 200, 130. I have seen same very minor varience's, like 199-201, 129-130. The only exception to this is the 1995 Mach 1 which had 203 ex. 134 transfer's. From my experience on the dyno with 670's and 1000 triple 670's is the port timing is just fine with 200,130. The only exception would be a 9000 r.p.m. engine where you might want a little higher, maybe 202-132. A lot of people seem to think higher is better, but it just does'nt work that way. Bear in mind the 670 already has fairly high port's compared to the newer series 3 engine, people have been raising the cylinder's on them and getting good result's, but remember the port's are low on that engine to begin with. Raising the cylinder on that engine usually won't bring the port timing up to 670 spec's, so you can probably see why there are gain's raising the series 3 cylinder's. Usually what you want to do is very carefully widen the port's on the 670. This involve's alot of work to do it properly. The key word is doing it properly. You usually just can't widen the port window only, you also have to widen the port passageway and that is alot of work, believe me I know. I have spent many, many hour's doing just that. There are considerable gain's to be had with a 670 engine. Usually you can get as much as 155-160 h.p. on a full race engine, and I have gotten 142 h.p. on my trail 670 with single pipe, that's at 7800 r.p.m. But everything in the engine has been modified to make that kind of power on pump gas, again many many hour's.
 
#3 ·
Thanks Science,

Great info. I was hoping there would be a lot of old rotary valve gearheads that would talk on this post. Why do you think it was such a popular thing to do on the reg 670. I know I did it on a half a dozen of them with a head shave, 501, short boots etc, and everyone loved them. I dont have a dyno to say one way or the other but I was curious why if it worked on the old 670(if it really did?) why you dont hear many putting in the .8 on the HO.
 
#4 ·
Hi H.O. RIDER. I was'nt aware it was popular to raise the cylinder's on the 670's. I know the 95 mach 1 had it's port's too high for the r.p.m. it ran (8100r.p.m.). That is why the hot ticket for that sled was to lower the port's, if you could get away with it passing tech. Generally removing material from the crankcase was the ticket. The tech guy's would catch it if you lowered the cylinder's because they were looking for it. I have spent a fair amount of time on different dyno's with different engine's, usually 1000 pro stock engine's, but I have been at mid west dyno 3 time's with 670 engine's and have learned some thing's. As I said earlier raising the port's is not the thing to do on a trail modified motor, the port timing is high enough for 7750-8000 r.p.m. If you want to get more air into the engine to make more h.p. widening the port's is the best thing to do in so far as porting is concerned. But as I said earlier it is hard to do properly, most people just widen the port without widening the passageway and this is not the thing to do, you should also widen the port passageway as well. This is very difficult to do and take's much time, which is why most people don't do it. I can do it for my cylinder's because I don't care how much time I have in my own cylinder's, it's a labor of love I guess. The work is very pain staking and exacting to do. I guess a cnc machine would be the ticket to do the work somewhat quickly. Tunnel porting is also good, but generally it's best to do with intake modification's. Putting larger carb's is not much help unless you enlarge the rotary valve cover to accept larger carb's (44mm carb's) although larger will help also so long as you keep enlarging the cover properly. This is'nt real easy to do either, again properly. Generally you can't enlarge the cover much more than 42mm, but this is much better than the 39.5mm that is stock. I modified my cover by shortening it first then enlargeing it to 44mm, I was able to make it bigger by shortening it, this help's in two way's first making it shorter help's power, I alway's see power gain's by shortening the intake tract on rotary valve engine's and enlargining to accept 44mm carb's so there is no restriction with 44 or larger carb's. Believe me there are good gain's in doing the modification to the intake tract. This is one of the reason's why I get good power from rotary valve engine's. My trail 670 that made 142@7800 r.p.m. was ported like I mentioned and tunnel ported and the intake was modified like I explained. The head was modified to run on 92-93 octane fuel, the airbox was modified (extensively) and I use a p.s.i. single pipe. Tried various pipe's both twin and single and the p.s.i. pipe worked best on my engine. The twin pipe's work best when you run higher r.p.m.'s. My race 670 use's jaw's twin pipe's and it run's 9000-9100 r.p.m., but I use the same principle's with the engine. The port's are larger and the carb's are larger, the cover is modified more and I use a brass wear plate and the compression is certainly higher, but everything work's together in the engine, which is key on any engine.
 
#5 ·
Science:

Great info again. I know that Don Emery, Rich Daly and others always went with a taller E-port, maybe this is why so many of us followed without knowing if it really worked or not, just that the experts were doing it. It always felt like it had more up top with a .8mm.

Thanks Again.
 
