Pontiac Firebird 1967-2002 - 1975 Pontiac 400 Engine

The casting numbers show its out of a 75 Trans Am 4 speed car. I want to put this into a 68 GTO clone.

Does any one know the factory rated hp for this engine?

Who knows about the heads and porting? Are the head and porting considered high flow? I know 454 Chevy heads came in three types. They used different port diameters to get better flow. I don't know if Pontiac did the same?

I would like to be able to get 360 hp out of this engine by installing a more aggressive cam, bigger carburetor, headers, better distributor and plug wires.

Is this engine a good candidate for modification?

Thanks
BT

I would order an Ohio Crank that would make that block a 455. That is the nice thing about Pontiac's, they are all the same external deminsions as each other except the 267 and 301. The 400 uses small mains than the 455. You can bore it .030 and use 455 pistons with the small journal crank with the 4.21 stroke and build the 400 into a 455. The 75 block is still a good one, I wouldn't go with a later year block. Pocket port the heads, put a Bullet cam in it and go have some fun.

You can get to your goal with that engine but you will need
1) New Heads
2) New intake manifold
3) New cam
And all the supporting mods, headers, etc....

What is the casting on the head? Should have a number/letter combination
such as 6X or 4H

The casting numbers show its out of a 75 Trans Am 4 speed car. I want to put this into a 68 GTO clone.

Does any one know the factory rated hp for this engine?

Who knows about the heads and porting? Are the head and porting considered high flow? I know 454 Chevy heads came in three types. They used different port diameters to get better flow. I don't know if Pontiac did the same?

I would like to be able to get 360 hp out of this engine by installing a more aggressive cam, bigger carburetor, headers, better distributor and plug wires.

Is this engine a good candidate for modification?

Thanks
BT

Send me the block and head casting numbers and I will c what I can find out. Also look for the letter codes on the front of the block on passenger side where the head and block meet. Depending on the head # u may not need a large amount of porting.

Off the top of my head the HP rating should be about 180 TQ is around 310.
Heads should be 5C castings and the compression will be approx 7.6 to 1

You will definatley have to change the heads, the stock heads on later model Pontiac motors had huge combustion chambers and just wernt that great.

The casting numbers show its out of a 75 Trans Am 4 speed car. I want to put this into a 68 GTO clone.

Does any one know the factory rated hp for this engine?

Who knows about the heads and porting? Are the head and porting considered high flow? I know 454 Chevy heads came in three types. They used different port diameters to get better flow. I don't know if Pontiac did the same?

I would like to be able to get 360 hp out of this engine by installing a more aggressive cam, bigger carburetor, headers, better distributor and plug wires.

Is this engine a good candidate for modification?

Thanks
BT

You may also want to check out the BRE Trans Am. The base Ban 1 has a 455 Pontiac V8 rated at 450hp. From what I remember reading you can buy a turn key motor from the place they get them from if your not wanting to do the engine work yourself. I'm not 100% positive as this was from a few years ago, but if your interested I can find the magazine and article for you because it lists the website to get them. No matter what, good luck with it. I wanna make a 69 GTO clone when i'm older and have the money/experiance to do it. The Judge is one of the sexiest Pontiacs

http://www.krepower.com/

http://www.allpontiac.com/

http://www.jbp-pontiac.com/

http://www.highperformancepontiac.com/tech/hppp_0410_butler_performance_aluminum_pontiac_engi ne_build/index.html

360hp is easy like falling out of your chair. Should have 5Cs like mentioned earlier, not a bad head just low compression. Still has screw in studs and 2.11 intakes, the 1.66 exhaust valves wont hurt your at all. If you plan to run manifolds you can find some 7K3 heads, they are big valve and have a bit smaller chamber so the compression comes in around 8.8 to 9:1, the problem with them is they are missing the end bolt hols so headers will leak. They did that in 72, not entirely sure why but they are still a good head.

96 heads are from 71, they are identical to the 7K3 but they have the end bolt holes, both of those heads are one year only. 6X is the easiest to find since they came on 350s, 400s, and 455s from 76 to 79. You dont want the heads from a 455, you want 350 or 400 heads, because the 455 head makes for very low CR on a 400. Great for blowers though.

