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  #16 (permalink)  
Old 03-08-2013, 06:28 PM
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When the car was on the dyno was it fitted with egt sensor in each header pipe. This will show the reason for the power dip if its reversion.

Try a one and a two inch or large carb spacer and run it again. More volume will minimize the problem if the intake is starving for a/f mix.

But like others have said its real common for cars on a chassis dyno to slip right at the top of the torque peak as the power comes on. When your dyno guy seen this graph did he tigthen the car down and run it again or say here you go and start unhooking chit. I would think that flat graph right around 5252 rpm deserved another look.

Also are your tires capable of putting 500 hp to the ground without slipping. Wide enough and had the right tire pressure. Another thing that should have been adjusted and retested after this run.

I like to put the cars on jack stands and check for drive line drag. If its rolls smooth and quiet its probably not your problem. If it takes two hands to trun the tires over then good place to start.

Hope this helps.

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Old 03-08-2013, 08:22 PM
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Those are 2 dino sheets right 1 flywheel dyno and 1 rear wheel dyno? They both show the same thing a rythmatic dip and rise! So the if that's the case, the tires drive train etc is not the cause of the dip in power!!

Most dyno runs are at least 3 , One cold eng run, one hot run, and one second hot run to check data against (best run of the 2). There is also data smoothing that takes tire & converter slip and throttle into consideration in printout on some dynos smoothing out the curve but some operators don't use it and read raw data!

Tightening tires down tighter on rollers after first run will drop power readings drastically on a second run! Tire pressure and any drag in drive train is very impotent as Hcomton said! Fluid temps must be at operating temps! Car must be in 1to1 final gear for a good reading.

Your torque dropped right off at 5000 rpm and came up again till 5500 and then dropped off steadily from 5000 on!

Valve train harmonics are whats sticking in my mind as the cause! When the resonance comes to its peak at 5000 rpm and as torque is reduced at 5300 rpm the resonance levels out and torque starts to resume momentarily till the torque band starts dropping off! I would check the valve train, maybe girdle it, push rods could be resonance flexing, Could be cam flex, ETC

Resonance can be through the whole rpm range and have a slow undetectable rhythm at low rpm with not much affect but as rpm and load build: the rhythm increases its wave cycle till it reaches a peak and something physically gives Like push rods, springs, rocker studs, cam!they will flex and absorb the harmonic distortion momentarily like a damper affect and then level out till the resonance begins again repeating the cycle!

Jester
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Old 03-09-2013, 01:09 AM
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Thanks for the detailed replies, keep them coming as I progress through this.

I don't think its valve train harmonics. The studs are girdled and pushrods are firm. I am almost convinced that the problem is a reversion. The intake has black soot inside and it goes all the way up to the bottom of the carb. I noticed the black before but never really took too much notice of it. Making changes to the intake system, i.e. bigger or smaller carb, air filter and no air filter, made changes to when the power dip occurres. Will connecting a MAP sensor and EGT confirm this 100%? Does the EGT sensor need to be right at the head to be a true reading, or can it be at the collector? Will also try a 2" spacer to see if the reversion will change significantly. What else do you guys think?

hcompton funny you mention the tyres. I am using 28x12.5 et street radials so yeah they have no problem putting down the power. Well on the track anyway, they do slip a little on the street. But the dyno guy mentioned that a semi racing tyre with a racing compound and side wall is the worse thing to use on the chassis dyno. He said standard street tyres are the best as they don't flex and twist or heat up under load.

So lets say it is plenum reversion, what to do from there? Is it a bad valve job or port job? Wrong cam timing?
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Old 03-09-2013, 07:41 AM
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What about a reversion dam I in the intake runners at the heads?
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Old 03-09-2013, 08:26 AM
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Originally Posted by HQHolden View Post
Thanks for the detailed replies, keep them coming as I progress through this.

I don't think its valve train harmonics. The studs are girdled and pushrods are firm. I am almost convinced that the problem is a reversion. The intake has black soot inside and it goes all the way up to the bottom of the carb. I noticed the black before but never really took too much notice of it. Making changes to the intake system, i.e. bigger or smaller carb, air filter and no air filter, made changes to when the power dip occurres. Will connecting a MAP sensor and EGT confirm this 100%? Does the EGT sensor need to be right at the head to be a true reading, or can it be at the collector? Will also try a 2" spacer to see if the reversion will change significantly. What else do you guys think?

hcompton funny you mention the tyres. I am using 28x12.5 et street radials so yeah they have no problem putting down the power. Well on the track anyway, they do slip a little on the street. But the dyno guy mentioned that a semi racing tyre with a racing compound and side wall is the worse thing to use on the chassis dyno. He said standard street tyres are the best as they don't flex and twist or heat up under load.

So lets say it is plenum reversion, what to do from there? Is it a bad valve job or port job? Wrong cam timing?
Oh yeah drag radials cant be used on the dyno they are not stiff enough to take the wieght of the car once its properly strapped down. A 500 hp car will need a good amount of force to hold it on the dyno. I would think vette wide rear rims and tires made for good handling would be best. Also sounds like 500 bucks. So maybe some loaners or trades may be needed to find a proper setup for your car.

