ChucksZ06

iTrader: (9)

Most folks believe that ports on the smaller side with higher velocity airflow rates make more hp/torque at lower rpms and as much or nearly so at higher rpms. AFR heads are touted for this a lot. Do not the ls7 and L92 heads go against this concept to some degree? They have much larger port volumes and still make good low end torque...the l92 is a truck head where low speed torque is necessary. Do you guys think the argument about 250 cc runner volumes being too large on ls1/6 heads may be bogus? I know the shapes are important too and there is obviouslly a limit on the size of the ports...but we have big block chevy size ports and valves on smaller engines and they seem to be working great. How much does this have to do with fuel injection putting the correct amount of fuel in the cylinder versus a carburator with low airspeed not vaporizing the fuel as it is pulled through the venturis? I built a solid roller 406 sbc a couple of years ago with holley port injection on a victor manifold. The engine has unbelievable low end torque compared to the same engine with a carb. It pulls strongly from 2,000 and from 3,000 to 7,000 it is like a blown engine. I think this has a lot to do with the low speed control of the a/f ratio. I would appreciate you guys thoughts on this.

DavidNJ

I think what you are seeing is that port volume and dry flow numbers are only a small part of what makes a head 'good'. If the runner length is the same (and with different valve angles or raised runners they aren't) port volume and CFM can be an indicator of flow velocity and to a lesser extent flow quality. However, they really don't tell the story.

And beyond flow you have the quality of the combustion chamber, the integration with the intakes (a bigger issue with carbs), swirl, and fuel separation. And the right selection is also going to depend on engine size, use, cam, exhaust, vehicle characteristics, and drivetrain.

In a perfect world the head with the best flow would also win on the dyno, on the track, and on the street. In the real world you have to see the completed result, and the others are only course indicators.

P.S.
You probably could have done a better job tuning the carb.

white2001s10

iTrader: (4)

I agree with you.
The potential of the head port on the running engine also has a lot to do with how well it is working with the intake manifold design.

A large head port that may not work well with a vic-jr intake and single carburetor may work very well with IR intake and multiple carbs.
It is a fact that a port that allows a loss in velocity will degrade the fuel suspension in a carbureted system.

Like you I think that most of this problem is avoided with port injection, leaving only the possibility of compromised ramming effect which again is dependant on how well the port is working with the intake manifold.

As was also meantioned, I feel the tune will also play a part. Running the same AFR across the whole RPM range isn't neccessarily what the engine will respond best to.

DAPSUPRSLO

iTrader: (14)

Port lengths of these ls7 heads and c5r heads are a good bit longer hence producing a larger volume given the same cross sectional area. The more I research into heads it just seems most of the stuff you hear is wives tales and it's best to just find a head porter who consistantly makes good power with their heads.

In general, get a head port which is too small with supposedly "good velocity" and it may become a bottle neck when turning a motor harder and or running a bigger motor causing pumping losses and fuel separation issues do to velocity which is too high. Get a head with not enough velocity and it going to want to keep on running and running into the higher rpms but the average power will be down everywhere. This is very general but you need to find a head with the proper dimensions for your combination and flow numbers vs. port volume are not a good indication in my honest opinion. There is just too many damn variable in a heads intake port to look at volume vs. flow.

There are some very good threads out there, alot on other boards, that discuss this very topic! I highly recommend taking a look, it's changed the way I think about cylinder heads for sure

SStrokerAce

iTrader: (2)

The problem with the notion that "More velocity= better head" is that it's a very narrow minded view of whats actually happening in the motor. Same with more CFM @ 28" throughout the lift curve is better. You can give me the choice of 10cfm more at .200" or 10cfm more at .600" and I'll take it at the top end anytime. More Velocity is not a good thing, the LS1 as a engine design shows that more and more everyday. The thing that I find funny is that you see the head "guru's" touting their heads where they can get more sales but never discussing them in depth in this forum.... interesting.

FWIW if you slow the velocity down in the LS7 head port and only slightly enlarge the runner, with a valve job change as well you can pick up 40+cfm on those CNC ported factory heads.

