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Header size?

2K views 15 replies 4 participants last post by  red65mustang 
#1 ·
Iv'e just finished making a new set of custom headers, it's the first time Iv'e made any and they were a particularly tricky set to make due to lack of room. They are a tri-y set up of 1 5/8" primaries into 2" secondaries then back to a twin 2 1/5" system. My engine is a 408ci sbc, 195 AFR Eliminator heads, Lunati cam with 241/249 @ 050 and 525/546 lift 110 lobe centres, victor jr intake, 750dp hp series holley or Demon (not sure on that one yet), HEI dizzy and MSD. My friend that works in an exhaust shop recommended the sizes that I used in fabricating my exhaust as he thought going any bigger could result in a loss of too much torque. Now that Im finished building them I think that could have been wrong thinking for this engine. The more research I do and more I read on this site, it seems torque loss in these 400+ci sbc's is the last thing I need to worry about! Your thoughts please!!!
 
#3 · (Edited)
Holden,
I plugged in a just roughly close to your combo into my DD (since you built your own headers design)...

1 7/8" could have gained you "up to" 6HP/6ft-lbs at the HP peak and lost 4HP/12ft-lbs at 2,000rpms (both at the flywheel) depending on motor details like cam advance/ign timing/etc....

in the "real world" that translates into "maybe" .05-.1 second difference at the strip and no difference you can feel on the street, so don't lose any sleep over it....

LOL...
using the correct stall convertor and deep gears needed by that big cam and parts combo.....
your motor is "NEVER" going to be at low rpms driving normal....
that's why "available" TQ/HP won't be a problem....
 
#5 ·
Holdon

red65mustang said:
Holden,
I plugged in a just roughly close to your combo into my DD (since you built your own headers design)...

1 7/8" could have gained you "up to" 6HP/6ft-lbs at the HP peak and lost 4HP/12ft-lbs at 2,000rpms (both at the flywheel) depending on motor details like cam advance/ign timing/etc....

in the "real world" that translates into "maybe" .05-.1 second difference at the strip and no difference you can feel on the street, so don't lose any sleep over it....

LOL...
using the correct stall convertor and deep gears needed by that big cam and parts combo.....
your motor is "NEVER" going to be at low rpms driving normal....
that's why "available" TQ/HP won't be a problem....
Ive got a 3000 stall and 3.25/1 diff ratio. I know the diff maybe a little tall but it is only a 3000lb car and I am intending to do a fair bit of cruising in it. I am still considering a new set of gears though. Out of interest did your program give any peak torque and hp figures? I would be very interested in those :) I was thinking that might be a big cam for the street, but have been assured it isnt.
 
#6 ·
Holdon

Notorious said:
I don't think you hurt yourself. I believe more people than not run too large of a primary tube for their application. I think your collectors should have been 2 1/2" though.
Thanks for your reply mate. You think my first collectors should be 2 1/2"? What size would you make the final collector then? If you have a look at my photo album there is a good pic of my engine with the headers fitted. Photos have not long been added so you can also see them on the home page.
 
#8 ·
Holdon

ericnova72 said:
I believe Notorious meant 2.5" final collectors. Power estimates w/CamQuest shows 480hp@6000rpm 485tq@4000rpm and :D , get this :D - 400tq@2000rpm and 440tq@3000rpm.
Yee ha mate, that is one wicked flat torque curve, sounds like its a pretty good starting point for a street/stip combo ay. Be a hard one to beat off the lights, if I can get that power down that is :D Thanks for running those numbers for me.
 
#9 ·
Holden,
Camquest is a free 20mb program on the Comp Cams site you can download and use.....

just a comment:
because that software is so limited for entering actual values for parts used and tuning/operating variables I wouldn't bet the bank on results accuracy....
I did chuckle at the 400ft-lbs @ 2000 results eric posted, (1ft-lb per cube with a 280+duration cam and open plenum at 2,000) because if that was true there is no need at all for a high stall/slip convertor and deep gears for decent acceleration from a stop light or at the strip!
 
#10 ·
Holdon

red65mustang said:
Holden,
Camquest is a free 20mb program on the Comp Cams site you can download and use.....

just a comment:
because that software is so limited for entering actual values for parts used and tuning/operating variables I wouldn't bet the bank on results accuracy....
I did chuckle at the 400ft-lbs @ 2000 results eric posted, (1ft-lb per cube with a 280+duration cam and open plenum at 2,000) because if that was true there is no need at all for a high stall/slip convertor and deep gears for decent acceleration from a stop light or at the strip!
It does seem like a lot doesnt it, although Ive got the Virtual Dyno programe and mine shows 480ft/lbs @ 2750 and 500ft/lbs @ 3750. Still, they are only programes after all, not the real thing ay!
 
