I have heard it mentioned on this site that Speed Density computers don't like cams with a lobe separation less than 114*. Why is that? I have a mildly built, carbureted 351W with a Comp 4x4 Extreme cam with 111* lobe separation. I built this engine awhile back for my faithful '82 F150, but alas, the rest of the truck fell apart around the engine! I bought a replacement truck and am in the process of converting the 351 to EFI to replace the gutless EFI 302 in my "new" '87 F150. I've got all the parts and am ready to swap it out, but this cam has me worried. Is it going to run at all? Will it have idle issues? What am I looking at? I LOVE this cam and really don't want to get rid of it. I'd like to keep the fuel injection for drivability and reliability (as well as retain the stock fuel system). Anybody running a Speed Density computer with a cam with less than 114* lobe separation?

Thanks!

Aaron

 

Lobe separation affects how much vacuum you have. The speed density computer makes it's calculations on the vacuum reading plus a couple other sensors.

I think you can run some 112 lobe separation cams with the mass air setup since it measures the amount of air coming in instead of vacuum.

Also to run speed density is is reconmended to have durations under .220 @ .050.

 

We're ok on the duration @ .050 (I believe it's .218). Does that extra 3* of lobe separation really make that much difference? ...stupid speed density...

 

It's not so much about those 3 degrees of duration, it's more about the stability of the vacuum signal. If the cam has really small lift.. like 0.380" total or so, then valve duration would probably be short enough that a cam with 110 deg seperation would generate a vacuum signal steady enough to keep the computer happy. But any cam that has a lumpy idle with a carb is pretty much doomed to run like *** on SD EFI.

 

The idle with the carb is actually quite smooth, not really that lumpy. Am I correct in assuming that the idle quality is where I'd see most of the issues with this cam and the SD computer? Since more throttle equals less vacuum (most of the time) would drivability be a problem, or just idle and off-idle?

Thanks for the help guys!

 

it could also mess with the computer and cause it to run lean because it messes with various sensors at this point. if you pulling the motor to drop it in another rig run a compcam 35-255-5 and your set. im sure that cam could be sold on ebay to make up some of the cost.

 

I figured that was my best option. What kind of hp/tq gains will I see with the 35-255-5 over the stock cam? At this point, I'm trying to decide if I really want to keep the EFI, or just put a 4BBL carb and intake on it, and run it oldschool. I know I'll have to re-think my fuel system, but at the same time, I don't want to tear my 351 down either.

 

the benefits of efi are huge compared to carb setup. and at these times of $3 gallon of gas every little bit helps. your swap is pretty strait forward really. and your half way there because you already have to replace the carb intake for an 5.8 efi upper/lower and i am sure your converting to serpetine belt setup right so that would mean pulling the water pump of your 5.8 inplace of a reverse rotation pump. the cam is sitting right behind the timing cover. i bet your talking 1 extra day at most

 

Yeah, I know what all is involved with a cam swap, and you are right, since I'm already in there, and will have to replace the w/p and entire intake setup, might as well. Question, will the reverse w/p from the serpentine 302 work on the 351? I've got the adapter bracket for the A/C and PS and plan to just transplant the serpentine system from the 302 on the 351. I know the 302's WP is good, and would like to re-use it if possible.

Thanks!

 

Yes.. water pumps are the same for both motors.
Depending upon the year of the motor, the 35-255-5 cam is anywhere from a healthy upgrade to a massive upgrade over the stock cam!!!

 

I think the engine came out of a '79 van (paid $100 for engine and C6). I had it bored .060 over, new pistons, etc. and put the Comp 4x4 extreme cam in it. This motor makes tons of power even with the '69 model 2bbl carb sitting on top of it. It has no problem hauling 1.5 tons...yes 1.5 TONS of dirt in the bed up the side of a mountain. BTW, steering becomes optional with that much weight in the back of an '82 F150 (at least I have a 9" rear.) Hopefully with the 35-255-5 cam and EFI intake, it'll be even better.

 

What are the specs on the Comp 4x4 cam you used?
An EFI setup using a stock truck intake will make lots more low rpm torque than the carb, but probably less peak HP if that peak was above 4500rpm. Using a short runner car style intake will produce more of a carb-like powerband, but it will still make more low rpm TQ then a carb.

 

I want to say it was this cam: CCA-35-239-3
Duration @ .050: 218 int./226 exh.
Lift: 0.493 int./0.512 exh.
Lobe Separation: 111*

It was either that cam or this one: CCA-35-235-3
Duration @ .050: 210 int./218 exh.
Lift: 0.477 int./0.493 exh.
Lobe Separation: 111*

It's been about 3 years since I built the motor, so I don't remember exactly. I do remember that I wanted my power band to start as close to 1000 RPM as possible, so it's one of these two cams.

Thanks!

 

I had a cam installed with my rebuild that wasn't computor compatible. 110 lobe sep. I'm still having to tweek it. Tweecert is a like a $400.00- $700.00 program and code reader so that the factory settings can be "Tweeked" to open up whats left of your engine.
It runs good if you don't know what you should expect from a 351. To me it runs like it's starving for air and fuel. Good luck.

 

Yeah that's what I was afraid of. I went with the Comp 35-255-5 that everybody is always talking about, and so far I've been happy. It's not quite as punchy as the old 351 in my other truck, but I did have to de-cam it a little. I should get much better MPG's with the EFI though, so it's a decent trade off. It's still WORLDS better than the 302 though!