Sounds good. Keep us posted on how it all works out, as I'll be watching closely. I'm still collecting parts for my rebuild, but it sounds like you're going a little more aggressive than me. Current plans are a 390 throw (of course), boring .060, ported and polished stock heads, Performer (dual plane, not the RPM) manifold, Stan's tri-Y headers with Jet-Hot, Pertronix Ignitor and coil. The C6 already received a performance rebuild. I've got or ordered most of those parts, already, though I'm still looking around for a good 390 crank and rods. The Ignitor and coil are already in, and the headers and Flowmaster 70s should go in soon, as I'm tired of my existing leaky exhaust. I'm pulling the engine in October and sending it to Southern Automotive for the rebuild.

Especially tell me how that carb and cam work out, as I'm undecided what to get. For the carb, I'm leaning toward a 570 cfm Holley street avenger, but it's kind of pricey and some of my friends think I'm being too conservative and should be running a larger carb. Anyone have any experience with the Demon Carbs "Street Demon" (625 cfm) on an FE? I'm building my truck for hauling and towing, not racing, and it has AC, ps, pb, etc.

ksd
69 F100 Explorer lwb 2wd FE 360 C6 3.25:1. WANTED: straight/clean styleside long box w/aux fuel tank for 67-69 F series.

 

I've got a straight, clean LB box for 69 F250 with aux gas inlet that I may not be using. I'll probably go with a flatbed.

Greg

 

I just wanted to point out that the RPM is actually a dual plane manifold too, and one that I'd recomend. In my opinion, that 570 would be way to small for the engine you want to build. I have the Stans with the coating and they are awesome, good choice.

If your knuckles ain't bleeding you did something wrong.

'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400+ ponies , 4-speed, custom suspension w lift, mud on black.

 

That may be a great carb but it depends on what your doing. It should give you lots of low end torque and good gas mileage. Of course if you're out to rev it over 4600 or 4800 then you may want something a bit bigger.

I run a 600 Holley on my 428 on the street so I don't wind it up too much and it runs great up to 4300; keeps me from getting too stupid and getting quite so many tickets. Of course I bolt on a 780 when I want to get serious and real stupid.

Bob
'66 7 Litre, top loader, 3.25 Traction-lock, Sidewinder Intake, SCJ Exhaust
'88 F150 LWB 4x2, XLT Lariat, 302, 5 speed, 3.08
'99 Ranger, 4x2, 3.0, 5 speed, 3.73 posi

 

I'm building for torque and driveability, though a good rumble and easy tire smokin' when empty should be part of the package. It's going to do more towing than anything else, and some serious highway miles. I don't expect to ever need it to run over 5,000 rpm. With my current 3.25 gears and 235/75R15 tires, I should only turn 2,500 rpm at 65 mph. Though I may go to 3.73 gears if I have to, I'd like to wait and see how she pulls with the 3.25's once the motor is done.

My understanding is that putting on too big of a carb will kill throttle response and low end performance, and too small will limit top end horsepower (by starving the engine at high revs). That being the tradeoff, I'm thinking of erring on the small side. After all, I'm build a truck here, not a dragster. It should also mean better mileage, right?

ksd
69 F100 Explorer lwb 2wd FE 360 C6 3.25:1. WANTED: straight/clean styleside long box w/aux fuel tank for 67-69 F series.

 

I went with the 750 carb because it is part of their performer RPM package for a 390. With help from the guys on this forum I ended up with Crower Cam instead of the ed cam (It was probably not right for a truck)and I gave up on the pricey Al heads which fell apart on me anyway (Heli-Coils came out while trying to install headers, too much trouble for that kind of money so I went back to my stock heads). The engine starts right up now and sounds great but the truck won't move. Shifter linkage moves but the truck won't move in any gear, nothing. I wonder what I've done now. I hope the 750 Carb is not too big, I think the crower Power Beast Cam and Ed RPM intake will use most of the carb. I realize the heads may be a little restrictive but the exhaust should be flowing well now with the headers. The engie sounds really sweet right now and I'd really like to see how much, if any low end power I've lost. With this cam running well in the 1200- 3800 RPM range I think I'll be OK for playing or trucking.
If I can just get it to move again, dohh!!!

