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Discussion Starter #1 (Edited)
This is for a 4.6 liter, 4 cam, 32 valve normally aspirated V8 engine. The GM Northstar to be exact.
I am using a 600CFM Holley model 4160. This is more than enough to feed a 4.6 liter to 7000rpm. I wont be exceeding 6500.
There is one intake manifold available to use a carb for the Northstar. Price exceeds $500. Too rich for me at this time. Plus I have always wanted to try and build my own from scratch.

So here I am.. mock-up engine on stand, carb in the mail and on the way. Carb is jetted for the 4.6 per my specs. Cams are stock. Valve train is stock with the exception of better springs to control the valves better at high RPMs.

The head has square intake ports, 2" in diameter.

The manifold I want to build is of a standard tunnel ram design.
something like this
http://www.profilerperformance.com/tunnelram-spreadport-187.html

But I am going to make the prototype from steel. As I can't weld aluminum just yet.

So my questions are this...

1. Plenum. How do I figure out the plenum size that I will need for this enggine?

2. Plenum size vs torque curve. How does plenum size effect this. Does larger plenum volumn move torque up the rev range or down?

3. Runner legnth. How does runner legnth effect power?

So the last question there I know. shorter runner makes more power up high. Longer moves the torque curve down. But does the plenum volumn effect this?

Since this is a multivalve engine, using a carburator and having gobs of power up top, I want to make the middle a bit stronger. Mainly due to wanting to drive this car on the street.
oh... the car. A 1985 MR2.

thanks in advance. I chose this forum as I know we have guys that know this stuff in here.

Oh ya, one more thing. I am using square tube for the runners. I talked to ChrisK and he said there would be no problems. It would make the job easier.

here is a sweet tunnel ram featured on this page using a dual quad setup and square feeds to the valves.
http://carcraft.com/techarticles/0304_ram/
 

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I am sure people with actual data will chime in.

THe one thing I've learned from people that do these things, that the plenum size should be close to the displacement of the engine, in your case around 4.6L.
From what I understand runner length, not plenum size, move tq up or down the rev range. The plenum is just your air storage for the engine to feed from, the runners tries to make the delivery smooth(er) to each port, and also speed it up by tapering the runners a little bit, I've heard numbers ranging from 2-5% pr. 4 inches of runner.

I know ChrisK has some neat calculus for making out the length from the back of the valves to the plenum floor. Has to do with resonance and shock waves moving backwards trough the runners.

Is your engine going to be turboed?

T
 

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Discussion Starter #3
This one will not.
I am trying to make this install as easy as I can. It is a proof of concept for mounting the tranny and engine in a MK1 without chopping the chassis like I did with the Toystar. The Toystar II will use the auto 4-speed connected to the all-aluminum Northstar. The only computers will be the Ford EDIS ignition controlled by the MegaJoltLite and the Powertrain Control Solutions transmission controller.
In other words my TEC3 is being used by my MK1.5 :)
The engine will use conventional mounts, not a sub-frame. I think I have figured out a way to use the MK1 suspension with the huge auto box. If this works, then the engine will be a bolt in (well, with a bit of fab work).

wow... 4.6 liters is a lot of plenum.
 

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i say screw the carbs and go itbs for the northstar. imagine a 8 throttle bodies. that instant throttle response in such a light body with the nice torquey engine. that would be so great.
here's jenvey's site. they have a lot of info on how far the tb should be from that valve face and etc.
http://www.twminduction.com/faq/faq-FR.html
twm's may be some what helpful also
http://www.jenvey.com/Tech_QA.htm

also you could use bike throttle bodies for them. gsxr750-1000s are 42mm plates, and busas are 44mm i believe. you may need bigger 1s but those are the easiest to mod, as each can be seperated and usually go for real cheap. for larger 1s, you'd have to look into bikes that are more exspensive, and rare. so really not much data on their tb sizes. but i think you could do it, considering all that youve done, why cut short on the build and not go all out? haha
and v8 itbs just sound monstrous. haha

oh, and if my memory serves me correct, the ideal plenum would be huge for a 4.6. i cant remember the exact ratio and formula to calculate it, but even for a 1.8 honda b series, the ideal is supposedly 2-3 times the size as most plenums from stock and aftermarket manifolds that are out.
 

