| Author | Message | ||
     Hams |
Buellers, I am seeking your advice and expertise before I dive back into another long night of tearing into top end. Recently did full resto on 96 S1. New thunderstorm top end and head work performed at reputable Buell/ Harley shop. Light CNC port, squish set at .30, heads decked .60, .60 shorter solid pushrods. Concerns: 1. Compression test at both cylinders produced 250psi, (seems a bit high. At first ride noticed detonation- had to back off timing quite a bit. 2. Bike has full race exhaust, (came with bike when I bought it, not sure of maker) No baffle- straight through. Bike coughs and stutters when on the gas below 3k rpm. 3. Stock S1 cams- did not upgrade to N4 due to reading they "should" be the same grind. Is this too much compression? Will back pressure on the exhaust give me the low end torque back and eliminate stuttering at low RPM? Do I need to reconsider cam selection? Shop that did head work said I need N4's to unload compression. | ||
| Hams |
Ok I may have answered one of my own questions. Just called my HD dealer, CAM part numbers for 96 S1 are exact same as 98 S1W. | ||
| Strato9r |
That is a lot of cranking compression; if you are going to leave the top end as it is, I would seriously consider cam profiles that would move the dynamic compression ratio up higher in the RPM range. Making compromises in timing just to get an engine to run without detonation is a a good indication that all your hard work won't come close to reaching it's potential without further attention. As far as exhaust system 'back pressure', not to oversimplify the phenomenon, but it is in reality, an averaged net pressure, with the low pressure pulses being what scavenges the remaining exhaust charge, the high pressure pulses being the leading wave of pressure as the exhaust valve opens. Very important to get your cam timing sorted out BEFORE dialing in a proper exhaust setup. | ||
| Bluzm2 |
Hams, I don't think the info you got is correct. If memory serves, the 96 S1 had standard D grind cams, the 98 has the same as an X1 or S3. | ||
| Hams |
Thanks for the comments and recommendations, looks like cam cover will be coming back off. | ||
| Sportyeric |
Check my math: Volume of a cylinder is 2 pi r squared times height. With the 60 thou removed from the combustion chamber by decking it that much you have: 2 times 3.14 times 1.75 squared times .06 equals 1.154 cubic inches equals 19 cc. Stock volume of T-Storm heads is 67 cc. Stock compression ratio is 667/67 for ten to one. Barring any other machining, yours is 667/48, almost 14 to one. Did the shop modify the combustion chamber to bathtub it and reduce the CR? If not, I think someone should have. Hah! Posted, then noticed how old the thread is. LOL. How did it turn out? (Message edited by Sportyeric on September 10, 2013) | ||
| Imadog |
If you multiply that by 14.7 psi (atmospheric pressure), that only equates to about 205 psi, Still high though. What is the highest compression ratio for pump gas? I always thought it was 12-12.5 psi. | ||
| Strato9r |
Soooo many variables regarding pump gas and compression ratios....there are literbikes pushing 14-1, but, cam profiles, well sorted rod/stroke ratios, small bores, nicely designed combustion chambers, and perhaps most importantly, liquid cooling, make pump gas usage a non issue. On the other hand, you really have to be sharp to get a 9-1 Ford flathead to run well on pump gas. An air cooled V-twin , especially a Sporty/ Buell type mill would be a tricky balance above 11-1, with any margin of safety for bad gas or high operating temperatures cut down to near nothing...IMHO, opinion, anyway. | ||
| Hams |
Haven't been around the forum in awhile. To answer some of the responses (thanks by the way), the shop did machine out the CC to bring it back to 67cc. After doing cam swap to N4's it did the trick. Really opened the motor up and now runs like a raped ape! Still get some minor pinging if I twist hard in high gear and low RPM. Still debating if aftermarket ignition (Dyna2000) can retard the curve enough to eliminate pinging completely. Or could be that adding octane to the fuel could do the trick. Non the less, for my first top end build I am very happy. Thanks to all for keeping up with this thread. | ||
| Sportyeric |
Just because. I was reviewing this thread for my own research interests and noticed my math above is wrong in several areas. First off, although the stock chamber is 67 cc, I neglected the effect of the piston dome, which, I think, creates an effective chamber of 62cc. (Chamber volume minus piston dome volume.) Secondly Formula for volume is not 2 times pi but only pi. However, the OP says the chamber was cc'd back to 67cc so its moot here. And to this, I'll hang a new question: with T-Storm heads decked 30 thou, that should create a CR of 11.4. I thought that should be OK with a tight squish but I've had to back off the timing quite a bit and still get pinging. I may try backing off the timing more but at what point does adjusting the timing to make up for high CR become a losing game? And another math question: a CR of 11.4 should yield a measured pressure of 167 psi, yes? What is considered max pressure for street gas on these engines? |
