605 HP Tunnel Port

[blykins]

Sorry Earl, I meant stocker.  Next time I talk to Robert, I'll clarify to make sure, I may have misunderstood.   I do know he said that Keith worked him up a Tunnel Wedge to match his heads and he slowed down.  I may have misunderstood in what he was comparing it to.

I haven't compared a 1050 4150 to a Dommie yet, last Dommie motor I did was a 529 inch BBF with a QFT 4500 1050, but we didn't try other carbs. 

[Barry_R]

I have done several single four barrel combinations and several 2x4 combinations.  I don't have the hard data in front of me right now - but general observations are similar to Jay's.  The 2x4 usually made more steam.  In every race environment where they are legal you will usually find some variation on a tunnel ram in the top flight efforts.

[blykins]

Have you done any back to backs?  2x4 vs 1x4, with the same combinations otherwise?  I've been able to do a few, and the results I have don't follow that trend, but again, it could very well be the way the dyno is set up. 

I have mentioned this to Dale Meers, the builder whose dyno I use, and he said that it followed the same trend that he sees when he dynos his own stuff.  He's a big Mopar guy, and while dyno'ing his 493 inch BBM last week, he found that a single carb intake was 30 hp above the dual carb intake he had (760-ish vs 730-ish).  Similar results, same dyno, so there could be a correlation, but he builds a lot more race engines than I do, so I would expect him to have more "real world" data. 

Apologies to Gary for hijacking his thread....don't want to detract from his moment in the sun. 

[CaptCobrajet]

Speaking VERY generally, a 2X4 FE will make more steam and go faster than a single carb.  It has a huge amount to do with how much a particular engine demands.  The bigger the mill underneath, the more a 2X4 will help, as long as it is a good enough manifold.  A tunnel wedge is borderline on being too big for a 427, and it is too big for anything smaller.  I have a customer with a 511 cube roller cam FE.  We did the engine with a Victor that I modified extensively, and a 4150 that most would call a "950".  It made 820 hp and a little over 700 ft-lbs torque.  When we freshened the engine,  I did a tunnel wedge in similar fashion, and two 850's.  On the same dyno, same cam, same everything, it then made 880 hp and 770 ft-lbs torque.  A huge difference.  By contrast, I did a 445 for a nice man with a LR 2X4 and two 600's that made 580 power and just under 600 torque.  The very same combo with a RPM manifold and a 750 DP made 560 power and a tad MORE torque than the 2X4.  If we go smaller and less, the difference will be smaller and less, until we finally get to the point where the single four suits the demands the best, and then the single will win.

On heads that are too big for an engine, a smaller (single four) manifold will crutch the issue often times.  My opinion is that on the combo Brent described, he has a "mid-size" FE by today's standards, with a "too big" port, so the single four with less volume helps the "big head" problem and the single four shines. 

A dual plane HR manifold with two 715's is 40 hp and no torque, better than the same engine with a dual plane and a single 780.  I did that back to back once on a 427 cube flat tappet.  The curves are different, and the single carb has better torque way down low, and falls off sooner with a different bell shape of the power curve.

It is hard to make a blanket statement about one versus two without looking at the other factors.  I did not mention T-rams for either one, but in that world, the dual carbs are absolutely better, as others have mentioned.

That should be a nice street driver at 10.25 C/R, and with that 106 cam it'll sound like John Force's funny car.

[garyv]

Nice explanation Blair !
Appreciate your insight into this topic.
garyv

[blykins]

Maybe I got into what Blair said I shouldnt have....generalities.

I have a degree in mechanical engineering and the first thing they teach you is that formulas and theories don't always work, so you have to go by data that youve gathered.  In my case the few instances that I've been able to do back to backs, the single has inched ahead.  Maybe it was the way the carbs were(nt) able to scavenge air...maybe the lengths of the single plane intakes favored the rpm range better, who knows....but those are my experiences.

I always try to not pass up an opportunity to gather more data, so maybe I can twist Gary's arm on a few things.... ;-)

[My427stang]

Dyno guys - any issues with vac secondaries opening when on the dyno?  I have never dyno'd anything other than mechanical secondaries, but have heard that some guys have to mechanically open vac sec carbs to get them to open all the way.

****Edited for misuse of the English language

[machoneman]

This has turned into one heck of a great series of posts!  Perhaps the real question here is the difference between OEM type dual carb intakes versus modern (or old) single 4 bbl intakes.

Think we'd all agree (?) that true drag race type tunnel rams, cast or sheetmetal, would out do single carb intakes. 

[blykins]

I'd agree with that.

[fastback 427]

Regardless of the single vs dual Carb discussion, Brent built a heck of a street motor! 10.25 to 1 compression with a little over .600 lift should be very reliable with great manners, (valve lash etc.). Like its been stated before its fun to see something a little different than the normal all aluminum, cnc head, 500 inch race motor. That motor, in Garyv's 3400 lb fairlane, will be an animal! With fangs! And fur on it and stuff! Nice build guys.

[My427stang]

Regardless of the single vs dual Carb discussion, Brent built a heck of a street motor! 10.25 to 1 compression with a little over .600 lift should be very reliable with great manners, (valve lash etc.). Like its been stated before its fun to see something a little different than the normal all aluminum, cnc head, 500 inch race motor. That motor, in Garyv's 3400 lb fairlane, will be an animal! With fangs! And fur on it and stuff! Nice build guys.

No doubt, I have known Brent for a long while now and all of his stuff is always very well thought out.

[cjshaker]

....will be an animal! With fangs! And fur on it and stuff! Nice build guys.

I hope you aren't talking about fuzzy dice hanging from the rear view mirror...lol

Ross, I'm not a dyno guy, but the only motor I've had dynoed was the old 2x4 MR in my Mach, and it had to have the secondaries pinned open. It just wouldn't build enough vacuum on the dyno with the dual fours. Strangely, they would open just fine when in the car. I never understood why that was the case though.

[My427stang]

Ross, I'm not a dyno guy, but the only motor I've had dynoed was the old 2x4 MR in my Mach, and it had to have the secondaries pinned open. It just wouldn't build enough vacuum on the dyno with the dual fours. Strangely, they would open just fine when in the car. I never understood why that was the case though.

Same as I heard

[jayb]

Yes, to really get the secondaries to open on the dyno you often have to either put in the really light springs in the diaphrams (yellow, I think?), or pin them open.  That is really easy to do with a tie wrap that holds the secondary actuation lever to that slotted plate that it fits into on the secondary throttle shaft.  Tie wrap them together when the carb is closed, and at full open throttle all four throttle blades will be open.

[Rory428]

Thats odd, when we dynoed both my old 428, and the current 454, I had my "old faithfull" Sidewinder and Holley 780 vac sec carb on both engines, and the secondaries opened up fully with no additional tinkering required. I don`t see why the airflow going thru the carb on the dyno should be any different than in a car (albeit in a car with a very tall hood scoop!)