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Discussion Starter #5
Did you even read my post?

I would need 48MM throttles. Find those for me.. a complete kit for $159. That is what one... one 4 bbl that can provide 300+ hp for a 4.6 liter engine. I am willing to bet that ITB will cost that... just for one. Plus I would then jhave zero bottom end. I know it is really cool for the drifters to throw these motorcycle parts onto 4AGEs, thus making them fly up high... but dogs in the middle.

oh ya... so how will I control fuel? go back and read my post.
 

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alright sorry, just throwing out ideas. i know itbs would cost more than the budget, but i think its a worthwhile mod. you wouldnt exactly lose your bottom end if you didnt go too big of tbs. just knowing that you do the unimagined, i thought you wouldnt mind some more brain storming. but sorry if i offended you or anything. i am searching the answers for your questions right now. if i find it ill post it. but i just felt like throwing out that idea first.

edit*
got 1 answer. just came up to me, if i remember correctly, the plenum should be around 1.5xs the size of the engine or so. but still dont see what the effects of it being larger or smaller does to the power and torque band yet.
 

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Discussion Starter #7
for my info:
http://www.hotrod.com/techarticles/2944/

http://www.jagweb.com/aj6eng/technics.html
Actually it isn't so much about flow - it is more about the interconnection of two chambers, the manifold plenum and the air filter box. The only thing separating them is the throttle and the larger the throttle the more these two volumes can act as one. When the V12 manifolds were designed over 30 years ago not much was known about the relevance of plenum volume although it was generally considered that if it should not be too large or responsiveness would suffer, which is still true. It is now known that in a tuned, single intake, plenum induction system, top end power increases with the volume of the plenum. Ten times engine capacity would not be too big - indeed bigger still would be better, though rather impractical. Obviously the V12 plenum volume, at around 1.7 litres, is much too small but increasing throttle area helps to couple it to the filter volume for a total approaching 4 litres. Also because the filter volume is always at atmospheric pressure there is not much delay in filling the manifold when the throttle is opened quickly which must help responsiveness (assuming the appropriate acceleration fueling is provided).
no problem, sorry I snapped. I have no issues posting these type of V8/Carb questions at other boards. But I trust the engineering skills on this board. I just snap when I see answers for questions I did not ask. I know all about individual throttles. I own a ton of them off of 20v 4age's and a box load of weber DCOEs.
 

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Discussion Starter #8
so knowing the above quote, building a tunnel ram with say...

.575 liters x2 = 1.15 liters

Now this is a little less than the stock injection manifold plenum is minus the intake runners that are wrapped around it.

To increase throttle responce I would add a second carb. 2x 400cfm Holley's would be perfect for that. I'll play with that later... :)
 

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with two carbs on your home made tunnel ram/plenum youll have much more even distribution. will two smaller carbs be similar in price to the single holley? im not up to spec on american prices. on my first car (1936 chev sedan) we used a weiand low rise tunnel ram and found it to restrict top end a little so we raised the carb spacers by about a inch and mid to top end improved dramatically. maybe just try having a bolted section on your plenum above the runners where you can bolt in different sized spacers? carbs onto this engine in particular hasnt been a "done" thing yet i presum so the results will be interesting. lastly id say go efi but i read your on a budget;)
 

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Discussion Starter #10
great idea! I was going to make one manifold with a bolt on lid. That way I could add twin carbs at a later date. Never thought about adding thicker spacers to increase volume!
Distibution does concern me. But if you look at that raised lid they seem to work okay..
the cost of twin carbs and tuning would be twice as much. Carbs are really cheap, especially old rebuilt ones on ebay. There are companies that specialize in this.
They are so cheap that I have even considered putting a Holley onto a 3sge I have. Ford OHC 2.0 liter Pinto motors put out over 200hp with a single 4bbl carb. :)
 

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Discussion Starter #11
here is a good writeup I found from Google Cache - the website was no longer around. It is a post from a Holden Commador board

PAWN 06-08-2004 12:57 PM
custom intake manifold info

ok guys i have some info that i found on the topic that i thought may be usefull for someone considering making a custom manifold........