Greg

 

You are correct in that too large of a carb will hurt throttle response, power and mileage. However, trust me when I say that with a built up 390 a 750 is not even close to too much carb. Besides, remember that the cfm rating is air flow, you can always put in smaller or larger jets to effect the amount of fuel mixed. I've researched this topic and played around with allot of different carbs in my time. Your planning on putting a 570 carb on a 390 with an edel rpm cam, heads, header, etc...it just doesn't fit. Oh, and I would be surprised to see serious tire smoking with those 3.25 gears, but you can always hope for the best. There are some good threads on these topics and I would suggest a search for more info...

If your knuckles ain't bleeding you did something wrong.

'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400+ ponies , 4-speed, custom suspension w lift, mud on black.

 

Thanks for the advice. The gears are going eventually, anyway, probably for 3.73's with a posi unit. You've convinced me that the 570 Holley is too small, though.

ksd
69 F100 Explorer lwb 2wd FE 360 C6 3.25:1. WANTED: straight/clean styleside long box w/aux fuel tank for 67-69 F series.

 

ksd,

I think I didn't get my message across. You original idea about the 570 was correct. The biggest problems with getting a truck to pull is lack of low RPM torque due to over sized cams and carbs.

Twenty years ago I built my Dads '76 F250, 360 into a 410 using a 428 crank, etc, and he pulled a large fifth wheel trailer. The carb was a 550 Holley. The GT 390s in 1967-69 used a 600 CFM Holley. The 428 Police Interceptors of 1966-67 used a 470 CFM Autolite, under carburated, yes, but they still made lots of power.

But you just don't want anything over 600 CFM if you want to maximize low end torque and get as good of gas mileage as you can.

By the way the Edelbrock carbs are easier to tune without having to disassemble the carb off the truck.

Bob
'66 7 Litre, top loader, 3.25 Traction-lock, Sidewinder Intake, SCJ Exhaust
'88 F150 LWB 4x2, XLT Lariat, 302, 5 speed, 3.08
'99 Ranger, 4x2, 3.0, 5 speed, 3.73 posi

 

I went back and carefully looked at the type of engine your putting together. I am adamant that (for example) the 670cfm Avenger would not be too much carb in the slightest. If you don't plan on carrying loads, and want a tire smoker, I'd actually recomend a 750cfm. This is a debate that will always continue, and many people do over carburate by (most commonly seen) putting a 750cfm on a mild 350 block. Its best to educate yourself on the topic and then figure out what meets your needs best. Fe's are large air pumps with a large airflow demand, if you are concerned about mileage you could put on a larger cfm carb, then jet smaller on your primaries. And if you want to cite old factory sizes to engine, Holley 780cfms were put on engines as small as the 350 and 302 (refrence: Holley Carburetor Handbook by Mike Urich, HPBooks). Food for thought.

If your knuckles ain't bleeding you did something wrong.

'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400+ ponies , 4-speed, custom suspension w lift, mud on black.

 

This may be way more information than you were expecting but this is simplified believe it or not.

Part I What a carburetor does

A carburetor meters fuel in proportion to the air flowing through it. A carburetor meters best over a specific air flow range and has extra circuits added to cover idle, power (acceleration), and cold starting. A fuel air ratio of about 12.5 : 1 is good for maximum power, 16 : 1 is good for fuel economy and 14.5 : 1 is considered a good compromise since too rich (12.5) wastes fuel and too lean (16) makes an engine run hot. There are other things such as air temperature and barometric pressure which vary quite a bit so a carburetor is not set up for peak power or maximum economy unless you are prepared tweak it like racers do just before each race. (The specific air fuel ratios given above are used by most mechanics for discussion purposes; the exact ratios vary depending on what axe you're grinding.)

