Why am I running out of breath at 4-5000rpm

[C0V3R]

Why am I running out of breath at 4-5000rpm
I've been running in my new 5.0 for a while now. I've noticed that it seems to run out of breath up top, seems to slowly stop pulling as compared to the 2-4000rpm range.

The build is as follows:

Brand new 5.0 roller shortblock
TFS Stage 1 Cam
TFS TW Heads
Weiand 7515 single plane
Edelbrock 1406 carb (600cfm)
Full MSD ignition & diz
Tri-y headers (and crappy press bent exhaust)

I am running around 15deg initial timing, and am up a couple of jets on the mains from stock.

Do you think the carb is too small, exhaust may be a problem or maybe something else?

 

[D.Hearne]

I'd suspect it's not getting full ignition advance. It's not the tri-y's or exhaust. I run the same thing on my V8 Ranger's exhaust and it was fine, even with the 400+ hp 331 sixpack motor. The right side collector to pipe has a horrible kink in it and the rest is std muller shop bends,the left side goes over to the right, then back to the left to exit the rear of the truck, it ain't pretty but it works. As for your carb, it's on the small side, but unless Edelbrocks are way smaller than I read they are in flow compared to a Holley, I don't think that's the bottleneck either.

 

[mustangdave]

I agree; check your total timing... Most MSD dizzy's are adjustable by changing springs in the dizzy.

 

[hotrodredneck]

you also might have a problem with the carb...my old man had the same thing happen with his truck, come to find out, the foats in the bowls had kinds stuck and wasnt letting enough fuel in at the higher RPMs

 

[ForceFed70]

To me it sounds like it's either the carb choking things up, or your fuel pump can't keep up. Could also be timing tho.

 

[C0V3R]

Is there any way to tell if its the fuel pump other than having the car on the dyno with an AFM?

 

[ForceFed70]

Just hook up a pressure guage.

Run it through the gears.. fuel pressure should not drop below about 4.5 PSI.

 

[D.Hearne]

I can tell when my Ranger's fuel filter gets clogged, it starves for fuel in full power runs. Kinda sounds as if it's got an ignition miss when it gets thru 1st and about the end of 2nd it'll start to miss.

 

[C0V3R]

Do you think a 72gph carter should be enough to fuel it?

 

[D.Hearne]

Do you think a 72gph carter should be enough to fuel it?

At 72 gph, your engine would have to be getting something less than 1 mpg. I ran my 331 with it's three carbs on a 30 gph pump. And it was plenty enough for full power runs for a few minutes.

 

[C0V3R]

Thats exactly what I thought. I know you need to have a bit of spare capacity but I'd hope the fuel pump is up to the job.

 

[ForceFed70]

Since it's such a quick and easy test, I'd still check the pressure at the carb to make sure. You could have some sort of fueline blockage or the pump could be failing.

 

[C0V3R]

I've got the smallest amount of total timing I can get in the MSD diz given the 15 deg base. IIRC this is 18 degrees making for 33 total. I'll have to double check later to make sure.

Also I have the stiffest springs so the mech advance will come in relatively slowly.

 

[D.Hearne]

You need more total advance and lighter springs.

 

[thumper460]

I agree on the timing... check the total, not what you should have!! the dizzy and your combo would like the advanve as soon as possible with the total about 38*.. haveing the heavest spring may keep the sucker from doing anything till 4500+ rpm!!

Yes check the fuel pressure with a gauge!! you need to STOP guessing and put a gage on it!!

I would do the ign.. and then the f/p first .. befor you go into the carb.. if the first two are good then check the carb float level!! ( low pressure at speed will indicate poss carb issues) all three of these are a pretty fast diags to do.. then you will have the base to start getting that mid to top back!! cool??
Just me...........................

Thumper

 

[67Dylan]

I also agree.... dial in the dizzy so the advance happens sooner i.e. lighter springs.
Also, thumper has a good idea about checking fuel pressure & timing before you tear into anything. If it's one or both being the problem the fix is easy and better than tearing into something needlessly.

 

[C0V3R]

OK turns out I was running a 21degree stop which would make it 36 deg total coming in at around 3800rpm.

I've read that with more efficient combustion chambers you can actually be better off powerwise with *less* advance, so I am going to give it a go by dropping out 3 degrees but bringing it in a little earlier.

 

[commonrail]

What valve springs, and camshaft are you running?

 

[C0V3R]

TFS-51402000
Valve Setting Intake: .000"
Valve Setting Exhaust: .000"
Advertised Duration (Degrees): 275 intake/279 exhaust
Duration at .050" Lift (Degrees): 221 intake/225 exhaust
Camshaft Lift: .312" intake/.319" exhaust
Valve Lift with 1.6 Rocker Arms: .499" intake/.510" exhaust
Lobe Separation (Degrees): 112
Intake Cam Timing at .050" Valve Lift:
Opens 3 degrees BTDC
Closes 38 degrees ABDC
Max lift: 108 degrees ATDC
Exhaust Cam Timing at .050" Valve Lift:
Opens 49 degrees BBDC
Closes 4 degrees BTDC
Max lift: 116 degrees BTDC
Firing Order: 1-3-7-2-6-5-4-8
Characteristics: Good idle, strong mid-range power. Aftermarket intake, heads, and headers recommended. Requires modified mass airflow sensor. Best with 9.0:1-10.5:1 compression ratio.


Springs are what came with the heads. Single not double set of springs.

 

[C0V3R]

Having backed off the timing and brought the advance on earlier it seems a fair bit better in the mid range and now the flat spot is more 4200-5000 so I am thinking maybe its fueling.