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Topic Review (Newest First)
09-16-2008 08:24 AM "Ram air" as you call it is just taking advantage of inertial of the flowing air stream and/or 'organ pip' effect. If you just suck the gas/air mixture from whatever length runner in a stock manifold, you will likely charge the cylinder w/ a variable amount of mixture, a little less than 100% and a little more than 100% depending on runner length, RPM, etc.

On the other hand performance manifolds attempt to take advantage of the physics of a flowing compressible fluid and obtain a 'supercharge' effect. When tuning using these principles, you MUST compromise. You can definitely achieve a significant supercharge and higher power at a given RPM but lose power and efficiency at off-peak rpms.

The first thing that must be done to tune an intake is to sized the length of the intake runners. In an intake manifold when the intake valve opens a negative pressure shock wave is sent up the runner at the speed of sound, regardless of the velocity of the flowing gas. This negative shock wave reaches the opening of the runner and collapses, sending a positive pressure wave back down the runner. You can size the runners to have the return positive pressure pulse hit the intake valve just as it closes which is a true supercharge => more power. As you can imagine, this only works at a specific RPM; all other speeds are likely to perform worse than a non-dynamic stock manifold. Also you can deduce that the shorter the runner, the higher the harmonic speed of the engine that receives the boost. Thus drag racing manifolds have short stacks/runners for a higher tuned speed. Stock engines have very long runners (witness the Chrysler 'Cross Ram' manifolds of the late 50s/early 60s with their ++foot-long runners).

Here is a discussion of the topic from another thread concerning design of an exhaust header but the principles are the same, just have to determine the velocity of sound in cool intake flow compared to hot exhaust flow.

"The main advantage gained in equal length, independent primary header tubes is from the strong negative pressure pulse that is reflected from the tube end when the strong positive pressure pulse form the exhaust valve reaches the collector. Other pulses from other header tubes are of much smaller magnitude in the tube of interest and can be ignored. Thus tuning length is very easy to determine once you have an estimate of the speed of sound in the hot gasses. A useful equation is

L = 120V/rpm

L = pipe length, less port length in head, in inches


V = velocity of sound in hot gasses. Values of 1300ft/sec to 1700 ft/sec are common.

Using V = 1700ft/sec the equations simplifies to

L = 204,000/rpm."

Here are a couple more threads on the subject;

Not only is the organ pipe effect working against a street engine running a racing tunnel ram, the size of the plenum under the carb is working against it too. The plenum is necessary to divorce the runners from the things going on in the carb throats which would kill the organ pipe pulses. At high (design) RPM, the flow from the carb is high speed and the mixture zips through the plenum to the proper runner w/ no fluid separation. However as lower RPMs the velocity of the mixture slows down and the liquid gasoline drops out of the flow and puddles on the floor of the plenum. This results in some very rich and some very lean cylinders. To avoid the latter which is bad, you compensate by over jetting and fouling some cylinders.

As you can see the is topic is very sensitive and emotions run high but the fact is that tunnel ram manifolds were developed in the 60s for the specific purpose of improving the performance of drag racing engines. Period. Can they be made to run on the street? Of course! No one is saying that can't be done. However since all engine design is compromise, a tunnel ram will not yield the all-round fuel economy/performance of a good street/strip hi-rise on a street light bandit but they will kick your butt and put on a band aid on the drag strip. It's simple physics.
09-16-2008 08:00 AM
lg1969 Don't forget, the beauty of a Holley 390 uses vacuum secondary. It will not use the entire 780 cfm. It will open as the motor needs more cfm as the rpm increases. You will get a strong throttle response at the low end.
450 are mechanical secondary. You punch the gas pedal and the motor will lean out causing it bog or backfire. The motor will run sluggish when you floor it. They are calibrated for low manifold vacuum. Having a strong manifold vacuum will cause the idle circuit too pull too much gas and causing the plugs to foul. Big block motors can handle a 450cfm. Small block motor are sensitive to carburetor size.
09-16-2008 03:29 AM
Alaskan Assassin That's what I was thinking. I used VE of 100% in the estimates I posted. I don't see how a ram-air would make your engine junk, and dual 390cfm should be enough.

Using more carb at any rpm is silly, no matter how much cfm your carb is pushing through your engine only has the capacity for a given amount of cubic feet of air in one minute. (trying to flow more air would decrease pressure, if my understanding of physics is correct) Increasing the pressure of that given amount of cubic feet of air will give you a performance boost. The ram-air intake increases pressure, the carbs determine the airflow. (right?) Which would also increase volumetric effeciency, as per volume there would be a higher charge due to the increased pressure, while not trying to force more airflow than can be used.
09-16-2008 03:14 AM
techinspector1 I have not read this whole thread, but may be able to help you out with CFM.

