|03-10-2013 12:21 PM|
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.
|03-10-2013 04:55 AM|
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.
|03-09-2013 11:47 AM|
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
|03-09-2013 07:41 AM|
|st3gamefarm||What about a reversion dam I in the intake runners at the heads?|
|03-09-2013 01:09 AM|
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?
|03-08-2013 08:22 PM|
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!
|03-08-2013 09:33 AM|
Am I the only one that sees the power wave?
Both Dyno sheets show the same problem at about 5100 rpm to 5500 rpm the line graph shows a drop at 5200 to 5600 and the torque on the other sheet shows torque drop at exactly the same 5100 to 5500. The HP gain was about 10 hp for every 100 rpm till it hit about 5000 rpm and it dropped to 4 or 5 hp gain every 100 rpm till it hit 5400 and then made another 10 hp gain fell off again to 4 or 5 hp every 100 rpm at 5500 and then gained at 5800 your sheet does not show a smooth gain in hp it shows a wave! If you make a graph of your hp at every 100 rpm you will see that wave in power gain! The first thing I would check is reversion pulses! Could be harmonics Ive seen this wave when harmonics created a resonance wave that robbed power and created a wave effect like this! If its a harmonic resonance in the engine it will only get worse! Ive also seen it when cold coolant entered the block every time the thermostat opened and a simple change to a hotter thermostat cured it!
What engine are you running and whats your set up?
Maybe I'm too picky and pay too much attention to minor details LOL years ago thet wave would bother me but maybe today thats considered a small thing
"People who run these chassis dynos like to talk about only 25 percent loss through the chassis and related components, like I wish, it doesn't take many installation sins to double that amount of loss."
Its actually an average of 1/3 or even a little more with air and power steering!
|03-07-2013 05:18 PM|
I don't know why the afr from the engine dyno isn't included in the chart, but fuel mixture was good at low 12's.
As for the cause of the loss I've been told everything from faulty torque converter, worn dizzy gear, incorrect pinion angle, tyres slipping on the dyno, you name it, even different ramp speeds of the 2 dynos was mentioned. I think the tyres slipping at a particular drive train speed is the most likely cause that I would like to eliminate at the moment. I used et street radials which I have been told can warp and flex a lot under the stress of the rollers. I am going to go back to the same dyno with some nice hard old street tyres, and also make sure the guy connects the MAP sensor so we can also eliminate reversion.
|03-06-2013 09:37 PM|
I guess I should wear my glasses more,,,,lol.
The dip is where the torque starts to roll over and the bsfc changes there.I guess thats why I thought 14.5 was the A/F:1
Maybe post the A/F ratios? thanks again shark
|03-06-2013 09:27 PM|
I do not see A/F listed on the print out sheet.
|03-06-2013 10:34 AM|
A/F @14.5 is a little lean for racing
the rollover in power happens at the point that torque is beginning its decline.
For racing only the A/F is quite lean.
Nice to see the same A/F all the way through the rpms.
as is should be a very fun drive.
carb size is adequate
|03-05-2013 11:45 PM|
1/4 mile time
the mph is decent,car should run 11.60s
you have tuning and launch issues,not hp issues
|03-05-2013 03:36 PM|
People who run these chassis dynos like to talk about only 25 percent loss through the chassis and related components, like I wish, it doesn't take many installation sins to double that amount of loss.
If this trend was on the engine dyno I'd be hunting reversion issues related to intake and/or exhaust tuning lengths and/or intake plenum volume, metering inclusive of the air correction jets and emulsion tubes, and ignition. This can be ignition as well, my Harley had a hole in the power curve that was cured with the addition of Nology plug wires all the messing with the carb jetting and ignition module did nothing but move it around a little or lesson it some, the Nology's eliminiate it with no other changes being active. Go figure?
The engine will respond differently to its tuning when on a chassis dyno than it will on an engine dyno and certainly different than either when on the street or track. Being a sucessful racer mostly revolves around identifying these problems and tuning them as much as the constraints allow.
About the best advice I can offer short of being there is to only work one tuning item at a time. This is something that grows geometrically if you start making changes to many things at once. The other is to start with the simple things then work to the complex.
|03-05-2013 03:02 PM|
The reversion can be limited by a good valve job. Also having the intake slightly smaller than the head so reverse pulses get hung up a bit. Lash will make almost no difference at only .002 tighter.
Just thinking out loud, but perhaps the Vic JR is too big for a 350, compounding the signal issue to the carb? Maybe even smaller headers that might scavenge better around 4000rpms?
|03-05-2013 02:51 PM|
Cam is 254/248 @ 050. 614/620 lift. 110LSA. 106IC.
Intake is super vic and headers are 1-3/4" but I don't think they are tuned length.
So is the reversion something like a resonance at a particular RPM that disrupts air flow or signal to the boosters? How do I tune the carb at a particular PRM only? Surely at WOT and 4K its flat out and well and truly on the main circuit.
Could this be a port match problem, i.e. the intake face may be cut too big in some areas and the head port is interfering with flow? Or would that effect flow more as RPM increases, not in a particular band.
Changing intake or headers is not an expense I want to go to at this stage. What about a spacer or super sucker? Or will that just make the dip happen at a different rpm?
You say big cam overlap can cause this, what about if my lash is 002 too tight - could that cause an increase in duration big enough, or is 2 thou stuff all?
|This thread has more than 15 replies. Click here to review the whole thread.|