Bret

MSURacing

iTrader: (1)

This is just like a cam coversation in that there is a different head that is "the right one" for every situation. I believe that if you are running a truck engine, you would like to have a smaller port because of the towing situation and the lower RPM's.

But, if you want to do all out racing and you can adjust your gears accordingly, you can go with the biggest baddest port and not worry about throttle response or low end power.

What the LS1 has done for the rest of the world is shift the power curve. I mean, think about it, until it came along, the peak engine speeds were limited to 5500 rpm. But being that the engineers new that the new LS series engines were going to have way better durability, they built around the higher rpm capabilities. So, if you make the same power higher in the RPM range, you get more work done which equals more horsepower.
Think about how the LT1 jumps out of the hole and has great reponse below 3500 rpm. The LS engines don't start kicking in until 3500 rpm.

So there is the trade off, you get more horsepower, but you need more reliability.
I think the direction of the LS7 is great for cars, but I think they may have some issues with converting the trucks to these larger port cylinder heads. The thing they have going is the fact that the engines are larger now to take care of the loss in low speed torque.

91Z28

I think that is why they finally did VVT.

MSURacing

iTrader: (1)

Forgot about that!!! There is another low speed bandaide!!!
All I know is that my brother in law, who hauls cattle with his 01 2500, was used to driving the old L31 and he was complaining because the truck didn't have any power. I asked him how high he was winding it and he said he never took it above 4000. I told him to put it to the floor the next time he was pulling the trailer and let the truck do the shifting. He came back with a smile on his face, but he was concerned that it wouldn't last long doing that.
I told him thats what warranties are for and he has over 80,000 on the truck now.
I think it comes down to driving habits. If you aren't used to shifting at high rpms, stay with a small port. If you like to beat the rev limiter by 50 rpm, go with a larger port.

Tony Mamo @ AFR

iTrader: (13)

I can only guess that comment might have been aimed in my direction...it certainly fits as I rarely look at this section and probably should. There is only so much time in the day.

The bottom line is I have written volumes of information pertaining to this very topic....although the information is probably spread over countless posts in countless threads in the "Internal" and "Dyno results" section so using the search feature might be challenging but the info is certainly available to those who are willing to spend the time to find it.

Is a "small port/high velocity" runner the end all in cylinder head design? NO....but the proper sized port for a given application that is on the smaller side in cross section and the larger side in flow will produce good results in the typical cars we are discussing (mainly street/strip applications) ten out of ten times. IMO, a 205's cross section (for example) is about perfect for a dual purpose performance build-up assuming that engine is in the low to high 300 CID range....the fact if flows on par with a lot of larger heads makes that situation that much sweeter because airspeed will be higher. That will help increase cylinder fill because the column of air has more energy providing an additional ramming effect, better atomization of the air/fuel intake charge, and it also reduces reversion which hurts both power and fuel economy.

Other important factors or things to consider are manifold limitations (is the larger head with more volume and medicocre low and midlift flow actually benefiting you when your manifold is possibly choking the "big" numbers anyway)....area under the curve (low/midlift flow etc.), intake/exhaust ratio (is the head balanced well?), combustion chamber design, and an efficient valvejob for YOUR application....meaning you wouldn't want a 55' seat unless you were prepared to run ALOT of valvelift to take advantage of it. Another topic thats hard to quantify is how "quiet" the port is which touches on the quality of the flow....not just the quantity. There are probably a half a dozen approaches to getting a certain CFM figure out of ports that might be reasonably close in volume, but I bet your performance results would not be identical at all.

In closing....small ports and velocity are not the sole criteria for a great pair of heads....but coupled with other good attributes, they are key ingredients to a winning formula if your looking for a well rounded package, especially one that is of a dual personality nature.

By the way, the small port/high velocity concept does wonders for part throttle response and fuel economy....just ask Patrick G. who has tested both (our 205's and 225's). Im sure he will be chiming in at some point to share his take on the situation.

Tony M.