Earlier 72 cc heads will raise your compression too high for pump gas, some people can get away with it due to elevation or ambient temps, but if you are in an area that gets hot and humid, you will need some dishes to run 91-93 octane. Those castings are 670, 13, 12, 48, and 62. If you want to run ethanol those heads will work great, and its easy to set that up.

I have had JBP, now Butler Performance Group do a few sets of heads for me, they do outstanding work but yes you pay for that work. Kaufman does good work and there is a guy in Canada that does excellent porting as well. You can either send in your heads or buy a set from them already ported and set up with springs. Iron heads usually run about $1200-$1400 fully ported and assembled. You cant get into a Edelbrock or KRE aluminum head for less than $2k. KRE does make a D port that will work with inexpensive headers, round ports require at least a $500 set of headers.

To hit your 360hp number, you could go with a set of ported 6X-4 (350 heads) a Bullet 280-288 cam, call them they will know what you are talking about its an older UltraDyne cam. A little bigger one that works great is a 288/296 UD cam. A stock intake, a built Qjet, some headers, and an HEI. You could go with an RPM intake on the ported heads if you want to run a Holley carb. The Performer isnt any better than a stock intake, even the EGR intakes like your engine should have. The only thing it will do the stock ones wont is allow a Holley to be bolted up. Whatever you do dont go with a single plane intake.

A higher budget would be an Eagle stroker kit making it a 462 like mentioned above, no need to go forged on the crank at your power level. KRE D port aluminum heads, and a cam with 228 intake duration and 230-234 exhaust duration @.050 lift. It would idle like a stocker and make well over 400hp, and get decent mileage if you can tune a carb.

I have a 455 in the 70 GTO with ported 5C heads from a 400, RPM intake, Qjet, and the 280/288 cam, its at 4100lb with 2.93 gears and a stock converter it ran consistent 13.50s. 275 65 15 street tires were a smoke show even at 35mph and a Th400. Its a silly amount of grunt from that combination, and a stock intake would work just as well. I ran the same heads and cam in a 400 in the GTO and while I never made it to the track it was very similar in performance.

When I had my 80 T/A I had a cammed 400 out of a 68 GTO with #48 heads, pump gas in WV about 3000FT.

As stated above butler performance is good, also look into Edelbrock heads they are very good for Pontiac motors.

360hp is easy like falling out of your chair. Should have 5Cs like mentioned earlier, not a bad head just low compression. Still has screw in studs and 2.11 intakes, the 1.66 exhaust valves wont hurt your at all. If you plan to run manifolds you can find some 7K3 heads, they are big valve and have a bit smaller chamber so the compression comes in around 8.8 to 9:1, the problem with them is they are missing the end bolt hols so headers will leak. They did that in 72, not entirely sure why but they are still a good head.

96 heads are from 71, they are identical to the 7K3 but they have the end bolt holes, both of those heads are one year only. 6X is the easiest to find since they came on 350s, 400s, and 455s from 76 to 79. You dont want the heads from a 455, you want 350 or 400 heads, because the 455 head makes for very low CR on a 400. Great for blowers though.

Earlier 72 cc heads will raise your compression too high for pump gas, some people can get away with it due to elevation or ambient temps, but if you are in an area that gets hot and humid, you will need some dishes to run 91-93 octane. Those castings are 670, 13, 12, 48, and 62. If you want to run ethanol those heads will work great, and its easy to set that up.

I have had JBP, now Butler Performance Group do a few sets of heads for me, they do outstanding work but yes you pay for that work. Kaufman does good work and there is a guy in Canada that does excellent porting as well. You can either send in your heads or buy a set from them already ported and set up with springs. Iron heads usually run about $1200-$1400 fully ported and assembled. You cant get into a Edelbrock or KRE aluminum head for less than $2k. KRE does make a D port that will work with inexpensive headers, round ports require at least a $500 set of headers.

To hit your 360hp number, you could go with a set of ported 6X-4 (350 heads) a Bullet 280-288 cam, call them they will know what you are talking about its an older UltraDyne cam. A little bigger one that works great is a 288/296 UD cam. A stock intake, a built Qjet, some headers, and an HEI. You could go with an RPM intake on the ported heads if you want to run a Holley carb. The Performer isnt any better than a stock intake, even the EGR intakes like your engine should have. The only thing it will do the stock ones wont is allow a Holley to be bolted up. Whatever you do dont go with a single plane intake.

