Any of you guys running single exhaust on your hopped up big blocks??? If so what size did you end up doing?? I cant run duals because of my narrow frame and dual t-cases. I was planning on running dual 2.5" mandrel bent pipes to a flowmaster Y pipe to single 3.5" . The muffler was going to be a Hooker aerochamber. Do you guys think this will be choking a 450HP BBC??

If you can I would try and get 3" right off the header and you should be able to go into a 3" single or go 3.5 if you want. I don't know how well those hedmans flow so I can't comment on it but I would check out Direct Natural Airflow, they are awesome!

Thanks for the reply! I think the blockhugger headers I am planning on getting have a 2.5" collector so thats why I was gonna stay with 2.5" head pipes. The Hooker muffler is supposed to be the best flowing chambered on the market but the DNA muffs look pretty sick. Are you running long tubes/mid's or huggers??? Single or dual exhaust??

I have a single exhaust on my 454....the sizing was not by choice but it was all I could manage. I have a dual mandrel bent 2.5's into a single 3.5 tail with a Dynomax Race Magnum muffler. Dual 2.5's flow the same as a single 3.5 - cross sectional area is the same. Just be sure to do a really well blended Y pipe.....when done correctly, you can actually have one bank of cylinders scavenge off of the other, making both headers work together...in essnece, your Y pipe will be one big collector. Whatever you do, don't dump into a single 3"! It is way too small and cross sectional area is way smaller than a 3.5. This exhaust sizing is perfect for around 400 hp but if you have a 500 hp + engine, this exhaust sizing will definitely be choking the engine. At that point, dual 3" and/or a single 4.5" are what you need to be aiming for....but good luck getting a 4.5" stove pipe to fit in a Jeep!

I'm not sure what has been done to your BBC but block huggers are usually terrible for scavenging with primaries that are too short and collector volume that is way too small....there are other headers that will fit that will be much better and will offer much better characteristics in these areas. If you are pumping a true 450 hp, these headers are a very poor choice!

I use some Headman 3/4 length Chevelle headers....the primaries still aren't as long as I would like but they are still over twice as long as a block hugger and have 3" collectors.

I'm not really much of a fan of chambered mufflers anymore....I have had many Flowmaster mufflers over the years and also owned a few different Race Magnum mufflers (non-chambered straight through muffler)and I can say with first hand knowledge that the Race Magnums are actually quieter than any two chamber Flowmaster....and since it is a straight through design, impedence of flow is a non-issue. Noise level is subjective though, they are both somewhat loud, the Flowmasters are just louder. I have no experience with the Hooker Aero Chambers.

CTD, Thanks for the reply. I dont want to run blockhuggers but I dont think I have any room to crossover the pipes exept for under the oil pan. I will take a real good look tonight. Where do you cross over your pipes to the Y pipe?? Here is some specs on the 454 I am putting together:

454
9 to 1 comp ratio
Ported closed chamber heads....2.19/1.88 valves....flow 294/206 cfm
Edelbrock Performer RPM intake
800 cfm Q jet prepped by "Carbshop"
1.7 full roller rockers
1 3/4" blockhugger headers
2.5" dual to 3.5" single exhaust..mandrel bent
Comp X4270H-.....270/278.....226/234.....050.......554/564 lift......111 lobe
Proform HEI
TH400....203/300 doubler...all 32 spline
2000 stall converter...might end up building a s/p 400

Desktop dyno says.....

447 Hp at 5500 and 529 tq at 3000 rpm

500lbs at 2000 rpm and holds over 500lbs from 2000rpm to 4500rpm

Ahh yes, the crossover....a challenge indeed! My crossover actually runs over top of my tranny and connects with a custom Y collector on the drivers side and right be side the tranny. I won't lie - it is tight.

As for your engine, while I'm not familiar with your cylinder heads specifically, I have to ask: what piston are you running to get the compression dropped down to 9:1 with closed chamber heads?!? And at what lift are you getting those flow #'s? 294 cfm on the intake is pretty stout and should be able to support more than 447 hp. That cam is somewhat similar to what I'm running other than yours is a dual pattern cam that favours the exhaust - which is a good thing in this case. And yes, you really need to rethink those block huggers....they really are way off the mark for what should be on your engine....it is unlikely that you will make 447 hp through those headers.

CTD,

The short block is a stock late 80's that has like 7.8 to 1 comp with open chamber heads. So with the closed chamber heads its bumps it up to like low 9's . Its a budget motor. But the heads are from a 68-69 427 motor. I got a score on them. They were ported buy shop called Gromm Racing Heads down here in california. He does good work. Here is the flow numbers on them:

Intake:

500-278
550-286
600-293
650-294

Exhaust:

500-180
550-189
600-199
650-207

Ok, that explains the compression....you have got some deep dish psitons that you would be lucky to have 7:1 on a stocker 125 - 130 CC peanut port smogger head! Now I see where the 9:1 and closed chambers are coming from!

Anyway, it looks like you have some stout heads there....lots of good low lift flow there. Another thing to consider when looking at block huggers is those 1.75" primaries....I don't even think they will come close to matching the exhaust port size on those 427 heads....I would be willing to bet the ports will want to see a 2" primary and at the very least, a 1.875" but even then, I doubt the 1.875" will be a good match. Don't take my word for it....measure the ports yourself - you might be surprised at how large they are.

Thanks CTD, I will measure the ports before purchasing any headers for sure. Could you send me some pics or post some of how you did your exhaust??? Any part numbers on the mid lengths you are running??


Hey by the way what torque converter are you running???

Well, I looked through the pics I have on my computer and there really aren't any that show how the exhaust is routed....it isn't exactly easy to photograph due to it's location but I will see what I can do. As for the header part #, I'll do some digging. And I replied to your TC thread as far as that goes.

CTD.....I laid under the rig for a while and there is just no way to have my exhaust crossover anywhere besides the oil pan. Just NO room at all. So.....I will have to sacrifice some power for fit. Oh well..

Yeah, I know how it is....that sux. But not all is lost. There is a street rod/vette header that is pretty much a block hugger but uses a special collector that allows for 2" primaries (I think) and a 3" collector. If you are interested, I think I can get more info for you....let me know.

Alright, nevermind. I found the info about the header in question.

Hedman part #68510, 55-57 Chevy "Tight Tubes", 2" primaries, 3" collector. This will be much better than any other block hugger style that I have come across since it will likely be a much better port match (you still have to measure to verify my assumption) with a much more reasonable collector volume - as good as it gets with this style of header, anyway.



Crap...that file extension doesn't work on this board! Ok, here is a link to the image.

Hedman #68510

Thanks CTD! I will check those out when I go header shopping ina few weeks!