Any engine out there is just a big air pump. The more air you can move through the pump, the more power you will make. You need to guesstimate the volumetric efficiency of the motor in order to tack any CFM needs to it. Most folks will agree that 75 to 80% would be a reasonable figure for a stock motor. As you bolt on better heads, install a longer duration camshaft, etc, you can improve the efficiency. Naturally aspirated motors tuned with the proper size and length of intake runners and exhaust headers can exceed 100% efficiency due to the ram packing of the cylinder that occurs just before the intake valve closes ABDC.

When sizing the carb for a warmed over street motor, I usually use a figure of 90%.
The formula is CID x RPM / 3456 x VE
327 x 8000 /3456 x .9 = 681 CFM
327 x 7000 /3456 x .9 = 596 CFM
327 x 6000 /3456 x .9 = 510 CFM
327 x 5000 /3456 x .9 = 425 CFM

Using more carburetor than is necessary to feed the motor at the extreme upper end of the rpm range is silly. In my opinion, you'll just slow down the airflow through the venturis and get lesser signal to meter the fuel properly at the lower end of the rpm range. I have no scientific facts to back up my opinion, just seat-of-the-pants logic.
09-16-2008 01:03 AM
Alaskan Assassin I'm attemptin to understand a little more about ram-air so help me with this and correct me where I'm wrong

The intake reduces the dynamic pressure and increases the static pressure, causing an increase of air density. Putting more air molecules in the same cubic inch of space.

Taking the 327 example, 470cfm should be enough at 5000 rpm, 660cfm at 7000 rpm, assuming the volume displaced on the intake stroke is half of the overall cubic capacity. 82.5 cfm per cylinder. This would make the ram-air intake more suitable for drag racing, as the engines air capacity per minute at lower rpms wouldn't be enough to utilize the carbs. 2 390cfm carbs would be delivering 97.5 cfm per cylinder, which would be enough at 7000 rpm, but would be best used at 8000 rpm. I can't find that to be a streetable condition to run an engine.

Or am I just crazy and my math is horrible?

(slightly new to this)
09-15-2008 09:20 AM
lg1969 hello...I'm back
I just love how people write about tunnel ram setup. NO, NO your 327 is going to run like a garbage. Lousy low end running too lean on a 390cfm or running rich on a 450cfm. I have had my Tunnel ram in my 69 Chevelle well over 30 years. 10 years on my 307sbc and 20 years on my 350sbc, and now the same tunnel ram I had for over 30 years is on my 406sbc. It's a pair of 390cfm Holley and yes 390 do run lean, just change the pri jets to 57. I did replace the sec metering plate to metering block from a pair of 780 holley. add 62 jets. 450 cfm will work, but it will backfire. A friend of mine had a 327 wth a pair of 450cfm on a 327 roadster. It would backfire once in a while and foul the the plugs. He switch the 450 for a pair of 390cfm and the problem went away. It ran a lot smoother. My chevelle passed the sniffer test before they changed it to safety inspection. That guy who going to put a tunnel ram I say go for it. But use a pair of 390cfm. Keep the metering plate. Oh... My 390 cost me $148 a piece. Times change, LOL. The tunnel ram I'm using is old TR1X Edlebrock. The same tunnel ram the Grump used in his 331sbc Vega. He was running low 9"s. Good luck
By the way, Look up on lg1969 photo album. you will get the message.
09-15-2008 08:25 AM
Alaskan Assassin wow....

this is a pretty old thread...

The guy who I got my block and heads from was going to do ram air, and I considered it to... but I'm thinking it'll be a bit more work and money than I can really put into it.... would be pretty sexy though
09-15-2008 07:48 AM
tunnel rams

One thing to remember: small block chevys like to run lean and small block fords liketo run richer. I ran the 390's on a tunnel rammed sbc and 450's with the 306 sbf. I found this combo works well.
03-06-2005 06:14 AM
Originally Posted by camaroman7d
The tunnel ram design is not torque friendly, nor is it low RPM optimal but, that doesn't mean they can't be run on the street. I doubt you are making the most of that tunnel rams potential but, you like it and that's all that matters. That is what hot rodding is all about.

That is a good statement, the tunnel ram is capable of going beyond 100% VE but you have to keep the rpm's high and that is not a good thing on the street in the eyes of the local law enforcement. I run a single 4 on my ram and it runs very well, but I know that for street use I would be better of with a dual plane intake...I just like how the ram looks and performs. Dyno figures show a much higher horsepower output with the ram at higher rpm over any other intake I have compared it with and I really don't care about the loss of torque on the bottom end for my application.
03-05-2005 02:50 PM
Lonestar I have some video of me with my 327 tunnel ram. PM me if you want it but I shot this holding an inferior camera in one hand and trying to drive the car in the other.
03-05-2005 12:44 PM
camaroman7d Thats' good to hear form the horses mouth. I know it can work and have always said that. The drawback is for the amount of money and time spent you can go just as fast or faster with a single carb. On a street car there is no "real" advantage of running a tunnel ram. I have always liked them and if someone want to run one because "it's their car and they like the look" I am all for it. In most cases people have a mis matched setup and can't get the tunnel ram tuned. Sounds like you have yours dialed in, right on!!

