Stock manifolds choking engine?

DJCarbine

New Member
May 4, 2005
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66 289 block with:

gt40p heads
Performer RPM cam/lifters
4105 600cfm carb
Performer 289 intake manifold

Its backed with a 5 speed and 4.11's


I did the head and cam change myself, set the timing to 16* BTDC and all was fine.
I didn't want to do the exhaust just yet, so I just bolted up the stock manifolds that have a cutout after the Y pipe going into the 40 year old single exhaust.

It feels like there should be more, the car runs great... but it feels like there is more power to be made.


Do headers really make a huge difference, say... over the stock manifolds? I know its a stupid question and the answer is YES, but on a modified engine are they really needed to utilize the full potential of all the parts?
 
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I think the stock single exhaust is your biggest restriction. Even stock dual exhaust systems have twice the flow capacity and even then they are far from overkill.

Stock manifolds aren't helping things much either, but for your engine, you could consider Hipo manifolds. From what I've heard, they fit the P heads (maybe with some slight clearancing for the plugs). Manifolds have a number of practical advantages over headers, including less risks for leaks, quieter, no ground clearance problems.

Powerwise, headers are better still, but there aren't many that fit the GT40P heads. It depends quite a bit on the engine. On a mild 289, (Hipo) manifolds are somewhat restrictive at the top end, but on a hopped up 347 they are a severe restriction, so keep that in mind when you're reading about comparisons.

Considering your intake and heads, I'd say that Hipo manifolds would serve you well, but since you're using an RPM cam (rather than a Performer), you may be better off with headers. Power-wise that is, manifolds are more practical.
 
Tri-Y headers and a dual 2 1/4 inch exhaust would be good for your setup I think. Definitely the stock manifolds and single exhaust are restricting you a lot, and you would notice a difference in the step up.
 
I am surprised no one mentioned that the Performer intake way too mild for that cam and 4.11 gears!
I bet that thing feels like a truck rather than a performance car.

You need headers and at least a Stealth intake... A mild single plane will work better though.

I ran a similar combo with 4.11s and T289 and it was great.
I swapped to a Performer RPM and it made the engine feel sluggish and lazy.

IMO, Stealth or T289 with full length headers will wake that thing right up.

The Performer is no better than a stock intake, so you are choked coming in and going out.

Edit:
There will be some that don't agree, so take it for what it's worth, but in my experience, a Holley will always trump an AFB (E-brock) as well.
When I have done the swap one for the other, it has always been day and night difference.
Your current combo is a little schizophrenic.
4.11 gears and RPM cam are pretty wild with all the other mild stuff.
You MUST match everything to get your moneys worth.
Dump the cam and gears, or bring everything else into line with them.
 
Slowing building it up on a college kids income, which is more or less negative :D

I will get an intake and headers, just wanted to see some suggestions on what/how much they will help. Thanks!
 
i agree with ratio411 for the most part. your current combo is rather off kilter, and improvements can be made. if you are on a budget, change to dual exhaust with 2 1/4" pipes at the very least as it will make a difference you can feel. the problem with the stock exhaust manifolds isnt so much the lack of flow, though that is a factor, but remember that exhaust gases have weight and header tuning takes advantage of that weight like the stock manifolds cant. you can improve the stock manifolds some by porting the runners, but headers will be your best bet. for the street i also recommend using tri-y headers. as for the intake, it is a bit mild for the cam you have selected, but will work well for the street. as for the carb, i prefer the edelbrock for the street, and the holley on the track. as for upgrading the intake, do it when the rest of the combo is done right. that way you can wait until you find a deal on a new one, for find a good used one for cheap.
 
I am surprised no one mentioned that the Performer intake way too mild for that cam and 4.11 gears!
I bet that thing feels like a truck rather than a performance car.

You need headers and at least a Stealth intake... A mild single plane will work better though.

I ran a similar combo with 4.11s and T289 and it was great.
I swapped to a Performer RPM and it made the engine feel sluggish and lazy.

IMO, Stealth or T289 with full length headers will wake that thing right up.

