Exhaust / intake flow ratios on LS heads

[randy331]

does the in/ex flow ratio have any meaningful bearing on the in/ex duration split/backsplit/no split you might choose?

No.

Randy

[Geoff2]

Factory Pontiac iron heads often have E/I ratios in the 70s, with 80% being quite easy to achieve with a little porting. Highest I have see for ported heads was 94%. It is not that the exh port is so good, it is because the intake port flow is so low! In contrast, 460 Ford alum aftermarket head was 49%. So, pretty obvious that a cam for a Pontiac might be single pattern, or even reverse pattern [ less exh duration ] , while Ford head will need extra exh duration.

[mag2555]

Hell, I can yank 220 cfm out of the badly bent up iron Pontiac center Exh ports with just one hours work, where as I have spent a combined 3 hours of grinding and flow testing on a iron Ford 460 late model head head just to yank 184 cfm out of it and this was aided by stepping up to a 1.75" valve!

[raynorshine]

If the engine would run better with a higher ex to in flowz ratio, just lower the intake flowz till it has the flowz ratio you desire, then it will run better.

Randy

that's exactly what i was thinking!!

[Newold1]

I am not going to act like an expert here, because I am not one when it comes to cylinder head port flows, there actual effects alone on engine power and torque production. I realize and believe as others that the engines complete air fuel induction, combustion and exhaust evacuation systems are one big system that along with the camshaft pretty much determine the power and torque output of the engine.
When some say the exhaust/ intake flow ratios do not bear any real effects on an engine's performance I have to disagree especially when I am considering the LS and LSX engine performance builds.
Granted these engines make great power in stock configurations and in performance builds, especially when compared to past years with the old small block chevy engines.
What makes me question the E/I ratio of percentage of intake flow that the exhausts generate is that the duration spreads that all of the aftermarket is using and promoting for these performance builds in "Normally Aspirated" applications without performance adders or nitrous use are spreads that appear wider than are used in many other performance engines such as the BBC or many others. This increased exhaust duration has to affect other factors in this engine especially those that change the relative timing of intake opening and closings to the exhaust openings and closings. We all know this always seems to have an effect on dynamic compression, cylinder filling, overlap, scavenging and reversion in all 4 cycle gasoline engines.

My thought is what effect would a higher exhaust flow relative to intake flow on the LS and LSX performance engines have? Why can't an LS-LSX exhaust port flow more relative to the great flows that exist especially in aftermarket or improved CNC versions of the heads? Is it just exhaust valve or port size? Is it Valve angles or placement? These improvements are made all the time in other engines and even in the SB2.2 small block engines the E/I ratios are markedly better and they seem to make better HP per cubic inch because of that in N/A uses.

This is the reason I offered this question and asked for input on this post. As I always feel , teach me! - Show me! I am open to the reasons, but there has to be a reason, but not necessarily an answer I guess?

[groberts101]

does the in/ex flow ratio have any meaningful bearing on the in/ex duration split/backsplit/no split you might choose?

well sure it does.. don't inflate it but don't trivailize and downplay thinsg to the point that they make 0 difference either. Change the flow windows size or volume potential as it relates to the cylinder volume/induction tract and the cam timing needs to be re-optimized. Some heads make more power with ratios you'd not expect.

IOW, increased exhaust flow volume and efficiency/whatever that means.. changes its relationship with the induction tract. Aside from pumping loss compromises.. I usually go for as high a velocity as the SSR and primary pipe sizing will possibly handle and adjust tube/collector size to make the most of it. Less cam timing and smaller merge are the payoffs. Course.. depends on the combination of parts and powerband width requirements too. Personally.. I've been listening to Ed Henniman(headers by Ed) and Calvin Elston(Exhausted on this site) long enough that I've started designing most of my ports from the outside in. Calculate needed primary pipe size for the flow/power level(obviously somewhat chassis restriction dependant too).. THEN.. translate that primary pipe size from the flange into the port. Do the valve/seatwork and then connect the dots in a decent enough way you won't lose much between the two points. 230+ exhaust cfm coming out a smaller pipe during a static bench test = plenty decent power capability.

My quick answer to this threads title is that the intake side of these head designs have been so highly refined and valves sized to the point that you'll never reach a really high bias unless you purposely port the living schnitzel out of the exhaust side and leaving the intake side untouched. But as these guys have already mentioned.. that motor sure wouldn't be a barnstormer at the track. Might sound a little burlier at idle with noticeably better cruise mileage though! lol

PS. nobody here has ever purposely left an intake side untouched and concentrated all effort only on the exhaust side? Even fooling around with higher ratio rockers too? DV had it right long long ago in one of his old books. Reduced pumping losses in and at the exhaust port alone has somewhat similar but slightly better than only installing a free flowing exhaust system. Put an oversized 1.6" exhaust valve, nice VJ/throatwork, and remove those smog casting bumps on any Ford or Chevy and follow it up with a rocker ratio increase. You surely won't be saying it does next to nothing for an engines powerband anymore. Not a far stretch to imagine that until that point of diminsihing return is fully exploited.. cam timing and lift range adjustements could be leveraged for even greater gains. Won't even get into the improved scavenging potential that could be leveraged.

[Morgo]

The exhaust valve in LS-head is so close to chamber wall that not much flow there cannot be.So it´s more lift..