PS....BTW, I feel the LS new truck head, while certainly great for us hot-rodders to fuss and tinker with, is really not an ideal head for a truck. If the "truck" we are talking about is just that.....a truck for towing loads and/or people....not a hot rod. Rumor has it GM missed their power goals with an "as cast" head (the truck head) and decided to step up to a CNC piece instead. Im sure alot of time was invested so why let it go to waste. That same raised runner architecture with a smaller runner would have been the ultimate truck head in my opinion, AND would have still been able to put up a strong HP numbers because a properly shaped smaller port with that geometry would have still flowed a considerable amount of air....especially for the needs of a heavy vehicle which places a paramount on low and midrange TQ.

Patrick G

iTrader: (14)

When I swapped from AFR 205s to theAFR small bore 225s, I found out some interesting things:

1. The larger port volume and flow of the AFR 225s did not hurt low rpm torque at WOT. In fact, the dyno sheet was an exact overlay of the AFR 205s up until peak torque where the 225s squeaked out an extra 5 rwtq.

2. Although the 225s outflowed the 205s by close to 20 cfm, this was only at .600 lift and above. The airflow was nearly identical between the 205s and the 225s up to .400" lift. At .450" lift, the 225s started to outflow the 205s. But because we bolted up a ported FAST 90mm intake manifold, the potential airflow gains of the 225s were not fully realized. With max flow in the 280 cfm range, the only way the 225s could have made substantially more power than the 205s, would have been to have a significant flow advantage at the lower lifts. It did not. As a result, we only saw a 5rwhp increase in power (even though we were running a .644" lift cam).

3. There was a significant loss in low rpm throttle response and fuel economy with the swap to the AFR 225s vs. the 205s. The high velocity ports of the 205s gave much better throttle tip-in. As a result, I had to significantly change my VE tables in the lower rpm (to maintain the proper A/F ratio) and my fuel economy dropped 1-2 mpg. For the typical dual-purpose street/strip racer, the small increase in power did not outweigh the loss in driveability.

Lesson learned? Although larger cross-sectional runners can still maintain excellent low rpm power and torque, often they compromise the part throttle driving manners to a point to justify staying with the higher velocity ports. In my experience, bigger wasn't always better. When bolting up a flow limiting intake manifold like the FAST 90, some of the best performing heads are going to be the ones with the best flow in the lower lift ranges. Basically, flow above 280 cfm show diminishing returns when the intake is bolted up, but quality flow in the .300-.500" range seems to make a stronger case for power and torque increases.

MSURacing

iTrader: (1)

Nice guys, looks to back up my thoughts. So, Tony, what do you think of an L-31 type port on a LS series engine? I think your heads do the closest to this comparsion. But just think of the incredible fuel economy and low speed response. I love the feel of my 383 L-31 and it gets 15 city, 18 hwy no problem.

ChucksZ06

iTrader: (9)

Thanks for the responses. By the way, the bait was swallowed big time. A lot of this comes down to how we use our engines and what kind of drivers we are. I want my wifes yukon to idle smooth and have good low speed torque. In my performance car I want it to pull like hell in the upper rpm ranges. I cannot understand why it is such a big deal what torque the engine makes under 3,500 rpms...people on this board talk about area under the curve till the subject is beat to death. Most performance cars never drop below 4,000 rpms when going through the gears...so why talk about area under the curve below where you use the engines power. If you race someone from a rolling start and you do not shift down to near your torque peak you are not driving very aggresively. I think it is an apples and oranges arguement...and a lot of this velocity thing is bullshit. If you want to build an lsx engine for fuel economy and great throttle response at low rpms...then go with little cam and a torque peak of 3000 rpms...make it very crisp like a diesel. ( i am being absurd to make a point) It is about getting the air into and out of the cylinder and the ports or "pipes" that do it the most efficiently are the best for producing power(torque). The ports on modern engines are much larger than they were 30 years ago and the low speed power production is not even comparable. Obviously there is a limit but I believe the facts are showing that bigger can be better as you can have both the low speed power and higher max horsepower.

Tony Mamo @ AFR

iTrader: (13)

Chuck...

You seem to have some pretty strong opinions for a guy that came on here seeming to be open minded and originally asking all the questions. If I'm the guy that took the bait I am certainly pleased others had a chance to view my thoughts and opinions.

