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Unread 04-11-2014, 09:20 AM   #1
rambo3489
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Carbed Stroker Questions / Suggestions

Alright. I am planning hopefully well into the future here seeing as how I currently have a perfectly capable 4.2. BUT when the time comes I am wanting to do a stroker. But am wanting to keep it carbed. But I have a few questions. Mainly on the carb choice. I currently have a 1.08 MC2150 and love the thing. I was thinking about going with a 1.21 2150 when I do the stroker. It's rated at 351 cfm. Now my question is will that be enough seeing as how the carbed stroker guys I see out there are running 400+ cfm carbs?

My understanding of the cfm rating is the carbs maximum air flow rating. This is really only an issue when getting into the 5000 RPM+ range. Now seeing as how a stroker makes peak torque well below that number does having such a high cfm carb actually benefit you? Because I am under the impression that if the venturi gets too big for the motor the velocity of air/fuel mixture passing through it at slower engine speeds is not enough and the air/fuel does not get mixed well. Thus causing a loss of low end performance. The exact thing I will be shooting for.

Also, side topic here. When I do build the stroker I will need to break in the new cam and need high RPM's quick. Does anyone see a problem with me using my 1.08 venturi 2150 to break in my cam seeing as how I know how to get that carb to respond the way I want it to? The 1.08 venturi should be enough to get me into the 2009-2500 RPM range seeing as how there won't be a real load put on the motor. Therefore not requiring a very rich mixture. Also I have a manual choke so I could set the air flow myself which might help with the first unknown start up.

Lets have a little talk about this. Hoping to get some good info on a carbed stroker build here.

Thanks,

Tyler

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Unread 04-11-2014, 10:08 AM   #2
Old4X
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The benefit of a stroked motor is low end torque and performance. If you want to run above 3,500 rpms all the time, you want short stroke, not long stroke geometry.

With the smaller carb, you will have better fuel atomization, quicker throttle response, more low end torque, and better fuel mileage. Seems like a no brainer to me.
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Unread 04-11-2014, 10:57 AM   #3
rambo3489
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Not disagreeing with you here. Just playing devils advocate a little. At what point does the carb become too small? I mean you cant run a tiny little carb and expect to run the perfect mixture. Wouldn't it run lean? Or does too small of a carb only effect the max engine RPM's due to it essentially getting choked out? If that's the case why wouldn't we all look for the smallest carb we can find?
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Unread 04-11-2014, 11:09 AM   #4
Old4X
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You run the smallest carb that will support the highest average RPMs the application is likely to see. You want velocity through the carb for efficiency. Some manufacturers tried variable venturies to do this, but they didn't work out well.

The 4 barrel carbs with vacuum secondaries are designed with this in mind. Small primaries where you run 99% of the time, but still can pull 5500 rpms+ when the desire hits you.

If you pull up the performance stats on all the 258s over the years, the one with the most balls at the bottom end ran a small, 1 barrel carb. It was in a J10, IIRC, and had significantly more torque than the 2 barrel variants. The max torque was all available below 2,000 rpms, right where you want it for heavy pulling or off road running.


eta: the 258 is a stroked 232
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Unread 04-11-2014, 11:20 AM   #5
Old4X
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Also, look at how you drive your vehicle. My engine has never seen 3,500 rpms. I drive in the torque band. Looking at the HP curve, I probably never see more than 55 HP from my engine, but I do run at max engine torque quite often. I want excellent driveability below 3,500, as that is where my engine lives. I do not give a flip about what it can do at 4,500 rpms, so a humongous carb and super high output ignition are of no interest for me (in my Jeep, anyway).

And people wonder why I get such good gas mileage.
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Unread 04-11-2014, 11:28 AM   #6
rambo3489
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The same holds true for a larger displacement (4.6) and free flowing intake n headers? Smallest carb that will run the desired rpms?

Also I've heard it said that exhaust back pressure helps the low end too. Any insight?
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Unread 04-11-2014, 11:52 AM   #7
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As you go to more free flowing exhaust, you develop more power, but the torque band shifts up (max torque comes in at higher RPMs). This may be what you are referring to. The 4.0 is a good high reving engine. It has simi-headers from the factory. The earlier 258s were better off road motors than the later 258s, mainly because they significantly lightened the crankshafts to give them more "seat of the pants" throttle response in the late motors. This was purely for street driving, it actually hurt off road use. The 4.0 gives the throttle response they tried to get in the light 258, and got back most of the low end torque. Plus made for good HP numbers in the ads.

The mac daddy of stroker engine design is the Diesel. It is a LOOONG stroke motor. But it is a very low RPM motor as well.

Don't think I have ever seen headers on a tractor.
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Unread 04-11-2014, 12:00 PM   #8
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Clifford engineering makes a nice 4 barrel intake that would fit a stroker. I do have a 600 CFM Holley with vacuum secondaries and external adjustable float bowls that would fit right on it. $75 and you pay the freight if you want "enough " carburetor. I am sure Clifford has the intakes in stock.

Would I put it on my 6 banger? No. But there are folks that are running that setup. Like I said, it is all in how YOU operate your equipment, and matching the hardware to what you want.
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Unread 04-11-2014, 12:02 PM   #9
rambo3489
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What are some "back pressure saving" alternatives to running headers? Other than the stock exhaust manifold?
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Unread 04-11-2014, 12:04 PM   #10
rambo3489
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Also I'm guessing that from your responses a 1.08 MC 2150 would be enough to get me to 2500 RPMs to break in my cam?
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Unread 04-11-2014, 12:07 PM   #11
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It should be. I would be running my two barrel little holley instead of this 450cfm four barrel if the mixture needle hadn't snapped off in the carb plate. The four barrel does OK too but that two barrel was the best
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Unread 04-11-2014, 12:10 PM   #12
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That's on a 4.5 though, not a larger stroker. I tried a 1.21 motorcraft on my 4.5 but couldn't get rid of the hesitation off idle
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Unread 04-11-2014, 12:10 PM   #13
rambo3489
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Quote:
Originally Posted by superj View Post
It should be. I would be running my two barrel little holley instead of this 450cfm four barrel if the mixture needle hadn't snapped off in the carb plate. The four barrel does OK too but that two barrel was the best
Lol. How'd you manage that one?

Why not sell the 4 bbl n go back to a smaller 2?
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Unread 04-11-2014, 12:14 PM   #14
rambo3489
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Quote:
Originally Posted by superj View Post
That's on a 4.5 though, not a larger stroker. I tried a 1.21 motorcraft on my 4.5 but couldn't get rid of the hesitation off idle
I'm battling the same issue on my curent 1.08. Small stumble when slowly accelerating after cruising or decelerating. If I close the choke plate 1/4 of the way the stumble goes away. Going to play with the jets a little.

Checked and its not the PV. Accel pump works fine. All that's left is jets or venturi size.
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Unread 04-11-2014, 12:16 PM   #15
rambo3489
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Also are there any real differences between a MC n a Holley? How does the Holley do at step angles? I've heard that's their Achilles heel.
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Current Project: None... I disgust myself.

[URL="http://www.jeepforum.com/forum/f22/my-yj-build-adventures-thread-1356941/"][SIZE="3"][B]BUILDING THE YJ MY WAY[/B][/SIZE][/URL]
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