Looking at upgrading my big block with a set of AFRs. I am torn between their 305cc as cast and 325cc as cast. Some may suggest 325cc as being a bit large, but I may plan to run a 496 down the road. My current set up is as follows:

'90-'91 Gen IV Block 7.4L
GM Forged Crank
7/16" Dimple Rods
Pistons are L2349F30

Cam is custom ground based off of 288AR cam on a steel billet core with iron dist gear.

actual cam specs are as follows:
Adv Dur: 291/291
Dur @ 050: 246/246
Lift: .626/.626

Intake is an Air-gap Performer RPM for rectangular port heads.
I am also running a FAST EZ EFI set up.

Heads currently are closed chambered heads that came off of a 396/375HP. Heads have been ported and polished. Intake runners are approximately 330cc. The heads have also been modified to accept the 2.19/1.88 exhaust valves. Piston to valve clearance was measured at .120.

Will I have any issues by going to a 2.25 or 2.30 intake valve?

What would be the best choice between 305cc and the 325cc?

Car is 100% street driven with the occasion spirited runs..

 

Afr 305 will be plenty....and likely have better power in the low-midrange. You can easily make 650-675+ HP with the 305. Ofcourse you can have them ported/bowl worked for for 750+...

Save the 325s for a 540" or bigger build.

I would atleast get the 305s with the cnc options. You didn't mention (or I missed it) if th cam was HYD roller or solid?

Should be a fun build!

 

A lot also depends on your pistons, are they domes? You may have to mill the AFRs to get the chambers down if you are running flat tops. You still may not get the chambers down enough compared to the closed chamber 396s.

 

If you insist on using the rect port intake, get the 305's. You might want to consider the oval 290's though. With better mid-lift flow numbers and smaller port runners, they would perform better than the 305's. But since they're a full CNC head, they're a bit spendier.

 

IMOE, it's best to choose the heads based on the engine you plan on building in the present. And, in the event you're not going to rev a 7.4L beyond 6000 very often (if at all), big ovals are really the better call. Concern is average port velocity. And, if rectangles do get the nod, I'd keep port volume on the small'ish side.

FWIW (not exactly apples to apples), the solid roller 427 BB I'm building up will be limited at 7200, and if not for dual purposes of street machine and track days (AX/RR), as well as the fact I already had them on hand, I'd be looking at rectangle ports with less than the 315 cc's mine have (RPM 454-R's). If making the decision today, and if based only for street duty, I'd likely be hard pressed to justify them.

 

Your cam is going to want to peak under 6000 rpm, you've got a dual plane intake. It's not a high RPM screamer combo. I'd use the 305's but at least shoot for the CNC chamber option. It will make a killer engine combo.

You might look into the Brodix line also..more good stuff over there too.

Not sure how you plan to use it, gearing, trans etc....so you might even be better off with a smaller head.


JIM

 

My last motor was a 468 2 bolt main block, Eagle rotating. ended up with 12.0-1 compression with the AFR 305 rec port heads. Cam was a cam motion solid roller .691/.684 lift with .262/272 @.050 on a 112 lsa. I used a soggy bottom dart intake and 950 hp carb. we made 708 hp and 622 tq. This was a street car too, i wish i would of went with the oval port heads and not the rec ports for the street. Ovals will give you a ton more torque, and if your not going to buzz the motor to 7000 its a better head. I also switched to a vic Jr intake to help pick up some air speed and bottom end.

 

FRSTR90. I am using Brodix Race Rite ovals on my 489 with RPM dual plane Air Gap and 950 DP Proform carb. Cam is a fairly conservative hydraulic 236/241 @ .050" with only about .550 lift. On the dyno this pulled 528 bhp @ 5850 rpm and 565 ft/lbs torque @ 3850 rpm. I wanted to be able to use the combo for city use with 27" rubber, 3.08 rear end and M21. I have achieved this to a large degree - the combo will pull from 1150/1200 rpm and cruise in town in 3rd gear at 30mph (4th at 40mph) comfortably and is actually better on fuel economy than the stock L36 390hp combo. Dyno figures at 2500 rpm of 225 bhp & 465 ft/lb torque show where its really all happening, yet it will still 'spin' happily up to 6500 rpm. Your addition of EZI I guess will allow the option of a more agressive cam, but still retaining good drivability. My own intention is to go a similar route myself but utilising 'old school' stack injection.

 

Thank you sooo much guys for your responses!! To those who responded early, I'm sorry I didnt write back sooner.. On vacation and tried to respond by phone. it would ask me to long in every time I would write my response...

I think I will go for the 305cc heads. Ovals are probably the best choice as the engine will spend most of its time below 6000 rpms. A couple things keep me from going to an oval port. The first is that I have no complaints with my current set up. And, second, there was a magazine article that ran the AFR 305cc heads, 288AR, in a 427 with 9:1 and made peak HP at 6500. Now, I'll be completely honest, I know nothing about oval port heads.. I have always been turned off by them as I heard they were only good for 5500-6000 RPMs. I am always open to learn.. In fact, I spend most of my time just reading through everyone's threads.

Alan - I had those same thoughts myself.. Cam is solid btw.

