What if your carb is too big?

[tdsmith]

Ok, I've been trying to track down a mysterious problem with my car since I got it. For some reason my power drops off really sharply @ 5200 RPM. I've talked to lots of people with different opinions. Some of the most common was too small of a fuel pump or the fact that I had a single plain intake. Both have been rectified. I now have an Edelbrock RMP Air-Gap intake, Performer RPM heads, and now a Holley Blue fuel pump set @ 6 p.s.i. My question now is could my carburetor be too big and if so, what would the effect be?

Full specs on my engine:
351W
KB181 pistons (22cc dish)
Edlebrock Performer RPM heads (60cc)
9.1:1 compression ratio
Edelbrock RPM Air-Gap Intake
Holley Blue fuel pump (6 p.s.i.)
Holley 750 CFM double pumper carb (#4779)
Cam custom ground (270 285 duration, 220 230 @ 50, 507 intake 532 Exhaust lift, 110 lobe center)
New K & N air filter
Distributor set 14 intial and 36 final advance


I've talked with the guy who built the engine for House of Cobras a few years back and he doesn't understand why I'm not getting the higher RMPs, so I'm turning to all of you. I would like to get to
6500 RPM if possible. I was on Holley's website and according to
their formula (Engine size * max RPM/ 3456 = CFM) a 650 carb would be plenty for my engine. So my first question is could the larger carburetor be part of my problem? I've had it tuned for air fuel mixture, but could it still be causing a problem? Should I buy a smaller carb?

Do you have any ideas what I need to look at? I'm technical enough to put an engine together on my own, but not to know what things I should and should not try for this specific problem.

Thanks in advance.

Troy SPF#578

[DAVID GAGNARD]

Troy;

A 750 cfm is on the "high" side for a 351-W like yours,but I would not think it would hurt top rpm or hinder it.... When you say it "drops off sharply at 5200" do you mean the motor "noses" over or just does it quit accelerating????? May be the valves,more so the valve spring pressure..... If you do not have enough valve spring pressure the valves will float at a set rpm and the car just plain quits pulling....trust me,I know, I did it building my motor,would not rev past 4800rpm,read the valve spring pressure wrong and did not have enough....Reset valve spring pressures to 125 lbs. on the seat and now my 484 lift cam will rev to 6500rpms with no problem....

With your cam and setup 6500 rpms should not be a problem,that's why I would look at the vavle spring pressures....

Just my .02 cents...

David

[tdsmith]

On a dyno you can see that the engine drops of like a rock @ 5200. I had 278 rwhp @ 5200 and by 5400 I had 140 rwhp. I've never seen a curve drop so fast.

I'll check the spring preasure, but I'm not sure what it should be. I'll have to call the builder again.

Troy

[DAVID GAGNARD]

Troy;

Actually a single plane intake is better for top rpms and a dual plane is better for low to mid range rpms,(I'm running a dual plane Edelbrock Performer Rpm),either of the ones you have should work just fine.... My setup is very similar to yours,timing about the same,I think I have a top end of 34 degrees and run a Holley 600 double pumper,but have had everything from that 600 to a "built"750 on it and two or three in between. I'm running more compression but I would not think that would matter that much.... Something seems way out of wack with your dyno #'s,dropping off too soon for your cam.... Also,ask about how the cam was installed,straight up/advanced/retarded,this could also have a big efect on top rpms....

As for as spring pressure goes to get to 6000 rpms you would need a minimum of around 120lbs. on the seat give or take 5 pounds or so....I'm running 125lbs. seat pressure and have no problem with 6500 rpms,once while doing a burnout at the dragstrip and not paying attention to the tach I had it would up to 7100rpms in second gear.....

Something surely seems off for your motor to fall off that quick at that rpm. Seems that cam should pull strong thru 6000 rpms.... Mine is only 272 duration and 484 lift,a lot smaller than yours and pulls good to 6000rpms,after that it falls off but I do not think near as fast as your motor did....

Hope you find the trouble and keep us posted....

David

[George Snyder]

Hi Troy,

Normal things I can think of off the top of my head to restrict higher RPMs are, fuel starvation, air flow inadequate, timing, or springs too light ( valve float)

When you say 6 psi fuel pressure, is that while making a dyno run or at idle.

If 6 psi is steady during run, and float levels are set correct, that seems OK.
Rpm Manifold will support 6K+ no problem.

Springs or distributor?

What brand and model distributor are you using?
I have little knowledgeable how much lift Edelbrock RPM Heads will support, as I never used a set. 532 lift doesn't sound excessive.

Bud

[tdsmith]

the fule pump regulator never drops below 6 p.s.i.

