Okay, so I'm looking at the melbourne header/exhaust sale in the C4 sale section and they only offer 1 3/4" primaries. I was planning on going smaller with 1 5/8" but wanted to do a little research to see how much I might loose in torque from stepping up. Now, I know most of us here (especially CFI-EFI!) are fans of the smaller primaries in long-tubes. And yes it's setup-specific, but are there dyno results to compare?

David Vizard seemed to think we're all going too small relative to cylinder head selection (see chart below). My trickflow CNC'd heads are "rated" right at 250/190 cfm Intake/Exhaust at peak lift, and this is on a 383ci superram engine with LPE 219 roller cam....

** the bottom line was for street cars, the top line for all-out racers, and the middle was somewhere inbetween. I only looked at the bottom line...



Now, this chart (in one of Vizard's articles and repeated elsewhere in my searches) as well as other performance orientated sources seem to steer me towards the larger diameter pipe (ie 1 3/4") based on the cfm of my cylinder heads. I did some searching over at thirdgen.org, and -even with less displacement - many of those guys want to run pipes 1 3/4" or LARGER...

So what am I missing? I know the average enthusiast is just smart enough to be dangerous (ME!), most of us select a primary size that is too large, but yet most reputable online sources that I can find suggest I use the larger pipe given my combo.

I know it's "smaller primary, more torque" and "larger primary, loss of torque - but some top end improvement..." With my setup, how much of a wash will it be between the two? Anyone with a stroker superram have comparisons or personal experience/numbers to share?

Again - 383ci SuperRam with LPE 219 roller cam and trickflow heads flowing ~250 CFM I and ~190 CFM exhaust (give or take) with a 2600 stall TC... and the last dyno I had on a bad tune showed something like 350HP and 470 ft-lbs of torque on the stock 1984 exhaust...



EDIT: I'd be okay with the 1 3/4" primaries anyway, since I have a 400"+ SBC in my mind for the future. But given the sucky search results and the entertaining spectrum of opinions on the topic, I was hoping to hear some real-world comparisons or dyno numbers...

 

I'm not touching this one publicly because is ALWAYS ends up is a sensless pissing match of theories rarely backed up by any data (but I have LOTS of personal experience)...if you want my opinion PM me and I'll galdly share it

 

I don't have to say much, mostly because I already have, and everybody knows that I've had extensive drag strip and street experience to back up my recommendations. My only comment, for now is, if you will be operating your engine to the 250/190 cfm capabilities of the heads, a LARGE percentage of the time, go with the bigger tubes. Otherwise, if you actually DRIVE your car...

RACE ON!!!

 

An after thought.If you end up viewing and studying dyno charts and numbers, make sure to pay attention the applicable portion of the curve. Maximum numbers CAN be impressive, but... Do you plan to drive the car or the dyno?

RACE ON!!!

 

I run 1 3/4, why, because they were available, on Island here in Hawaii and they bolted in without modification. They seem to be very complimetary to the 396 LT1 I have, and the majority of folks run this size due to what we have available for reasonable prices.
I am seeing people with LSx engines going for 2 inch headers. My 454 Big Block had 2 1/8 inch, but the heads flowed close to 350 cfm intake and 275 exhaust.
Too each his own. I think we will never really know what size is better, unless we do extensive testing with both on the same vehicle, same day, dyno, track , driver etc etc. But the variables on your engine size, capabilities makes a test inclonclusive to most people as most have different combos of parts.
You will make better power with headers, regardless of size, compared to stock manifolds.

 

You have plenty of tq, why not go with the 1 3/4 and build more HP, especially if you are going to go to a bigger block later.

 

Well that's pretty much my line of thought. I've been paying attention to these threads for a while and it's very difficult to come up with an applicable gameplan - there seems to be some vague threshold of "once you're over XX cubes or YY power, then the 1 3/4" primaries are a plus." But it's so speculative to fill those values in.

Honestly, I've come to the opinion that it's best to err on the smaller side for that extra torque (re: CFI's comment on actually driving the car) and I would take 1 5/8" in a second if it was actually offered. But it seems that the demand is for larger headers, so that's what is being made and offered; hence I'm almost forced to go larger simply from the cost perspective (the sale I'm looking at only comes in the larger size)?

Perhaps a better question is: Can anyone form an educated opinion on what I'm LOSING by going to the larger primary size? With an output heavily slanted towards torque anyway (with a restrictive, narrow stock exhaust), would I actually *lose* torque or just not gain as much relative to the current output?