#6 ·
Hi H.O. RIDER again. Raising the port from .5mm to .8 mm will raise the port's .012 which is 1.1 degree for the ex. and 1.4 degrees for the transfer's. On the dyno that might show 1 to 2 h.p. if it in fact it did gain h.p. If the pipe still worked with that port timing. It's alway's a can of worm's when you change port timing to see if it will still work with the pipe you are using. Only a dyno can tell this. I would think that if a engine did gain 1-2 h.p. it would be very difficult to tell by the seat of the pant's. There are so many other thing's that can affect the speed of the sled that would have bigger consequencies than 1-2 h.p. About the only why to tell would be to time the sled before and after and hope that the condition's were similar during testing, talk to you later H.O. RIDER. Regard's Rick
 
#7 ·
sciencemagic, Could you comment on doo's switch to flat piston and small chambered head on the HO. Was there any power there? or did it all come from the new pipe and more agressive timing in the (then new) MPEM? I guess I'm still trying to figure out why rotax went that way with the combustion chambers?
 
#8 ·
hillpoounder:

I dont have any dyno info on this , but a buddy of mine runs std. 670 heads and pistons in his ported HO and feels they work better than the flat tops. I guess another nice point on this is the std. pistons are available by more manufacturers and in more sizes. Doo only makes a .010 over in the HO style.
 
#9 ·
Hello hillpounder, you ask a very good question. Ski-doo changed to a different head design and flat top piston in 1998 for summit X and 1999 for h.o. I have to think that rotax thought that design was better, especially since they continued to put it on subsequent engines. Why in the world would they spend the engineering money and manufacturing money to change the head design if they thought it was inferior? And then continue to use it on later models if they again thought it was inferior. Fortunately I was able to go to the dyno and get some answers, especially after all the bashing that is done on that head design. What we found was a very small gain at peak H.p., 1 to be exact, but a very significant gain at mid range, 7 H.P. To me that was a win win situation. Admittantly it was minor at peak, but the gain at mid range can't be denied, and when you consider a trail engine spend's 90% of it's time in the mid range it seem's a good change. When these change's were made on the dyno we did back them up, (2 run's with old style head 2 run's with top hat design head) to make sure we were getting good data. The newer style head diffinately require's different ignition timing, (the squish velocity is higher) and the c.d.i's. are mapped for that. All the dyno work was performed on a 670 rotary valve motor, and that motor was trail modified. It made over 140 H.P. at 7750 R.P.M. on pump gas on mid west dyno and that dyno is noted for stingy number's not high number's. So in all fairness I can't factually say how it would compare to a old style head design on a series 3 engine(newer rotax design). I suspect that people who are modifying the newer head design and possibly having trouble are not getting the squish velocity right and are not getting the ignition timing right after a modification. Again I can't see why rotax would continue to use the top hat design for as long as they have used it now (6 year's) if they thought they had a inferior head design after they changed it back in 1998 (summit X). I guess the only sure factually way to know is to talk to a rotax engineer. Be interested to here other factually documented result's, or other hard evidence on the subject hillpounder.
 
#10 ·
Sciencemagic, thanks for sharing your 670 experience. I like to the give the factory engineers the benefit of the doubt, hate to try and second guess them. Some different heads I've run on the series3 and their squishband/bore area. Stock=66.7% , brand A=52%, brand B= 39%. As the squishband gets narrower the squish needs to be set tighter to get the same msv. My results have been better with the narrower bands, BUT ONLY if the squish is accurately set. IMO the stock wide band design is more forgiving of the sloppy squish specs of production line (something rotax has been famous for years, king of the basegaskets). Just a theory, I dunno. I just saw the same tophat design show up on a new aprilla race bike, kind of threw me back, I never related it to perfomance. You mention timing for the tophat, the curves I've seen seem quite advanced in the midrange, does this not suggest slow combustion?
 
#11 ·
Hello hillpounder again, yes the timing curve on the new (not really now) top hat design does have more timing in the mid-range, the curves I've seen are about 18-20 degrees at 5000-6000 r.p.m. and start at 22-23 degrees and go down to 13-10 degrees at WOT. These timing numbers give good torque at low and mid-range, great for the trail, but will not work at high engine speeds, and I speak from expereience (detonating pistons). I would like to know why rotax did this (factually) but I would surmise it was for better low and mid-range torque. The big question I had was would that design work with a very high r.p.m. engine (race engine) and the fact that you say an aprilla race bike with that design I quess answers that question. What size engine was it? Any compression numbers? I have seen alot of the honda, yamaha G.P. spec's (as much as they will alow anyway) and they have conventional chamber designs, (I don't know the squish ratio's) but they can't have real high compression now with a mandated 100 octane rule. It is very interesting that rotax went with a top hat design for their race 2-stroke's. So if this is true they are making it work for very high r.p.m. hillpounder. Be very interesting to know the squish clearance's of that engine, if you have any spec's I would appreciate knowing them. In my mind that was the only fly in the ointment so to speak regarding high r.p.m. and a top hat design chamber. On the dyno there were diffinate gains in the mid-range and low end, but like I said very minimal at the peak. What h.p. numbers have you seen with different head designs, and what compression ratios were they? I'am assuming you made the comparisome's with equal ratios. Any opinion as to why there were gains with the different heads, again assuming same ratios? By the way hillpounder, do you have any other info on the aprilla (rotax) engine? Talk to you soon, thankyou, Rick.
 
