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z31maniac HalfDork
1/5/09 9:47 a.m.

Previous discussions (on another forum) have indicated the long runner length of the stock manifold as a one of the obstacles to high RPM flow. Basically my idea for the intake manifold is a Dbilas style setup, minus the individual throttle bodies of course.

My two main concerns are: 1. Size of the airbox 2. Shape of the airbox

  1. I want to make sure the airbox is large enough and will replenish fast enough for each intake stroke of the crank. I don't know how many of you are familiar with sportbike stuff and so called "Big Bang" bikes, but for instance the new R1 uses a wildly timed crankshaft to improve smooth running etc, lots of discussion about piston speeds and throttle response and E36 M3, but the main reason they don't just have a piston firing every 90° of crank rotation is that the airbox would not be able to refill fast enough for each intake stroke.

  2. A simple rectangle with the throttle body on the side, ie exactly like the Dbilas setup or a "house" shape with the side of the rectangle slanting outwards to long side, or even more a 666fabrication style intake (which also matches the E46 M CF airbox I think).

The plan is to start researching and gathering parts so I can build a 3.1L high compression N/A M20 for the E30.

Which I know what parts are needed, the last variable to research is the intake manifold.

pinchvalve SuperDork
1/5/09 9:53 a.m.

I place intake and exhaust design in the same category as turning straw into gold, but there are some on here know volumes about airflow! I stand in awe.

JoshC New Reader
1/5/09 11:03 a.m.

Yes, longer runner length sacrifices high RPM flow and power; however, it also enhances low RPM torque. Obviously cam selection has a lot to do with power as well, but look at a truck intake compared to a car intake for a given engine (F150 and Mustang manifold for the 4.6L for example). The truck will typically have longer, smaller diameter runners and a correspondingly lower power band, but typically higher torque. Intake manifold design is a constant compromise since a given manifold is essentially optimized for one RPM. The exception would be variable geometry designs which alter runner length or plenum volume.

Finding the right compromise will be the key. Performance gained on high speed straights could easily be negated by sluggish response out of low speed corners.

z31maniac HalfDork
1/5/09 11:07 a.m.

My thought is for shorter runners.

The headwork, cam and exhaust will all be optimized to operation in 4000-7100 RPM range, so I think optimizing the intake for that RPM range makes sense as well.

To try to use the intake to get a flat torque while everything else is designed for mid-high RPM power, seems a bit counterintuitive.

JoshC New Reader
1/5/09 11:16 a.m.

I agree, but determining how short, what diameter and how large to make the plenum will be challenging. Probably an iterative process. Is any intake design allowable (I'm not familiar with the rules for E30)? If I were going to build a custom intake, I'd incorporate a plenum that could be increased/decreased in volume with the addition/removal of spacers between the base and the cover. This would allow some tuneability for different venues.

Also, check out some of the materials available here. No affiliation and I have no experience with these components, but if I were to fab a custom intake I'd seriously consider these as a starting point.


z31maniac HalfDork
1/5/09 12:14 p.m.

Definitely not an E30 race car or anything of that nature.

Mainly a fun street car that will be auto-x'd and taken to HPDE's. The only class an E30 can be REALLY competitive in is DSP and I don't really want to build my car to a particular spec for Auto-x when I drive it all the time.

Mainly, I have the lofty goal of building a 200 whp 3.1L M20. And from some of the other stroker builds, the intake seems to be the last place to try to get some more upper RPM performance.

But you are correct, determining the runner length and plenum size is going to be challenging.

I was hoping one of the smart people could help guide me in the right direction!

JoshC New Reader
1/5/09 1:30 p.m.

Have you established data for the stock intake (runner length, diameter/cross sectional area, etc,)?

I am somewhat of an intake aficionado, but not an expert by any means. I've never really found any definitive text in relation to design. For most subjects, there seems to be basic information and equations available; however, intake design seems to be one area that manufacturers have guarded somewhat closely. It seems most start with the factory dimensions and tweak from there.

twentyover New Reader
1/5/09 2:11 p.m.