My distributor is a MDS 6AL.

Troy

[aumoore]

Troy
I had a similar problem with my Explorer 5.0 motor with GT-40P heads and a trick flow cam. The motor would nose over at about 5500 rpm and I thought I was getting valve float. The Cam is supposed to peak at 5500rpm and I thought maybe the cam was just out of gas. I was running a Mallory Unilight distributor driving a MSD coil with no ignition box(the unilight will drive the coil without a ignition box).

I added a Mallory Hyfire digital ignition to help cure the rich smell at idle and it did clear that up. Now the motor will rev all the way up to the 6250 rev limiter in the Hyfire box(it is adjustable in 250rpm increments from 4000 to 10000rpm by changing dip switches) I guess the spark was not strong enough without the ignition box to support the cylinder pressure at high rpm.

Are you running a ignition box? It may be hitting a malfunctioning rev limiter or if no ignition box you may be blowing out the spark.

[tdsmith]

I have the MSD 6AL ignition box. and the revlimitor is set @ 6600.

[Bob Putnam]

Originally posted by tdsmith


On a dyno you can see that the engine drops of like a rock @ 5200. I had 278 rwhp @ 5200 and by 5400 I had 140 rwhp. I've never seen a curve drop so fast.

Troy

I can't think of anything but valve float, ignition breakdown or a rev limiter that would create a precipitous dropoff like that. Fuel problems will create rough running, and cam profile will gradually taper the torque off.

[trularin]

I remember an equation that gave you a ball park CFM based on cubic inches and maximum RPM.

(Cubic inches X max. RPM)/ 3456

Take your max RPM the engine is expected to see, times the cubic inches and then divide that number by 3456.

So, if you have a 351 and you want to see 6000 RPM;

351 X 6000 = 2106000, Then 2106000/ 3456 = 609.375.

Round to the next hishest available CFM = 650 CFM. I imagine if you shop around you could find a 625.

If you go lower, you will bottom out on available mixture transfer.

Just my $0.02

[McCobra]

TD,
Altitude may be a contributing factor.

I have heard great things about Walt and Chip Hane in Evergreen.

They dyno tune for our altitude and seem to be real good guys.

Good Luck,
McCobra


http://www.eps-hane.com/

[rbray]

If your carb is too big don't waste money buying a smaller one.
Buy a bigger engine!

[NeedAntiVenom]

Troy,

I have seen a few posts about MSD rev limiter accuracy. One person removed the limiter chip and it immediately wound out without problems. You may want to try either removing it (for the dyno) or installing a higher value. There may be variances in the components within the MSD box that cause the actual rev limit to be approximate.

Hope this helps,

Jim

[tdsmith]

I'm on Walt's wait list... but it will be mid Sept. at the earliest before my number comes up.

My MSD rev limit chip was a 6400 for the dyno run, but I'll try again without a chip.

Basically, I'm jsut at a loss now.... I'm very close to trying to find a supercharger to push more air back into my engine and pretend that I'm @ see level. :-)

Troy

[McCobra]

How about Blue Oval in Denver?

[George Snyder]

Can a few of you Guys help with posting replies on this topic and keep it on the front page.

My e-mail notification doesn't work and I really want to know what the problem was. I'll know forget where it was posted and maybe never hear the solution.
Thanks,
Bud

[Doug I]

an off the wall suggestion ...

could the air filter be reaching a limit at 5200 and choking the engine?

Plug wires breaking down at that load?

Did it run rough when it ran out of puff ? Off the top of my head I'd have said valve float would cause erratic running.

I'd put a vote in for the rev limiter chip being off and shutting it down early.

[Mr.Fixit]

Try it without the rev limiter chip and remove aircleaner.

If not that, I'd guess valvesprings are weak and floating the valves.

I am assuming you allready gave it an ignition tune up by replacing the sparkplugs, cap, and rotor. Checked the plug wires, and the coil is OK.

[gfridland]

You must look at not only the fuel pressure, but also the hose/line sizing in order to determine weather or not you are starving the engine. You may have 6psi in an 1/8th inch line, but it still will not be able to deliver the volume of fuel necessary at the high RPM. Make sure there are no restrictions in the fuel filter or the carb inlets.

[ford428]

crank engine around with wrench to 36 degrees total, the pointer is then pointing to the 36 degree mark on your harmonic balancer, this is then a static full advance position....then go to number one terminal on dist. cap..mark its C-C down the side of the base so u have a reference point when u remove the cap...a lot of times the dist is installed on tooth away and when the centrifigal advance goes to full the rotor is too far away from correct terminal to fire...