Intake manifold design discussion & facts thread

well heres mine in the the process of adapting su carbs to a chevy 292

 

The equal length runners work well with direct injection-mechanical or electronic. But is not necessary on a carb setup. You need a common plenum with carbs to equalize pulses. Something to also add into your equation is the camshaft you will be running. The intake opening events, overlap etc. all play a part in pulling the fuel mixture into the cylinder. Also exhaust header size, steps in tube diameter, collecter size etc. play a part in this. Now you no why it takes all those engineers and a lot of testing to make a car perform from the factory.

 

Here´s the article out of the old magazine I mentioned in my initial post, for all who haven´t seen it yet:

 

Okay, so runner length and plenum volume must play together, but how to accomplish this, try and error?!

So the runners should all be of equal length, but there are also other approaches to this. Some say that cylinders that fire short after each other should not be neighbors in the intake design..don´t know how to express that better..This can of course only be accomplished when using two plenums.

So what do you think, is it better to have equal runners or unequal runners but an intake designed with the firing order in mind?

 

Well, everybody else has thrown up a bunch of scientific equasions and stuff. I have made and modified intakes for sbc (and others) for drag racing. The runners were kinda short, due to hood & scoop clearances and height restrictions. The runners were probably about 10", you weld two runners (like 1 & 3) together at the head end before welding into the flange; at the plenum end you space them enough to tig weld all around. In my case the seperation was for the hole of the carb so that with a 2 - 4bbl set up each hole was directly above the runner opening. The runner opening protruded about 1/4" above the floor extending into the plenum area in an effort to direct the reversion pulse away from the other runners. As others have mentioned you can simplify construction by making a valley plate, mine had a step flange milled into the edge to join the intake flange and was held to the block with 10-32 screws and had a lip front and back extending down to keep the front and rear seals from squirting out. We played a lot with plenum volume by glueing pieces of wood inside the plenum and shaping them/refining/molding etc with PC-11. This was for the sbc, other engines i have done different. Making intakes is fairly common in drag racing since cylinderhead development is way ahead of intake manifolds and there just aren't off-the-shelf intakes for the newest generation heads.
Remember this was a race motor, but it did idle good around the pits and each hole of a carb fed a particular cylinder so it was easy to tune. Good luck.

 

Have got another, probably again dumb, question:

I found these three pics of crossram intakes:

GM:


Offenhauser:


Edelbrock:


All of them seem to be for the sbc. In the flanges there are the holes for the intake ports and six smaller holes. What are the smaller holes for? For the water I guess? (Sorry for that really dumb question, I´m 17 years old and never built an engine )

Why doesn´t the Edelbrock intake pictured have these holes? The GM has it and the Offenhauser also (at least 4 the last 2 look closed somehow?!)

Thanks!

Florian

 

The center two holes are for exhaust heat to the carburator. Heat riser. The front and rear holes are for water. Few people would want exhaust heat on a competion engine which is what these manifolds are mostly for. The water cross over is so the water can flow back to the raditor and is necessary in front. A person cpuld tap into the rear ports if he wanted water flow for something.

 

Here are some threads from 2004-05. Few pics left but still have some related content. Please forgive my newness to the topic in 05. Some terms were not correct.

http://www.jalopyjournal.com/forum/showthread.php?t=18464&highlight=build+intake
http://www.jalopyjournal.com/forum/showthread.php?t=39460&highlight=build+intake
http://www.jalopyjournal.com/forum/showthread.php?t=129850&highlight=build+intake

 

Florian, What harmonic is that number you gave figure out to be? You can use the 3rd or 4th harmonic wave and still get some ram effect and at the same time have a shorter intake track. If you do the math, sbc's use this method to keep the carb low to the engine.

Intake theroy books take you through the math. Good luck and have a good pencil and lots of paper handy.

 

I've been doing a little reading on this subject. Some interesting info here. I know it's almost a year old, but would like to bump it up.

Any more info? Any other general Rules of thumb? Resources to read and learn from?

 

Food for thought. Although definitely not a street manifold cause you couldn't get under the McDonalds drive thru. LOL. Lippy

 

Food for thought. Although definitely not a street manifold cause you couldn't get under the McDonalds drive thru. LOL. Lippy[/QUOTE]

yes, the original tunnel ram. In reading the "We Were the Ramchargers" book, it seems they had some tuning issues. Mancini kept fattening up the AFB's because there was a flat spot around 3500 rpm that they couldn't get to go away. When all was said and done, they figured out the headers (8 megaphone pipes) were over-scavenging the cylinders at that rpm and no amount of extra fuel was going to fix it.

Something to keep in mind, how the intake and exhaust tuning work together.

 

For sure, you should get David Vizard's book "How to build Horsepower Vol II, carburetors and intake manifolds"

It is widely available at big book stores.