Matt B (fs)
Matt B (fs) UltraDork
8/27/20 2:50 p.m.

So I'm going down the rabbit hole of a theoretic engine build and I'm noticing a lot of aftermarket performance cams have a dual pattern design (intake and exhaust are different).  Searching around the web for the strategy behind these doesn't seem to be well understood.  Maybe I'm looking at the wrong sources of info.  It wouldn't be the first time.  The best explanation I've read has been some domestic (read: V8) motors benefit from a longer exhaust duration, mostly due to how poorly the exhaust side of the heads flows.  So much so that they're now seeing gains from going back to a single pattern design with better flowing systems.

Ok, but I'm looking at building a little Japanese four cyl.  Does any of that apply?  Well, turns out a lot of companies making cams for the 4AGE also make dual patterns, but in their case it's the intake side that has more duration & higher lift.  I guess the reverse could be true and the 4AGE 16V intake cfm sucks, but that seems tenuous at first glance.

So... what do you guys think? What's the use case for dual pattern when the stock cams are single?  Why would you run a more aggressive intake profile vs exhaust?

frenchyd
frenchyd PowerDork
8/27/20 3:01 p.m.

In reply to Matt B (fs) :

The short answer is yes. Dual pattern cams are a way to compensate for  design compromises. 
 

To see if you need dual pattern build yourself a cheap( free ) flow bench.

take a bare cylinder head and Hook your shop vac up to the intake port and hold the valve open  however much the max lift is.  Rude and crude you can put some rubber around the valve stem  and use a pair of locking pliers ( vise Grips) to hold it that open. 
  Use a unisyn or some other cheap flow  meter ( British car guys all seem to own one)  to measure how much flow 

Repeat for each cylinder 

Then compare flow numbers. but use a little judgement here. Is it past time to do a valve job? One valve bent or burnt?   
If they all flow the same  go to the exhaust side and repeat. 
If they are all the nearly the same. You don't need dual pattern cams. 
exhaust flow should be 80% of intake. 
The logic is it's harder to suck air in that blow it out with the piston coming up. Plus the fuel occupies  space that the  the exhaust doesn't 
 

 

SkinnyG (Forum Supporter)
SkinnyG (Forum Supporter) UberDork
8/27/20 3:07 p.m.

But isn't the exhaust supposed to flow less?  I have never seen intake and exhaust flow the same.

frenchyd
frenchyd PowerDork
8/27/20 3:09 p.m.

In reply to SkinnyG (Forum Supporter) :

80% of intake is the formula. 

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
8/27/20 3:14 p.m.

Yes, they should be different.

On one side, you are trying to ingest air (~79% N2, 20% O2 and stuff) and fuel at 1/14th of that but high density, at pressures from .2ATM to 3ATM (more for more extreme boost).  And thet temps tend to be near ambient up to mabe 200F/100C.  

On the other side, you are trying to push out a gas that's now  70%N2, 15%CO2, 14%H2O, and stuff and pretty high pressure and very high temps- like near 900-1800F or 500-1000C.  

Gas properties are very, very, very different, so the needs of the flow path are incredibly different.

And that is ignoring the want to balance the amount of peak flow (lots of cam) with overlap (exhaust into the intake) with acoustic dynamics in both the intake and exhaust, PLUS any trade offs to game the physics by lowering the charge via pushback (increasing expansion ratio to compression ratio).  

Patientzero
Patientzero HalfDork
8/27/20 3:33 p.m.

10-15 years ago we were running bigger exhaust cams on SR20's and RB25's.  256 intake/264 exhaust or a 264/272 setup.  Everyone said it helped spool but I just ran what the fast guys were running.

frenchyd
frenchyd PowerDork
8/27/20 3:47 p.m.
alfadriver (Forum Supporter) said:

Yes, they should be different.

On one side, you are trying to ingest air (~79% N2, 20% O2 and stuff) and fuel at 1/14th of that but high density, at pressures from .2ATM to 3ATM (more for more extreme boost).  And thet temps tend to be near ambient up to mabe 200F/100C.  

On the other side, you are trying to push out a gas that's now  70%N2, 15%CO2, 14%H2O, and stuff and pretty high pressure and very high temps- like near 900-1800F or 500-1000C.  

Gas properties are very, very, very different, so the needs of the flow path are incredibly different.

