Im using hooker 1.750 super comp in 65 mustang 351w vic jr heads and the car runs 11.20 at 122 mph with 200hp nitrous it runs 9.82 at 138 mph if thinking about new headers i will sell mine at a good price they are used i can send pics
I think that this question is more complicated than just a horsepower number. For example, my 429 made 365 horses stock and a 1 5/8 header would have been too small for it. On the other hand my 289 is making around 375 horses and I am still running a 1 1/2 tri-y header(I know ... I need better headers).
I believe that the right sized header question involves the cubes of the motor and the rpms you are going to spin it at. The first factor has to do with what the volume of each exhaust pulse is and the third one has to do with how many exhaust pulses you are making per unit of time.
If the header is too small it will take too long for the exhaust pulse to evacuate the system and as your rpms go up the pulses will begin to stack up in the system. That will adversely affect scavenging. On the other hand if the header is too big it will create weak exhaust pulses that won't have sufficient velocity and will again adversely affect scavenging.
It would just be really great if Ford Muscle could do a header analysis of some sort and splain all this stuff.
As was mentioned above, it depends on your combination. Bigger engines and engines with bigger heads and valve sizes, need larger diameter headers to make good power. Smaller engines that RPM higher but do so with slightly restrictive heads and longer duration cams can get away with smaller diameter primary pipes. There are no hard and fast 'HP versus header diameter' rules.
N2Omike was running almost 130mph using 1 5/8th headers. He was running a 302 w/289 heads and a 200 shot of nitrous. His cam was also heavily exhaust biased.
On another site I go on someone brought up that doing a lot of exhaust port work was a waste of time. He brought this up because of a friend that just dyno'd at 600hp(if I remember correctly) with a small chevy 400 that only flowed 175cfm on the exhaust. Another member of that board played around on EA and switched exhaust effiency percentages and saw little or no gain by upping them. The original poster also brought up a man by the name of Billy Glidden who states that "the exhaust will find a way out", to further show some plausability to the notion. About that time I was reading car craft when they had the big block shootout. Curiously enough all the heads in that comparision flowed close to the 175cfm on exhaust number. They also all made around 500 hp. So I guess the question is have we been fed a bunch of BS about the intake/exhaust ratio all these years?
I did not say that engine ran with 1 5/8ths. What I was trying to point out is that we have been taught that we need big flow numbers to make lots of power. In this case and in the case of the car craft article all heads flowed in the 175 range on the exhaust. All engines made 500hp. Now if a big block can make 500hp with only 175ish cfm on the exhaust side why are we hogging so much out on the small block. Now look back at Mikes combo and you will see he ran 1 5/8ths a cam heavily biased to the exhaust(by 12 degrees) and ran 129+mph. I am pretty sure he told me his car weighed in the 3200lb range. That puts his horsepower over 500. Hopefully the machine shop will call with his flow numbers pretty soon. That would validate this even more if it was around that number(which most small ford heads are with the work he has done.) Now how does this translate into header size? well if flowing 175 cfm can get you over 500hp I would think a good 1 5/8th header should be good for more than your average street car.
As far as the lift the 175 came at, being the poster was not the owner of the car I have no reason to not believe him that it was peak flow. Do I think it happened? Probably but not nessicarily, the important thing about him posting this was it made me watch the exhaust flow in buildups I see and the 175 number seems to come up quite a bit.
You really have to look at many things to consider whether your header is too small. If you've got a well-designed long tube header that actually uses the exhaust pulse from one cylinder to help evacuate another, than you can get away with smaller tubes. On the other hand, if you've got headers like I do (351w 1 3/4 custom) in a maverick that are fairly restrictive, than the smaller 1 5/8 tubes will restrict it. My 1 3/4 tubes are restricting the 351 due to their design constraints (not much room in a maverick chassis), but I have no other choice without going to a mustang II front end. Just something else to think about, if it makes any sense
My car weighs 3300lbs with me in it. The analyzers say it's pushing around 550hp with the nitrous on. All drag strip runs were made with the 3" dual exhaust hooked up.
The smallish headers, along with the nitrous induction and full exhaust were all big reasons for going with such a 'lop-sided' cam with 12 extra degrees of exhaust duration. It's a flat tappet solid that measures 236/248 @ 0.050" with a 110 degree lobe seperation. I would really like to try out a set of 1 3/4" headers, but they just aren't readily available for my application. At least the Super Comps (which have an outside diameter of 1 5/8"
are made of really thin metal, which opens their inside diameter a bit over other makes.
My 'new' (most of the parts have been on the shelf for years, but the old engine just wouldn't die) engine combo will have a set of TW heads that I've been grinding on, and should be a tad hotter than the last powerplant. We'll see if the 1 5/8" headers keep it from making any more power. I'll be happy as long as it runs as good or better as the old engine.
Those new 1 3/4" Hooker #6208 headers for 351W swaps have got the wheels turning. They would probably work with a 302, as long as it had raised exhaust ports (TFS hi-ports) or a stroked 351W...
A 400ci 351W would probably tear the car in half...
9.50s at 143MPH with 351w in 3200# car and 175HP nitrous shot using 1 5/8" headers and full 3" exhaust. The cam had 10 more degrees of duration on the exhaust and almost the same lift. I say high exh flow and large headers is more a bragging right than a necessity. Sure 1 3/4 probably would have helped out some but the 1 5/8 headers with full exhaust surpassed the "HP to header diameter rule of thumb". BTW, I saw a STOCK unported CJ head 472 make 680HP on the dyno with only the stock 156CFM exhaust port using 1 7/8" primary headers with 3" exhaust, a solid roller cam, and tunnel ram with 2 850 DPs. It will push the exhaust out. Intake flow is more important.
**** your talking about the small block shoot out. The one I was refering to was the big block shootout. The big block one included a pontiac, olds, and buick, along with the chevy, ford and mopar. All heads flowed around the 175 cfm number I speak of.
I agree with you all saying that the size depends on so many things.
Just to simplify :I still think a naturally aspirated engine should use, if possible and practical, bigger than 1 5/8" headers if the goal is to make more than 450 horsepower...
Ville though I don't disagree with you, having a larger diameter header is of even more importance on a power adder car. Neither poster above was making a little bit of power N/A and then spraying. They were both making very good power for their combo's and then putting a 175-200 shot of nitrous on top of that and made even bigger power. Don't get me wrong, if I am starting from a scratch build, I would go with a 1 3/4 on something making that kind of power. But if you already have 1 5/8ths that will fit properly, the money to upgrade would probably be best spent elsewhere.
<font size=-1>[ This Message was edited by: jdsgallops on 4/21/02 12:13pm ]</font>
Definitely use the 1 5/8" if you got them already. Too big hurts you more than too small but 1 5/8 will handle nearly 600 naturally aspirated HP if the exhaust ports aren't bigger than the header flange. 9.50s at 143 with 175 horse nitrous hit in 3200# car was done already but 1 3/4" headers weren't available at that time otherwise they would have been used. I know of a Mustang II that ran 11.20s off the bottle with a 12:1 289 and fully ported 2.02/1.54 size valves in 351W heads and solid flat tappet cam with 4500 stall and 4.66 gears using 1 1/2" headers. Greg Cook is running a 13.5:1 6600RPM 557 while only using 2" headers. Sure, bigger would be better but the fact is those small 2" headers are handling the job just fine.