Intake volume and its effect on horsepower 3

[ErikU19]

I currently have a 5.4L DOHC motor with a 1700CFM centrig blower on it. The intake on it is a short runner intake with a volume of 2.3L (total). My question concerns whether the volume of the intake would have a detramental effect on the performance on the engine being that the intake volume is less than half of the displacement of the motor. Would i be better off with an intake with a larger volume? say at least 5.4L?

I have read that a general rule of thumb is to have an intake volume equal to that of the displacement of the motor.

Thanks in advance!

 

[Azmio]

Erik

Larger intake surge tank volume does make a naturally aspirated engine generate more power. However, the response is longer because the tank requires more air to fill up the volume.

For turbocharged or supercharged engine, you do not need to have large surge tank volume.

 

[Lou Scannon]

Azmio, I'm not saying I don't buy your comment about forced induction engines, but what is the theory that makes them different from NA engines in this regard?

 

[Warpspeed]

What can sometimes happen when an original n/a induction system is used with a blower, is that cylinder to cylinder air distribution problems can be greatly magnified.

What works well n/a may not have such equal air distribution at three times the air velocity and flow. Exhaust gas temperature readings will tell you if all is well or not.

A different plenum may sometimes help, but you need to understand and investigate the problem first before trying to fix it.

 

[patprimmer]

Tuned length manifolds are dependent on the speed of sound in the manifold. This is fairly constant in a NA motor at WOT, but varies considerably in a supercharged motor at WOT.

Also, slow engine reaction times makes cars harder to drive. This is more critical at higher power to weight levels, so is normally more critical on a supercharged car.

i.e. small power gains are relatively more critical on a low power to weight car, but small drive-ability gains are more critical on a high power to weight car.

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

Last time I checked, the speed of sound was a function of temperature only. I wouldn't expect intake manifold temperature of a boosted application with decent intercooling to vary a great deal more than an N/A application.
The drivability comment makes some sense. Do you know of intances where this was a reason not to use tuned length ports in a boosted engine?

 

[crysta1c1ear]

A speed of sound caculation:-

Temperature 15° centigrade
convert to Kelvin
T=15+273.15=283.15

gamma=Adiabatic index of air 1.402

R=Universal Gas Constant 8314.47 Joules per Kelvin per kmol

M0=Molecular weight of air 28.94

R/M0=287.31
gamma*R/M0=402.8
T*gamma*R/M0=116068.04
sqrt(T*gamma*R/M0)=340.69

Speed of sound at 15°C is 340.69 metres per second by my calculations.
1117.74 feet per second
762.1 miles per hour

==

So while the formula seems to be independent of most things apart from temperature, gamma, the adiabatic index of air, assumed to be constant, is defined as the ratio of a couple of other constants, which I believe actually vary a bit.

And how accurate is the formula?

 

[GregLocock]

Oh, that formula is more accurate than any dyno you will ever use. Gamma is a very stable property of a given gas at normal temperatures, it is somewhat related to the geometry of the molecules, from memory (fundamentally).

I'd guess it falls apart at about the point where the gas starts to behave in a non ideal fashion - very cold.


Cheers

Greg Locock

 

[patprimmer]

I have seen many turbo applications where the manifold volume was kept on the smallish size, including short runners. I can only speculate on the reasons, these being packaging, response time, weight, cost, air flow distribution.

I have used minimal length runners on a Roots blower manifold on a SB Chev in a drag race. The reasoning was to keep the blower as low as possible without impacting on power and at the same time improve aerodynamics.

Not all forced induction engines are intercooled, and not all intercoolers are perfect.

Most positive displacement blowers are not used with an intercooler as intercoolers do not effect the VE in this case, but they still do effect the detonation threshold and parasitic losses to the blower.


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Lou Scannon]

I hear you regarding the roots blowers. I'm on the page that an aggressively tuned induction system is less important on a supercharged engine, since tuning/adjusting the boost system is so much more effective than ram tuning. Except for a racing application that always runs in a narrow rpm band, I would choose short, non-tuned intake runners for a supercharged application.
A contrary example is the Ford 2.3 litre turbo engine of the mid-1980's, which had 21" runner lengths in its tuned intake manifold. Many enthusiast owners have cut these down to about 1/2 that length, taking advantage of the 2-piece design of the manifold.

 

[Azmio]

Hemi

We did try several intake plenum volumes. As I wrote before, the big one produces the biggest horsepower.

As for the intake manifold length, NA depends a lot on the tuning effect from long intake manifold to generate higher volumetric efficiency.

With boosted engine, the intake plenum has positive pressure and the positive pressure range is quite constant for reasonably long rpm band.

So my guess is, for boosted engine why do need long intake manifold length for ram or tuning effect? With positive pressure in the plenum, one does not need to have a large overlap to make use of the scavenging effect. You will end up wasting fuel going into the exhaust port.

Long intake manifold will have longer surface frictions. Long intake manifold is also known to maintain good air flow at low rpm but with supercharger or mild turbo positive pressure, you can have good airflow at the valve seat even with short manifold.

Lastly, long manifold is known to choke the air flow after certain rpm. With boosted engine, it is not a good idea to restrict the ample flow of air by having long intake manifold.

 

[patprimmer]

Good one Azmio

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Azmio]

To cut it short, if you can maintain constant positive pressure inside the intake plenum for reasonably long engine rpm range, it's better to have short intake manifold length.

For NA engine, you need relatively longer intake manifold for :
1) Good air flow velocity at the valve seat in low rpm especially at idle
2) To use the long intake manifold for tuning and ram effect to get high volumetric efficiency

 

[larrycoyle]

ErikU19

Sorry to wade into this discussion so late but did you get your question answered to your satisfaction? There are several factors other than total volume you have to consider for a manifold to meet your needs.


Larry Coyle
Cylinder Head Engineering, LLC

http://www.cylinderheadengineering.com

 

[Azmio]

Larry,

That's interesting, appreciate if you can share with the rest of us