Weird issue on fresh build
Originally Posted by 09g8
Spark plugs are flawless, so clean I thought it was running super lean even though my wide band settled in right at 14.5 at idle and 12 something at WOT. The engine ran flawless before I pulled it apart and I did not have the heads machined. I am not against the thought it just seems unlikely
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Since I put this post together I have put probably 500 miles on it with no overheat or loss of coolant. I can do a hydrocarbon test soon but I also do not see that caising low boost.
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I do believe I just had a thought that would explain it. I came across an article that discussed vacuum on a cammed engine and a random comment talked about exhaust backpressure and LSA and how they are related. The same engine with the same cam with stock exhaust would have 15 in/hg, with smooth exhaust and a high flow cat around 13 and with straight out of the headers 9. The LSA of say 112 on an exhaust with any restriction would be decent vacuum. I however have 112 degrees of lsa with a longer duration on the exhaust side and kooks headers to 3" customer exhaust that I built and spent a lot of time making sure it was very smooth and the least amount of restriction as possible. No tight turns and a 1/4 resonance pipe that adds to the volume the pipe can store.
Sound resonable?
Sound resonable?
Well the exhaust pressure thought is easy to test. someone mentioned it earlier in the thread, actually. Drop your Y pipe and run the motor, see if vacuum improves. It will not explain the lack of boost, though. Your cats would have to be nearly plugged solid to cause that high of a intake pressure at idle. I don't think your exhaust is so high flowing that it's going to affect your idle vacuum. I run a dual 3" off road (no cats) exhaust with a 256/270 cam and idle with 8" vacuum. You're nowhere near that aggressive. TO be honest, I think you'd gain more vacuum raising the idle speed 100-200 rpm than you will from dicking around with your exhaust.
How is your PCV run? You could well be pressurizing your crankcase.
Have you tried disconnecting the intake pipe from the blower to just let it run full on NA?
I suspect you do in fact have a boost leak and a vacuum leak. I suspect it's going into and out of the crankcase, which is why you don't see it on a smoke test. .
How is your PCV run? You could well be pressurizing your crankcase.
Have you tried disconnecting the intake pipe from the blower to just let it run full on NA?
I suspect you do in fact have a boost leak and a vacuum leak. I suspect it's going into and out of the crankcase, which is why you don't see it on a smoke test. .
Last edited by Darth_V8r; Jul 29, 2022 at 06:49 AM.
Thread Starter
Teching In
Joined: Jul 2022
Posts: 11
Likes: 0
Well the exhaust pressure thought is easy to test. someone mentioned it earlier in the thread, actually. Drop your Y pipe and run the motor, see if vacuum improves. It will not explain the lack of boost, though. Your cats would have to be nearly plugged solid to cause that high of a intake pressure at idle. I don't think your exhaust is so high flowing that it's going to affect your idle vacuum. I run a dual 3" off road exhaust with a 256/270 cam and idle with 8" vacuum. You're nowhere near that aggressive. TO be honest, I think you'd gain more vacuum raising the idle speed 100-200 rpm than you will from dicking around with your exhaust.
How is your PCV run? You could well be pressurizing your crankcase.
Have you tried disconnecting the intake pipe from the blower to just let it run full on NA?
I suspect you do in fact have a boost leak and a vacuum leak. I suspect it's going into and out of the crankcase, which is why you don't see it on a smoke test. .
How is your PCV run? You could well be pressurizing your crankcase.
Have you tried disconnecting the intake pipe from the blower to just let it run full on NA?
I suspect you do in fact have a boost leak and a vacuum leak. I suspect it's going into and out of the crankcase, which is why you don't see it on a smoke test. .
I have tried eliminating the blower and no difference
All vacuum lines where capped of including brake booster, pcv system, evap system, and BOV and then the car was ran NA and I saw maybe a 1 in/hg increase in vacuum.
