69 mach 351w engine rebuild question
hey guys,

i am planning on getting my 351w 4barrel rebuilt. It is #s matching stock but burning smoke. I mentioned this on another thread and after replacing valve seals and deep creep in combustion chamber, it still smokes. You can see black oil specs on rear of car.

My question is, the restoration place said they can bring the 351w to 400HP but they also quoted adding shift kit to my FMX trans. FMX trans is #s matching as well and in good shape. Do you think I need to do this?

Should I also add headers and replace original exchaust maniforld?
Yes Thmbsup I would agree with all the upgrades, as well as an Edelbrock RPM or Performer Intake manifold. It should make excellent power. Just keep all the original stuff and if you were to ever sell it, you would have all the stuff to make it original again. No harm no foul. JTS 71 Mach1
Yes, the FMX should be made a little stronger to handle the new horse power.

As JTS mentioned, keep all the original parts.
I would have to go with that.
The shop is also suggesting replacing my edelbrock 600cfm with Holly 750 cfm to go with 400hp. If I don't do this he says I would have to rejet it. Does this make sense?
I don't like Holleys anyway shape or form. But he is correct that it could use 750 cfm. And that you 'might' have to rejet the 600 edelbrock. The 600 Edelbrock will do more then you think, but rejetting may or may not be required. You'd simply have to try it, and go from there. JTS 71 Mach1
I prefer a Holley double pumper, but to each his own. Some of the older Holleys can get finicky over time ie: blown power valves, metering block leaks and sticking floats, but I haven't had any issues with mine - going on 5 years now....... Anyways, CFM = cubic feet of air/fuel mixture per minute. Jetting up a carburetor (any make) will only richen your fuel/air mixture, it won't make the carb flow any better. If you're objective is to provide your engine with the optimal air/fuel charge it can handle (or perhaps a little more), I use this simple, easy-to-remember formula: Optimal Carburetor CFM =Cubic inch displacement multiplied by your engine's maximum R.P.M. , multiplied by the engine's volumetric efficiency - (usually a well-tuned V-8 runs at around 80 to 85 percent efficiency) divided by 3456. This number will be the approximate c.f.m. requirement for your engine. So lets plug in some numbers and see what that tells us.
I will use 356 for cubic inches assuming it will be bored at least .030 oversize, I'll also figure a safe operating r.p.m. of 6,000. Next, we'll say your engine will operate on the "high" end of the efficiency spectrum so we'll use 85%, and divide all that by 3456.
356 X 6,000 X .85 divided by 3456
= 525.347 CFM
So, as you see, an engine's requirements are not as high as most guys believe. There are variables such as compression ratio, camshaft selection, and exhaust system to consider as well. A 400 H.P. 351 is pretty stout, but I would choose a 650 C.F.M. carb. Over-carbureting an engine won't make more power, and may give you driveability issues. Like J.T.S. said, you'll probably have several trial and error episodes, and you'll have to see what works best. Efficiency is key here, efficiency = horsepower and torque. GXR
In my opinion most guys have problems tuning holleys. They really are a fantastic carberator and if you notice most serious horse power engines use them in one form or another. They really are easy to jet and tune with some patience and basic tools-IE vacuum guage, timing light and a magnifying glass to read the plugs. Most of the early issues have been fixed -IE blown power VLV do to an engine backfire. The newer holley style carbs are very user friendly and quite good right out of the box with only minor tuning. I don't like double pumpers on the street as they can be finicky if not tuned correctly and the same goes with mechanical secondary carbs. By all means use them if your horsepower and engine/trans cobo requires it but vacuum secondary carbs in the traditional style would probably be a wiser choice and more street friendly. Obviously it depends on your goals and plans for the car. most deffinately beef up the FMX again in my opinion over the years racing these cars (mostly clevelands but with some FMX trans) anytime you up the horsepower the FMX in particular can use some help-especially if you go to a larger stall convertor. You wont be disappointed doing it. If you choose the holley style carb many on here Im sure are / would be happy to help you through the tuning process. I am a little skeptical in the 750 CFM size though, I would think a 650-680 CFM would be more suitable for your needs. especially if the car is mostly driven on the street. I think you would find you run into a quite rich condition with the 750 but a lot depends on cam, intake, head selection and other modifications. I would think the 750 would work well in the mid to upper RPM's 3500-7000 but typically that is not street friendly RPM's. Usually a street engine is 2500-6500 max and I think the 750 would plague you with bogging and running rich but that's just my opinion. You can jet a smaller carb and make it appear larger but it is very hard to jet a larger carb smaller and not have issues-FYI. Best of luck and if any of us can assist please don't hesitate as there is a wealth of knowledge on the site.
With 400 HP all you need is a 650. I have a 600 cfm edelbrock on mine with a air gap 2 intake. 351C Edelbrock Crate Engine W/ Ozzie 2V Heads rated at 425 HP. Does just fine. I do love the new style Holly's too. A lot of people think bigger is better or go as big as you can before you have problems. With a 750 cfm carb you won't have problems until you get to high altitude, steep hills, or if you do more than just drive normal or straight.lol Personally I wouldn't go to a 750 below 500 HP. Now if you were stroking that motor as well I would recommend a 750. Highly recommend upgrading the trans and Engine. Great post GXR02190! Very Informative.
I agree, this is a great post. It has a lot of personal opinions on carburetor choices and good advice on carburetor size. I do already know that choosing the right carburetor size is actually very important to the overall performance of your engine. Personally I am very interested in learning more about automotive carburetion so I can tinker around more with properly setting up a carburetor. I know there are a lot of factors involved.

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