The Performer is no better than a stock intake, so you are choked coming in and going out.

Edit:
There will be some that don't agree, so take it for what it's worth, but in my experience, a Holley will always trump an AFB (E-brock) as well.
When I have done the swap one for the other, it has always been day and night difference.
Your current combo is a little schizophrenic.
4.11 gears and RPM cam are pretty wild with all the other mild stuff.
You MUST match everything to get your moneys worth.
Dump the cam and gears, or bring everything else into line with them.
Ditto plus two !!:nice: The Weiand is good, so are the older unequal length high rise dual planes like Ford's own A321 (current offering) The older C9OX, Shelby "Cobra" and Edelbrock's old F4B. I favor these over the single planes, but Dave knows what he's talking about here.
If you're strapped for cash, shop for a used intake, as long as no one's milled it let it get corroded thru the coolant passages or broken the bolt holes, they last forever.
 
I didn't mention the intake, just to keep his question on exhaust, but you're absolutely right, a stealth or better will help out too. Though, I would say that the exhaust is the bigger choker right now than the performer 289.
 
Finding headers for the GT40P's is the problem I'm having too. The FPA headers are a little steep at over $450.

Supposedly a place called Central Coast Mustang also makes and sells Tri-Y headers that fit the P heads. Anyone ever heard of them? :shrug:
Their website is down, and I don't have an exact price on the headers, but I'm seeing people saying $280-$320.

I'm going to give them a call in the morning, and if they're still selling them(or still in business), I'll pass on the info.
 
The guy from Central Coast Mustang does not have a good reputation.

I can tell you my personal dealings with him tended to confirm what I already had heard.

The stock manifolds, um.....are super restrictive. If I had to change one thing, that's the part I would change first.
 
the P heads are your biggest problem, it would almost be cheaper to buy better heads, I have heard some headers work but dont know which ones. Otherwise open up a set of hipo maniflolds and go a 2 1/2" dual exhaust system, that will make a big difference but not as much as long tubes and a 2 1/4- 21/2 exhaust. Add a steath or rpm intake and add a 750 cfm carb, on a dual plane it wont hurt torque a bit.
 
For exhaust I'd say either try the hi-po manifolds or a set of 69/70 351w manifolds and port them. They will flow just as good as headers. Then slap a 2.5" exhaust on that sucker. That will take care of your exhaust setup.

Lets hear some opinions on the RPM air gap intake manifold ?
 
I heard a rumor that tri-y's would fit but I can't confirm it.

When I looked at going that route, I saved $400 by using P heads and spend an extra $500 to do the exhaust again so I could make less power than just buying the GT-40x heads. That was ten years ago. Today, I'd just suck it up and buy AFR's.

I'd rather have non ported stock low compression smog era 1966 289 heads and headers than P heads and stock manifolds. The heads aren't that much worse, the manifolds are far worse.
 
i've heard as much as 10% increase in HP with longtubes and dual exhaust over stock manifolds . .

i might be wrong, but i noticed a considerable change when i threw longtubes and 2 1/2" with glass packs on my old stock 289
 
i've heard as much as 10% increase in HP with longtubes and dual exhaust over stock manifolds . .

i might be wrong, but i noticed a considerable change when i threw longtubes and 2 1/2" with glass packs on my old stock 289

There is no set number or percentage. Exhausts benifit motors differently. A stock motor will see less of a benifit then a heavily modified motor.
 
i'd start with jut getting rid of the single exhaust and maybe going with hi-po or 351w manifolds for now, you can always do headers later if needed. the P head is about the best box stock OE head there is but you also have to keep in mind that the chambers are so much better than a stock 289 head that the timing requirements are much different. in fact the 16* initial is probably way too high assuming you have have about 38* total. you'll probably pick up quite a bit by backing it down to around 10-12* initial for 32-34* total which is more in line with where the P head likes to run. the intake is probably a restriction as well but for now i'd play with the timing and at least go with dual 2-1/4" if nothing else and the hi-po or windsor mani's if you can swing it. i wouldn't go any bigger than 2-1/4" exhaust right now with the 289, though.