And that is ignoring the want to balance the amount of peak flow (lots of cam) with overlap (exhaust into the intake) with acoustic dynamics in both the intake and exhaust, PLUS any trade offs to game the physics by lowering the charge via pushback (increasing expansion ratio to compression ratio).  

Very well said!  

Curtis73 (Forum Supporter)
Curtis73 (Forum Supporter) GRM+ Memberand MegaDork
8/27/20 6:38 p.m.

Indeed.  Frenchyd hit it.

Many manufacturers (especially the big V8s from the US) came with some compromises on the exhaust side.  In an SBC for instance, the exhaust port has a very short path with a relatively sharp curve.  Most chevy heads aren't up to the task of providing that 80% golden ratio.  Some are as bad as 64%, but most fall into the 70-75% range.  The crutch is to hold the exhaust valve open for a proportionally longer time to give it adequate means to push out what it needs to.

Smaller hole, so to get the same mass  through it, you leave it open for slightly longer.

Matt B (fs)
Matt B (fs) UltraDork
8/28/20 10:26 a.m.

Ok good stuff here (as always).  Thanks to everyone who's replied so far. I do want to address a couple of posts directly though.

frenchyd - cool DIY flow bench idea. I assumed I would need something fancier than that.  I've also seen a technique where you can use a flow bench to determine your maximum effective lift, but that's getting off on a tangent.  This guy on youtube explains it in detail, plus other stuff.

alfadriver - also appreciate you breaking down the atmospheric science.  Some of that makes sense to me, but I'm not going to pretend I can extrapolate all of the consequences to the 4 stroke cycle.  That said, thinking about this in layman's terms it seems that since all molecules are simply chemically converted in the combustion process that we're not losing significant volume for a given pressure/temp.  In fact, even with the same molecular mass, pressures should be much higher due to vastly increased heat.  It's been 20 year since college so I'm sure I'm overlooking something and oversimplifying, but if the gist is true then that begs a couple of questions.

  • Why would any manufacturer design single pattern cams?  Same old story of engineering and manufacturing costs?
  • Is the intake side of the 16V 4AGE so awful that it's actually advantageous to do the opposite of what the combustion cycle "wants"? Seems antithetical to what the engine was designed for, but I guess it's possible.

    I realize these questions probably don't have easy answers since they're influenced by the decisions of designs of individuals not on this board.  Just trying to tie up loose ends. 
asphalt_gundam
asphalt_gundam New Reader
8/28/20 11:50 a.m.

I'm no cam expert but I built enough high performance engines and spent lots of time picking cams (or lobes) for many of the engines. Even ordered the first 4 pattern cam for a small block Mopar to put in a customer engine.

Single vs Dual pattern: IMO a single pattern seems like it was just easy and no real effort went in R&D. I come to this conclusion because most cams that did get extensive R&D are dual pattern. The dual pattern is less about flow and more about the timing of peak flow/velocity and its ability to generate favorable overlap conditions. 

Someone much smarter than me at developing racing heads and cams once said "don't worry about the exh, It flows enough, focus your time on getting the air in". Think about how hard it is to get air into a cylinder when it's relying on a few inches of vacuum to do it...now how easy is it to get the exhaust out when it has a hundreds if not over a thousand PSI forcing it out. There are studies out there that point to 90% or more of the exhaust flow is completed in the first few degrees of exhaust valve opening. Long before it ever reaches peak lift. So the logical question is why are there even big exhaust lobes in use? Why not just have small lift and duration exh? Well it has to do with getting air in the cylinder on the intake stroke. Think about it...it there's still 10% exhaust gas in the cylinder...you now have that much less room for fresh air/fuel. If the exhaust valve is already closed, then there is no overlap effect. Then there is the residual pressure that would be there if the exhaust was closed before reaching TDC and starting the intake stroke. That pressure would try to go up the intake as soon as the intake valve opened up. (this effect is also why you don't want a restrictive exhaust system)

This is not to say that the exhaust flow doesn't matter, just that its only requirement it to be enough in regards to flow. What's way more important is the velocity and pressures involved. Every flowed a head backwards? A port that resists reversion by say 30cfm is going to make more power than one that flows 5cfm more but only resists reversion by 10cfm. During R&D if I made a back-cut angle change to a valve that picked up 3-5 cfm on the intake and ALSO lost a few cfm of reverse flow, that is a good improvement! If it gained more reverse flow then I'm testing a different angle or cut width or whatever I need to. Air moves through an engine like AC current (back and forth) but we really only want it to go one way. The entire engine combination affects this. Especially the cam, head, intake, exhaust.

