Discussion in 'Early CJ5 and CJ6 Tech' started by Roberts1969CJ, Jun 22, 2016.
What is optimal performance carburetor for an offenhuser (cfm, vacuum, mileage) (V6 225)?
I have always ran the 2G on my V-6 225......that being said, with the new motor I am running a Holley 390 4-barrel (8007) carburetor with an off-road kit installed on an Offenhuser manifold.
So far I really like this carburetor. Its easy to adjust/work on, as long as your comfortable working on a Holley and know the basics on how it operates.
It's 390 cfm
Pulling 17 inch of vacuum at idol (650 RPM's)
Mileage?.......that's a tough one.....The first tank of gas was burnt getting everything dialed in and breaking in the cam. I have about 98 miles on the tank in it now with the gauge showing about 1/2. Plus its not going to get optimum gas mileage until the motor is fully broke in.
Generally the Holley 8007 is considered to be pretty close to the perfect match. That being said, I'm running a TA470, and I've got no complaints.
Thanks guys, you helped a lot.
I run that carb on an offenhauser 360 intake too. Electric choke is fine once i set it right. The first one I got was faulty from the factory. Holley told me to send it back to summit. Called Summit and they said send it back. They shipped out a new one the same day. Been fine since. I like it because it requires nothing from me. I have no run it on washboards or significant hills for any period of time, but so far zero issues.
I'm running a TA470 on mine too.
Pretty sure it would run upside down!
Looking at the TA470 it doubles the CFM over the Holley 8007. If i am not mistaken, the higher the CFM the more HP/Performance you get, but I also read the TA 470 is designed for off road. I drive my Jeep on the road a lot more than off road. I want to have as much HP I can get from my V6. Is there a big difference in gas mileage between the two?
The 8007 is 390 cfm...the TA470 is 470 cfm (there's also a 670 cfm version, which is far too big for a V6).
There is a limit to the amount of CFM that the engine can consume. Lots of guys have used the 470 and have had no issues that I have read. When I purchased my 390 it was considerably cheaper than the TA 470. Now they are 10 bucks difference..
I have plenty of power from my 225 with the Holley 390, fuel mileage.. not so much. I dont own a jeep for fuel economy though. Like I said, no issues. Starts up instantly. I am VERY happy with it. I drive my jeep 99% on road.
If I had to pay full price for both the manifold and the carb, I would have really looked into fuel injection. Mcruff says there are platforms that will work without spark input from megasquirt.
Maximum cfm doesn't help driveability at trail speeds, etc.
I ran an Edelbrock 500CFM on my other V6 w/ Offy intake. I took the time to tune/ jet it properly, and it ran pretty darn good, but 500CFM is really too big IMO. It can be tuned to run fine, not too rich etc., but the venturi size is too large to achieve proper fuel velocity on a 225. I run a Holley 8007/390 on my current V6, and it runs very well.
As the CFM goes up, vacuum for a given speed goes down. Carburetors use vacuum as both a control signal and a a power source to control the mixture. The general rule for carburetors is the less vacuum you have, the less accurate fuel control becomes. You can overcome this to a degree by richening the mixture, at the cost of poor fuel economy.
Fuel injection gets around this (mostly) by using vacuum as a signal and not as a power source. Thus the throttle body holes can be big so long as there is enough vacuum to measure reliably.
While we are on the subject of carburetors, here is a formula I was looking for and which Timgr above gave me. This keeps one from "overcarbureting" an engine. When I was growing up, a lot of my friends thought that a bigger carburetor was the answer in trying to beat my '67 GTO. They couldn't understand why an engine with a smaller carburetor was kicking their tails. Doesn't take a rocket scientist to figure it out. I am running a 150 cfm 2-bbl on my F134 and it works great. By the way, Glenn, I got it in the hills the other day and it works fine, up and down steep slopes and on side hills. Now if I can just get my other set of brake shoes, I will be back on the road.
Pretty easy to calculate the CFM at a given RPM and displacement. Use dimensional analysis.
134 in^3 volume (“134” is the cubic inch displacement of my 4-banger)
2 revolutions/volume (4 –stroke engine)
4000 revolutions/minute ( I used the redline value for the F-134 engine for max possible air usage by the engine)
1 foot / 12 inches (at least it did last time I looked – this is just to make the units end up at CFM)
Now put what you know together so that the dimensions give the result that you want - CFM, or feet^3/minute
134 in^3 / volume * 1 volume / 2 revolutions * 4000 revolutions / minute * [1 foot / 12 inches]^3 = 155 CFM.
This assumes 100% volumetric efficiency, that is, that the intake path is so smooth and large that the cylinder fills completely. We know this isn't true, but we can conclude that I should not need more than 155 CFM to get everything out of the F134 that is possible without forced induction. Has anyone seen a blower for the 134? Maybe a turbo?? A blower might work well, but given the long stroke and small bore, it wouldn’t lend itself well to forced induction, I think.
I might as well join in but with something different. I run a Motorcraft/Autolite 2150(1.08 venturi) on a Offy 360 Dualport using an adapter plate. I cannot recall what power valve is in it but I have #52 jets in it. I don't have another 225 to compare it to as nobody else has one that I run with. It seems quit snappy and responsive and delivers 14-18mpg, not that I keep track I was just curious. I have thought about switching to a Holly to get the most out of my Offy but I've never really liked them. The 2100/2150 are simple, mine performs great to me and I already had it along with some spares. Not looking to change anybody's mind just more info.
More CFM and any attributes that one may associate the added air and fuel may have in additional Power is only possible if you have the correct intake track, cylinder heads that can flow higher numbers , compression and Ignition that can burn the fuel and exhaust that can get it out of the way for the next charge................225's do just fine with the 390 cfm Holley and since it has vacuum secondaries it will act as a 2 barrel all day long until the engine rpm signal and demand is great enough under load to open the secondaries.
I almost never open the secondaries, but when I do the 225 can handle it and takes off. Really runs well in my junk engine setup.
I'm right with you. My MC 2100 with 1.08 venturi seems perfect to me. Last weekend my wife and I went from 500' elevation to 7500' without any problem. We cruised loaded @ 65 on the freeway and just shift in/ out of overdrive on the hills.
The 21xx would work fine on a 225, especially if you pick the smallest venturi model (1.08"). They are simple, easy to rebuild, and work well on the trail. There is a float damper spring available (factory equipment on Jeeps after 1974) that helps prevent bounce flooding. Get a 2150 from a Jeep and it includes an altitude compensator (aneroid) that changes the mixture automatically with altitude.
If I had to fault them, I'd point to the power valve which is vulnerable to puncture by backfire. They also seem to corrode a lot (no zinc chromate?), and it seems there are a lot of worn-out 2100s out there. Jets and other hard service parts can be difficult to find.
I know you are looking at a 4bbl, but for what it's worth, I had a 470 TA that I got dialed in really well. Off road, it was hands down the best carb I've had. I always thought it could be a bit better as a driver though, and tinkered with it a lot. On a whim, I put on a Rochester 2G with matching iron intake. My driveability is great, it works well off road, and the mileage is incredible. It's funny that the engineers who built this engine were right in their carb choice for daily use! I am sure that if my engine was anything other than stock it would run out of steam on the top end, but I am more of a lugger than a wide open guy so for me this combo works great. Just giving you an option to think on.
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