Comparing 2 heads...

[travis]

Am I understanding correctly how this all works together?

Street performance application, so idle-6000 rpm is important here.

2 sets of heads, with very similar intake port dimensions and flows across the board, but one set has a great flowing exhaust port for its size and the other set a mediocre flowing exhaust port but similar dimensions, valve size, etc. Say i/e of 250/200@.550” for one set and 250/150@.550” for the other set.

Without any other changes affecting flows, is the peak HP POTENTIAL of both sets of heads about the same?

Every formula I have seen for calculating HP potential only uses intake port flows.

I assume that by adding exhaust duration, you can get the peak HP numbers of the heads with the poorer flowing exhaust port pretty close to those of the better flowing heads, correct? But...with the longer duration exhaust lobe, do you always lose torque at the bottom end of the rpm range? Say under 3500 rpms or so?

This is all assuming everything else is the same, only changing heads and the cam.

[mag2555]

Compression ratio / cylinder pressure are big factors that can help a under performing Exh port to work as needed during the blow down cycle, that being getting a good size chunk of the spent gasses out of the motor even before the piston starts its way up the Bore on the Exh stroke!

The better the Exh port works overall and especially in the .050" to .300" lift range the smaller the Exh lobe you will need to make power.

One thing that can shine a light on things is to add up and then average the flow numbers between the two heads of there respective Intake and Exh ports up to the max lift you will be running and then decide.

Also take into concideration that your hole Intake system may not pass the amount of air your seing from the head alone, and also that a set of headers that are slightly bigger then your Exh ports will pick in most cases atleast 4% more flow then the bare port starting at lifts as low as .350" on some heads, but surly by .400" lift.

Motors of under 11 to 1 comp need all the low lift Exh flow they can get 95% of the time!

[randy331]

I assume that by adding exhaust duration, you can get the peak HP numbers of the heads with the poorer flowing exhaust port pretty close to those of the better flowing heads, correct? But...with the longer duration exhaust lobe, do you always lose torque at the bottom end of the rpm range? Say under 3500 rpms or so?

If more ex duration added power to the one with less ex flowz, I'd bet money it'd add power to the better flowing one too.
Optimum ex duration has nothing to do with cfm on a flowZ bench.
There's more to an ex system than just balancing blow down for the top end vs tq production at the bottom end.

Randy

[CGT]

If more ex duration added power to the one with less ex flowz, I'd bet money it'd add power to the better flowing one too.
Optimum ex duration has nothing to do with cfm on a flowZ bench.
There's more to an ex system than just balancing blow down for the top end vs tq production at the bottom end.

But then things don't fit into neat little boxes!

[randy331]

But then things don't fit into neat little boxes!

If you want that, I'd get a job at Fedx.

Randy

[travis]

So other than actually purchasing and dynoing several sets of heads, which is obviously not realistic for a hobbyist such as myself, how do you choose?

[CGT]

So other than actually purchasing and dynoing several sets of heads, which is obviously not realistic for a hobbyist such as myself, how do you choose?

What 2 heads are you looking at?

[mag2555]

If we are talking abou Aluminum heads for a SB Ford or Chevy motor of not more then 383 cid then go with a Airflow research head of no more than 180 CC's and then sleep well at night , as for the average build your just not going to do better for the average guy!

[steve cowan]

So other than actually purchasing and dynoing several sets of heads, which is obviously not realistic for a hobbyist such as myself, how do you choose?

Travis,
here is the million dollar question,unless you are prepared to purchase the necessary equipment and parts and test and tune and retest 99% of us have to follow the pack on average trends available.i am sorry that i cant answer your question but in my mind it is a can of worms.
i am sure you are like most people here on this forum who read pages and pages of threads and posts trying to grasp everything available but to me the more i try and learn the more complex it becomes.
if you have not already i suggest finding old threads from randy 331,CGT etc
these are threads with so much information available including cam events,port sizes and shapes,in/ex port % and exhaust duration and extensive dyno and track testing to prove there theory works in real life and not pie in the sky dreams.
i will be the first person to admit having bolted parts together with huge expectations and been a little disappointed by the results,thats the difference between the (pro engine,gifted hobbyist) and mere mortals like myself LOL
i am sorry for the rant but thats how i see it,time, money and alot of experience over time and data and trends from your own testing,just because i see or read one person do it does not mean i can duplicate the same,have not helped have i ??

[PRH]

It would seem to me the only reason not to buy the head with the better ex port is because of cost.
If it’s an aftermarket performance head, it’s likely you could pick up some easy ex flow with a different valve job and a little tweaking of the port.

[travis]

So other than actually purchasing and dynoing several sets of heads, which is obviously not realistic for a hobbyist such as myself, how do you choose?

What 2 heads are you looking at?

I’m actually trying to make a decision on several different sets, and the I/E differences aren’t as drastic as the example I gave...more like 20-25 cfm on the exhaust side. So maybe I’m just splitting hairs again for what I am building.

[groberts101]

So other than actually purchasing and dynoing several sets of heads, which is obviously not realistic for a hobbyist such as myself, how do you choose?

What 2 heads are you looking at?

I’m actually trying to make a decision on several different sets, and the I/E differences aren’t as drastic as the example I gave...more like 20-25 cfm on the exhaust side. So maybe I’m just splitting hairs again for what I am building.

I think you're selling yourself short on the knowledge you have acquired through study and life of trial and error.

Couple things to think about rhetorically.

Does your low rpm engine have the capability of fully utilizing a 200cfm exhaust port?

If not.. what happens to the ports velocity profile when it's larger than the application requires for the task at hand? Sure thing about the cams ability to adjust that time area relationship, evolving into shorter duration less overlap and smoother idle, but will that fix a slightly too large exhaust ports slower velocity profile? Maybe a smaller better matched flow rate/velocity adjusted primary pipe sizing would become more critical for such a combination? Or would the shorter duration alone be enough to combat the potential for an increase in reverse flow/reversion? Maybe lower rpm combos would show gain from a very fast port combined with a smaller primary without the need to kill as much duration?

Personally speaking, for such a combo I'd take a smaller exhaust port cross section and work/streamline the guide boss along with some minor transitional blending.. over a bigger as cast port with full guide boss restriction even if the smaller port configuration flowed slightly, maybe even considerably less cfm. Remove some of that smaller ports biggest obstructions while keeping velocity as high as possible. Then incorporate a smaller primary and secondary along with a reverse pattern cam having less lift and slightly higher duration so power carries better past peak. Less reversion to enable better torque higher manifold vacuum and still hit decent peak power and not fall off a cliff past it.

[Steve.k]

Do you think you leave engine alone after its together or keep ramping up? If the latter go with the bigger exh.

[af2]

Just interesting................... The exhaust is mach speed? so are trying to slow it down?