Ben - I am glad you made that suggestion! Those CNC'd heads that you're speaking of flow almost as well as the 305cc heads and flow slightly better on the exhaust. Like you said, they are "spendier" than the 305s. I just need to way the cost vs performance. I'll also need a new intake.. Decision, decisions.. haha

TSW- Thank you for the advice... I think Ill go with the 305cc heads, but I'm definitely open to suggestions. Those heads that Ben mentioned go well with your comment. There is no doubt that those might possibly be the best head for my set up. You mentioned something that I do not hear most people talk about which is the port velocity.. One thing I love about AFR is that their heads seem to flow more air with a smaller port than the competition with a larger port. That is one of the reasons why I chose AFR. I was in between AFR and the DART Pro 1 310cc heads. Now I'm on the fence between the oval 290 and rect 305.

Jim - Knowing that you have experience with the 288AR cam, do you think that your results of a 7 HP loss from 5800-7000 was because of your ovals? If I remember correctly, you were running a set of ported Merlin ovals... I believe that you might have maxed out your heads in regards to flow at 5800/5900 which could have resulted in your loss of 7hp. I would like to know your thoughts on that..

Also, I was wondering what kind of heads you are running now? I had looked into the Brodix as I had seen that you had recommended them many times before.. In fact, the post I saw was from 2005. However, when compared to the AFRs, they just do not seem to flow as well. Is there a particular head that you think I should be looking at?

Just trying to pick your brain a little as I value your knowledge.. Your recommendation and description on this cam were spot on with what I have experience from this engine... I do want to thank you again for helping me with that!

Grumpy - Those are some impressive numbers! Where were your peak #'s? Any problems with the 305cc heads on the street?

Rosco - It sounds like you have a nice set-up. It delivers all that you want which is the best part. I will have to agree about the EFI.. The EFI has done wonders to my engine. The engine idles about 300 rpm lower and sounds more docile or at least idles smoother. Definitely keep us informed on your new set-up! That will look awesome!

 

Also, a couple things I did fail to mention.. trans is currently TKO 600, rear end is 3.55. I am thinking about moving this engine to another '73 and put behind a Magnum T56 with a 3.55. Also, my goal with the car is drop the weight down to 3000-3100 lbs. The car will be primarily a street car.. Possibly a DD in the late spring/summer/early fall. goal is to make it much faster than a stock Z06 and competitive with a ZR1, in a straight line of course.. As I have been thinking, I may decide to drag the car a couple of time a year.

One of my main concerns is has anyone had any trouble with running a 2.25 intake valve with a stock replacement piston? My pistons are replacement pistons for the original LS6... There was .120 valve to piston clearance. Do I have anything to worry about?

 

Stay away from ProComp heads. Not quite as high quality as most stuff sold at Harbor Freight. They're counterfeit Chinese junk and by the time you have all the work done to them that they need just to be ready to run, you'll be very close to the price of a set of decent American made heads like Brodix or AFR. RaceRites might be just the ticket for your mill, I don't think I'd go much bigger and expect as crisp of throttle response as you'll get from these.

 

If the cost of the 290cc heads is not a problem for you, that's the way to go.
The 290s are really nice looking heads and manage to move more air through a smaller, more efficient port.
They should make more power than the 305's everywhere.

 

My old Merlins flowed 324 cfm on intakes and seems like 84% on the exhaust. The 244* cam peaked around 5850 on mine...but just hung on well. I just did a 427 Tunnelport Ford dyno test with a 242* cam (flat tappet) that peaked at 5900-6000 rpm. The huge heads allowed it to hang on pretty well after peak..but could have used more cam for sure to really use them. I think some more cam on my Merlins would have allowed it to peak HP higher,,,,but it was perfect for what I was doing. TQ was strong down low and you could wind it to 7000 and not fall off any. VERY broad powerband. If it had been tanking after peak HP I would suspect head flow issues.

For example my 540 used Brodix as cast heads at first (long before CNC stuff) and it fell off rapidly after 6500. After porting them they just hang on forever. Not that they stall...they just don't drop off any really. I could keep throwing cam at them to move peak power higher, but I like the broad powerband. I'd rather run a little less cam with great heads than use a huge cam with poor heads.

My current heads are Brodix CNC Oval 3Xtra's with 365cc ports. There are smaller 332 cc version of them as well as some of the *Headhunter* line. Even the Race Rights do very well with the right cam. Flow numbers aren't everything. AFR's are always a decent choice..so you can't go wrong there. The 290's are real nice..but get pretty pricy. You might Ck with Mike Lewis Racing for a good deal on the heads...whichever you choose. He usually saves folks some $$$.

The CNC stuff is a great choice today if you can swing it. The heads will be able to support those added cubes you're going to throw at it later!

JIM

 

FSTR90, I have the 290 AFR's and was previously running them with a solid roller until my mishap. But anyway, they performed very well. I can't say I have any comparison to another BB head, but I was directed to these since my original LS7's iron heads I was told were around 320- 330cc' and flowed terrible and would only come alive at 5500 rpm and higher.
Contact Mike Lewis with Lewis Racing Engines, he should be able to get you a good deal vs going directly with the manufacturer. Jim knows Mike. Be worth getting an estimate.
I did not realize they were more than the 305's...