These may prove helpful...

http://mgcgti.com/Page40.html http://mgcgti.com/Page39.html http://mgcgti.com/Page38.html

44Dwarf Reader
1/5/09 2:17 p.m.


Very good book, Dated info yes but the math is the same.


TrentO New Reader
1/5/09 4:32 p.m.

I've always wondered at the throttle body positioning on the end of the plenum. Wouldn't this cause uneven distribution of the intake charge? I've been debating a custome RX-7 setup for a while, planning on basically building a reverse equal-length header design with a plenum where the collector would go. Thoughts?


kreb Dork
1/5/09 4:37 p.m.

I've wondered why folks don't make intakes that look like headers - i.e. equal length and radiused bends. The plenum chamber style intakes always look like they'd have a lot of unhelpful turbulence and resistance to air flow.

Unfortunately, I don't know E36 M3 in terms of actual tech.

Appleseed Reader
1/5/09 11:47 p.m.

The R1 reasoning seem to deify logic. If the intake cannot fill back up to work at 90 degree firings, how can it work when two pairs of pistons are firing closer together than 90 degrees?

z31maniac HalfDork
1/6/09 6:20 a.m.

^Because that's not how the cranks used to fire and it's not how they fire now.

Check it out and see, the new crank timing is wild.

44Dwarf Reader
1/6/09 6:24 a.m.
kreb wrote: I've wondered why folks don't make intakes that look like headers - i

Exhaust has heat and is pushed out buy the piston so there a diffrent flow pattern.

Intake flow is not an easy thing to do with todays small cramped cars.

Look at a 413 cross ram intake if you want to see a header like intake.

Paul_VR6 New Reader
1/6/09 7:28 a.m.

For a quick stab at runner length:


It works well enough to explain why my engine chokes up after it's 6k peak so quickly no matter what I do to everything BUT the intake runner length.

Here you can see the effects of 'wave tuning' in the torque curve.

That's a car with throttle bodies, but a tuned runner plenum manifold will have similar peaks and valleys.

Your best bet is to mock something up that you think will work with a few sets of 'adjustable' runners. Go to the dyno see what each length does for you. Once you make a choice, make it all fit.

For the plenum, .5-1.5x the displacement is the range you should be looking into. The optimum size usually depends on tb size, throttle response needed, where you want your tq, etc. The bigger you make it, the better it'll be at high revs. I don't know of any way to calculate the exact effects, however.

pkrstr8 New Reader
1/6/09 7:57 a.m.

Ok, please forgive my lack of full comments on this. I am trying to jumpstart my brain after a very long break!! And definately not enough coffee!!!

First you need a manifold to displacement volume for total power. 1.5 is about the standard, but I have seen 2.0 and above. Over 2.0 you get into idle stablity issues and you have to have a robust idle control. Somewhere I have the calculations it get you started on this.

Runner length to plemumn ratio. I have seen the length of runners discussed. What it does is rocks your power curve about the peak point. The longer runners favor low rpm, and shorter, higher rpm. The ideal is to calculate the Mach number for your intake valve and runner size for that. Unless you want to to a split port which was mentioned here.

TB location, the quick answer is with port fuel injection, location is not too important. With the provisions of common sence and it still needs to flow well.

If you start talking carbs, well that is going to be a whole nother pot of coffee!!

Jensenman SuperDork
1/6/09 8:31 a.m.

I'm not even faintly up to speed on intake design as far as volumes etc, but I know a little bit about headers (nowhere near what Andy knows, though!). Basically, the reason exhaust headers are so much longer than intake runners is when the exhaust valve opens and the piston pushes the exhaust gases out they are expanding as well. These combine to create a 'wave front' which travels down the tube that can be used to good effect because as it hits the collector it expands, creating a low pressure area. This helps 'pull' the next exhaust pulse out of the next tube, creating a scavenging effect and that's why when designing a header it's important to get sequential cylinders right next to each other in the collector. There's no such expanding pulse on the intake side meaning the design of a header is no real help when designing an intake.