#12 ·
Rick, the bike was a 250 twin. I have the specs lost somewhere but can say the portimes were fairly mild but the areas were big. Compression around 15.5. Just under 80hp at 11,200. Squish was one of the variables depending on course, around 1.2mm though.


The timing you describe is close to what I've seen also, depending on which box (mxz or summit, 700 or 800). Wonder if the advance is due to the physical distance the head puts the plug from the piston? I dunno? I assume the detonation you talk about is on a wfo pull? Where I get in trouble is when you have 4-5 mile of deep snow to pull up the road to the "good" riding. It's too deep for full throttle, you'll just trench in. If you back off you'll also be stuck. So your forced into the midrange. The advanced timing and the (normally) lean jetting combine to keep building more and more and more heat as you go, until.......you get the picture.

The 3 heads I mentioned were NOT set all the same as you ask, so it's not really a fair comparison, at least powerwise. The narrower tighter bands seem to require less fuel to make power, which should mean something. I am still curious about the top hat design though. I've never seen a HO670 head, does it look like the series3? What was the factory squish on the 670HO, and how tight were guys cutting them for pump gas? I know the series3 can become a problem under .065, but the 800HO is like .038, did they cut the band back some to do this? (what I've seen do with the non HO)?
 
#13 ·
Hello hillpounder, the squish on a 670 H.O. is about .070 and about .050 on a Summit X. I have not timed a 670 H.O. but I have timed a Summit X and it is 22 degrees at 3500 and 10 degres at 8000. According to the ski-doo factory the H.O. should be 22 at 3500, 12.5-13 at 8000. I now from experience that my 670 with that style head and 140+ trailable H.P. at 7800 r.p.m. will detonate at 13 degrees very quickly. I know the engine is making allot of cylinder pressure (97 lbs of torque) and that has a big influence on the timing/detonation combination. My squish is .055 and that is not the answer. I'am currently changing it to .060 on one head and .065 on the other. My compression is 12.75. The detonation I mention is at WOT, and I ride at 1500 ft. The flame speed is by the way very high with that design (top hat) at the mid-range, but rotax incorporates big timing also at that point to make good torque, I believe they can get away with it because there isn't allot of cylinder pressure at part throttle in the middle r.p.m. range. I know I have no detonation problems at that r.p.m. range at all. As a matter of fact no problem at 7200-7300 partial throttle, go full throttle though and instant deto. That is the reason I'am going bigger on the squish. Yes lower timing will also work, but I ran out of time to experiment. I know the summit X works at 10 degrees at sea level at full throttle with .050 squish AND 13.0 compression. I personally c.c.'d one and personally timed one with a light to make sure I got good data, I know that works. I would suspect that maybe 11.5-12 degrees might work with my engine combination and .055 squish. It would be interesting to me if you could find those Aprilla spec's hillpounder if possible, (port timing). The spec's I have seen for the Honda, Yamaha G.P. bikes are also low for the exhaust, but somewhat high for transfers. Be interesting to see what rotax is doing, especially when their engineer's are given green light to make H.P., not trail power for average 87 octane trail rider with warranty issue's. It is interesting though that they are using the top hat design head, would not have thought that hillpounder. With the Aprilla at 15.5 ratio and roughly .060 it would be very interesting to see what ignition timing they have? I know they can use higher octane fuel and that helps the deto problem with high timing and that design head, if in fact they are using somewhat high timing. I know when we go the dyno with are 1000 pro stock engines that make 245-255 H.P. it takes big timing 15-17 degrees to make those numbers, but that is with a conventional head design, maybe 45-55% squish and .050-.055., and very high compression. Talk to you soon hillpounder, Rick. Oh by the way, the 670 H.O. head is the same as a series three engine, and what ignition timing is ski-doo using with the 800 H.O. and an incredible .038 squish?
 
#14 ·
The stock squish on my HO's was in the range of .075 to .085. The domes are always off center and have yet to see one with four equal squish measurements in parallel with the piston pin and on both sides of each piston. The average was more like .80 on my HO's. I knock them down to .072. I know of a a few guys that ran .065 like we did on our std 670's and they all went down. This is @ sea level.
 