A higher budget would be an Eagle stroker kit making it a 462 like mentioned above, no need to go forged on the crank at your power level. KRE D port aluminum heads, and a cam with 228 intake duration and 230-234 exhaust duration @.050 lift. It would idle like a stocker and make well over 400hp, and get decent mileage if you can tune a carb.

I have a 455 in the 70 GTO with ported 5C heads from a 400, RPM intake, Qjet, and the 280/288 cam, its at 4100lb with 2.93 gears and a stock converter it ran consistent 13.50s. 275 65 15 street tires were a smoke show even at 35mph and a Th400. Its a silly amount of grunt from that combination, and a stock intake would work just as well. I ran the same heads and cam in a 400 in the GTO and while I never made it to the track it was very similar in performance.


Great post...

I've seen the stroker option mentioned a few times.. and if the OP was looking for 500hp I'd be on the bandwagon, but the kind of horses he's looking for can be done on a budget ( Pontiac budget... not Chevy budget).

The 400 set up in my sig makes the kind of power he's looking for and it was done fairly cheap.

It costs me around $2000 to build either a 400 or a 455, add about $700 for porting. Plenty cheap power if you know how to do some of the work and pick the right cam and tune it right. No need for deep gears or high stall converters, and a build like the OP wants can be done easily with stock rods and cast or forged Speed pro pistons. Get your rings, bearings, oil pump, gaskets, and pistons from Summit or Jegs, no need to buy the kit, but you can go that way too.

Next week I'll be dropping a 400 like he wants in one of the Trans Ams, ported 670 heads, .040 over block, balanced, stock rods, forged slugs, and a 288/296 cam. Its going to be an ethanol engine so I wanted more compression. It should easily make 360hp and a bunch of torque.

Guys:

The deal with the 68 LeMans & 400 5C heads fell through. I now have a 70 LeMans with 400 engine and 6x heads. The boss to left of the 6X is either a 6 or 8 I can't tell which. This number is the combustion chamber head size and should be 4 or 8. Did Pontiac make a 6 size?

The head date code is XE317. This came from the passenger side head front. When were these head made?

The block number is 50055. This came from passenger side rear below distributor where the head and block meet. When was the engine made?

It has a Rochester Q jet 4 barrel but where do I find numbers?

It has a Pontiac intake manifold number F207 near top radiator hose bracket.

I would like to get 360 hp out of it since that is what the 70 GTO was rated at.

Is the 6X head better than the 5C head?

I was told the engine came out of a 72 Grand Prix however I don't know if the engine is original to the Grand Prix.

I need to check the spark plug wiring back to the distributor. Is there a mark on the distributor that identifies #1 and does the numbering increase clockwise or counter clockwise?

Thanks,
BT

No 6X-6, it's either a four or a eight head ( sounds like you have an 8).

I believe my 6x-8 heads are D port. What's the difference between D port and O port? Is one better than the other?

Thanks,
BT

I believe my 6x-8 heads are D port. What's the difference between D port and O port? Is one better than the other?

Thanks,
BT

The Oval port heads came in the high performance heads, the D ports were the regular design.
The 6X's actually had decent flow numbers ( for stock heads of the era )...
BUT... you're probably at 7.6:1 compression

The only thing that has changed from my original post is I am using 6x-8 heads which have a CR of 7.6:1. So is a 366hp goal from a Pontiac 400cid with aftermarket cam, aftermarket distributor, aftermarket intake, headers, and aftermarket carb reasonable? Is there any reason to go with an aftermarket carb as opposed to staying with the Q-jet?

What is the CR of the 5C heads? Is it similar to the 6X-8 head? If I find 6X-4 heads would this require higher octane fuel or can I stay with 87 octane?

Thanks,
BT

I believe my 6x-8 heads are D port. What's the difference between D port and O port? Is one better than the other?

Thanks,
BT



the O ports only came on the Ram Air IV and Super Duty's. All the rest are D ports. Most aftermarket aluminum heads are O ports. The headers won't interchange between D and O.

The only thing that has changed from my original post is I am using 6x-8 heads which have a CR of 7.6:1. So is a 366hp goal from a Pontiac 400cid with aftermarket cam, aftermarket distributor, aftermarket intake, headers, and aftermarket carb reasonable? Is there any reason to go with an aftermarket carb as opposed to staying with the Q-jet?

What is the CR of the 5C heads? Is it similar to the 6X-8 head? If I find 6X-4 heads would this require higher octane fuel or can I stay with 87 octane?

