Preformer rpm

[t!mortician]

So I have a 1986 chevy short bed with the eddelbrock preformer rpm top end kit on a 78 454 bored .30 over 9.5 to 1 compression cam specs are  Duration Advertised   335° Intake/329° Exhaust
Duration @ .050''   236° Intake/245° Exhaust
Lift @ Valve   .625'' Intake/.639'' Exhaust
Lift @ Cam   .368'' Intake/.376'' Exhaust
Lobe Separation Angle   112°
Intake Centerline   109°
Intake Timing @ .050"   Open 6° BTDC
Close 51° ABDC
Exhaust Timing @ .050"   Open 54° BBDC
Close 11° ATDC
 Also I have 9 to 11 lbs of vacuum at idle
Carb is 17080212. I am thinking recipe # 2 out of the book but kinda new. If there is a thread on this already  I can't find it so any help would be great

[t!mortician]

Also headers and 3 inch Exhaust. I am going to order ax rods, .035 needle and seat, full taper primary rods to .026 tip, cam and spring set, and B secondary hanger. I will follow recipe 2 and post how it goes any comments or suggestions are greatly appreciated

[Cliff Ruggles]

You will find that carb already pretty generous for idle fuel.  We can discuss the specifics when you call in the parts order.  I'm here 7am-4pm, M-F EST...tks......Cliff

[t!mortician]

Ok thank you I will give you a call tomorrow 🙂

[t!mortician]

So after talking with cliff I ordered a kit with 72 jets,  his primary metering rods, a new choke pull off, and it had an orange or red spring for the power piston. I also opened idle tubes from. 036 to .038 and the idle down channel fromb.050 to .055. I put a set of ax secondary 

[t!mortician]

So I now have some control over the idle screws but i can't seem to get the power I want or expect at any point in acceleration the only thing I have noticed is that the power piston won't seat until about 2000 rpm. I think I have the lightest spring already any suggestions 🤔

[Cliff Ruggles]

The power piston is controlled by engine vacuum, so it is a load sensing device just like the vacuum advance in the distributor.

It will always be applied or in the down position at idle and "normal" driving at light throttle/light load.

If you aren't making enough vacuum at idle or when you are driving the vehicle gently it's there is something wrong someplace unrelated to the carburetor.

Most well thought out engines, even those that make a lot of power will make at least 10-12" vacuum at idle speed and a LOT more than that at light part throttle cruising.  They will nearly bury the vacuum gauge coasting. 

Take some vacuum readings at idle, and during "normal" driving scenarios and post the results......

[old cars]

Curious, What type of cam that is. Roller or flat tappet. seems like a big spread between advertised duration and @.050 specs.
Assuming its an automatic trans, do you know your vacuum at idle in gear?

[Cliff Ruggles]

Edelbrock doesn't make camshafts, they source the from CMC just like Speed Pro, Sealed Power, FM and about a dozen other companies have for decades.

The cam is most likely rated at .002" tappet lift instead of .006" like most Cam Companies rate them, so you're going to see a much wider spread between advertised and .050" duration numbers.

What the engine needs here is about one more full point of compression to be happy.

I have a 455 engine in my Ventura with CNC ported aluminum heads and 236/245/112 LSA cam with the ICL at 109, lift at the valves is .639", so a very similar build.

It idles just about smooth with a very slight hint of "attitude" making 13.5" vacuum at 750rpm's. 

The BIG difference is that my engine has 11.3 to 1 compression instead of 9.5".  It would NOT idle well clear down at 9.5 to 1.  It would also have poor throttle response and lackluster performance until I got it past 3500rpms or so.

What is really needed here is more compression for that much camshaft in this particular build.....IMHO.......

[old cars]

Too many variables here. Street vehicle or track warrior? Pistons,Rear axle ratio, stall converter, tire size?

If this is a true street vehicle that cam would be better suited to more cubic inches. If those are stock flat top pistons it might not be making even 9.5:1 compression.

A 454 (true street engine) could make More power, better drivability, and use less fuel with less duration and tighter LSA camshaft.

Edelbrock makes excellent intake manifolds and cylinder heads. There camshaft choices not so good. Low intensity (too slow lifting the valve) and wide LSA to get the engine to idle better with more duration. This especially does not favour low compression.

[Cliff Ruggles]

The worst cam you will ever put in a big CID engine with moderate compression will have short seat timing and tight LSA.

Sure they will pack some punch and wake things up by yanking the power down some, but you can very quickly run into detonation issues even at relatively "low" compression ratios.

Case in point below with an engine I got involved with:

The engine builder (owns a speed shop and dyno) called Comp for a cam, and they sent him an XR276HR for his 455 engine with 250cfm head flow and 9.3 to 1 compression.  Sounds like a great choice, right?  Not much more seat timing than a 350SBC 300hp cam, 224/230 @ .050", 110LSA, etc.  It didn't make chit for power and pinged pretty hard on the dyno resulting in rod bearing replacement.

I was contacted as everyone at the dyno facility right down to the guy taking out the trash was blaming the Q-jet for "not keeping up with their engine".

Instead of messing with carbs I suggested a cam change with more seat timing, 114LSA and ICL at 110.  The engine builder thought I was nuts!

He did it anyhow, no other changes and a couple weeks later called and told me that the engine was a completely different beast.  It made nearly 90 MORE horsepower on the dyno, idled smoother, and no detonation whatsoever.

Once the engine reached the vehicle the owner reported that it was very "mild" at a glance, but excellent street manners and more power than any type of street radial could ever hold!.......Cliff

[old cars]

those are some pretty vague statements. Not sure if you understand seat timing

[Cliff Ruggles]

What's not to understand?

I helped a customer who couldn't make 425hp with a 455 CID engine at 9.3 to 1 compression because he trusted the tech and Comp to supply his cam and took him to nearly 520hp with more power (torque) at every RPM and it quit pinging on pump gas.

The engine builder and the owner also were remarkably surprised at how well the engine idled with the larger cam in it and absolutely love the end result once it made it into the vehicle......FWIW......Cliff

[old cars]

Maybe there was more wrong with the initial build then the builder told you.

[Cliff Ruggles]

No, they only made ONE change.

The engine builder actually owns a machine/speed shop, and dyno.   He specializes in Big Block Chevy and Ford engines, but this was his first Pontiac build.

He bought a complete rotating assembly to stroke a 400 block to 467CID, zero decked, tight quench, ported the factory iron heads to 250cfm, etc.

The missing piece of the puzzle was the camshaft, picked it too small and LSA too tight.  One has to realize that a 455 Pontiac is a poor design from the start, although it is capable of big power.  The stroke is long, piston speed high, heavy internals and fed by cylinder heads with no more cross section in the intake runners than a decent small block Chevy head.

They are excellent "mid-range" engines if you know hot to exploit them, but using tiny cams on tight LSA's just makes them too good at what they already do best, which is making all the power right off idle.

I suppose the XR276HR cam may work OK in a 454 Chevy built with peanut port or PASS round ports on it, but it's not nearly enough cam for a 455 Pontiac build, or at least far from ideal as the dyno chart and results we got with a much larger cam on a wider LSA clearly show.......FWIW......