The way I understand it, if you have two ports right next to each other inside the plenum which are sequential in timing (say for instance 3-4 on the standard 1-3-4-2 4 cyl firing order) then the first cylinder to draw a charge (in this case #3) will 'steal' intake air charge from #4. Longer intake runners make this worse because the larger volume of air inside the runner takes longer to get moving.

fabron None
4/9/09 12:48 a.m.

I built a 450HP turbo corolla that used basically a box bolted to the intake side of the engine with blended, radiused, and port matched inlets going into the extensively ported head. Very short runners indeed! Developed incredible topend power but cut into bottem end grunt. Long runners are to develop low speed velocity, like a header for the intake. Under boost the atmospheric pressure in the intake allow the head to breath with no restriction. If I had used the stock intake my boost temps and pressure would have been higher causing me to cut back on boost. But the turbo would have spooled up quicker. Less restriction=less boost pressure=lower boost temp=more CFM. To build a tuned intake you need to know exact air consumption at the RPM window it'll be designed to work in. You need to know exact volumetric efficiency of the combustion chamber, the cam, the stroke, exhaust, the firing order and timing, the btu's and stoichrometric value of fuel, etc.. A wider RPM range will necessitate a law of averages and sacrifice flow from one end to another. Toyota implemented something called TVIS, in HOTROD's hotrod of the year winning 16 valve GTS corolla. It had one intake port per intake valve. Next to the head, a RPM activated throttle valve would open both ports to the combustion chamber. By the way the same 16 valve block was offered in a first ever mass produced 20 valve configuration with short individual throttle bodies that made 50 more horsepower over the 16valve.

MacDubois New Reader
4/9/09 9:31 a.m.

I've always been a fan of this site, because I sorta understood how they explained everything... Scuderia Topolino

It's all pretty simple math as long as you know your engine and where you want your torque peak to be...

jcanracer New Reader
4/9/09 1:46 p.m.

I am glad that somebody actually bumped an old topic that i didnt know about.

I have debated making an intake manifold for my engine for a long time since my engine came with a plastic intake manifold.

My specs: Toyota 1ZZFE 1.8L with I/H/E and 272/272 duration cams. my last dyno: http://img444.imageshack.us/img444/9568/recentdynoqz0.jpg

I have on hand all the makings of an ITB setup http://img357.imageshack.us/img357/4915/throttle5xn2.jpg (enclosed with plenum so that i can keep the MAF) but i do not know what length runner to pursue. Alternatively I might sell those bits and build a new intake manifold from pieces ordered from Rossmachineracing.com. I have my calculations in order for the plenum chamber but not the runner length.

jcanracer New Reader
4/9/09 3:26 p.m.

damn, i seemed to have killed another thread....

Dr. Hess
Dr. Hess SuperDork
4/9/09 3:42 p.m.

Hey fabron. Welcome to Da Board. Yeah, we kinda know about the 4AG and its various versions 'round these here parts. What crank were you using to get 450 HP?

jcan, any idea what a stock 1zzfe would do on that dyno? Do you have a "before" run? How does a 1zzfe with vvt compare?

John Brown
John Brown SuperDork
4/9/09 4:08 p.m.

jammy you didn't kill the thread, we were all just patiently waiting for your awesomeness to wear off of our computer screens a little more.

jcanracer New Reader
4/10/09 11:42 a.m.

Dr. Hess: Stock 1zzfe with vvti dynos between 112-115 whp

Mr. Brown: my apologies!

fabron New Reader
4/21/09 11:50 p.m.

I purchased a low mileage 1988 MR2 supercharger 4AGE engine from Pacific Auto Salvage in Vallejo. I used basically a freshened stock bottom end that I balanced. I ported the head and installed new valve springs and seals. I used a turbo from a diesel Chevy Suburban. I custom made both manifolds. Since it would run on propane, I used an IMPCO 425 mixer (carb) from a 454 Chevy and a Borg Warner/ Century industrial 450HP LPG regulator with electric primer and a balance line. I modified the distrubitor to limit advance under boost to somewhere around 5 total degrees due to excessive pinging, even with 115 octane propane. I found that my exhaust temps were actually lower with the mixture slightly lean for some reason I don't understand. I had to limit RPM's to 9,500 because the regulator would starve out for fuel at 9,600.

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