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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.
 
#27 ·
Science, here's some aftermarket inserts that are simular to the head I saw. Notice the area that lips up around the sparkplug, the stock head was more pronouced and squish angle was steeper as oppsed to flat. These head are more "conventional" in comparison, but you get general idea.


http://www.vhm.nl/ click on aprilia

http://jha.p-man.net/goods003.php?no=S10049-00


can't remember but I;m thinking something like 178 117? A lot of power is made with the pipes and timing. The factory makes a lot of differnt jugs though, kind of like, good, better, best, depending on who are, LOL.
 
#28 ·
Hi hillpounder, of the two chambers I saw on the two sites you mentioned the second site or JHA has a chamber very similar to the 670 H.O. summit X. The first site VHM, the chamber is very different from that of a 670 H.O. That design seems very similar to the many snowmobile after market designs, Polar, Crankshop, Union Bay. But like I said the second site you gave (I believe it is called JHA company) their design seems just like 670 top hat design, little hard to tell exactly from the photo. The port timing numbers that you gave me hillpounder seem confusing to me. Most port timing numbers for hi performance bikes seem to be around 195-200 for ex. and low to high 130's for transfers. So the numbers you thought sort of through me for a loop. If I understand you correctly though, the aprilla 250 engine had a head chamber design like that of the JHA design, similar to 670 H.O. and not like the one on the first site VHM which supplies to many bike companies including aprilla. Thanks for the info hillpounder, Rick.
 
#29 ·
The 670 X motor only has 1 head- 670HO and Summits were the same engine (except slight pipe changes and intake boots for the lay down MXZ), in 1998 the Summit X motors had an 'X" on the heads to distinquish it from the STD. 670 head, if theres a difference in compression its from production tolerences and condition of the motors- I run a smaller gasket in the Summits (5 hole) it shows better on the dyno and the hill VS a thick gasket and head cut-IMHO-BJ
 
#31 ·
Thanks for reply BIG JOHN. I would be surprized if the summit X and 670 H.O. had the same compression ratio. I c.c .'d a 670 summit X and it had 13.0-13.1 and .049-.051 squish. I have never been able to c.c. a 670 H.O. but from what I hear on this forum , the squish is around .070-.075. Be surprized if ski-doo would allow 13.0-13.1 for flat land sledder's. But I don't know for sure. I know the timing of a 670 summit X is (with a timing light) 10 degrees @ WOT (7900 r.p.m.) The spec sheet I got from the ski-doo race shop in Canada said the H.O. had 13 degrees again at 7900. I'am quite sure you can't run 13 degrees timing @ WOT with that combustion chamber design AND summit X high compression and not deto pistons. You can run 16 degrees with old design chamber, 15 was stock setting. Flame speed is allot higher with top hat design so that dictates lower timing. BIG JOHN, if you know of compression ratio of 670 H.O. and ignition timing in degrees @ WOT of 670 H.O. please let me know. It's possible the spec sheet I got from ski-doo is not accurate, (13 degrees) and I would like to know factually what the timing is, along with compression ratio. Thankyou Rick.
 
#32 ·
Anybody have combustion chamber cc volume specs comparing a 583 head to a ‘old’ style 670 head? If not a comparison then how about just the cc volume of a 670 combustion chamber? I have a 583 head I could measure, but need to know what a stock early 670 head volume was for a little project.
Reviving an antique thread, I know, but figured my question fit here better than it would to start a new thread.
 
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