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-   -   What is causing this flat spot in power? (http://www.hotrodders.com/forum/what-causing-flat-spot-power-230337.html)

HQHolden 03-04-2013 11:06 PM

What is causing this flat spot in power?
 
Had my engine dynoed a while back and all went well. Got everything up and running and seemed to be going well. Took it to the track and was not running the times I had expected. So took it to a chassis dyno and found a huge dip in power towards the top before it picked up again.

Attached is the printout of both dynos. As you can see the engine dyno shows a smooth power delivery all the way through. Everything I am running on the car was used for the engine dyno, eg water pump, alternator, fuel pump, reg, etc. On the chassis dyno we tried dropping the exhaust system in case there was too much back pressure but it made no difference at all.

So what do you think?

http://i46.tinypic.com/29zps8i.jpg

http://i50.tinypic.com/a1jt4j.jpg

HQHolden 03-04-2013 11:09 PM

btw. Its a 350 sbc. 750dp carb. What else do we need to know?

Also I think over all its down about 50hp or so as I feel 520 odd engine hp should have shown around 400 at the wheels. The 260kw atw is 350hp.

prumora1 03-05-2013 12:53 AM

what transmission, what is weight of car, what fuel, and what are your times on the track

HQHolden 03-05-2013 01:49 AM

Quote:

Originally Posted by prumora1 (Post 1653637)
what transmission, what is weight of car, what fuel, and what are your times on the track

th400
3300 race ready
98ron
11.8@114

Landshark928 03-05-2013 11:58 AM

The dip around 4000+rpms is typical for a SBC with a large overlap cam. The large overlap can cause higher pressures in the intake manifold that can reduce the signal to the carburetor and cut fuel flow. Your BSFC backs up the theory. If you can adjust your carb to fatten up that area only you may get back 15-20ftlbs. With EFI it's a lot easier to get it back.

What are you cam specs?

ap72 03-05-2013 12:04 PM

Quote:

Originally Posted by Landshark928 (Post 1653769)
The dip around 4000+rpms is typical for a SBC with a large overlap cam. The large overlap can cause higher pressures in the intake manifold that can reduce the signal to the carburetor and cut fuel flow. Your BSFC backs up the theory. If you can adjust your carb to fatten up that area only you may get back 15-20ftlbs. With EFI it's a lot easier to get it back.

What are you cam specs?

an intake or header change can also help pick up signal in this area and reduce reversion.

HQHolden 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?

Landshark928 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?

oldbogie 03-05-2013 03:36 PM

Quote:

Originally Posted by HQHolden (Post 1653630)
Had my engine dynoed a while back and all went well. Got everything up and running and seemed to be going well. Took it to the track and was not running the times I had expected. So took it to a chassis dyno and found a huge dip in power towards the top before it picked up again.

Attached is the printout of both dynos. As you can see the engine dyno shows a smooth power delivery all the way through. Everything I am running on the car was used for the engine dyno, eg water pump, alternator, fuel pump, reg, etc. On the chassis dyno we tried dropping the exhaust system in case there was too much back pressure but it made no difference at all.

So what do you think?

http://i46.tinypic.com/29zps8i.jpg

http://i50.tinypic.com/a1jt4j.jpg

The chassis dyno only shows what power the tire is putting into the ground (roller); this may or may not reflect anything the engine is doing. A lot of mayhem happens inside torque converters, transmissions, u joints, rear axle, and finally the tires.

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.


Bogie

vinniekq2 03-05-2013 11:45 PM

1/4 mile time
 
Quote:

Originally Posted by HQHolden (Post 1653640)
th400
3300 race ready
98ron
11.8@114

that sounds like 480-500 hp
the mph is decent,car should run 11.60s
you have tuning and launch issues,not hp issues

vinniekq2 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

Landshark928 03-06-2013 09:27 PM

Quote:

Originally Posted by vinniekq2 (Post 1654106)
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

14.5 is not A/F it's pressure at the Fuel Pressure Regulator, which is most likely not hooked up to the dyno, therefor the consistent default setting.

I do not see A/F listed on the print out sheet.

vinniekq2 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

HQHolden 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.

painted jester 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:P years ago thet wave would bother me but maybe today thats considered a small thing :confused:

Jester



"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!


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