shaneandrew

im building up my 305,416 heads with 1.94 valves,hand porting,268 cam,weiand dual plane intake.i have flowtech shorty headers with 2.5 collectors.i already installed a cat-back with a 3 inch intermediate.keeping the engine in mind,should i stick with the flowtechs and get a normal 2.5 ypipe,or dump them for hedman shorties with a 3 inch collector and a fabbed 3 inch ypipe.would 3 inch header back be too big?

five7kid

iTrader: (14)

Seems like it would be worth replacing the headers to me. 3" worked well with my built 305, although it was probably more than it "needed".

shaneandrew

i might go to a 350 down the road,so it would be nice to have that there,i just wanted to make sure it wouldnt be detrimental to the 305

subroc

iTrader: (2)

I also have a 3 inch system on the 305. If it were true dual i would say dont but since its not go for the 3.

Air_Adam

The Hedman Y-pipe is a good pipe, but has the 2.5" intermediate pipe. Good news is that its easy to chop it off in the middle of the Y and replace it with a peice of 3" pipe. I did that with mine. Worked great. The headers are nice too... easy plug access, and good clearance all the way around. I have these headers on a 350 and they work well. 1-5/8" tubes, 3" collectors.

For the catback - go 3". It may cost slightly more, but will help get the most out of what you have, and gives you lots of room to grow.

shaneandrew

I have a 3 inch catback,thats why i wanted to go with a 3 inch header and a 3 inch y.so your saying the pipes coming off the headers are 3 inch then necked down to 2.5 inch?that does sound like a very easy problem to remedy

Air_Adam

The collectors are 3", the pipes off the collectors are 2.5" IIRC, then into the Y, then a single 2.5" out. Change the outlet of the Y to 3" and your golden.

subroc

iTrader: (2)

I dont think they make a 3 inch collector header for our cars but i could be mistaken. Also u dont want to go that big. You will lose all your backpressure and your car will run like poo.

Air_Adam

Backpressure is bad.

myvmax1

The way I always understood it, was that your amount of back pressure is relative to your specific tune. A little back pressure can help build torque, and keep your peak power lower in the RPM range. But too much can have very negative effects on an engines efficiency and longevity. Long story short, Full length headers with 1 5/8'' primarys and 3'' collectors backed by a full mandrel 3'' cat-back is most likely too much pipe for a bolt on 305. Exhaust velocity will probably suffer, but not to the point that you will notice any big losses; and your plans for something of a few more inches (350+) will take full advantage of this increased flow capacity.

Max,

gregsz-28

iTrader: (1)

Look at:
http://store.summitracing.com/egnsea...4294791244+115

subroc

iTrader: (2)

Wow never saw those. Ok i stand corrected. But i still dont think that would be a wise idea to run a collector that big. You would lose low end.

shaneandrew

so i see alot of people have alot of info.at this point,would it be a good idea to pick up the hooker shorties,because ive heard nothing bad about them,and stick with the 2.5 hooker y-pipe.that way im not losing backpressure on the 305,and ill have room to grow should i decide to go with larger displacement down the road?

five7kid

iTrader: (14)

Backpressure is what you want to lose. It's the whole point of exhaust improvement.

subroc

iTrader: (2)

Yea to a point. Once you cross over a certain point however you hurt your vehicles performance. If we were talking a boosted car then yes lose as much backpressure as you can. But since we are talking an N/A SB you want to go big but not too big. The optimal exhaust, as i understand it, is an exhaust system that has a venturi effect, pulling exhaust out of the cylinder completely. If you go too big you lose this effect and engine performance suffers.

hotrod7687

[QUOTE=subroc;4006099]Yea to a point. Once you cross over a certain point however you hurt your vehicles performance. If we were talking a boosted car then yes lose as much backpressure as you can. But since we are talking an N/A SB you want to go big but not too big. The optimal exhaust, as i understand it, is an exhaust system that has a venturi effect, pulling exhaust out of the cylinder completely. If you go too big you lose this effect and engine performance suffers.[/QUOTE]

Thats true, but its loss of exhaust velocity, not backpressure that causes that.

