i have a bbf 460. just put 5,000 in the motor to restore it. big pistons mild cam roller rocker everything running about 500 to 550 hp. can get 5700 rpms when opened up wide open. has a tunnel ram with dual carbs. 2 holley 450's vaccum secondary. runs great. i think. maybe a little lean on top end. no hesitation. gps average speed 67 to 68 mph. question is the 450's enough. i want to put 2 750's. is that to much. should i stick with holley or eldebrock. vaccum or mechanical secondary. or double pumper. what do u guys think he should run or should he just leave it alone.

alright newbie.

thanks for the info tom. my manly instinct just wants to go bigger but at the same time do not want to waste the money. can i get more hp if i went with 750's. i think i want to try holley 750's vaccum secondary. do u think that will make that much of a difference from the 450's.

I run two 1050's, 8 #99 jets and four 50cc squirters. Plugs run clean no idea how it runs on the top end, never been past a quick stab of the secondaries....but thats just me. If ya wanna unload the 450's lemme  know.

I run 2 750 Holley Marines, no problems. Still runs a little lean on long high RPM runs, but throttle response is fine. Still need more tuning, but we'll see how it works out after a tuning session at MHP here in Phoenix this winter.

Ray

ray thanks for the info. those 750's vaccum or mechanical double pumpers.

ray thanks for the info. those 750's vaccum or mechanical double pumpers.

Vac Secondary dual feed single pump.

Ray

You're probably fine, as Tom suggested with the 450's set up properly. I used the marine 750's because I got a smoking deal on an as new pair of them. That owner dropped down to 600's on his sled when they couldn't get these tuned. I think they had some vacuum leak issues, but wasn't gonna tell them that buying the carbs.
 
Be careful with Vac Secondaries on a tunnel ram with a high duration cam. Vacuum could be a real problem and the slightest leak, well, goes without saying. No secondaries. I have barely enough to pull the secondaries open, and at that, doesn't happen until 4500 rpm or so. All is good and works fine, but keep retorquing the intake, carbs and carb adaptor until you're sure you'll have no leaks.

I have a pair of 660 centersquirters here somewhere, thought I might rebuild them and try them on another tunnel ram engine later on.

Good luck with yours! Let me know if i can help. That linkage affair was a couple week project figuring that out mounting the carbs the way that I did (well, had to).

Ray

I did a back to back test on my Pop's old 402BBC. Turned an A impeller 5000rpm. We had two 450dp's on it (tunnel ram) and I threw on my 660 center squirters on the boat ramp to see how they did. I picked up 3mph without any tuning on the 660's. At the time I didn't think the motor would want 1320 cfm being only 402 cubes and only turning 5000rpms, but it sure did work out. Maybe tunnel Ram's like more carbs up top compared to a single  I dunno.

My 2 cents.

thanks for the info pw_tony.damm like those titts.

good info gt. i really dont know to much on carburetion or anything. your saying i dont need more cfm as much as i think i need. or with the tunnel ram i should use more cfm. i just feel the motor could use more fuel.

Well, lets just say I have seen the math done like this;

If a certain engine uses 850 CFM on an open plane high rise manifold and you were going to go with two fours on a ram, the take the 850 CFM and multiply it by 125%....Sooooo 850 CFM X 1.25 = approx. 1060 CFM (or 2 X 550's).

If it used 1050, then 1050 CFM X 1.25= approx. 1300 CFM (or 2 600's or 2 X 650's). And so on, soforth.

I am not saying you need 1100 CFM, but if you knew what the CFM requirement is for a single carb, then this is how the math seems to work out...

I personally dislike tunnel rams for the type of driving style I posses, but they will make some ponies.

GT

Here's my take on it, Glenn and Tony both touched on how these things work breifly. Tunnel rams increase air/fuel mixture velocity, mostly reserved for full throttle applications, and, important! cool the intake charge tremendously due to that velocity.  Basically meaning the air and fuel will both be denser, (More fat). Lean conditions with 2 450's surprises me, a bit, but you said you put 5k into the motor. I'm going to assume this wasn't just a rebuild and it's been hot rodded quite a bit (If not, you were ripped on that rebuild), but even at that, I would put a vacuum guage on the thing and try a few spring kits on what you have first. (HDRider, Tom at JBP, Boost, or EngineDoctor would be the best source on info, data on doing that). I think it can be tuned just fine with what you have, IMO.

More specifics would help as to cam, C/R, etc.

What I call a High Duration cam is anything in the 292 and up range. Those are going to be an immediate vacuum problem in many cases (I'm running one a LITTLE bigger than that). On a Boat, not really a huge, insurmountable problem. No Power brakes, Pollution contral crap, heater cores or anything else to rob vacuum. Hence the vacuum leaks comment.

The sheer volume of the tunnel hurts throttle reponse, in many cases, it's intended purpose is WFO. But the vacuum can be made up from gravity and velocity. The push is if your primary's are too big, you'll never develop enough vaccum for the secondaries to open, or open fully. spring kits can help this to a great degree.

Here's when you KNOW it's running  GOOD! Will idle fine, cruise all day long, no lag when flat footed, and after a hard full throttle run for a mile or better and it has frost on it like your freezer develops, and the plugs read good and grey! That is utopia on a tunnel ram. (Kids freaked out and couldn't beleive, at the AVI, when after a mostly full throttle, up-river 5 mile or better blast they coulda made a slurpy from the frost on the top end of the engine at 90+ degrees outside)

Hard to come by. I'm very close, but as I said still running a tad lean and hesitation still a slight issue. Some of the hesitation is my pump, but not all of it. Price you pay for going against the grain and trying vacuum secondaries on one of these things. BUT, they can, and do work, just not so easy. Keep this in mind though, mechanical secondaries have a totally different set of issues to deal with, and I'm not sure they are easier, IMO.

Long and short of it is this. Which is too late. Sounds like your dead set on the tunnel, which I totally understand. Though it really seems to me more performance, and less hassle, can be had with a large single carb/single plane setup (850 to even a 1050 Dommy on a Victor Jr, Torquer, or any single plane intake).

I have 5 boats here to try anything I want, and beleive me I will. The Moo's gonna stay Tunnel Rammed just because I can. Whether it's gonna perform better than the rest? Next summer will tell. As it is, she runs out just fine. Ask Nordie, lol.

Ray

Need to correct that last part as it reads very snobby to me. Didn't mean to sound that way. Just tripped across smoking deals on parts, boats, etc, all making this possible. I'm not rich, nor have an attitude like that, but as I read that, it came across that way. Trust me, I'm as frugal and scraping as the rest of us are. I just happen to have tripped across a situation that warrants this experimentation.

I am sure you can email them to Brad or someone on here and they will help you out