#11 ·
Red65mustang,
Yes, CamQuest is a simple and somewhat limiting but does allow you to input "custom" head flow figures so I used the AFR Eliminator 195 #'s from their site and their XE grind that is virtually the same as the VooDoo, and used the small tube open header option. While the HP #'s generated may be a little high, I've found them to be not far off from the fancier Engine Analyzer software I have, just faster, and will show the trends. Only bad thing is you can only test cams from CompCams, but all the major companies are pretty cookie-cutter in this respect.
As far as the high low end TQ #'s, Have you built many 400's with really good flowing heads?? I have, it's my favorite engine, TQ is the last thing to worry about,and you will usually have more than you can use from a dead stop unless you have a well-prepped suspension-- I practically won't give the time of day to the thought of a 350, it's a weinie engine-- 383 is my minimum even for a race engine. The 400 has plenty of TQ to spare, and a 280+ degree advertised camshaft is middle of the road to slightly small in a well prepped 400.
Holdon is using the new AFR Eliminators, and they are head and two shoulders above any other head company's off-the-shelf 23* head.
 
#12 ·
Holdon

red65mustang said:
Holden,
Camquest is a free 20mb program on the Comp Cams site you can download and use.....

just a comment:
because that software is so limited for entering actual values for parts used and tuning/operating variables I wouldn't bet the bank on results accuracy....
I did chuckle at the 400ft-lbs @ 2000 results eric posted, (1ft-lb per cube with a 280+duration cam and open plenum at 2,000) because if that was true there is no need at all for a high stall/slip convertor and deep gears for decent acceleration from a stop light or at the strip!
Just in reply to what you chuckled at before, perhaps you might want to have a look at the AFR website and some of their dyno figures. Even a 383ci with the same heads as mine, 9.5/1, 93 octane and a very similar cam was making 450ft/lbs at 2500rpms. It was running performer rpm, but in saying that I have got more cubes and higher comp. So I dont really thing 400ft/lbs at 2000 is out of the question after all. Having a high stall does get you up into a better hp area though, even if there is loads of torque everywhere.
 
#13 ·
Holden, eric,
to clarify why I chuckled:
yes you can/do have 400ft-lbs+ TQ actual at 2,000 with your motor combo....just lock out the timing at 36* and stomp the peddle to the floor for WOT 13/1 A/F ratio from a stop sign.....
(hard to do with stop and go intown traffic!!!!)

accelerating from a stop at anywhere near a normal rate the TQ is maybe possibly/approx 300ft-lbs (max?) due to that cam's overlap low Hg signal causing exhaust reversion into the open plenum intake at low piston speeds....
(simply stated the mix quality and quantity of actual cylinder fill for making best TQ power is yuch )

if it did make just 250ft-lbs at 2,000 (very probable) "at part throttle" that is still a tremendous amount of acceleration force applied to the tires from a stop sign:
250ft-lbs at the flywheel x 2.46 first gear(?) x 3.25 rear gears = 1998 ft-lbs twisting the hides!!!

so I do chuckle when people use WOT dyno results for describing a street cars actual driving performance.....

just a difference of perspective on the subject....
 
#14 · (Edited)
Red65mustang,
I agree with your last post, and I can be a little blind-eyed as I'm a drag racer(part throttle?!?!? oh ya, that's what I use up return road :drunk: ) and I don't really place a total trust on software programs, but they are more useful if you use them to plot the trends that show as you play with different parts specs in the same engine.
Your example is a good one, and also don't forget that the torque converter is multiplying at a rate somewhere between 1.8 to 2.3 the engines torque at rpms below stall. (I can't remember if this is a WOT issue tho) :thumbup:
 
#16 ·
Holden,
your welcome,
last thought/comment:
to work around the 3.25 gears for best part throttle street performance (to make the most low rpms Hg and flywheel TQ possible) do try to use as much base timing as the motor will tolerate....about 20-22 base and 16-14 cent....
(if that causes a hot start problem rig a simple single pole ign on/off switch so you can turn on the ign when it's already cranking)

http://www.gnetworks.com/v4files/barrygrant/Ign timingwithimages.pdf

eric,
I totally agree, any of the DD programs are excellent for comparing and seeing what a change does do...for gain and loss!
it also does bust alot of the hot rodding motor myths and help you see the "bang for the buck"
 
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