Volumetric efficiency is a measure of how easily the intake side of engine allows the air fuel mixture to flow into the combustion chamber and well the exhaust ports and manifolds allow exhaust gases out. 100 % volumetric efficiency is rarely seen except on racing engines (and tuned intake and exhaust systems can get up to about 115% over a narrow RPM range.) The average engine headers and air cleaner, etc has a volumetric efficiency of about 90% at wide open throttle over a power band determined largely by the cam and to some extent the intake and exhaust systems. The other 10% is usually called pumping loss since the internal combustion engine is just a special purpose air pump and the intake and exhaust ports etc are just big pipes the air is being pumped through. The intake manifold, exhaust manifolds (I hate most stock FE manifolds!!), and camshaft are key to volumetric efficiency. Volumetric efficiency variations are one reason why you can't remove a carburetor from one engine and install it on another and expect it to run well before you tune it.

Part II Choosing a Carburetor

Almost any carburetor will work on any engine. But one that is too small will limit the RPM and peak power (volumetric efficiency) and one that is too large will hurt low RPM torque. A carburetor that is too small is usually easy to detect; the engine accelerates well at low RPM but does not reach the RPM expected.

An over sized carburetor is harder to detect. A carburetor meters the fuel based on the pressure difference between the air outside the engine and the manifold vacuum and the volume of air flow. At low RPM and wide open throttle there is less difference and not much air flow; the larger the carburetor the smaller these differences. So if you want the carburetor to work well at low RPM you select a small carburetor.

The 1967-68 390 2 BBL and 4 BBL (non GT) engines provide a good example of carburetor size impact. (There were two 390 2 BBL engines but the one that had the high compression of the 4 BBL engine is the one I'm referring to in this example and both used the same heads and valves, cam and exhaust manifolds (I hate the stock FE manifolds!!, but I think I may have mentioned that). The 2 BBL engine had a 351 CFM Autolite carb and the 4 BBL engine had a 446 CFM Autolite carb. (Some books give slightly different Horsepower or torque numbers but the RPM numbers are always the same.)

2 BBL 351 CFM HP of 280@4400 Torque of 403@4400
4 BBL 446 CFM HP of 315@4600 Torque of 427@4600

Notice that the larger carburetor made more HP and torque but this was at HIGHER RPMs. Note that the RPM given above was where the highest torque or HP occurred. As you accelerate through the gears, normally you could rev above the HP peak by 400 to 600 RPM before the power drops off enough to require shifting. This is definitely true of well built engine.

On August 3rd you said, "I'm building for torque and driveability, though a good rumble and easy tire smokin' when empty should be part of the package. It's going to do more towing than anything else, and some serious highway miles. I don't expect to ever need it to run over 5,000 rpm. With my current 3.25 gears and 235/75R15 tires, I should only turn 2,500 rpm at 65 mph." Your plan to use a 570 CFM carburetor fits what you said on August 3rd.

So pick your carburetor to fit the RPM range you expect to use. A 750 CFM carburetor would peak several hundred RPM higher but would have a large torque loss at low RPM. (Your idea about 3.73 gears would help your towing capacity and permit the use of a bigger carb if you want to rev it higher.)

Part III Adjusting a Carburetor

The carburetor should be adjusted or tuned for the engine it is installed on.

Changing the jets (or on a Edelbrock carb metering rods and/or their springs) is done to adjust for any unusual air flow optimizations or inefficiencies for the engine the carburetor is installed on. Changing the jets in a carburetor adjusts the air fuel ratio and we normally want 14.5 or so. Re-jetting the carburetor for the load and driving style (economy or max acceleration for jack rabbit starts) is part of tuning the carburetor also. Re-jetting does not change the amount of air. It just changes the amount of fuel; all it does is modify the mixture ratio.

Re-jetting to save fuel is often done but the adjustment range is limited and effects how well the engine runs. Remember a lean carburetor mixture usually makes the engine run hotter; if it doesn't run hotter it may have been too rich. And re-jetting will not make a carburetor flow more or less air.

