ram pipes/velocity stacks

[Brooky]

I have a set of throttle bodies and im wanting to make a set of ram pipes for them. The throttle bodies are tapered from 40mm to 36mm with 38mm butterflies.
Would the attached design be any good?



Thanks, Tom

[SirYun]

i would think that if at all possible the rollback could be extended a bit more.

Blair and Cahoon have produced an interesting article on this very matter

http://www.profblairandassociates.com/p ... lmouth.zip

[RedLexus]

I started reading all that but got bogged down... could you perhaps summarise it for the less technically minded like me?

[SirYun]

basicly:

No radius :bad
Any radius: loads better (80% of the max gain) provided it about 3mm radius.
you don't need a full round edge but at least 60 or better 180 degrees.

if you design in a taper the need for a generous roll back and increased radius increases. without any end radius a tapered pipe is actually a lot worse than just a straight piece of pipe.
With the proper end radius, the taper does yield a gain of a few % over a straight pipe with a simple 3mm roll back.
i'd say R=6mm would be sufficient or for inside plenums a 180 degree with R= 1/2 D.
it looks like sliced in 1/2 donuts
an eliptical flare is quite hard to construct without the proper tooling and the gains over a simple large radius are quite small).

if you have problems with spitback and standoff ( a cloud of fuel in suspention just in front of the carb) i would suggest (based on the RET article and others (vizard comes to mind)) an exponential or eliptical flare.

using CAD and CNC those shapes should prove quite easy to make.

hope this helps

[Brooky]

Thanks for the replys people. Very helpfull

60mm O/D is about as big as i can fit to the throttle bodies, any bigger and i probably wont be able to get the filters on.

So would keeping the pipes straight and decreasing the radius but extend them round abit more help over my current design??

I do have a Vizard book but its stored away

Thanks alot

[SirYun]

Keep in mind this is theory and flow measurement..your design will work fine

If you are using filter socks and that is why you can't go bigger than 60mm:

have you considered using something like a large ITG dome filter to cover all the pipes?

that way you can integrate the radii in the plate and get some extra room furthermore the plate does the same thing as the more extended rollback and it will probably be worth something over socks on trumpets. and look cooler as well..

[Brooky]

Space is very limited (Fiat Cinquecento engine bay) so the dome filters might not fit either

Ive done a few more drawings with smaller radii but they roll around more

8mm radii


6mm radii


Thanks alot guys, v.helpfull

[SirYun]

it there a specific reason why the pipe is 60mm long (calculated lenght , space for injectors) ?

[Brooky]

A maker of induction kits/remapper recomeded I make them 60mm long. Thats the only reason.

[Guy Croft]

A good thread, well done.

GC Q&A is pretty quiet so I'll just add a bit. I have done a dwg of an optimised design for you, based on my own back-to-back dyno test exp (p131 GC manual if you have it), the flared and roll-back design shown was way better than anything else, I published the key data, the best was a Pipercross type (I've drawn a similar style).
The race engine on test with no rampipe (or velocity stack as some call it) was hopeless due to the sharp-edge entry losses at the carb face (injection systems behave the same way) as I have indicated at some length in my head prep dvd; the Weber OE rampipe was better than nothing but gave significantly less torque above the peak value. So a good design gives, well, to be honest, cheap power..

The most important features are:
1. The flare (or taper, see dwg) - enhances torque over parallel bore but outside 15 deg incl angle you are going to get separation and turbulence (flow loss). Flared is definitely much better than parallel.
2. The roll back radius - I've shown only R8 but a big rad is way better than a small one (you should make it as big as space will permit) and it should ideally extend right round to radius depth as shown.
3. Length - well, you're never going to get the perfect length without back-to-back testing or very advanced simulation. The optimum length depends on the cams, cam timing, bore/stroke, required powerband, header length and configuration (4-1/4-2-1) so most folks just go for something that fits and that's cool. But for a given engine it's pretty well true to say that if the pipe is longer, the peak torque will be lower down and vice-versa. (For given length of rampipe - well, believe it or not the same is true of inlet manifold length and header length, so you can see everything on a reciprocating unit is interlinked).

Worth mentioning that if you can mount the injectors in the rampipe and spray onto the the throttle plate it will give better top end power. Inboard injection is better at lower speeds (below peak torque-ish..) but given a choice always go outboard. The issue of fuel mixing (even with well atomised injection spray) is critical to good power (and something I have started exploring intensively as part of my head development work).


Hope this helps, well done for all having a go at this,

GC

[Brooky]

thank you for that Guy, very helpful.

Would it still work if i shortened them a bit but retained the 15degree taper to get an o/d of 70mm or under.??

Thanks alot

[Guy Croft]

Sure, you can do that no problem,

GC

[pacman]

What about turbocharged engines?
The principles for flow in the inlet should be the same for n/a and turbocharged engines, but is the increase in flow with rampipes the same for turbocharged/supercharged as for n/a?


PeterC

[Guy Croft]

In terms of reducing entry loss, yes, definitely worth using on pressure-charged engines.

As far as pressure wave effects are concerned in any inlet tract configuration inboard of the turbo or blower (ie: downstream) you need to think-thru the individual systems.

1. Turbo engines (on boost) and supercharged (which are always on boost): It is true that on valve opening, with the higher pressure differential across the valve throat (compared with a normally aspirated engine), you'd tend to get a more powerful negative wave propagating up the inlet tract. That, along with it's subsequent harmonics, is the one that really makes engines light up. But, whereas on a n/a engine that neg wave would reflect as a positive (ram-charging) wave when it sees atmospheric at the open end of the rampipe (or end of the runner where it enters the plenum), on a boosted engine it sees high pressure directly on the other side of the valve and will tend to be reflected, I guess, almost immediately without carrying any charge with it. Does that matter? No, because the charge is being blown in by the boost medium anyway. You may well ask, 'are there going to be conditions of boost where the negative wave is in fact beneficial in enhancing the charge momentum?', well, maybe..!

2. Turbocharged off boost, well the engine is effectively working just like a normally aspirated unit so the rampipe length will definitely have a beneficial impact during that rather weak torque phase.

Most turbocharged engines run a common plenum with single throttle plate and fitting rampipes (in this context, I'm not just talking about radii) into that architecture can mean building a special airbox. My view on that is given that modern plenums have pretty good runners and radii, it wouldn't be worth it, excepting in the case where the OE plenum just won't fit in the car. Where individual throttle bodies are used, well, sure you'd want them and predicting the best length is always going to be rather 'hit and miss' unless you can model it on advanced software, in which case you'd probably be best treating the engine as a (low compression) normally aspirated one with very short header lengths. Rampipe length is critically affected by header length and configuration.
The 4 cyl supercharged engine might run, say a contemporary 4-1, 36" long header (and in my exp independence 4-1 types are the best). What about rampipes there? It depends how the supercharger is oriented. If the carb is mounted outboard of the blower (suck thru) a rampipe is definitely needed to help overcome the aggressive reverse flow/pulsing out of the blower intake side.
If the carb system is inboard (blow thru) - injectors likewise - whether mounted on split or paired runners (with a common plenum, to feed from the blower) again, for the same reasons outlined above I think there is going to be no more benefit from a rampipe in terms of length than with the turbo unit with the same setup, except, again, to note that a good entry radius helps flow.

That's my take on it, other views based on dyno exp are very welcome, it is a rather opaque subject, mainly because few but the biggest firms can afford the software modelling to examine this and feedback from them is, as far as I know, pretty well unpublished.

GC