What's optimal? Depends on everything in you combo. Change the intake manifold? Its likely the perfect cam is now different too. But since we're not building Formula 1 engines getting in the ball park is good enough for improvements. Get your entire engine combo in the ball park so that it all works together and now you have a combo that works. Perfect example is crate engine. Lets say it makes 450HP, I could pull it apart do a little extra machine work, change the valve job, add a back-cut, pick a different cam, maybe change the intake or port it, clean up the ports in the head, and many other small changes. Now it makes over 500hp, gained torque everywhere, idles better and runs smoother. With nearly all the same original parts. 

Getting back to the main topic. Single vs dual pattern is barely looking in the rabit hole. Want to go deep you need to start looking at and figuring out how to calculate pressures, velocity, timing of those in relation to the cam, the head, the intake, the exhaust. Then now lets look at lift, duration, overlap, centerline....change one, it all changes. Might be a small change or a huge one. Not enough for you? Lets look at asymmetrical lobes (different opening and closing ramps) and 4 pattern cams. 4 Pattern cam? On a V8 to get the cylinders evenly running it has been found that the longer runners (1,2,7,8 on a GM) need 2-8 degrees more duration to get the same power production as the short runner cylinders. A whole n'other can of worms to look at when designing on of these.

 

frenchyd
frenchyd PowerDork
8/28/20 12:13 p.m.
Curtis73 (Forum Supporter) said:

Indeed.  Frenchyd hit it.

Many manufacturers (especially the big V8s from the US) came with some compromises on the exhaust side.  In an SBC for instance, the exhaust port has a very short path with a relatively sharp curve.  Most chevy heads aren't up to the task of providing that 80% golden ratio.  Some are as bad as 64%, but most fall into the 70-75% range.  The crutch is to hold the exhaust valve open for a proportionally longer time to give it adequate means to push out what it needs to.

Smaller hole, so to get the same mass  through it, you leave it open for slightly longer.

Yeh, what few really understand is the complex compromise made when adjacent cylinders firing order are 90 , 180, or 270 degrees from each  other. In part that causes some of that second order harmonic issues.  
It's also why a 60 degree V12 with adjacent cylinders  firing 120 degrees  Apart feel so smooth and there is so little gain with headers. 
 

Paul_VR6 (Forum Supporter)
Paul_VR6 (Forum Supporter) Dork
8/28/20 1:01 p.m.

To quote someone much smarter than I am, "it's complicated." The general trends that exist for some of the examples for 2v motors above still hold true for 4v but they are generally more likely to have good exhaust flow and may already be factory 'over cammed' on the exhaust side, and it's just for design ease and there's almost no downside to doing this (especially on dual vvt engines).

The original VW 16v engine is the opposite with a perpendicular exhaust valve angle and a very short, sharp turn coming out of the port. On those engines a bigger exhaust cam helps more than it should, but similar performance can be done with similar duration cams, better valve job and some port work. 

That being said the cams I just had made have four different lobe profiles based on intake and exhaust length (VW VR6 with long/short ports and 'not quite equal' port lengths). 

alfadriver (Forum Supporter)
alfadriver (Forum Supporter) MegaDork
8/28/20 1:16 p.m.

In reply to Matt B (fs) :

The big thing is about balancing compromises.  

Do you have an exhaust manifold, or headers- that matters.  Do you have an intake manifold or ITBs?  What about the volume of the manifold, or even the volume of the area ahead of the throttle body.  Are you going to use VCT on the exhaust manifold to make EGR?  Are the valves direct acting buckets, or some kind of rocker?  If it's a rocker, do you have different lift ratios between intake and exhaust?  What about valve size?

Single cam = cheaper production- that's why you would have one cam profile for each side.  Sometimes that's lost in the thousands of dollars of the engine, sometimes that's a big deal for a couple hundred dollar engine.

For sure, thanks to the major property changes in the gasses, the exhaust does not need the same net area to exhaust vs. intake.

Matt B (fs)
Matt B (fs) UltraDork
9/8/20 7:46 a.m.

In reply to asphalt_gundam :

I hadn't thought about flow vs. reversion, but that makes sense in terms of net flow.

Also (to everyone here), I understand that the design of a specific cam profile shouldn't be assessed outside the overall system & use-case it is intended for.  That said, you guys still gave me a lot to chew on and more than a few things I was overlooking (like rocker ratios).  It's much appreciated.

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