A good way cheap way to measure if the mixture is OK is to put in new plugs and make a run down a nearby flat stretch of highway under load. Just accelerate up to highway speeds and hold it for a few miles. A slight up-hill grade is OK but down hill grades won't work for this. Pull off on the side of the road or into a rest stop but do not idle for more than a few seconds. Remove your spark plugs and see what color the white tips are now. A light tan is ideal. Brown is rich. Black is way too rich. If the tip is still uncolored (white) the carb is too lean. You may not have a lot of color build up if you only went a short distance but the color will be there.

Proeliator is not wrong about running a big carb for horse power. And if that's what you want, go for it. But if what you wrote on August 3rd is correct then you may be a lot happier with something around 600 CFM.

Bob
'66 7 Litre, top loader, 3.25 Traction-lock, Sidewinder Intake, SCJ Exhaust
'88 F150 LWB 4x2, XLT Lariat, 302, 5 speed, 3.08
'99 Ranger, 4x2, 3.0, 5 speed, 3.73 posi

 

I just want to comment that Bsprowl included allot of good information in that post, and allthough I do sit on the other side of the fence he has many points to consider. Actually, if you want maximum mileage and lowend power he is correct, although I maintain that compromises can be reached by re-jetting. Basically I guess I don't understand why someone would put on the edelbrock rpm (empathis on rpm) package including the very expensive heads and then restrict the whole tamale with a puny carb. The bottem line is you want mileage, low speed power, and yet also a tiresmoker...in essence having your cake and eating it too. It seems like the 670 avenger falls between the 570 and 750 size we've been throwing around, and I stick with that recomendation for what you want. Let us know what you end up with...after all these posts it will be interesting what you do and how you like it.

If your knuckles ain't bleeding you did something wrong.

'72 F-250 "Hi-Boy" 4x4, Dana 60/HD44, FE428 @ 400+ ponies , 4-speed, custom suspension w lift, mud on black.

 

Thanks again, guys. Excellent info and advice from both of you. I'll keep you posted. In the meantime, I've decided to find a new FE motor and start my buildup on it instead of rebuilding what I have. That's going to delay my need for carb selection for some time.

ksd
69 F100 Explorer lwb 2wd FE 360 C6. WANTED: straight/clean styleside long box w/aux fuel tank for 67-69 F series.

 

I'm jumping into the fray a little late, but you can get good throttle response and fuel economy with a big carburetor. If you go putting a smaller carb on that motor, you're going to strangle it beyond 400 rpm, which I believe is when that rpm package starts to come on strong. I recommend following Edelbrock's suggestions and getting at least a 750 cfm carb. For low-end response, fiddle with the metering rods and accelerator pump. You should be able to charge off the line pretty hard, and you'll really notice when the secondaries open. I went one stage richer on my Edelbrock 750 cfm-equipped Galaxie in both the primary and secondary circuits, and it really roars along now. I put the carb on an otherwise stock '69 390 GT and I get about 18mpg on the highway if I don't play around or get in a hurry. The trick is to fatten up the power circuit and secondary jets while staying conservative with the idle and cruise circuits, all of which is really easy with the Edelbrock carburetor.

 

All,

Looking back over what has beccome quite a discussion, I thought I would make a few points:

KSD: the Ed RPM intake is a dual-plane intake. And I think it's going to work well with the Crower Cam. Both parts were reccomended at several places on this forum. It sure sounds nice, running with only the headers. Not as loud as I would have thought.

All: I was surprised that the Ed 750 carb ran well right out of the box. Once I got the distibutor timed in reasonably well the throttle hooked up and seems to be responsive throught the lower rev range. I haven't had the guts to really get into it as the truck isn't actually running. (Thanks to my misguided efforts to reinstall the engine). I really appreciate all the advice I've recieved so far, if only I'd have asked about reinstalling the engine????