Thanks,
BT

The 360+HP you are looking for is going to be a stretch with those heads...my 400 is very close to your goal and I have ported/milled 6X-4's.
You're gona need better heads.
My 6X-4's work awesome on 87 still.

What kind of budget do you have for the engine? Figure out how much you can spend on it and I will give you links to parts to get what you want, or close to it. :)

It's really not a budget at this time. I plan on replacing the intake manifold, cam, and adding headers. I would like to stick with the Quadrajet if possible. I realize the 6x-8 heads are limiting me. I can look around for 6x-4s and add those later. Would it be possible or cost effective to mill my 6x-8 heads down so they approximate 6x-4s?

Go ahead and give me some links to parts.

Maybe some one can tell me about the Quadrajet carb? I noticed two adjustment screws on the front of it. Is the screw on the passenger front the fast idle mixture screw and the one on the driver front the idle mixture screw?

Thanks,
BT

It's really not a budget at this time. I plan on replacing the intake manifold, cam, and adding headers. I would like to stick with the Quadrajet if possible. I realize the 6x-8 heads are limiting me. I can look around for 6x-4s and add those later. Would it be possible or cost effective to mill my 6x-8 heads down so they approximate 6x-4s?

Go ahead and give me some links to parts.

Maybe some one can tell me about the Quadrajet carb? I noticed two adjustment screws on the front of it. Is the screw on the passenger front the fast idle mixture screw and the one on the driver front the idle mixture screw?

Thanks,
BT

The two screws under the front of the carb are the idle mix, they allow fuel to flow when there isnt enough signal for the main circuit to flow fuel. If the main is flowing fuel at idle, the float is too high and it will run extremely rich, you can tell if the boosters are dripping fuel at idle. That is the same with a Holley or Edelbrock carb as well. A good place to start with those is 1.5 turns out, turn them to get the best idle quality, not necessarily the highest vacuum.

Qjets have enormous adjustability, it makes them seem complicated but really they arent that bad. They get a bad reputation from people not knowing what they can adjust or how to do it. Rather than giving you a huge wall of text about Qjets, I highly recommend picking up this book. You can get it from Amazon too.

http://www.cliffshighperformance.com/buy_book_2.html

That book will answer your questions and show you how to rebuild and tune a Qjet. Its worth the price believe me. I dont get paid anything for passing that along, its just the best book I have found on the Qjet and Cliff is willing to help out when he has the time.

The stock intake works better than the Performer, and unless you have ported heads you dont need the RPM intake. Trust me that you dont need a single plane intake, all they will do is load up with fuel, cost you mileage, and make it idle poorly. If you have a stock 4 barrel intake, keep it there is no need to buy another one. It isnt a chevy and the stock intake isnt a POS.

This company has budget based parts that arent overkill for a street engine. You can get complete rotating assemblies from them, and the 4" stroke is an easy swap for a stock block 400 and it will increase your torque considerably. Im planning a 4" stroke engine early next year using one of their crank kits.
http://www.pacificperformanceracing.com/index.html

The rods they sell under the Tomahawk name are a step up from the stock rods, and cost about the same as having the stock ones resized with new bolts. Cheap insurance and you can use the press fit forged pistons with them.
Rods:
http://www.pacificperformanceracing.com/products/pistons-rods-rings-bearings.html
Pistons:
http://www.summitracing.com/parts/TRW-L2262F30/

Good cam for a street 400 even with the 6X-8 heads. There is a really nice Crower cam, I can look that up on the Performance Years Forum if you want to know, its a bit more pricey but makes really good power in low compression 400s and 455s.
http://www.summitracing.com/parts/SUM-K2802/
You can use it with stock springs, but I would suggest Comp cams 995 springs.
http://www.summitracing.com/parts/CCA-995-16/

And a gasket set.
http://www.summitracing.com/parts/FEL-KS2617/

That cam with a set of headers, recurve the distributor, and a good tune on the Qjet should put you quite close to the hp mark you want. Its a budget build that if you keep it under 6000 rpm should live a long long time.

