Mr Gadgets

Ok, lets take this a step further. What about the EFI setup that Merc has with individual throttles for each cylinder. With no plenum will that kill the low end? I have only seen this setup on race boats, so I make the assumption that it would be a high rpm friendly deal??
My next set of heads will cause me to throw my pretty sheet metal intake in the trash. I hate the thought of paying for a new one. But besides a slice and dice on the present one to make it fit a new one will be needed. Just want to know my options..

Thanks

Dick

Dennis Moore

The reason why plenum volume is so critical with carburetors is because you are using engine vacuum to pull fuel out of the carburetor. The larger the plenum, the less vacuum the carburetor will see (from the engine) and the weaker the air will pull on the carburetor (this is particularly harmful to low speed performance).
With fuel injection, engine vacuum is much less important because the fuel is being forced into the engine under fuel pressure. It is really comparing apples and oranges.
Dennis Moore

robyw1

You’re absolutely right Dennis. The problem with getting a tunnel ram to work right was the user. Although each ram was still a little large to feed 4 cylinders they would still sacrifice torque when compared to a good dual plane manifold. Everyone seemed to compare them to a single plane but those are not known for torque either. The reason why they were so hard to get power out of is that everyone would run dual 600-CFM or larger carbs on them. Sonic30ss says he saw a dyno shootout on the tunnel ram and it won out HP & torque over other manifolds. I saw a comparison (don’t know if it was the same one) and the tunnel ram was impressive but it had two 390 CFM 4150 Holley’s. However it was Edelbrock’s Performer RPM manifold that kicked all of their butts to 6500-RPM and still made good power to 7200 RPM. It was tested on a Small block Chevy 355 cu-in. It was the clear winner overall and only fell short of the tunnel ram by 11 measly HP at 7200 RPM. Up to 6500 RPM not a single manifold was within 60-HP of the Performer RPMs maximum.

Now based on what I have read from this list I haven’t seen many marine engines spinning over 6000 RPM. I’m sure you guys are out there but I’m talking close to stock Magnum Mercurisers. If you are normally aspirated and staying less than 6000 RPM then there is really no need for a Tunnel Ram.

Remember the formula for the correct carb size is:

cubic-inch times maximum RPM divided by 3456 = CFM.

And that is running at 100% VE. You normally aspirated guys can deduct at least 10% off those CFM results. Do the math on your engines and tell me if a couple of 750 Holley’s on your tunnel ram is too much or too little.

Later
Roby

bob

Gadgets/

I ran the Merc/Kinsler EFI race system for one year with 14 degree Big Chief heads and a solid roller as a pleasure boat. The intake had 8 -2.10" throttle plates. System was fantastic above 1500 rpm or so. The throttle plates are so large they make idling sometimes tough. But, with that particular cylinder head and the EFI, you make some pretty respectable hp at 6500+!

Mr Gadgets

Thanks Bob,
that was what I was looking for. I guess I am looking at what type of manifold I can build the easiest. A sheet metal one is a work of art, but if I could bolt on a bunch of air valves and fill in the hole in the middle.. well I am thinking.
Thanks

Dick

ALFSTER

I've run a tunnel ram w/ 750's on my 540 for several years with no problems.No problems with idle,comes strong on plane,good mid range,excellent top end.

jdnca1

I run a Dart tunnel ram with 2X775 Race Demons on my 632" BBC. CR is 10.5:1; Dart 360 fully ported heads; 280/288 @.050 .730" 115CL solid roller. Idle is 900, 650 in gear. Motor never loads up hesitates or stumbles ANYWHERE in the rpm range. The key is to making a TR work is to have your cam and carbs correctly matched. A correctly set up TR will always make more TQ in the mid range and always make more power up top. LONG RUNNERS with little restriction. Usually a single carb will be better down low. 900 HP @6500 on 93 Octane, 845HP @5800. TQ is 820 @5000.

SMALL CARBS is the key to making a TR work. 1050 Dominators would probably make another 25hp up top, but low end and idle quality would go away. Now if you were going to turn 8000 you would need some monsters to keep the motor fed.

WETTE VETTE

JDNCA1,
You need 1 more or those 632's for your 28!!!

jdnca1

I Know!! Its cheaper to put on whipples though.. Plus the whipples are stealthier, you know the 632's are nasty when they're idleing..

Bulldog

I had an L-88 427 chev in a 21 ft jet a few years ago. It was topped with 2 660CFM center squirter carbs. Full-time four barrels, with 1 50CC accelerator pump if I remember right. The accelerator had 1 nozzle in the center that shot all 4 barrels at the same time. The TR was a Weiand Hi ram which has a small plenum compared to the Edelbrock. The engine had torque everywhere.

Once when I was running it on the trailer I noticed that the manifold had condensation on it. I often wonder if the plenum/long runner system allows for some expansion of the wet fuel & air mix, and cooling itself like in a refrigerant cycle????

Bulldog