#15 ·
Hello H.O. rider, you can get equal squish clearance but it is a pain. You can move the cylinders around slightly and the head slightly and pretty much get the clearances real close. It is a pain like I said though. Interesting about the people that had .065 squish and deto'd their pistons. I believe the ignition timing is about 12.5-13 degrees on the H.O. , compared to 10 degrees for the summit X and that has 13.0 comp. and .050 squish. I know I was'nt able to run 13 degrees at .055 squish, it would deto instantly. But from what you tell me .072 squish and I presume stock timing will work. What is the compression at that clearance? Have you ever timed your engine with a timing light H.O. rider? It would be interesting to see if in fact you see 13 degrees or there abouts at W.O.T. If you degree the flywheel or clutch, I prefer the clutch because it's easier to see and look at a tach that I have on top of the carb's for that purpose. Let me know if you can H.O. rider if you have ever timed your engine with a light. Have you ever increased compression on a H.O. 670? And do you know the stock compression on a 670 H.O., thanks Rick.
 
#16 ·
Science:

I didnt mean to say that .065 squish wouldnt work , I just know a couple of guys that went down when they tried. I found the stock squish on mine set at an average of .080 out of the box with 142 pounds of compression with a deadweight tested gage. One was stock with a .5mm and the other a .6. I knock them down, conservative as it may be, .008 and end up with 148-150 psiand .072 squish(avg).
I personally find the domes are not square in the head as measured by the drop from the machined surface of the head, thus making it almost impossible to square up the squish, on my heads at least.

I find .072 and stock timing to work fine. The guy I spoke of earlier running .065 squish with std 670 head and pistons has had no deto problems.

I have not verified the timing but I can and will let you know the results.
 
#17 ·
Hello again H.O. rider, I guess I misunderstood you when you said .065 guys had problems, I assumed they had the top hat design head (H.O. summitX) not the old style head with very little squish band. The old style head a guy can go down to .055 and not have problems, at least I haven't had problems with that squish number and friends of mine with that same number haven't had problems either. And that is with high trail compression (12.5-13.0) and a little ignition timing increase to boot. If that number of .065 was with the top hat head (H.O.) let me know again please. It seems to me that .065 would be border line with that design head. Do you know what compression in numbers, (12.0-12.5-12.75 etc.) a stock H.O. is? I have c c'd a summit X and it is 13.0, which you would expect for mountain riding. I really don't relate to a gauge reading because they can be so different from gauge to gauge H.O. rider. I have seen more than 10 lbs. reading from gauge to gauge and then you get into different oils and different engine temperature, it all makes a difference. If I knew the stock number for an H.O. it would give me some idea of how I can modify my mod 670. It already is high, and will remain higher than an H.O., but it can possibly point me in a derection I need to go H.O. rider. I know what I currently have is marginal, and I just have to back off of either squish, timing, compression to make it work all the time. Look forward to hearing from you, Rick.
 
#18 ·
Science:

You were right the first time. I know 2 guys that deto'd with .065 squish on their 670 HO's (top hat design) and other wise stock motors.

I know another guy who runs std 670 pistons and heads on his ported 670 HO with .065 squish and has no problems.

I dont have the CC'd data but I can get it from a friend.

I would not personally run anything less than .070 on a top hat head(670HO) without race fuel or at elevation. You are correct in saying .065 is dangerous.
 
#19 ·
Thanks for the reply H.O. RIDER. Yes it does seem .065 is about the limit for a top hat head. I would appreciate it if you could get c c data for a stock H.O. head. I do have data from a stock Summit X 1999, and it is like I said 13.0-13.1 with only .049-.051 squish, and it works because the timing is only 10 degrees at WOT. It has 22 degrees at 3500 r.p.m. I can only assume the timing that ski-doo list's for the H.O. which is about 12.5-13 degrees again WOT, 22 degrees 3500 r.p.m. is correct. And it works because of the lower compression and looser squish of the H.O. You have already given me the squish of your H.O. stock, now if I could see what the compression is I could have good data to determine what I need to do precisely, not just winging it. Thanks, Rick.
 
#21 ·
You guys are absolutely correct on gauges, however mine is correct.
It is calibrated on a deadweight calibrater used for setting up pressure switches and finding offsets in pressure transducers. The schrader valve is actually epoxied into the threaded fiiting where the gauge goes into the head. With the schrader valve that close to the end there is no extra volume added to the combustion chamber. As you mentioned this doesnt really matter as long as you use the same gauge on the same sled under the same conditions it will give you the answer you are lookng for.
 
#26 ·
Hello Hillpounder, I'am using a h.o. piston to match the head. I guess a guy could cut the squish band to match a more conventional piston dome though if he wanted. I'am not sure of the results though if that was done. The only way to know for sure would be to dyno the engine before and after the head cut and piston change. Is there any chance hillpounder you could speculate on the port timing in degress of the aprilla bike engine for both ex. and transfers? I'am very curious about those numbers is the reason I ask. Thanks Rick.
 
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