The point you might be missing is that the area under the curve I refer to doesn't imply weaker power numbers (peak) than a larger headed combo, it implies that the smaller more efficient port (and perhaps better sized for the job) will make the same or better peak number, only give you more everywhere else. Its been done a hundred times in the dyno section of this board, Take a similar set-up....add AFR's...make more peak power and alot more more TQ and HP approaching peak power....IE "area under the curve" as some others have questioned what that phrase meant. If you take all of the "tuner" and "shop cars" and put those results all to the side (including my own), look at what the average Joe's and shops not looking to prove anything (regarding their own cylinder heads) are putting out. The "small" AFR 205 cars are some of the best performing cars out there whether they guy is running an X3, and F13 or something as large as the T-Rex, OR he's got a Stealth cam or some other very streetable package like our 224/228. The end results look all the same, all of which bode well for the smaller, very efficient design utilizing commonly available complimenting parts (manifolds, headers, etc.) which is another big piece of the puzzle. Whether you want to pay a premium for those benefits is another topic entirely.

Hopefully some of the people reading this thread that haven't formulated their opinions yet might actually have something to think a little harder about (that is what these boards are really about...sharing information). We (AFR) could have easily just produced a "big" head with sexy peak flow numbers for the masses....considering what we have done with much smaller runner volumes it's obvious we have the talent to do so. Ultimately results are what we are measured and judged on, not flow numbers, and we will stick to our guns confidently knowing the heads we produce will provide the best results in 95 out of 100 combinations. Some people will buy them and others will not and thats what makes this country a great place to live....

Peace...

Tony M.

hammertime

iTrader: (1)

Keep in mind that the majority of people in these forums are looking for dual purpose solutions. I think even Tony might admit there are heads available that make more power at 5000 - 7000 rpm than an AFR 205. But if you are using them on a true dual purpose machine, something that you have to drive in traffic, around town, etc an a regular basis, then you are giving up the response and drivability that is available with a properly matched setup.

Sure, low end power delivery is a moot point for a race car. You are only optimizing the operating range for a very small window, and only for wide open throttle. A real street car has to perform under many other circumstances. Think about Patrick's comment above - is 5rwhp worth 2 miles per gallon in your daily driver?

DavidNJ

Keeping the thought of proper sizing, you have to realize the peak flow at 28" of water (about 1 psia) is only a small part of the story. There is cam timing, intake tuning, swirl, combustion chamber shape, etc.

At peak VEs, the exhaust starts a positive flow before TDC. And it continues postive to valve closing. Below that range, you can get reversion at IVO and reverse flow before IVC. Having the correctly sized ports for your displacement, engine speed range, intake tuning, cam timing, etc. is critical to keeping those VEs (above 100% on non-restricted motors) high over a wide range.

Second, what good is it to have the mixture if you can't burn it or burn it to early? Having to run more timing causes pressure to build before TDC, working against the engine creating power. And incomplete combustion is just that, you are pumping unburned air fuel mixture, lost power, out the exhaust. Combustion chamber shape, piston shape, quench, plug location, swirl, etc. all conspire hre.

So your best numbers are from the combination that works together.

Now lets look at some of the biggest limitations. Number one, the hydraulic cam. Basically, you are probably done before 7k because your valve train can't keep up. Even if you could keep the lifters from floating the valves, the hydraulic lifter is limiting your spring pressure.

Number two, dual use. Basically, a race car doesn't even have to run below 4k. Ever. On yellow laps I need to keep my car above 3500 or it doesn't cool. The fan is electric. A street car does. What you consider acceptable on the street may vary from others. However most peope drive on the freeways at relatively low revs and get stuck in traffic every now and then. For instance at the gate getting into the race track!

Tony's heads have some of the most efficient ports (which the ratio of volume vs. flow is for similar shape ports) and a well designed chamber. BTW, Tony, how big a valve spring can be fitted, and how tall a valve?

Patrick has been running probably the best hydraulic cam lobes out there...the Comp LSKs. They take advantage of the big 55mm cam bearing and lightweight valve train components and produce lifts that rival some solid lifter cams and still maintains an aggressive 50° major intensity (for an hydraulic roller, solid rollers could be closer to 30° are below). BTW, Patrick, do you have dyno numbers for that latest 223/227 110/+0 LSK cam?

