Did a little simulation on Engine Analyzer Pro with different header sizes on my current setup.

I am not claiming these Engine Anaylzers are 100% accurate but none the less it cam up with some interesting results. With just changing from 1-5/8 to 1-3/4, and 1-7/8 and then 2 inch. Keeping everything else the same.

Here are the results: Picture might be large and some of the colors are not real easy to see, but you can get the picture.



The interesting thing is that with the 1-7/8 it lost average HP and Torque and with the 2" it lost even more.

From this you can almost conclude that on a 350 or equiv motor you can easily get away with 1 5/8 heads.

 

very cool, ski. header size is always an interesting subject.

looking forward to watching you tear up the track this spring!

 

Even with the moster motor you are running??? I would almost find that hard to believe... but there it is estimated on paper (sort of). That is very interesting... Thanks for posting that.

 

If the only thing you changed in EAP was the primary size, it won't give accurate results. You need to change the flow coefficient. Try a flow coefficient of 3 for the 2" and 1.4 for the 1.75". These are values I've measured on my flow bench working with AFR competition square exhaust ports and various pipe extensions. I imagine with your large brodix heads the flow coefficient may be even smaller with 1.75" as the mismatch in port size to header size would be even worse. For those that aren't familiar with the EAP inputs, flow coefficient simply accounts for how well the exhaust port flows with a given header bolted on. ie you may have a large 230 cfm exhaust port but it only flows 180 cfm when a small header is bolted on. For accurate results you need to measure it with your heads and headers. I don't know how you input header size but EAP uses inside diameter and many headers are specified by outside diameter.

There are many factors that come into play with header size. Try shortening the exhaust duration with the bigger headers. It will bring up the low rpm power and may not hurt the top end. You can try taking it off the opening or closing side or both. There's a good reason that people can't agree on optimum header size - it's all in the specific combination. As you and corkvette have proven small headers will work really well with the right combination of parts even on a 434 small block.

 

I tested your theory when I read it. I did not change the flow coefficient from 2.5 for the first test. This time I took the 1-5/8 and went from 1-3 for the F.C. Here are the results: I also put the 2" test up with FC at 2 and 3 and it did not make much difference. In both cases the 2" lost power. Not what I think most would expect on such a motor.



Pretty neat stuff..

 

You just made CFI-EFI's whole day I'll bet!

 

I'm keep'in my 1-3/4's though..

 

yah! i agree!!

 

I agree if you compare the 2" header to a 1-5/8" header with the same flow coefficient the smaller header makes more power but it's not realistic to expect the small header to have the same flow coefficient(FC) as a large header. You need to compare a 1-5/8" header with FC of 1-1.4 against a 2" header with a FC of 2.5 to 3.0.

Note that the FC is not a linear function with respect to power. The power output is hurt much more dropping from a FC of 2.0 to 1.0 than dropping from 3.0 to 2.0.

The 2" headers in your graph make more horsepower above 5700 rpm than the 1-5/8 with a FC of 1.0. The 1-5/8 header, even with a FC of 1.0 makes more power below 5700 rpm than the 2" headers. It is interesting and you can see how the smaller header sizes might give equal or better times than the big headers if your car is setup to take advantage of the midrange torque and not high rpms. I suspect a 1-3/4 or 1-7/8 might be optimum but probably wouldn't make much of a difference with your auto trans.

 

Excuse me...

I just plugged you into an ET to HP calculator to compare to the graphs you have here.

The results were.....601HP by ET......597 by MPH

Pretty darn close to the graphs.

Pardon me....

 

Good god, is this never going to die?

 

I did compare the 2"@FC=3 (second down in legend) and 1-5/8@FC=1 (last in legend).

Sure the 1-5/8 is going to be lower on my big azz motor - but its not THAT much less. MAybe 20 less peak HP, but with more torque through the entire run.

Relating this back to a 350 or 383 motor => I think 1-5/8 would be the ticket. Remember what Corky ran on his 406. I took mine off at the track and we switched on the trailer for back to back runs. The 406 did not yield anything better. He did it again this year with the 434, and I think it was less than .1sec gain. Around .08sec if I recall correctly.

None the less its interesting.

FSR402 - not sure I follow your comment?

 

I wonder if this could be added to the tech sticky? I've always agreed with CFI-EFI that 1 5\8 headers are the best choice for 'most' of our cars... but I figured once you start building a 600hp monster larger primaries is the ticket. I guess not.

 

Aren't you the one who is always preaching real world results and not dyno/cpu numbers?

Now lets see a swap ....

 

Ski, I misinterpreted the quote below. I thought you meant "power" as in horsepower or that the 2" lost torque throughout the entire rpm range simulated. You meant peak torque. After looking at the graph a little harder I noticed an interesting thing - the 2" makes about 20 ft-lbs more torque below 4200 rpm. The 1-5/8 has about a 10 ft-lb torque advantage from 4200 to 5700 rpm and the 2" has an advantage everywhere else. It would be interesting to input both power curves into a drag race simulator, let it pick optimum shift rpms for both cases and see which gives better times.

fsr402, I'm not sure what you mean, for me this is just a discussion about the most accurate way to use EAP and interpreting the results.

 

i said before and i will say it again regarding header size is to better error on the small size than on the larger size
what you get in ultimate hp at a extreme rpm you will never make up in tq across the board

 

Reread the second sentence of my first post.

It is fun to play with these analyzers, but I take it with a grain of salt.

When you look at these graphs its easy to overlook the better choice. As corky mentioned peak numbers don't mean much. Its the averages that make or break you. There is another screen called history on this analyzer, it tells you in a summary - you gained or lost peak and also most importantly what I watch "average" hp and torque. The 2" lost averages in both hp and torque. As mentioned this is not something you would think you would see.

 

I also take simulations with a grain of salt. You've proven with real drag strip results that the 1-5/8 work about the same as 1-3/4 on your cars. I also agree that it's generally better to err towards the smaller header especially on a street car.

As far as EAP predictions (which don't mean much sense you have real drag strip results), look at the graphs again. There's no way the average HP of the 1-5/8 with a FC of 1.0 is greater than the 2" when using a FC of 2.0 or 3.0. unless the EAP screen you're referring to is ending the data for average HP calcualtion at 6000 rpm or less. Plot just those two runs and it will be more obvious. You would need to shift the 2" header configuration at 6600 rpm or more to take advantage of it's HP.

 

Don't pay any attention to it, that's just the first thought that came into my head when I read what this thread was about... The whole 1 5/8 against 1 3/4 header sizes.

There is a lot of good info in here that I'm having fun reading and trying to understand. please continue..