Blue Thunder Intake: What carb size?

[Machlovin]

I am buying a Blue Thunder intake for my 351c 4v. What size carb should I be looking at? I know George Pence recommends a 750 cfm carb, however I think that may be a bit extreme for my engine as it appears to have only been lightly modded (I didn't build it but it has full length headers, MSD coil, Mallory distributor...). Currently my engine has a 600 cfm Holley carb on a Holley Street Dominator intake.

Thank you.

 

[Don65Stang]

On my 71 m-code 4-speed car I had an Edelbrock 600 that did fine. I sold that one and installed a street avenger 670. That seems perfect IMHO. The street avenger 770 may be an equally good or even better choice. I struggled between the two. I have a boss 351 clone engine that I'll definitely put a 770 on it.

I think the street avenger carbs are excellent right out of the box.

 

[marks73]

This formula has worked well for me in the past on different size builds.

http://classicinlines.com/CFMcalc.asp

 

[Machlovin]

This formula has worked well for me in the past on different size builds.

http://classicinlines.com/CFMcalc.asp

Hi Mark,

Thanks for the link. I've tried using the formula before, which suggests I use a 600 cfm carb. The reason why I was thinking of going bigger is because George Pence recommended a 750 cfm presumably because of the increased air flow that the Blue Thunder manifold intake provides.

I'm wondering if I would benefit by also increasing the size of my carb, however 750 cfm may be too extreme for my application (although I'm not entirely sure as I am still learning). I just want to make sure I'm taking full advantage of the intake by using a proper carb.

Thanks.

 

[wwhite72]

Too much carb and you risk washing out the cylndrs. Want to go faster just push down further on the pedal on the right. My 351c has a 650 that had custom jetting done and pulled 375hp on the dyno with stock exhaust manifolds and dual exhaust and plain mufflers...until it started having some bad vibration when i punched it or got up in the rpm's. arrrg.

 

[MotoArts]

Every choice regarding engine components is and will be a compromise.

Desired performance comes from:

- Cam and its power range

- Header diameter and length

- Cylinder heads

- Compression ratio

- Rear gear

- Transmission and converter

... the list is endless.

Unfortunately, there is no right or wrong answer.

IMO, an average, stock displacement, relatively stock cammed. dual plane intake'd street Cleveland should be fine with the aforementioned 600-650CFM vac sec carb. A properly tuned 750 vac sec will run fine also. May not even be enough difference to stress over, honestly.

What George means (I believe) regarding the intake is the POTENTIAL increased airflow the BT allows. An average street cruiser is not turning 6500 R's all of the time it is running. The carb is merely a "valve" regulating airflow. To me, it sounds like max CFM may not be needed in this case. You'll get a wee bit more street "snap" out of the smaller carb bores in a 600.

I have the same intake as yours (Shelby), and several different carbs. Unfortunately I'm nowhere near completion of my Cleveland, but have experimented in the past on other engines. None of the engines have been barn burning, mega horsepower stump pullers, but it was fun to see the difference in swapping various parts nonetheless.

I'd be most interested in hearing how the BT compares to the Street Dommy, back to back with your current carb.

Depending on the rest of your combo, you may like it or flat out hate it (doubtful )...

 

[Don65Stang]

Agreed the above post is correct. Plus for a street engine that will spend more time at lower rpm than higher rpm, the smaller venturi causes a higher velocity of airflow at lower rpms. This translates into better throttle response over the higher volume, slower velocity carbs. That is why I picked the 670 cfm carb over the 770 cfm.

 

[Machlovin]

An average street cruiser is not turning 6500 R's all of the time it is running. The carb is merely a "valve" regulating airflow. To me, it sounds like max CFM may not be needed in this case. You'll get a wee bit more street "snap" out of the smaller carb bores in a 600.

I think I understand. A smaller cfm carb will generate more power in a lower RPM range because of the increased air velocity due to the smaller inlets, but max power will be limited because air flow will be limited in the higher RPM range.

So if I'm driving on the street, and floor it once in a while but still keep it within relative legal speed limits then I'm better off with a smaller carb. Does that sum it up correctly?

My next question is whether the Blue Thunder intake will make much of a difference for me. I read that the Holley Street Dominator is in some cases worse than the stock intake manifold, which is why I wanted to buy a Blue Thunder intake as I have an opportunity to purchase one at a good price.

Thanks.

 

[mweeps]

Any suggestions on a replacement manifold and carb for a 72 R Code to make it more reliable?

 

[1971mach1]

Any suggestions on a replacement manifold and carb for a 72 R Code to make it more reliable?

If you want reliable go with an Edelbrock carb. Their website says they are compatable with modern gas including ethanol. I've been running an

Edel Performer 750vs on a relatively stock 351c-4v and it's been running great for 5 years now on 91 Calif pump gas. No rebuilding or even tweeking.

