Confused on carb sizes.... Thermoquad 750/800cfm vs Holley 650cfm

[Super Blue 72]

Ok, so if you see in my other posts I have a Themoquad that is leaking badly and I'm hopefully in the next week going to tear it down and check out the possibilities of either leaking jet wells at the epoxied areas or O-rings, cracked/warped body or who knows what...

Any hows, without having to change my intake I was looking into other spread bore carbs.  Roachester Quadrajets ( That's a typo, but I thought it was funny so I'll let it fly...  )(had one on my 318) seem to have the same issues with the jet well area so I have read but the Holley 4165/4175 does not due to design.


So, from what I understand, my TQ was a 750-800cfm carb.  The Holley comes in 650cfm, double pumper.  (heard they don't get good gas mileage, a recurrent theme, as compared to the Q-Jet).

---Will the Holley at 650cfm be "Too small" for my engine?  It is a bone stock 340ci, 8.5:1 compression, stock everything, auto transmission.

---Any issues with it being a double pumper?  Too much gas off the line? Will it bog? 

---Why do some not have fuel bowl vents?  (The big tube hanging off the passenger side of carb.)  Is that ok?  My TQ has them.  Just plug the tube?


I see a Holley on craigslist for $50 so it might be worth the mickey mousing of brackets and vacuum lines to actually have a carb that doesn't leak gas and flood the engine!


Thanks in advance for any input!   

[polarbear123]

The 650 would work just fine. Rule of thumb 2xCI = 680

[Super Blue 72]

Thanks, polarbear. 

Just weird how stock it has alike a 750cfm carb.   

I wonder if it has to do with it being a spread bore? 

[cudabob496]

http://holley.com/applications/CarburetorSelector/CarbSelection.asp

[Super Blue 72]

Thanks for the link cudabob! 

Interesting, they do not recommend the mechanical secondaries for my car.  They say only for cars 3100lbs or less with a manual tranny? 

The carb I'm looking at tomorrow is a Holley 4165, double pumper with mechanical secondaries.

Would this be bad for my application?      It is a bone stock 340ci, 8.5:1 compression, stock everything, auto transmission.

[Bullitt-]

I've always heard mechanical is best for manual tranny or all out racing....below from Jeggs carb FAQs

2. Should I use a vacuum secondary or mechanical secondary carburetor?

Answer: For street cars the vacuum secondary carburetor works best on most cars with an automatic transmission. They are more forgiving than a mechanical secondary because they work by sensing engine load. The mechanical secondary carb is best on a lighter car with a radical camshaft, a lower gear, and manual transmission. A mechanical secondary is the best choice for most high RPM race purposes.

[Bullitt-]

Another spread bore to consider...T/Q legacy lives on    http://www.demoncarbs.com/1900.asp#1

[Super Blue 72]

Thanks for the info, Bullitt.   

Nice looking carb, but still confused if I should get that Holley.  That TQ Legacy and the TQ old version had mechanical secondaries.  I dunno... 

Another issue is cost.  For $50 plus $35 for a rebuild kit the Holley may have the edge.  Still thinking...I think too much...

[Bullitt-]

Thanks for the info, Bullitt.   

That TQ Legacy and the TQ old version had mechanical secondaries.  I dunno... 

NO...T/Q is vacuum secondary's.   The flow of air is controlled by the spring loaded air valve on the upper air horn.

[Super Blue 72]

Aha, gotcha.  I was confused by the secondary butterflies! 

Thanks again! 

[Bullitt-]

Aha, gotcha.  I was confused by the secondary butterflies! 

Thanks again! 

Technically not vacuum secondary's as it does rely on the mechanical opening of the butterflies but it is the vacuum of the engine that opens the air door/valve. Same approach on the old Carter AVS (Air Valve Secondarys) as well as the current Edelbrock Performer series and apparently from what I read these new Demon pieces.   

[Super Blue 72]

I just read somewhere that if you anything over 30mph (I would imagine figuratively speaking) that the secondaries are opening.

Stinks for fuel economy but the person posting says they are a lot of fun.

Maybe the vacuum secondaries may be the better way to go.  Thinking too much...

[hooD]

I just read somewhere that if you anything over 30mph (I would imagine figuratively speaking) that the secondaries are opening.

Stinks for fuel economy but the person posting says they are a lot of fun.

Maybe the vacuum secondaries may be the better way to go.  Thinking too much...

My cuda has the stock 850 cfm TQ.  The rear barrels only open when you nail it...otherwise you run on the 2 smaller front barrels giving you better fuel economy.   I'm very happy with my TQ.

[Super Blue 72]

Yeah, I would stick with my TQ if it wasn't leaking... 

Just looking for a cheap alternative.

I may have found the issue.  I think the air horn was dropped at one point in time, causing the O-ring on the passenger side not to seal.  I noticed it the first time I took the carb apart.  I think with small variances in the O-rings combined with the ethanol in the gas it caused the leak that was a little noticeable to be more pronounced.

I did a double gasket after filing the bottom of the air horn to make a better seal as the passenger side look a little mushed in. You could see that there was not a smooth mating surface.

We'll see what happens when I install it this weekend hopefully... 

[Chryco Psycho]

I would just grab the 650 Holley spread bore , tyr it , you will like it