Custom Intake Manifold Design
An engine constantly starts and stops its intake airflow as the intake valves open and shut. When the piston descends, it creates a low pressure in the combustion chamber that causes a negative wave to race along the intake port and manifold runner. In a naturally aspirated car, the wave is below atmospheric pressure while in a forced aspirated car the wave is at a pressure of less than manifold pressure. When the wave reaches the plenum chamber, it is reflected back towards the engine - but this time as a positive wave. This returning wave has the potential to help ram more air into the combustion chamber, but only if it reaches the valve when it is again open! The trick is to design the system so that these positive reflections arrive at the intake valves at the right time - helping to push air into the engine.

The gains available form a well-tuned intake system should not be underestimated. Pioneering work done by Jaguar on their mechanically injected racing engines showed that it was possible to gain more than 100 per cent volumetric efficiencies (VE) by using very long intake runners. This means that the cylinders actually breathed in more than their swept volume! Many manufacturers have since developed efficient tuned intake systems that give very high volumetric efficiencies. As an example, Ford in Australia developed a superb dual-length intake manifold for their straight six, a system that achieves a VE of 100 per cent at the peak torque rpm of 3000. Power and torque gains of up to 20 per cent have been seen in some engines - and that's as much as is achieved by low-boost turbo charging!
The three variables that can be changed in designing an intake system are runner length, runner diameter, and plenum chamber volume. Jaguar found with their experiments that the longer the intake runners, the better were the peak torque outputs, and other manufacturers have seen similar results. However, very long runners cause an overly great pressure drop and are not properly tuned in length for peak power, so explaining the changeover characteristics used in some sophisticated intake systems.
A starting point for working out the length and diameter of intake runners can be gained from the following equations. In a Helmholtz Resonance system (one with runners connected to a common plenum), US-based engineering guru David Vizard suggests that a runner length of 17.8cm at 10,000 rpm makes a good starting point. (In this context, "runner length" refers to the distance from the inlet valve to the plenum chamber.) Add to this length 4.3cm for each 1000 rpm less that the system is being tuned for. Tuning for peak torque (not peak power) is the norm, and so if the engine were being tuned for 4000 rpm, a runner length of 43.6cm would be required. You can see that for an averagely-sized engine bay, the longer the runner that can be fitted in, the better!

One equation for runner diameter is to multiply the engine volume in litres by the engine's volumetric efficiency, then by the tuned rpm, then divide this sum by 3330. The final figure is square rooted, giving the runner diameter in inches. As an example, a 5 litre engine with an 80 per cent efficiency (expressed as 0.8) and tuned for 3000 rpm, will have a runner diameter of 1.9 inches, or 48 mm. The volume of the plenum should be around 80 per cent of the volume of the cylinders to which it is connected
 

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So are you realy going to use old school carbs?

That'll be wild....

If you ask me, do hanging injectors man, above the trumpets, and use two throttles in parallel into a two chamber plenum.

But I am crazy.....

T
 

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Discussion Starter #13
yep... old school. I want to see those bad boys sitting above the AW11 hood. I have also always wanted a Black anadized Moroso air cleaner. Something I could never afford back in 1980. Now I can.
I thought of that as well. Make my own venturis. But it would all need toi be controlled by some computer. I need this baby done intime for Kingston.
 

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Out of curiosity.. could you elaborate on the reasons for your decision to running a big single carburetor instead of the OEM induction system?
 

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Discussion Starter #15
$$$
Cost.

I can get a carb working good for a lot less and make about the same power, if not more.
Hot rodders do it with good success.
Plus it is something new and different. I haven't worked with a Holley 4bbl since 1979. FI is quick and easy. It just costs a lot of cash to get it tuned properly. Carbs are just so much easier.
 

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Bill Strong said:
Ford OHC 2.0 liter Pinto motors put out over 200hp with a single 4bbl carb. :)
A dozen good one-liners spring to mind, like
  • Yeah, but that's because they had such a good, even burn...

4.6L sounds awfully big for an open plenum. You might want to start smaller and experiment with different spacers. Seems comical now, but I remember cutting a bunch of spacers out of birch plywood for my brother's '66 SB Nova.
 

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If you are asking about calculating runner length, you have probably not yet run across this information. Don't forget that the runner starts inside the cylinder head

You're gonna run carbs? I fart in your general direction :)
 
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