As for carb size, I concour with George about 750cfm, even for the street. It really woke this 351c up, especially 3000 to 5000 rpm (and I never go over 5k RPM). Even low-end seems stronger. But you want vacuum secondary, not double pumper. A VS will only use what it needs so carb size is not so critical. Gearing is the best cure for low-end power anyway.

 

[TommyK]

An average street cruiser is not turning 6500 R's all of the time it is running. The carb is merely a "valve" regulating airflow. To me, it sounds like max CFM may not be needed in this case. You'll get a wee bit more street "snap" out of the smaller carb bores in a 600.

I think I understand. A smaller cfm carb will generate more power in a lower RPM range because of the increased air velocity due to the smaller inlets, but max power will be limited because air flow will be limited in the higher RPM range.

So if I'm driving on the street, and floor it once in a while but still keep it within relative legal speed limits then I'm better off with a smaller carb. Does that sum it up correctly?

My next question is whether the Blue Thunder intake will make much of a difference for me. I read that the Holley Street Dominator is in some cases worse than the stock intake manifold, which is why I wanted to buy a Blue Thunder intake as I have an opportunity to purchase one at a good price.

Thanks.

Not trying to discourage you from the Blue Thunder but for your combo a regular Edelbrock Performer with either your current carb or a 670 SA would probably be a better choice. If you are contemplating a cam change in the future then the Blue Thunder makes sense.

 

[cazsper]

How about an Air Gap from Edelbrock? It's suppose to be great for both 2v and 4v Cleveland's with a relatively broad power band.

 

[Don C]

I really like the way my Shelby/Blue Thunder with a 750 Holley performs on my 351C-cc, throughout the power band. The cam is a little warmer than stock.

 

[MotoArts]

An average street cruiser is not turning 6500 R's all of the time it is running. The carb is merely a "valve" regulating airflow. To me, it sounds like max CFM may not be needed in this case. You'll get a wee bit more street "snap" out of the smaller carb bores in a 600.

I think I understand. A smaller cfm carb will generate more power in a lower RPM range because of the increased air velocity due to the smaller inlets, but max power will be limited because air flow will be limited in the higher RPM range.

So if I'm driving on the street, and floor it once in a while but still keep it within relative legal speed limits then I'm better off with a smaller carb. Does that sum it up correctly?

My next question is whether the Blue Thunder intake will make much of a difference for me. I read that the Holley Street Dominator is in some cases worse than the stock intake manifold, which is why I wanted to buy a Blue Thunder intake as I have an opportunity to purchase one at a good price.

Thanks.

Not trying to discourage you from the Blue Thunder but for your combo a regular Edelbrock Performer with either your current carb or a 670 SA would probably be a better choice. If you are contemplating a cam change in the future then the Blue Thunder makes sense.

Agreed.

Machlovin, another word to consider here is "theory".

In "theory", the smaller carb and smaller cross section, dual plane intake will work better with your combo and intentions.

In theory, from what I have read, the Air Gap *may* work ok also for your combo.

Maybe.

Some things to consider is that it's rather tall, and has a higher (remember I mentioned the word "compromise"?) power production band (that I believe is unnecessary for your combo).

I happen to have a Performer in stock, too... just in case my Shelby doesn't work as expected... according to my years of hoarding and relentless theorizing (that in turn leads back to the hoarding... ).

If the BT is a juicy price, why not pick it up? For the price of a gasket set, try it out, really can't hurt. Be our guinea pig, let us know the results. You could always resell it if it doesn't produce in your combo.

 

[Machlovin]

Thank you everyone for your input, you've been all helpful and I sincerely appreciate it!

I've been cramming so much engine knowledge in the past week it is ridiculous. I've only scraped the surface, however at least it's not a complete mystery to me know.

 

[goodnigh]

On my 71 m-code 4-speed car I had an Edelbrock 600 that did fine. I sold that one and installed a street avenger 670. That seems perfect IMHO. The street avenger 770 may be an equally good or even better choice. I struggled between the two. I have a boss 351 clone engine that I'll definitely put a 770 on it.

I think the street avenger carbs are excellent right out of the box.

When Don of OMS sold me the Cleveland long block we were

running an Avenger 600. The fact the block had been rebuilt

with a "mild" cam (it has yet to be determined how "mild") lead us

to install a 670. After driving the car for a month we decided to

install a 770 Holley Avenger. The car just felt like it wanted more air.

mike

 

[wwhite72]

If you search on the name Dan Jones you will find many entries on his testing of intakes on Clevelands used for Panteras. the guy has done a LOT of testing of components on Ford engines. everything produces different results. dont spend lots of money on parts you wont be happy with.

 

[F16Dad]

Looks like this is the resounding intake recommendation - from Mustangs Unlimited??? (Blue Thunder Intake Manifold Dual Plane 351C-4V - BLUE THUNDER IM-351C-4V)??

Thanks Gentlemen for the input !!!

 

[Hemikiller]

Just a reminder - a 351C-4V engine does not conform to the norms of the engine calculators. They want way air and fuel. It's tough to overcarb a 4V engine.

If George Pence or Dan Jones say to use something, you can go to the bank on that.