The tunnel ram design is not torque friendly, nor is it low RPM optimal but, that doesn't mean they can't be run on the street. I doubt you are making the most of that tunnel rams potential but, you like it and that's all that matters. That is what hot rodding is all about.

03-04-2005 10:30 PM
lostpony I found this thread on a Yahoo search a few months ago.

I am running a dual-quad Weiand #1988 street tunnel ram with sync'd, electric-choke Holley 390's on a .030 over 1978 302 with '66 289 Shelby heads and a built C4.

Nearly everything I was told when I was asking around about this and spec'ing it out, in person and on forums, including but not limited to:

- needing a bigger cam
- fouling plugs
- making headers glow red-hot
- idling at 1500+
- no power below 2500 rpm
- no power below 3000 rpm
- no power below 4000 rpm
- needing to rejet carbs
- needing to run off one carb
- stock motors will valve float in ideal tunnel-ram rpm band
- needing to block off 4 of the runners of the T/R
- less power than with a good single quad
- impossible to tune
- needs tuning every weekend
- runs like crap when cold
- runs like crap when hot
- needing a hotter fuel pump
- needing high-octane gas
- needing a bigger coil

All of the above is WRONG. It swapped right in (once I cut the hole in the hood) and all I had to do was replace the cap and rotor (clearance issues.) Starts on the third stroke, runs killer, idles at 600 when warm, comes off a dig like a cat with its *** on fire, and gets double-takes everywhere I go. It sounds KILLER!

It is ridiculously, irresponsibly, sphincter-clenching, what-the-hell-was-I-thinking fast.

Maybe it's the 390's. Maybe it's that we rebuilt the carbs completely (stock Holley 4160 rebuild kit). Maybe I got the linkage exactly right (home fabbed). Maybe the Horsepower Gods accepted the sacrifices of knuckle skin and blood and finally had mercy on me.

Or maybe it's that the naysayers either never tried it, or went too big on their dual carbs. (Most of the horror stories I heard came from someone who heard of someone whose friend tried a dual-quad tunnel-ram. I did hear from a guy who tried dual 750 cfm's on a 350 Chevy and gave up after months of tinkering because he "could never get it to run right." Big surprise.)

Don't give up. Just find 390's for a small-block.

If you're still around (or if you found this thread, as I did, through a search), check out my build page at . Or email me through this site if you have questions. Good luck.
09-27-2004 10:09 PM
Super Streeter Actually,if you set up a tunnel ram correctly the first tiem,it wont foul plugs at idle either.The tunnel ram has the upper hand on this one since with 2 carbs to idle the engine off of,you can keep both carbs closed more then a single 4 barel would sit,and in doing so you kep the idle transfer slots from being exposed.If you are an idiot and try to run non tunnel ram carbs on a street application,or if you try to idle the entire engine off of one carb it will run like a pig,but with the right setup it wont be a problem.The bigest issue with a tunnel ram is warm up time since a race type tunnel ram runs very cool and takes a long time to warm up and never gets very hot up near the plenum.A better solution is a tall "street type" tunnel ram with long runners that are webbed togehter and dont have an air gap between or under them.These setups have long runners that favor a 2500 to 6500rpm powerband,but you still have to give them a decent amount of warm up time.Dont just jet the heck out of the carbs to make it run better while warming up.If you wanted to do something really trick you could fabiracte a set of "stuffing blocks" to epoxy into the plenum in an area that doesnt effect flow{between the carbs would be good as long as it doesnt block the passage entirely},and this would give a little bit beter throttle response,and would allow the engine to run better in cool air since the accelerator pump shot would be more concentrated.If you could bolt a mini plenum un place of the large open plenum{some street rams come like this} it would be even better.Other then that,it is a piece of cake.
09-25-2004 10:07 PM
tunnel rams

We all realize that a 2 fours tunnel is not going to be an everyday driver . I Mean if I were a rich person , could afford to replace plugs real often and gas was no issue I could enjoy driving one . As for a vehicle driven around on say weekends to cruise a little up town : no problem , thats what most of us do with our hotrods anyway. I have an 82 Chevy truck Im driving tonight just during the weekend : it feels pretty good tonight coming to my girlfriends : another Chevy truck came zipping around me ;;when we came to the light , he just sat there. He could hear the cam rocking my truck. All week I drive my other one with a v6 . Ive also got a Samurai with a mild 350, 63 Chevy 2 convertible in the works 283 to be a cruiser in the spring. It all depends on the purpose of the vehicle.
09-25-2004 09:18 AM
Lonestar Your so right willys36, over jet to get an idle...hence why my plugs were always fouling.

Definately is a fun ride above 4 grand, but the public doesn't care much for that!!

I am in the process of trying to make my '85 Bird street legal again, it has a single 4 tunnel ram on it. I have to go out and run the thing to 7,500 to clean the plugs!
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