Im running these headers on my 70 GTO with a 455 that has ported 5C heads and a similar cam to the 2802 from Summit.
http://www.summitracing.com/parts/SUM-G9020/

For $1580 you can get fully assembled street ported iron heads in almost any casting from KRE. Scroll to the bottom, they do really good work as does Butler Performance and a number of other Pontiac gurus/specialists. Most will run about the same price as KRE though.
http://www.krepower.com/Pontiac_head_prices.htm

That's good to know I can keep my stock intake and save a few bucks. Now to start saving and eventually spending! Thanks to all who contributed.

BT

That's good to know I can keep my stock intake and save a few bucks. Now to start saving and eventually spending! Thanks to all who contributed.

BT

Hey Bro where are you located? If I see better heads or what not pop up in your area I'll let you know...
I always seem to run into good heads that are too far for me to get so I pass them down.

I am located in Houston, TX.

Thumpin455 posted:

Good cam for a street 400 even with the 6X-8 heads. There is a really nice Crower cam, I can look that up on the Performance Years Forum if you want to know, its a bit more pricey but makes really good power in low compression 400s and 455s.
http://www.summitracing.com/parts/SUM-K2802/

Thumpin455:

What do you think of these cams I found on ebay. Are they any better for my application than the SUM-K2802 you mention above?

http://cgi.ebay.com/ebaymotors/Pontiac-Ram-Air-Cam-Lifter-Kit-lifters-389-400-455-GTO_W0QQ itemZ190286805913QQcmdZViewItemQQptZMotors_Car_Tru ck_Parts_Accessories?hash=item2c4dfa3b99

http://cgi.ebay.com/ebaymotors/Pontiac-Ram-Air-Cam-Lifter-Kit-lifters-camshaft-400-455_W0Q QitemZ190314026701QQcmdZViewItemQQptZMotors_Car_Tr uck_Parts_Accessories?hash=item2c4f9996c d

http://cgi.ebay.com/ebaymotors/Pontiac-350-455-400-RV-Torque-Camshaft-Cam-kit-lifters_W0QQ itemZ190327914012QQcmdZViewItemQQptZMotors_Car_Tru ck_Parts_Accessories?hash=item2c506d7e1c

Thanks, BT

I think all the links are broken or removed. The XE cams work pretty good in a low compression 400, but for the most part almost any cam has less than 228@.050 intake duration it will work for what you want, but you want at least 218@/050 to see any improvement. There are lots of guys with 2802 cams in their 400s and 455s, its a good street cam. If you want the stock 068 cam or the 744 will work too, the RAIV cam makes it a bit harder to have power brakes in a low CR 400, but it works too. Rhodes lifters tame the bottom end on the RAIV stick. The Edelbrock RPM cam is a RAIV copy, but you can find it cheaper than Edelbrock. Crane makes copies of the stock cams, and those three are the most popular stock cams. The best cam I used in a low CR 400 was the UltraDyne 280/288, you can get it from Bullet Cams, just call and ask for it. Had a 400 with ported 5C heads, similar to your 6X, and the 280/288 and that thing was a beast, almost as much power as the 455 and a nice idle.

Thumpin455:

Here is ram air spec cam I found on ebay. It doesn't mention if it's ram air 3 or 4. It's intake duration @ .050" (INT): is 212 since this outside the range you gave (218 to 228@/050) I will stay away from this one.

Ram Air spec cam

Performance hydraulic flat tappet camshaft and lifter kit for 350 389 400 428 455 Pontiac Ram Air engines, 3/2carb hi output engines, 4bbl hi output. SPC7** and 16/HL1951S. Kit includes M-Select camshaft (**temporarily we are shipping Elgin equivalent or Comp Cams equivalent**) and Topline Hylift lifters. APPROX Cam specs are as follows:

* ADV Duration (INT/EXH): 285 / 298
* Duration @ .050" (INT/EXH): 212 / 225
* Valve Lift (INT/EXH): .408 / .406
* Lobe Centerline: 113/118
* Power Range: idle-4500
* If we ship Comp Cam, specs will be different but still offer similar performance
Upgrade your stock cam to a Ram Air spec cam...........

**
So a RAIV cam results in low engine vacuum? I can see how low vacuum is not good for power brakes. I have the selection narrowed down to three cams; SUM K2802, Ultradyne 280/228 or RAM AIR 3 or 4 cam. Can you tell me more about Ram Air cams, can I use stock springs with these? Are they a good choice for my application. Is there a difference between Ram Air 3 & 4 cams from a performance standpoint? I am concerned about the power brake issue with Ram Air 4 cams. Are the Ultradyne 280/228 and SUM K2802 considered ram air spec cams?