Net...for a FAST intake, 90mmTB 347, an AFR 205 or ET 215 with the right LSK lobes will probably work best. With the Dart as cast heads just behind.

Note: Some shops like Cartek and TEA have done phenomenal stuff with stock heads, however they haven't modified the combustion chamber. Which is not as good in the older LS1/2/6 designs as it is with current designs.

Note 2: This is just for intakes. Exhausts have their own issues, for which 28" H20 numbers with 70°F air are a flawed model for 100psia pressures and 2500°F exhaust.

CoronaL

iTrader: (6)

well I'm no engineer, but I've done a fair amount of reading/research into the issue. I went with Patriot heads for my TA b/c of price performance needs at the time, and I'm happy with the product and service I recieved. Very much more than expected, and I'm one to expect ALOT.

That said, everything I read about AFR is AMAZING. Tony is on the boards and always replies with quality posts/information. His reply here is another example of what AFR is about. Not just go, but the support and knowledge to better the community as a whole. Honestly, who can discount the fact that a head flowing x# CFM with a smaller port and higher velocity would be better than a higher volume head with the same cfm. I've seen Z06's with AFR heads and a 224/224-114 cam making 425+hp while even with ported LS6 or GM type castings would be making 20 HP less or so. Patrick G was making SIC #'s with a cheater cam and AFR 205 heads. That's nuts.

To get back to the original question of this post ie the bait . I think the new heads are interesting in that alot of people are starting to run 402+ strocker LS2 based motors. In that case, these would be an attractive "budget" head and offer more flow than traditional LSx based GM heads. I"m sure porters will have fun with them, but let's not forget the advantage the LSx series heads offered ofer the
lt-1 type heads prior. We're heading back to that port shape, and although runner volumes and fow have increased, is it really for the better? Possibly, or else why did GM make the move? But alot of other factors come into play here such as intake, cam, and motor CI/efficiency that will vary vs.our LS series motors. And let's not forget the 6spd auto's that will change the powerband requirements for these new cars.

All in all, it's all relative! Match the port volume and design to the purpose. If you want a 7k rpm screamer that makes power from 5k past, then by all means use a big port etc.... IF you want a car that makes tons of power all over, then you need a vastly superior design that offers the best of both worlds, high velocity mixed with high cfm flow, matched as closely as possible to the other complimenting parts.

My 231/237 cam makes good power up top, but for all out fun, my 224/224 cam in my Z06 is awesome at shredding tires and impressing my friends . All in the eye of the beholder, but don't shortsight yourself

Tony Mamo @ AFR

iTrader: (13)

My thoughts exactly....very well said and captures in a sentence the overall gist of what we are trying to promote and convey in our products and combinations we feature.

CoronaL

iTrader: (6)

rgr that Tony. I admire your work and that of AFR. I may not have bought your heads yet ;p. But doesn't mean I haven't wanted to

If I won lotto tomorrow, dream motor would be a nice stroker LS motor, AFR heads, single turbo and no traction

DavidNJ

Actually, a 347 trying to optomize 5000-7000rpm may not have a better alternative than an AFR 205 or ET 215.

airflowdevelop

Seems like some truth is coming out...Maybe by this time 2065 people will start to forget about marketing heads by flow #'s....or even port volumes..

The first and foremost question I believe should be asked is...what is the velocity over the ssr apex and the outer ssr radius...This has always been one heck of a problem with the cathedral port heads (atleast for me and most of the head porters that I converse with). If you can keep the air stuck...and stuck at any amount of depression, the DC will come around on the heads. These heads are notoriously "lumpy" on their velocity profile, and if you fix the itch at one place...you most certainly will find yourself scratching somewhere else.

I am a little confused as to what "flow vs. volume" has to do with anything...I have heard this theory a good bit, but have yet to be educated on it. Sometimes I guess it is hard to educate a redneck, but I am all ears!

Next time you call about a set of heads...ask what the DC is? ask them what the peak and average velocity is...then ask why? And if they say higher velocity is better...hang up the phone.

Dennis