What kind of compression ratio do you have? Are you using the flat-top with 4 valve reliefs and no dish? The 268H is a good cam from what I hear but there are some other cams out there that have more lift and duration or more lift with about the same duration. I like the looks of the 941 but wish it had more duration and maybe a 108 lobe seperation (I can't remember what the Crane has but it is a higher number). I really kind of like the idea of getting a custom ground cam so...

 

The problem is that the intake port on the head is smaller than the intake manifold port on the Edelbrock Performer RPM intake. What this does, is creates a air dam and a lot of turbulence as part of the air is blocked from entering the port. This is undesirable. What you're saying would be true if the intake manifold ports were smaller than the head ports. Then yes, this would help stop reversion and you shouldn't port match the intake manifold. When it's the other way around though, it is beneficial to match the head itself to the larger intake manifold ports.

Basically, look at which way the air flows and port match accordingly. Going from a smaller port to a larger or same size one is ok, but going from a large port (like the Performer RPM) to a small port (like the stock heads) is NOT ok. The Performer RPM was designed to work with Edelbrock Performer FE heads, that's why it's ports are bigger. When used with stock heads, it is beneficial to smooth the transition between the large ports of the Performer RPM to the smaller ports of the stock heads.

 

oh boy... I'm no expert.
my heads are C6ae-r's, which are large port(pre emmissions) car heads(someone correct me) and are milled, ported, and have the larger cobra jet valves, they also recieved bowl work (guess that's part and parcel with the larger valves anyways). I've got the car pistons, and static compression is 9.5:1 by math.
I'm running a 500cfm 2bbl and the stock 2bbl manifold.

For "me" and the way "I" use the truck, "I" probably would have been better off with the 260H or even the XE256H... but my "guess" is on the xe stuff I'd have needed to invest in springs and rocker assemblies due to the aggressive profiles.

The truck seems to make plenty of low end which is what I wanted, would have liked even more, however, it does suprise me how it will rev when I stand on it. I'm not saying it's a race truck, but for something that seems so torque'y, it rev's pretty good... enough that I let up so it'll shift as it runs out of breath, I'm not sure if that's carb related or cam related, or likely a combination of both. On the dyno, it shifted itself at 4800rpm... which is beyond where it's making usefull power... again, I don't know the the carb/intake choke it, or if the cam is too tiny to pull air thru the runners at those rpm... ? long story short, I do not have realistic dyno numbers so I cannot provide those.

If I were doing it over again, I would contact a custom cam designer and work things out. I'm sure there is plenty of low end and fuel mileage sitting in my combo not being utilized with an off the shelf cam, which was likley designed for a 4bbl, and more restricted heads...

I know that the age old conception is that Ford heads need an exhaust biased cam, but, the entire exhaust system needs looked at, not just the intake/exhaust cfm. On top of that cross sectional area of the port needs consideration to determine the velocity, header size/length gear ratio bla bla bla... all determine valve events, which in the end control cylinder pressure... all stuff I know goes into designing a cam, but none of it I can do the math on. From custom sticks I've seen in the mustang world(age old conception of dual pattern required), a single pattern cam is often spec'd when 'stockish' iron heads are used in conjunction with good exhaust (big headers, no cats etc)

Man I sure can ramble... and there's more .
Rusty ran a desktop dyno on my engine in this thread https://www.ford-trucks.com/forums/s...no#post2133063 and basically came up with a very low reving power plant that made it's tq early for a gaser... at least in my book.

Bla bla bla...
I'm sure these guys can help you in you cam selection... I used Buddy Rawls to design a cam in my mustang, and got everything I wanted, and now wish I'd have spent the extra coin for a custom stick in Ol' Jessie.

I only know enough about this stuff to likely mix things up and misinform you, so, possibly some of these guys can proof read my post. .

 

Rusty Is right.. You get Reversion either thru the Cam being out of time or non ported heads..Or running a Larger port into a smaller port!! Which you dont want to do!!

Trick heads on the Exhaust side will look like a Mirror...The Intake side is only kept rough enough to Atomize the fuel.... What the fastest way to any point?? A direct Line!! Right!!

I can take a Flash light and look down the Plenum of my Offy T-ram and see the Intake stems?? And that Intake Ported and Port Matched Make's, 50HP over the Victor Intake...and because its a Stroker motor..I loose 6 hp on the bottom end and gain 100lbs of TQ..and 50hp....

So dont **** in my boot and tell me there's a flood!! I aint buying it!!

