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Bowtie or Die!
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Discussion Starter · #1 ·
Im building a 388 stroker for my 67 nova. I got a deal on the headers I have. there headman they go down by the engine not over the fenders like some nova headers. The thing i was wondring about is that my headers tubes are 1 1/2 ". We are tring for at least 430 to 450 hp for this engine. Flat tops, pro 1 dart heads, .480 lift 230 [email protected] Performer rpm, 700 dp holley,Are theses headers going to kill my HP? I know resricted exhaust kinda helps tourque. Any ideas?? Thanks for any replys
 

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Those headers are not big enough. The minimum for that engine would be 1 5/8. If you really want over 400 hp, then you'll be better off with 1 3/4. If you compare 1 3/4 headers to 1 1/2, you'll see there is quite a bit of difference. The bigger tubes are a lot harder to make fit, that's why they cost so much. But if you really want to make power, you gotta match it up properly.

Other than the headers, your combo looks pretty good. In my opinion, it wouldn't be a true 450 hp though. More like 375-400. I would think a bigger cam would be needed. Just my .02
dh
 

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Bowtie or Die!
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Discussion Starter · #3 · (Edited)
Thanks for the reply. I know that the 1 1/2" headers will cut into my hp Im just wondering how much. Ive researched whats available for my 67 and theres not alot out there as far as headers go. Hooker makes a super comp thats 600.00 from jegs that runs over the fenders and cant use a heater core its 1 3/4".I guess I may be stuck with hedman. as far as HP goes 400 hp should be no problem with tons of Tq well over 400. Check out these dynoed build ups, awesome stuff .........http://www.ryanscarpage.50megs.com/combos1.html
 

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headers

The small tube headers wont hurt you that much. For a street engine, as a rule of thumb, its a waste of time to go with primary tubes that are larger than your exhaust valve, since that is the biggest obstacle. Its difficult to get more to flow through your exhaust system than the valve seat will allow.
Most likely you have 1.6 exhaust valves, which means you have a 1 1/2 inch seat diameter anyway.
Going to large tube headers (read that 1 3/4 and over) on your engine can hurt your bottom end a lot more than the 1 1/2's will hurt your top end.
Were I buying the headers and didnt have any, I would opt for the 1 5/8', but since you have the 1 1/2's, I would go ahead and run them.

I ran your engine through my dyno program. Since I didnt have all the cam specs, I had to find a compareable cam in one of my cam books.
Basing your engine on 10:1 compression, with closed exhaust,

Peak [email protected] RPM
Peak Lb Ft [email protected] RPM.

I would say you are if not at it, extremely close to your goal.

OOPS!!!!!!!! The HP and Torque figures I gave you were for a 350 instead of a 383.

Your 383 reads:
Peak HP 410 @ 5500 RPM
Peak Lb Ft 424 @ 4500 RPM

From where I sit, it looks like you made your 400+ goal.
 

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I understand your dilemma and when the alternative costs $600 I can totally see why you want to use the 1 1/2 headers. Without a direct dyno comparison (real dyno, not virtual) it's hard to say exactly how much power they will be costing you.
I'm not sure I'm convinced by the statement that the primary tubes do not need to be larger than the exhaust valve. First of all, 1.60 valves are much closer in size to 1 5/8 (1.625") tubing. The 1 1/2 tubes are significantly smaller than the 1.6 valve. More importantly though, the flow around the valve and the flow in the tube are totally different and I think comparing them is an oversimplification. The reason is because flow restriction is also related to length.
Most importantly though, according to Poiseuille's equation, resistance of flow (R) is directly proportional to the length (L) and the viscosity (h), and inversely proportional to the radius to the fourth power (r4).

R ~ (L * h)/r4

This means that the diameter affects flow to the power of 4! To put this in perspective, tubing 1 1/2" will have almost twice the resistance of 1 3/4" tubing. 1.86 times the resistance by my calculation. With the headers having almost twice the resistance your flow will be significantly hindered. If this was a daily driver or an engine made specifically for torque, this wouldn't be much of a problem. But if you want a real 450 hp, this will be a significant hindrance. I'm not saying you shouldn't go with the small headers, but just be realistic in what you expect from them. If you go to the drag races, you won't see many 12 second cars with 1 1/2 primaries.
-disclaimer: I'm not a physicist. I think my calculations are correct, but I'm wrong all the time...
dh
 

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Bowtie or Die!
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Discussion Starter · #8 ·
I think given the alternative of, no heater core and spending 600.00, I will roll the dice and run the 1 1/2 headers. I can always upgrade later.My car weighs in around 2800lbs with my 3.55 gears and 400+ hp and 400+ tq 12s will be NP!:mwink:
 

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virtual dynos

As for virtual dynos, they arent perfect but they do give some very strong guidelines to work with, if its a good program.
The figures I gave you are, of course, flywheel ratings. Differential readings would be somewhat lower, due to parasitic drivetrain losses, as much as 50-70 hp, depending on the drivetrain.
As I said earlier, were I purchasing the headers anew, and didnt have any, I would have opted for the 1 5/8ths primaries, however, you arent being hurt that much on the upper end by the smaller tubes, and since you already have them, thats a major factor in the equation. Im not an engineer or a scientist either, but having been around racing and hot cars for some 40+ years, I have seen a lot of stuff tried and proven as well as tried and going down the toilet. From years of observing and experience as well, I have a good idea what works and what doesnt, but I am far from being an expert on it.
As for the comments about the 1.6 valve being bigger than the 1 1/2 inch primaries, I already covered that with the throat size of the valve seat.
I remember many years ago, scientists saying it was impossible to build a car capable of going the 1/4 mile from a standing start in 8 seconds flat; then 6 seconds, then 5 seconds, and today we have street driven vehicles doing it in 7 seconds.

As a matter of record, there have been many published dyno tests on headers. Probably the best one I ever saw was in Circle Track magazine back in the mid 80's. They did a very comprehensive test of headers of all different sizes of primaries, on a typical test 350 Chevy engine. Power band wise, stock car engines for short track, arent all that dissimilar to hot street engines. You could plainly see in the test where the headers had their most effect with their power curves. While 2 inch headers look awesome and all that stuff, unless youre going to be turning 4000 rpm plus all night long, they would have no advantage over 1 1/2 or 1 5/8ths headers and almost no advantage over 1 3/4 primaries, until you get into the areas of over 7000 rpm +. The 1 1/2 inch headers showed the best power between idle to 5500 rpm. At about 2500 rpm the 1 5/8ths were the same until about 7000, and on and on. The losses using the 1 5/8ths on the lower end vs the 1 1/2 werent that drastic.

But as I said previously, the 1 1/2 inch primaries wont hurt you as bad on top end as 2 inchers will hurt you on your lower end.
 

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Hooker makes 3 headers for your combo: 2122HKR (1 5/8"), 2243HKR (1 3/4") and 2214HKR (1 3/4"). I checked around on prices and found the 1 5/8" around $339 and the 1 3/4" around $405. They are all Super-Competition headers. Check Summit Racing and they have pictures of all 3 headers along with what exactly they fit.
 
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