[Newold1]

I realize that on small bore LS engines with let's say bore sizes from 3.865 to 4.060" the stock location of the standard 1.560-1.605" size exhaust valves are about max to limit shrouding from the cylinder wall but how does one explain why on an LSX bore size of let's say 4.125 to 4.185" bore sizes the heads might benefit exhaust flows if a 1.70" exhaust valve and a larger exhaust port size would increase the exhaust flows and raise the E/I flow ratio percentages? When I study all the aftermarket LSX cylinder head offerings as well as all the CNC'd. and ported LS or LSX offerings will not produce E/I ratio percentages above about 68% for exhaust flow versus intake flow. This also includes the heads where the valve angles varied between the stock 15 degree and even those with 11 degree valve angles. The thing that makes me scratch my head is that even with the greatly improved intake flows on LSX-DR heads for example where the intake flows are now up to about 430 cfm @.700 lift which is a 80% flow improvement over a stock LS1 Gm head that flows about 240 cfm @.700" lift, the exhaust port on this same LSX-Dr head only flows about 250 cfm @.700" lift or just a 20% flow improvement over a stock LS1 exhaust flow of 210 cfm @.700" lift. My question is in a normally aspirated LS or LSX performance engine does this low percentage increase in exhaust flow have an affect on power and torque production "POSSIBILITIES" in these engines or does the camshaft profiles that are being used which show as much as a 35-40 degree duration splits on exhaust valve timing totally negate the decreasing exhaust flow percentages, or is more hidden power being "Left on the Table" from not improving LSX cylinder heads exhaust flow numbers? This is the question I am trying to answer more specifically in my head.

Maybe Darin Morgan or some of our other cylinder head specialists may have already addressed this question or could share their expertise on why it is as it appears and why it either does or does not really make any difference?

[70GS455]

Keep in mind the environment inside a running engine. The exhaust gas is 1000+ deg F while the intake is ambient (roughly). So the densities are vastly different. The lower density exhaust won't move as much mass anyway even with 100% I/E. So, not sure how important or revelant that is.

[Newold1]

Then why would these current LSX naturally aspirated engines have such big intake versus exhaust splits on their camshafts ?? If we are saying it does not matter and increased exhaust flows relative to increased intake flows are not needed why would they increase these normal duration splits?? Let's think about that more carefully.

[pamotorman]

if this was necessary I am sure the folks that spend money to bring out a aftermarket head would have done this.

[RW TECH]

why would they increase these normal duration splits?? Let's think about that more carefully.

Normal to what measure?

[randy331]

Let me try to understand this??

We take a static flowz measurement of the ex port at peak lift, with some sort of short pipe on it (or maybe no pipe) at a lift the engine will have at maybe 110* ECL or so, at one fixed depression of 28". We then divide that number by a static flowz test at a fixed depression of 28" of the intake, at a lift the engine will have at maybe 106* ICL with no manifold/carb etc. on (or maybe so), and the results of that math determine how much before BDC we will open the ex valve, and how long after TDC we will close it, and those ex open/close positions will be "as compared to in duration" or intake valve open/close that were based on what ? FlowZ????


Seems simple enough, just a couple questions??

How do you go about deciding the FIRST valve event??
Which one is first?
How do we decide the intake needs reduced, or ex needs more if ex/in flowz ratio is lower than "optimal" whatever optimal is ???
How do we decide the same thing if the ex/in flowZ ratio is higher than "optimal" whatever optimal is ??
How do we decide the intake is "good" and the ex is weak, or the other way around??
What's "optimal" flowZ ? I mean there's gotta be a standard to compare to for something to be weak or strong.
What do we do if bolting the intake on drops cfm by 25???
Does that count??
How about if a 1.8" primary tube flowZ more than a 1.75" tube on the flowZ bench, but the car is faster with the 1.75" header?
Do we add or reduce ex/in duration??

Randy

[shawn]

Lots of assumptions being made that you can actually gather any useful data "flowing" an exhaust port.....

shawn

[Newold1]

Here is a fact on the LS and LSX aftermarket performance and racing version heads. On quite a few of these heads flow about 300cfm @.650 lift with a 2.10 intake valve and flow about 240 cfm @.650 with a 1.60" exhaust valve. Quite a few of the racing version LS heads produce about 450cfm @ .650"lift with a 2.10" intake valve and about 250cfm @.650" lift with a 1.60" exhaust valve.

These higher flowing intake LSX cylinder heads represent about a +50% increase in intake flows but only about a +5% increase in the provided exhaust flows.

Question: On a naturally aspirated LSX engine does this mean that the exhaust flows that the engines equipped with the substantially lower flowing on the intake side cylinder heads do not need all their possible exhaust flow and it's just the lower level of intake flow they provide that is the single greatest factor in lowering the horsepower they are capable of producing? Seems that way if you just look at the numbers from this standpoint.

Did GM originally over design the exhaust flow characteristic of these heads to start with or is exhaust flow just not as big a factor as we have sometimes been led to believe?

When I consider these points and questions I am still drawn back to the point that in the BBC heads and engine performance improvements the exhaust flows of the aftermarket heads tend to increase exhaust flows nearly at the same percentage increases as those increases that are on the intake side of the heads? This is also just considering cubic inch and approximate bore sizes nearly the same in the 427-496 versions of both type of engines.