Have a great day!!

Russ

 

yes, I'm glad he cleared that up, i am not sharp enough to know that the aftermarket intake has larger runners than the stock heads. Only stands to reason... The theory is correct, the application of it to this situation was completely wrong... my appologies.
The intake on my 302 is matched to the heads as well, and I too can see the intake valves, but carefull attention was paid to cross sectional area from one end to the other, not just at the head intake interface. Sometimes, folks will port out the intake to match heads only to create a bulb shape in the runner, as the cross sectional area increases to accomodate the larger head port, a sort of negitive pressure area, is created, I forget the proper terminology... and had assumed the advice given was in an attemp to avoid this.
Russ, I know you know this already(likely better than I), I only offer it up to folks that may not be aware of the mistake and may be considering home porting.

That '**** in my boot' thing... that's funny stuff.

 

I have the flat-top 4 valve relief with no dish (hypereutectic if it helps). My machineist said with the numbers he decked my block and heads to I should be around 9:1 compression. I think I will probably just stick with the cam and see what it does after I get my heads done. If its not enough, well then I'll just have to step it up. You guys rock!!!!

 

Well Russ it's runnin down my leg, not in your boot. Sorry, I, in my vast amount of supposedly accumulated knowledge did not know that the Performer Rpm had a larger runner than the C8Ae head ports. That is what is so good about this forum. If someone makes a mistake there are others to help out.

 

If you're still looking for someone else that has a Comp 268H cam, here I am. My 390, with the 268H cam, all of Comps accessory parts like roller chain, dual valve springs, large retainers, high rev lifters, adjustable cam timing gear, etc. I use my truck, 1974 F-100, for pulling trailers and for that useage, it is fantastic. I does run out of air around 4500 rpm with the 700 Carter Superquad sitting on the Edelbrock Performer intake. But anything below 4500 and it's a beast. I will be doing a rebuild this summer and most likely will be following Rustys specs on his engine in regards to cam. My cousin has a parts shop/machine shop so parts are reasonable and machine work is free. He has a set of GT heads for it but I'm not sure that they appropriate for this application. I have a feeling that as I get started, it is going to go from trailer puller to tire burner. If you feel like you need more cam at higher rpm, Comp has an adjustable timing gear that will move the power to a higher rpm. It will effect your low end drivability though. I had originally set mine for scream speed but I enjoy the A/C and power goodies too much so I set it back to zero. This may be a cheaper alternative for you rather than a new cam.

 

You should be at more than 9.1:1 compression with those pistons and no decking of the block and heads that are in the 70cc range. Maybe you have the larger chambered heads (not sure though, were they a D-shape = wedge or a kidney bean shape).

 

Have you thought about maybe putting a solid lifter cam in your engine. I have been told that if you drive them on a rather consistent basis that you don't have to lash the valves as often as everyone says, but then I have heard that when you get to winding the **** out of your motor they will need adjusted soon. Some of the more experienced fellows in here will probably clarify this.

 

0Your right the actual compression is higher aroudn 10:1, but the elevation and lack of oxygen up here in Montana warrants a drop to about 9:1 (or so says my machineist). I have C8 heads and honestly can't remember the bowl shape. At any rate the compression is actually higher than 9:1, but I haven't had a chance to get to the dyno and work any numbers, we just had our second crazy snow storm of the month, dumped 6 inches on us here in valley.

 

I have some good bud's that I gradutated high school with that go to college up in Bozeman. I was up there a few weeks ago for a college rodeo. If you ever see a nice looking red and white 73 F-100 2x4, it's probably me. And if you can hear heavy metal music blasting, then it's almost a given that it is me. Maybe I will run in to ya up there sometime 'cause I do get up there from time to time. How often do you drive your pickup?

 

No kidding, I go to MSU, Electrical Engineering. Went to the Rodeo too. If ya ever see a red and white 1970 F250 its probably me, its my daily driver. Hit me a message next time you come up, we'll have a beer and jam too.

Preston

 

What a small world. I would definately e-mail ya if I get the chance to head up there and am near a computer. You may know some of my buds up there. Nick McGregor, Dusty DeVries, Carson Durr. Those are the guys I graduated high school with, I have some other friends up there to but those are the ones that I can remember their last name!

 

Can't say I've met them, but still kinda funny you were up here not too long ago. Let me know when you come up, as long as the gas doesn't cost too much. I'd love to meet some of the people that help us poor chums out. Thanks again everybody!!!