Intake to Exhaust Head Flow Ratios

[SappySE107]

What is ideal, and why? How is this conclusion come to (powerband, max rpm?). Ive read so many different ratios for different heads but nothing on explaining how companies come to their conclusions.

[Guest]

I've heard different things as well, along the lines of 60% exhaust flow to intake flow, on a mild N/A engine, 75% exhaust flow to intake flow, for a built N/A engine and in the area of 80% exhaust flow to intake for power adder engines, this would all be relitive to the entire area under the curves in comparison. It kinda makes sense when you think about the projected air flow going, the efficiancy of each type of engine and what all will need to come out in the end.

I'd be interested in reading/disscussing more.

[SappySE107]

I havent had ANY luck finding definitive answers. What you said makes perfect sense. The whole reason I want to know if because DKOV is selling DOHC head stating 65% for exhaust is perfect. Sounds really low IMO, especially for a DOHC engine. I would expect the powerband dictates something here, with higher ratio for higher RPM use. thats a guess though. Im still looking for info though. I didnt see anything in the head porting books I have...at least I dont remember anything.

[RacerX11]

Here's my take on it: I think the ratio is mostly dependant on the efficiency of the combustion process. There are really two ways to get more power out of a NA engine. The most common way is to increase the amount of air flowing into the engine (volumetric efficiency). With more air flowing in, you need more air flowing out, so my guess is the ratio shouldn't change much based on this.

The other way is to increase the efficiency of the combustion process (IE, raising the compression ratio, increasing quench, higher octane fuel, advancing timing, etc.). You aren?t moving more air into the engine, you are simply squeezing it harder, burning it better, and getting more energy out of it. For this case, you would need a higher ratio of exhaust flow to get the higher energy content out of the engine.

Most race engines have both the volumetric efficiency optimized, as well as the combustion process, so they would need the higer flow ratio.

Marty

[SappySE107]

Higher octane is more of a fix, not a power adder.

Ok, so how does the ratio change on VE changes or compression then? I know how to make more power, but with head porting there has to be a reason for ratios.

[RacerX11]

True, higher octane isn't a power adder, but it allows you to run more compression and more advanced timing, both of which are methods of increasing power without changing the VE of the engine.

I guess the point I was trying to make was that for an engine with just VE enhancements (outside of the head), you would want to keep the ratio pretty close to stock. If you increase the compression ratio, you would want to increase the exhaust flow ratio to compensate for the higher combustion efficiency.

Marty

[SappySE107]

what are you basing this on though? I want to know the math, or experience with results in changing the intake/exhaust flow ratio of the head. I doubt GMs stock ratios are optimal, especially considering the casting flaws and sharp edges around the bowls as heads come stock. In their original design, did they even consider the flow ratios? I mean, they came up with the valve sizes from somewhere, either by what fits, or what works best.

Pushrod as an example, the exhaust valve is 82.5% the size of the intake valve. How did those values come into use over 1.6/1.3 valve sizes. We all know the new values are better because the valves are larger...Im sure I have seen something about valve sizes for an engine, but that is just a similar discussion to the actual flow (but worth discussion all the same).

[DarkKnight]

I think that the whole reason for having smaller exhaust valves (compared to intake) is to increase exit velocity of the combustion gasses. That in turn should help scavange any remaing gasses right after TDC. Then with a high enough velocity, when valve overlap occurs, the suction generated by the faster moving exhaust gas will draw in fresh intake charge. It seems to me that this effect is more important at lower RPMs than higher. At higher RPMs the combustion gasses have much greater velocity and so it isn't necessary to try to increase that with smaller valves. At higher RPMs smaller valves become a restriction rather than an enhancer. And since typically built N/A engines usually run radical cams that are only good at higher RPM use, they don't need a high delta in the I/E ratio. So, choose your ratio based on your driving habits .....At least thats what I think I meant.... If I'm way off, don't make fun of me...

