HELP! Stumped! Need some help! 26lbs of boost and still slow!
06-23-2017, 09:44 AM
#42
I worked on a turbo bike that made less power with a turbo than it did NA because of exhaust back pressure. Had to pull fuel out relative to stock on the top end because so much exhaust gas backed up into the intake ports/manifold. The cam is a big player on turbo stuff.
06-23-2017, 09:59 AM
#43
If he was running 250 duration on a 105 then I could see back pressure causing massive power loss. His cam isn’t that radical. 6* of overlap at .050 isn’t horrible. You don’t need a perfect part combo to run semi-well with a turbo LS. Sure a cam with less overlap or less back pressure would help…(not that we know how much back pressure there even is) but it’s probably not killing 250+hp unless something else is seriously wrong.
06-23-2017, 12:18 PM
#45
That's just an example of what happens when the cam mismatch is very poor. Between that and 100% optimized his is somewhere in between, albeit worse than the average. High chamber volume also provides more dead cylinder volume in addition to less flow restriction to reversion.
06-24-2017, 04:07 AM
#48
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A comparo... 
06-24-2017, 03:23 PM
#50
Sure the cams too big and not optimal for that setup. But you really think that’s the main cause of the car trapping 12-20mph slower than it should? Something seems way off with that combo, other than an overly large cam. Powers gotta be going somewhere. I’d find out what springs are on it and double check push rod length is correct. Are you running aftermarket lifters? Build a simple boost leak check rig. (lots of videos on you tube for this).
Think you could have something collapsed on the hotside? Flex coupling maybe? Rig up a quick back pressure gauge ($4 push to reset tire gauge works great) on the exh system and see where you’re at. Also turn the thing down. Try 15ish then bump a pound at a time watching gains. As soon as they level off, stop adding boost. When you stop seeing gains keep adding boost, things tend to break.
My Cam was cut fine, the key way in my crank was about 8* retarded. So when I installed my cam with the factory timing set it was 8* retarded from where it should have been. Seems a few newer gen4 engines have this issue, most do not. If the cam hit the card and was degree’d properly you should be fine there. If the “Professional engine builder” said they degree’d it and you have doubts, check it.
Think you could have something collapsed on the hotside? Flex coupling maybe? Rig up a quick back pressure gauge ($4 push to reset tire gauge works great) on the exh system and see where you’re at. Also turn the thing down. Try 15ish then bump a pound at a time watching gains. As soon as they level off, stop adding boost. When you stop seeing gains keep adding boost, things tend to break.
My Cam was cut fine, the key way in my crank was about 8* retarded. So when I installed my cam with the factory timing set it was 8* retarded from where it should have been. Seems a few newer gen4 engines have this issue, most do not. If the cam hit the card and was degree’d properly you should be fine there. If the “Professional engine builder” said they degree’d it and you have doubts, check it.
06-24-2017, 11:56 PM
#51
Have you tried adding timing? What do your plugs look like? All that being said I've been chasing the same issues and it looks like it's a very restrictive exhaust causing my problems. Your timing seems low compared to what I've seen others do on e85
06-25-2017, 12:12 AM
#52
Ya you need to read the plugs. Everywhere I read online said I needed to be at 14-16* with e85 on sbe. But the car really wanted a lot more. I ended up at 24* on 15lbs. Every setup is different. I added 2* until it stopped picking up a ton, added 2* picked up 75hp added 2 more and picked up another 75hp another 2 and picked up 50hp and that's where I stopped. I'm not telling you what you should be running but just pointing out that you can't go off of other setups you really need to know what yours wants.
07-11-2017, 10:41 AM
#55
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Check flex couplers for leaks or expansion?
07-11-2017, 11:21 AM
#57
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07-11-2017, 11:23 AM
#58
07-12-2017, 07:52 PM
#60
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