Large Tunnel ram will fix the reversion issues same as a 4-7 swap cam. But since the cam is installed and working i would think about changing the intake out for different versions. I would recommend a victor jr if you have one handy.

I would also like to add that edelbrock has some good experience with these issues i would think a quick call to them might present a simpler solution like larger 1050 race demon carb or large spacer. Would lower the vacum level at wot in the intake. But i would look for there recommendation since the victor series is designed to lower chances of reversion.

If you do take the intake off or have an inspection camera handy. You will want to take a good close look at the back side of the valves and see if they are leaking even a small amount will dirty up the intake somthing aweful. This may be the issues. A leak down test may show the problem.
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Old 03-09-2013, 11:47 AM
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Sounds like the valve train is not the problem

The further soot travels up the intake manifold from the head, the worse the reversion problem.

But, your problem gets worse at higher rpm? and reversion usually decreases as RPM increases That's very curious to me

Can you give us a run down on the engine mods especially head and valve work and cam degree install, spring pressures etc

How well your manifold controls reversion pulses and how well it is able to equally distribute fuel and maintain fuel and air flow velocity will determine how large the carb can be and how much power you can make. You can put a band-aid on it with different spacers like sheer or merge or anti reversion plates but that's still only blocking the pulse from reaching the carb the reversion pulse is still there it takes the proper intake or the correct cam to cure the problem!

I'm still curious why the reversion is at the upper RPM range your intake charge and exhaust should be at peek velocity and negating a reversion pulse

My thought that my curiosity draws me to is: If the intake valve is closing on time at lower rpm but slower (late) at higher rpm that would cause a higher rpm reversion pulse! (intake lifter loft, intake spring pressures, intakes bouncing off seats????? It really puzzles me LOL

Jester
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Old 03-10-2013, 04:55 AM
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Quote:
Originally Posted by painted jester View Post
Sounds like the valve train is not the problem

The further soot travels up the intake manifold from the head, the worse the reversion problem.

But, your problem gets worse at higher rpm? and reversion usually decreases as RPM increases That's very curious to me

Can you give us a run down on the engine mods especially head and valve work and cam degree install, spring pressures etc

How well your manifold controls reversion pulses and how well it is able to equally distribute fuel and maintain fuel and air flow velocity will determine how large the carb can be and how much power you can make. You can put a band-aid on it with different spacers like sheer or merge or anti reversion plates but that's still only blocking the pulse from reaching the carb the reversion pulse is still there it takes the proper intake or the correct cam to cure the problem!

I'm still curious why the reversion is at the upper RPM range your intake charge and exhaust should be at peek velocity and negating a reversion pulse

My thought that my curiosity draws me to is: If the intake valve is closing on time at lower rpm but slower (late) at higher rpm that would cause a higher rpm reversion pulse! (intake lifter loft, intake spring pressures, intakes bouncing off seats????? It really puzzles me LOL

Jester
I am puzzled too. I wonder how many engines have problems like this and go unnoticed.

Heads are [almost] full ported sportsman II's. Valves are 2.08/1.60. 3 or 4 angle valve job. Intake is a port matched super victor. Who would have thought a super vic would have problems with reversion in the top end? Maybe its the headers not being proper tuned length causing the problem?

Cam is 248/254 at 50. just over 600 lift and 110 LSA. Installed at 106 IC as directed by comp.
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Old 03-10-2013, 12:21 PM
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We don't know specifics on your build? What ratio rockers are you running?

You could try 1.6 ratio rockers on the exhaust only! It may increase the cylinder evacuation and velocity! If your running 1.6 now on all your valves you can try going to 1.5s on the intakes!

You can increase the lift of an existing cam profile by going to a higher rocker arm ratio. A small block Chevy where the cylinder head runners are not maxed out may benefit from moving up from the stock 1.5:1 ratio to 1.6:1 rockers. But be careful going up more than one tenth in rocker ratio can lead to trouble; there's a limit to how fast you can move and accelerate the valve before the valve spring (that's correct for that cam) can no longer control the system. If a profile was a good design with 1.5:1 rockers, and performance increased at 1.6:1 it'll probably be unstable with 1.65:1 or higher rockers. The correct solution is to design a custom cam profile from the ground up for use with higher ratio then 10% 0ver normal ratio! Over the counter cams are designed for a standard ratio or 10% over rocker any ratio over that design changes spring flex, harmonics, compressed spring pressures seat speed etc!

Special custom ground cams can slow the speed of the valves that high ratio rockers create on over the counter cams!

Myself I run high ratio rockers on the exhaust only that's just my preference but it depends on flow, I start with standard rockers run it and then go higher in ratio run it again, and if performance picks up I'll stay with the higher ratio rockers, if performance doesn't increase Ill drop back to a lower ratio! I don't build an engine and order a cam and put higher ratio rockers on it without knowing the downside or benefits of the rockers! Ive seen too identical builds and higher ratio rockers will benefit one engine and not the other.

Jester
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