Twin can 88 compression test, whats acceptable
So, Ive got two FXDX bikes. One, a 99, the other an 04, carbed, and both have comparable mods.
The 99 has 37k miles, a Thunderheader, Big Sucker intake, stock pistons, and SE 204 cams, and made 86 lb ft and 79 hp on the dyno.
The 04 has 56k miles, a Supertrapp 2-1 with closed end cap and 20 plates installed, Big Sucker intake, stock pistons, Mikuni HSR42 carb (50 AC pump nozzle, AC pump set to engage immediately and full travel, 98 needle with clip in middle, 17.5 pilot, 160 main jet) and, according to previous owner, SE 203 cams, which are close to SE 204s, from what I can tell by the specs.
The issue is that the 04 has noticeably less torque than the 99, so Ive been investigating why. First, I was thinking perhaps cam timing was off, so I did a compression test to see if there was a big difference in readings between the front and rear cylinder on the 04, as Ive been told youd see around 30 psi difference IF the cam timing was off a tooth. I also did both bikes, so that Id have a basis for comparison.
Firstly, 160psi is perfect for the 88ci motor from what can tell, with these cams installed and OEM headgaskets and pistons.
04 has 140 psi in both holes. 99 has 155psi in both holes. I dropped about half an oil cap full of oil into both holes on the 04, and the compression reading went to 150 psi.
So, if 160/100= 1.6, and bike should make 79 ish hp to the wheel and 79/100= .79 and 160- 140= 20/1.6= 12.5% loss which would theoretically be 10hp if the correlation between hp and compression is linear.
IS 140 psi enough to make this bike a slug in comparison to my other bike with 155 psi? I also was wondering if the crank case was filling with oil due to sumping and that was slowing it down, but I took it for a rip and then promptly pulled the plug from the bottom of the crank case, and only 1-2 oz of oil came out, so its not that.
Im going to perform a leakdown test tonight to see if the valves are sealing properly next. Im just wondering, the head gaskets are clearly stock, so Im sure adjustable pushrods were used in the cam install, and if theres major leakdown from the valves, could it be that the rods were adjusted too long and are preventing the valves from seating fully? If the rods were adjusted too short, could they be keeping the valves from opening fully, thus giving a false lower compression reading and also hindering performance? If either of the previous mentioned issues were present, wouldnt the valve train be noisy and perhaps have grenaded by now?
Last edited by fxdls110; 09-20-2019 at 01:48 PM.