ACCLR8N

iTrader: (8)

Yes, too big and you lose velocity driven by the engine pulses. No velocity, no scavenging vacuum helping empty the next cylinder. Be certain you are going to a built 350 (400HP) to even consider the 3" exhaust. Its loud.

shaneandrew

thank you for all the info guys.i think im going to go with the hedman headers referenced earlier in the post,and the y-pipe recommended for it.ill wait until i drop the 350 in to start thinking about a full 3in setup

Fullsizewagon

Having a 2.25" system on my "190hp" L69 305 kills torque over 3500rpm whereas the 2.75" it's supposed to have allowed it to pull strong to 5000.
So I say definitely go for 3" from the Y-back.

shaneandrew

I already have the 3 inch from the y back,so im all set there

stroker_SS

There is no Venturi effect in the exhaust system, you would have to have a venturi in the exhaust system which would be restrictive. What your thinking about is the Helmholtz law, there is no "vacuum"(in fact there is no such thing as a true vacuum Except in outer space) in the exhaust, just a "lower (less than the exhaust pressure leaving the cylinder head) pressure induced at the header because the momentum of the exhaust leaving one of the header primary tubes was enough for the following exhaust pulse in the next primary tube to sort of ride it's coattails out the collector. This is where your exhaust velocity counts, at the collecter, although having the rest of the exhaust "tuned" is also beneficiary.

What's important is that the exhaust pulses don't get in the way of eachother, but you don't want the exhaust losing velocity in the exhaust pipe either and stagnating(this would force the following exhaust pulse to push out the preceding pulse).

subroc

iTrader: (2)

Yea sorry i had the wrong term lol. It was the only way i could describe it at the time. For some reason i couldnt think of the right way of describing it but at least you all got what i meant to say.

cavazos31

iTrader: (4)

Its my understanding, that exhaust gas is hot and you'd like to keep it hot throughout the exhaust system. Thats one of the reasons why people use heder wrap or a ceramic coating, to keep the gases heated. Cold air is dense air, and dense air is heavy air. Furthermore, as gases cool down, velocity starts to slow down. You don't want your engine to be "pushing" a heavy mass of cooler exhaust gas out of the tailpipe. Too large an exhaust pipe, one which cools down the gases, may actually decrease velocity, since your engine is working harder to expel the cooler gases.

Also, in an attempt to keep exhaust velocity high the pipes off the 3" collector, reduce to 2.5". Many exhausts systems work in a similar fashion. Much like the ducting used to heat a large warehouse, the farther away the "pipes" are from the "engine", or the farther away the ducting is from the heater, the smaller the "pipes" or ducts. This is done to keep velocity up the farther down the system you go and is very similar to the way a car exhaust works, at least in theory. Not only are the smaller pipes helping by creating a "funnel" of sorts, which helps with velocity, the gases stay hot, further helping exhaust velocity.

I agree with what your doing by holding out on full 3" until you get your 350. That engine would benefit much more from it.

subroc

iTrader: (2)

Header wrap is primarily used to keep underhood temperatures lower. It does the same thing as ceramic coating basically...except with ceramic coating you are not welcoming moisture to rot your headers away to nothing.

cavazos31

iTrader: (4)

I said it was one of the reasons people use header wrap, it serves multiple purposes. While it is used to keep under hood temps down, header wrap is also designed to keep heat in the exhaust. Wrapping headers maintains hotter exhaust gases that exit the system faster through decreased density. I don't drag race, but I've seen a lot of drag cars that have wrapped headers. From a drag race stand point, I'd think keeping the exhaust gases from cooling and maintaining a high exhaust velocity would be the primary purpose. I was assuming header wrap would primarily be used by people interested in drag racing. I'd never wrap headers on a street car, I've heard they rot the headers as well. I'd just go with the ceramic coated, like you said. Perhaps I shouldn't have paired ceramic coating and header wrap together. Truth be told, I was thinking more along the lines of header wrap, but I'd just imagined they serve a similar purpose.