408 high rpm woes
Thread Starter
Staging Lane
Joined: Aug 2005
Posts: 69
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408 high rpm woes 97 corvette c5 408 stroker.
lunatti billet rotating assembly, custom je pistons 8.5:1 compression.
comp 230/234 591/598 113 lsa installed straight up
comp 918 valve springs installed @1.800 130lbs seat pressure
comp chromoly pushrods 7.150 length
comp shaft rockers
comp 875-16 pro magnum lifters
ported lq4 heads
6.0l iron block .031 over
60# mototron injectors
tuning with hp tuners.
Now the problem, motor runs solid when cold. Once temps reach 140+ it develops a misfire over 5500 rpm. Once motor is fully warmed up (220) it is like it hits a rev limiter at 5500 rpm. I have played with every spark, and fuel option there is in the tuning software. Air fuel ratios look great, no misfires detected on snap-on scanner, or hp tuner scanner. Just put in fresh coil packs, cam sensor, and crank sensor.
The engine builder had a similarly setup motor on the dyno last week with the same exact problem, and could not figure it out. My only thought is that maybe comp has a bad run of 918 springs out there? Or something in the valvetrain geometry is not right once the oil heats up and thins out? I wanted to think it was just a tuning issue, but two motors doing the same thing say it isn't.
I am open to any and all suggestions. My next stab in the dark is to put 921s and stock length pushrods in the car. This is a tedious and frustrating task at best.
lunatti billet rotating assembly, custom je pistons 8.5:1 compression.
comp 230/234 591/598 113 lsa installed straight up
comp 918 valve springs installed @1.800 130lbs seat pressure
comp chromoly pushrods 7.150 length
comp shaft rockers
comp 875-16 pro magnum lifters
ported lq4 heads
6.0l iron block .031 over
60# mototron injectors
tuning with hp tuners.
Now the problem, motor runs solid when cold. Once temps reach 140+ it develops a misfire over 5500 rpm. Once motor is fully warmed up (220) it is like it hits a rev limiter at 5500 rpm. I have played with every spark, and fuel option there is in the tuning software. Air fuel ratios look great, no misfires detected on snap-on scanner, or hp tuner scanner. Just put in fresh coil packs, cam sensor, and crank sensor.
The engine builder had a similarly setup motor on the dyno last week with the same exact problem, and could not figure it out. My only thought is that maybe comp has a bad run of 918 springs out there? Or something in the valvetrain geometry is not right once the oil heats up and thins out? I wanted to think it was just a tuning issue, but two motors doing the same thing say it isn't.
I am open to any and all suggestions. My next stab in the dark is to put 921s and stock length pushrods in the car. This is a tedious and frustrating task at best.
TECH Addict
Joined: Dec 2001
Posts: 2,907
Likes: 1
From: Bergen County, NJ
I wouldn't think it's a bad 918 spring set, especially if it runs well cold. Only things I can think of are valve lash too close (try just backing off a little bit on the rockers) injector or injectors binding with the added heat. Are you running open or closed loop?
Thread Starter
Staging Lane
Joined: Aug 2005
Posts: 69
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Originally Posted by DaddySS
I wouldn't think it's a bad 918 spring set, especially if it runs well cold. Only things I can think of are valve lash too close (try just backing off a little bit on the rockers) injector or injectors binding with the added heat. Are you running open or closed loop?
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TECH Senior Member
Joined: Mar 2004
Posts: 14,620
Likes: 17
From: BFE
Hot is when the engine is warm.
You test your preload with a dial. Comp "R" have been known to act up on too much preload. Yours could be borderline when cold and when parts warm up and expend, you have too much.
Who put it all together for you?
You test your preload with a dial. Comp "R" have been known to act up on too much preload. Yours could be borderline when cold and when parts warm up and expend, you have too much.
Who put it all together for you?
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TECH Fanatic
Joined: Jan 2002
Posts: 1,205
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From: harrisburg, pa
1/4 turn preload is not right.
According to Comp.. .008-010" with a dial indicator after zero lash cold in my combo. In my combo it worked out to 1/5 turn. Your need to measure your combo. Dial indicator on the rocker tip. Maybe you want to call comp.
According to Comp.. .008-010" with a dial indicator after zero lash cold in my combo. In my combo it worked out to 1/5 turn. Your need to measure your combo. Dial indicator on the rocker tip. Maybe you want to call comp.
The Comp Rs are supposedly very picky about the preload. You should probably get your engine builder to actually check the preload on them rather than just giving it a 1/4 turn.
Thread Starter
Staging Lane
Joined: Aug 2005
Posts: 69
Likes: 0
Interesting, what causes the crank sensor to not be close enough to the reluctor? I was leaning towards something like this due to the scan tool not seeing any rpm above 1400; however the tach and hp tuner scanner still work fine.
I would point at Comp R lifter,.598 lift with heavy roller rockers and comp 918 springs not mixing.If the power suddenly drops then I would say valve float which will feel like it hit's a wall.
On The Tree
Joined: Feb 2005
Posts: 178
Likes: 0
From: Odessa, TX
Had same experience (40+dyno pulls two different sets of lifters stock and comps same result), no matter how hard you look it is valve float, it is not the comp r's fault it is the preload or a combination of heavy valvetrain parts with not enough spring pressure.
Thread Starter
Staging Lane
Joined: Aug 2005
Posts: 69
Likes: 0
Originally Posted by MAC4264
Had same experience (40+dyno pulls two different sets of lifters stock and comps same result), no matter how hard you look it is valve float, it is not the comp r's fault it is the preload or a combination of heavy valvetrain parts with not enough spring pressure.