 

had i known of a source with a fair price at the time (over 2 years ago), i would have flat-out purchased 1 5/8". i was worried about it from the beginning. but so be it, i got 1 3/4'". like i said in another thread, my next vette will have 1 5/8" no matter what. if it takes me a year to get them, so be it. yeah, i read david vizards book on building small block v8's. my setup according to him is borderline beween the 2 primary sizes. he didn't get into cubes, he discussed horsepower output. but according to conventional wisdom i went against it, and opted for the slightly larger rather than the slightly smaller tubes. again, its due to what's readily available out there. i don't know what i have foregone in terms of torque, but throwing a number out i might say 10-15 tq, probably not a wildly incorrect number.

in the end, at least for now, do i care? nope, not really. i don't race for dollars, SOTP is absolute king to me that's what feels good and what it's about, so its really inconsequential at this point.

 

Come on Jay, be a trouble-maker!

 

I have a 95 LT1 6 speed, and I recently installed EM 1 3/4 LTs and random tech cats. I was concerned that the primaries were going to be too big for such a mild, relatively small motor. However, my car went from a best of 13.29 at 104+ to a 12.69 at 109+ with the headers and the randon tech cats and a tune (both times on DRs). The SOTP feel is improved but not overwhelmingly so and in fact does not seem to be in line with the quarter mile ET improvement. Maybe this is because of the size of my primaries.

 

[QUOTE=Ramanstud]I know it's "smaller primary, more torque" and "larger primary, loss of torque - but some top end improvement..." QUOTE]

Seems to me the loss of torque at the low end would be relatively small, if in fact there is a loss.........so the gain at the top end means more hp........more hp means the potential for more rear wheel torque with the correct gearing......so what is the problem guys...bigger is going to be better......

What do you guys think the crossover rpm is from when 1-3/4 becomes better than 1-5/8???? If you don't know the answer to this then this whole discussion is pointless...
??????

 

In my opinion.. what matters when im racing is more important to me. if im losing 20 ft-lbs of torque at 2000 rpm.. who cares? Do i really need the extra 20 ft-lbs while drivng home with a car full of groceries? When i am going through the gears the rpms never drop below a certain point anyways so i would much rather have that extra top end.

 

Careful there! Don't tempt me!!

 

Should you be using the chart with flow at your exhaust valve?

 

chart say's Ex,

 

Unless the torque/hp tradeoff is huge Id use the size that gives you the most useable torque in your style of driving. Most 1-3/4 will lose 20lbs min. for +5, maybe 10hp up top? Youll never feel it IMO. You are at lower rpms longer/more often then higher, so I would go 1-5/8 for a street motor.

 

I really have to ask, where did you get those numbers? I have seen numerous LTx cars dynoed with 1 3/4" headers (and nothing else, save for the possible cat-back and K&N type air intake) that never lost a single ft/lb of torque over the entire RPM range. Now, granted this is not an LTx engine in question...but I can't believe there would be that much of a difference.

 

Go with the 1-3/4 with the 383 superram combination you will not have a torque problem. I ran 1-3/4 headers on my 350 superram 219 cam combination before I went to the 406 and I never had any torque problems. The 350 superram 219 car made 343 RWHP and 382 RWTQ and ran a best of 11.82 at 114 in the quarter.

 

This relates very much to my above post... again

What do you guys think the crossover rpm is from when 1-3/4 becomes better than 1-5/8???? If you don't know the answer to this then this whole discussion is pointless...
??????

 

To be short, the question would be:

How much torque/HP would I GAIN with 1 5/8"? The torque would likely be notable - but how much over what RPM ranges?

Versus:

How much HP would I gain relative to the smaller primaries with a 1 3/4" - and how much torque would I be leaving on the table over what RPM range? Of course the only way to determine is either ET (I don't do 1/4 miles yet), or a dyno.

I think this discussion is more pertinent because of my larger cubes, the "extra" unusable torque I have and then these issues complimented by the availability of the smaller primaries. I can see the advantage of more torque on a street engine (which mine is), but again what would be the biggest gain when it counts (ie hauling a s s?). I can spin my 315's now if I floor it with a modest stall... it seems that, barring all other arguments of "peak" numbers, I'd want to focus on the upper RPMs where it would be more usable (ie traction)?

If I can afford it... maybe I'll do a before and after.