[ezrollin]

Here's a little info. on exhaust / intake that might be helpful. Performance E/I are in the 75-85% range and too much on the the exhaust side can be harmful. If you have the intake and exhaust flow rates you can divide the exhaust by the intake to get the %. The stock 350 vette H.O. head has a very good E/I, 82% although the overall flow rating isn't real high, 186cfm/152cfm. A cyl. head with a good E/I will usally perform better with a single pattern cam. Don't know if this applies to a DOHC head.

[Gemmer]

Here's a little info. on exhaust / intake that might be helpful. Performance E/I are in the 75-85% range and too much on the the exhaust side can be harmful. If you have the intake and exhaust flow rates you can divide the exhaust by the intake to get the %. The stock 350 vette H.O. head has a very good E/I, 82% although the overall flow rating isn't real high, 186cfm/152cfm. A cyl. head with a good E/I will usally perform better with a single pattern cam. Don't know if this applies to a DOHC head.

Thats cool and all...but I think hes asking what the logic behind the assumption that 75-85% or whatever is better than anything over or under

[Corpse]

Well, there are a few variables behind the % though. You have to remember that A/F mixture and how well it is burned will have an effect on what size the exhaust charge is. Of course there are other factors aswell.

Look at it this way though, the valve size ratio is important, but, the valve size is also very important. You may have a ratio of 80% but, on the other hand, the valves may bee too small for the application.

I've been wndering if the valve sizes in our engines are up to par. I don't know about the T-DOHC setups but, a 3.4 with an overbore is getting close to the size per cylinder of a 305.

A 350 with 1.94 intake valves is seen quite often, but, many people go larger. The 1.72" valves in most of our heads seem fine but, I think if you are trying to get a lot of power out of a N/A situation they may prove to be a restriction at high RPM's.

More than likely, the stock valve size was chosen for a few reasons. More than likely, fitment, emissions and torque.

I can't say that I know too much about the ratio between the I/E valve sizes but, there are many other factors that effect the necessary size of the valves.

I'll try to find some info, but, I've read some numbers on the I/E gas size for optimum combustion. I would just change the size by a certain percentage, just figuring, who is really getting perfect combustion.

-Dave

[SappySE107]

Im sure its a very comlex situation for determining the sizes themselves, let alone the ratio. Anything relating to this topic is very helpful, and hopefully in the end we can come up with the reasoning for the 80-85% flow ratio. For the pushrods, the 1.94" intake valve is gonna be tough fit. Aren't the 305 valves inline, instead of splayed?

[Will'sFiero]

I'm going to take a stab at this...

The ideal flow ratio idea is for using a single pattern cam, which is ideal for performance. A dual pattern cam is not ideal for performance because one side is always smaller than it might be with ideal head flow.

The intake port has atmospheric pressure pushing gas into the cylinder. The mixture is pushed into the cylinder by the ~15 psi pressure differential with an average moles/crank degree rate that mimics the engine's VE/RPM curve.

The mass of mixture in the combustion chamber does not change materially during the combustion process, so the exhaust port has to have the same mass/unit time flow characteristics as the intake port.

The difference is that the pressure difference across the exhaust port is different than the differential pressure across the intake port.

In order to get the mass of gas out the exhaust in the same time as it came into the cylinder, and not have "wasted" flow capacity that would lower port velocity, the exhaust port needs to be smaller than the intake.

The difference with different compression ratios is this...
An engine with 15:1 compression will have less waste energy in the exhaust than an engine with 8:1 compression. Because the 15:1 engine has less exhaust energy, the differential pressure across the exhaust port is less and the exhaust port must have a larger fraction of the intake port's capacity in order to get all the exhaust out in the required time.

In a similar fashion, we can look at blower engines... The pressure pushing mixture into the cylinder is higher than with a N/A engine, but the pressure difference across the exhaust port is the same as N/A... so the intake port can be smaller relative to the exhaust (or the exhaust larger relative to the intake) to match flow capacities.

LMK if you don't understand something...