Thanks
BT

That cam is a bit small, it will grunt ok but run out of steam around 4000, you can make more torque with a little more cam, and the torque is what moves you. Maximizing the torque without moving the RPM range up is what you want.

The RA cams like the 041 and 744 are factory cams. The Ultradyne cams have custom ground lobes for Pontiacs, not a chevy lobe profile like most cams. The 2802 is similar to the 744 RAIII and just a little smaller than the 041 RAIV, it has 224/234 .465/.488. The RAIV is 230/240 .516/.516 with 1.65 rockers, its about as big as you want to go with stock low CR heads on a 400. You can run stock springs with these, but to get the most out of it you should change the springs. The Comp 995 works quite well, I run them on all my smaller hydraulic cams. Not too expensive and they will handle more rpm than your bottom end will. The RAIV will severely tax your stock springs.
http://www.summitracing.com/parts/CCA-995-16/


Except for the 041 most will perform relatively close to each other, they are big enough to make more power, but not so big as to move the RPM range or need the head ported. Pontiacs have a high velocity port, its longer than the sbc from the carb flange to the valve, so you need a cam that will work with the ports. You can fill the cylinder pretty well under 5000 and there isnt much need to rev over that, but since you have a 400 you can run a bit more duration than your typical 350 sbc. The RAIV will still work and make good power, but some people dont like the loss of bottom end and choppy idle.

The nice thing about the Summit cam is its cheap and makes good power.

I dont know if your are interested. but i currently have a 67 pontiac 400 that I will be parting out in the spring time. Its too much of a PITA and I will be hopefully putting in a LS1.

Ill have 143 heads, holley 750 carb, a comp cam (really lopey, thats all i know, i can get specs if i ever take it out) and other various things if you interested.

My power goal is 360hp. I am not sure which head to go with but I am looking at 6X-4 or maybe 143. I will probably have to mill the 6X-4. Is there a better head to use? What CR or chamber size should I be looking at? I would like to be able to use regular gas. At my power level is it necessary to port the head?

Thanks,
BT

Yeah, I am interested in the heads. Email me at gphjstep3@juno.com.

BT

[QUOTE=rtpassini; I dont know if your are interested. but i currently have a 67 pontiac 400 that I will be parting out in the spring time. Its too much of a PITA and I will be hopefully putting in a LS1.

Ill have 143 heads, holley 750 carb, a comp cam (really lopey, thats all i know, i can get specs if i ever take it out) and other various things if you interested.[/QUOTE]

Almost any head with a 2.11 intake valve will work, but the ones that would work best are 64, 7K3, 96, and the 6X-4 and the small chamber 5C and 4X with big intake valves. 360hp shouldnt be too hard even with a 6X-8, you just need air flow. The D ports flow pretty much in the same range no matter what casting it is, that leaves compression as the main difference. 11 and 15 would work but most of them dont have screw in studs. One thing guys used to do to lower CR with the 72cc heads was to cut a 3 inch diameter .010 deep dish in the top of the TRW style forged slugs, that drops it enough that you can run pump gas no problem. Its simple to do by chucking the piston in a lathe. You can find heads for sale over at PY and almost all the other Pontiac forums.

the 6x heads are a great port shape. clean them up and you'll be in great shape.

+1 on the Q-JET no way i would ever get rid of it.

The 360+HP you are looking for is going to be a stretch with those heads...my 400 is very close to your goal and I have ported/milled 6X-4's.
You're gona need better heads.
My 6X-4's work awesome on 87 still.

Ironfist, what hp is your 400? How much did you have milled off the head? Did you use a thin head gasket? If yes, what was the gasket thickness. What are the available gasket thicknesses?

I have found a set of 6x-4 heads that have not been worked on. What CR should I target to get 360hp? I read 6x-4 chambers are 93cc. I also read from Jim Hand's book a valve job will add 2 or 3cc. I don't quite understand why a valve job will add ccs?

I have been experimenting with CR calculators. One parameter is the gasket bore diameter. How much larger is this than the bore; 10 to 20 thousands inch?

I have a stock intake manifold that has D secondary ports. Since I am replacing the cam for a more aggressive one should I change the intake manifold to one with O secondary ports? At my hp level is there a significant difference in flow between the two intakes?

Thanks,
BT