Without going to a specialist manifold fabricator, how much do I increase the volume of the port runners at the plenum end compared to the head end? Royden
Well I have just finished the CAD of my soon to be tunnel ram. I will fabricate this myself and have a friend weld it for me. I havent finished the top plate yet because I dont have the carby base sizes yet. Cheers Royden
Looks good! What length are the runners? My intake track is 14 3/4" which puts the peak ram charging at 6300. What's the plenum volume? 100-110cui would be ideal, mine measures 104.
Don't worry about the water. You can plug the water holes and drill the front of the heads and run the water out of them there. I did that when using a Chrysler intake. Worked great. Good luck on your intake Jim N.
I had planned to run a cross over with braided line at the back and 2 x 90' angle fittings plumbed straight into the radiator with no thermostat. The runners are 92mm long including plates. I will be using stg2 SE heads. Plenum volume is around 120cui. Are these figures sound right? Royden
I would say the plenum is plenty big and the runners are short. This would make it a great 7-8,000rpm manifold. With short runners I think you need to look at the 5th or 6th reflection. Your at the 6th reflection now around 6500. I would look at runners around 145mm and hold the overall height and reduce the plenum as needed. The tapper would reduce slightly for the longer runners which would have more of a constant velocity. The sixth reflection and your current lengths could work well. I have a Weiand but I'll give you the specs. runners - 220mm with plate very little tapper after porting 104cui plenum carb spacing is 9 5/8" mounted on SE heads.
Hi Royden I agree with David, your runners are short at 3.6", But in your drawing they look longer . I've attachd a jpg of apx 6" long runners and the spacing is similar to yours. Also roughly gaging from your drawing and what I've built in the past I would say your plenum is going to end up north of 300 ci. David I'm interested to know what formula you use to get your optimum plenum volume. Thanks Dave
I really appreciate your guys input as I only want to build this once and hopefully the engine responds to it ok. I made an error in the length of the runners. They are from plate to plenum 107mm. Dave I measured 348ci for the plenum after bringing the ends in 50mm. I dont know how I can get down to 110ci like David suggests. Royden
There is no easy answer to what your looking for. At 107mm your over all runner length from head to plenum will be around 129mm or 5.08" , still looks short too me . What's your intended use? Size of motor, Valve size , int port cc , cam , planned rpm for peak hp? Dave
I plan to run with a pair Greg Gesslers level 2 TA stg2 SE heads. A bit undecided on cam, but will prob get some advice from Greg as to what cam would run best with those heads. Intended use is weekend driver with as much drag racing i can get. So my plenum like David mentioned is still too big? Royden
Re plenum size, I've heard everything from 25% engine displacement to 200% Very generaly speaking , large plenum makes more power hurts throttle response. What I've built in the past started at 319ci , I now run over 700ci but it's injected so throttle response is not the same issue as carbs. I also launch at 62-6400 rpm so my situation could be alot different than yours. That's why I asked for more info. You won't be able to get as small a plenum as David is recomending based on your current layout but there are many tunnel rams running out there with similar plenum sizes to yours. Runner length, size and taper will affect power more than the plenum size . The more you know about your combination the better choice you can make on the runner size as well. Dave
Plenum size is one of those things I couldn't find formulas for. What I did find is a range of sizes which have functioned well for various apps. 20-40% (of engine size) is what I find in alot of designs. This really applys to carbs since injection is not effected by the lack of booster signal. I have two 750s, and the motor starts and idles cold like a large single carb. I wish I could post a mpeg of me starting the thing cold so you could get an idea of the kind of streetable manners I was after. Don't get me wrong it sounds like a prostock in person. I don't have mufflers yet. Make the runners the correct length and don't worry about the plenum volume. You build a box under the top plate which reduces the volume as needed. The issue you will be fighting is having enough signal and vapor for proper idle. I would use carbs with 1.72" baseplate and a four hole spacer with 1.75" bore(850/950 size). This gives you a fuel shear edge which reduces the fuel droplet sizes and produces more vapor for ignition. They make killer anti reversion fuel shear plates but they are crazy money. You may be able to add this feature into your top plate.
I wish you could send an mpeg too!! Thanks for your guys input I will make the modifications. With regard to taper of the runners, that will increase the air speed in the runners. Is this good or bad? Royden
I'll try and get a mpeg made. I've been using my phone to record. It overpowers the little mic. Tapper is used for shorter runners to bring the velocity up. When velocity doubles the pressure is squared. You will trade some innertia for the velocity. Personally, I like the longer runner to increase the fill after bottom of stroke.
You may find it easier to buy something like this http://www.racingjunk.com/post/1122750/Mopar-Indy-tunnel-Ram-Dominator-.html and then cut the flanges off and weld on the Buick flanges. There is a tubbed silver 68 Skylark at the Buick events sometimes that did this with a M1, 383 tunnel ram. Just and idea.
