Ok guys, I have a question...

Lets say you had three options for heads/intake. Lets say your first option is a set of stock LT1 Aluminum heads and intake and you sent them off to LE (for the sake of this argument) to have the LE2/LE3 package done to them with the cam that is recomended.

Lets also say that you had a set of stock LT4 heads and intake that you again were going to send out for the equivalant of the LE2/LE3 with again the cam recomended.

Lastly, lets say that you are again using the same LT4 intake (ported), but now stepping up to a set of AFR 210cc heads. Now, these already come with 2.08/1.60 valves so just some minor port work would be done again by LE and with a cam he recomends.

Now, I have all three of these combos sitting in my garage, one of which is going to get sent out to have the work done and go onto a 396ci LTx bottom end.

Now the kicker. Lets say that this is going into a land bardge of a car, an Impala SS. Weight is signifigantly higher than an F-body obvisouly, so low/mid range power is a little more important.

I would like to know which you would pick, and why. Not just a basic answer either. I would like to know why you chose one over the other FOR THIS APPLICATION. For instance, at what point would larger valves slow the port velocity down enough to really hurt low end? Would the 2.08 intake valves of the AFR's be too much for the 396, again, in this application?

This application again is a 1996 Impala SS (4400lbs) with a forged 396 short block backed by a built 4l60E with a 9.5" 3500 stall converter with 3.73 gears. Shifting will be within the range of the stock PCM (tuned of course), somehwhere around 6500-6800rpm or so.

I want to hear some logic, physics, etc behind these answers, not just "because AFR's are what people do."

Thanks guys!

 

I'm far from an expert on the subject but since you asked here is my two cents just for giggles. Seeing as this is a 396 I would use the AFR 210 heads. The 396 will need the flow. As for the cam maybe something in the 224 range on the intake. Reason is the 4400 lbs weight of your car. You will need some torque in the lower ranges to get the car moving.

What you really need to do is call Lloyd and give him the specs if that is who you are going to use and let him decide.

 

The 396 needs the airflow of the 210's. They will outflow the lt1+lt4 castings. As for the cam LE makes impala grinds with low to mid range tq. to get those boats movin.

 

The cam would depend on the gear and stall.

The heads would depend on the piston. The larger valve is only going to help you here, i've put 2.125" valves on there.

Bret

 

You are really testing me here...it has been three years since I worked with the specs of this motor...

But, if memory serves me, they are flat top pistons with 5cc reliefs and I want to say that they are sitting .006" in the hole. I need to find my paperwork on this thing, but it has been with me for three moves now so who knows!

Gears are 3.73 with a 3500 9.5" stall converter.

Is there anyone that thinks that the air may get too lazy on an N/A motor with that big of a valve?

Troy

 

Depending on how much air your heads are moving would dictate the valve size somewhat I would think. 2.125 sounds big, but if your heads are flowing well over 300 cfm and your duration isn't that long you would need a big valve to stuff as much air into the cylinder as you can before the valve closes.

 

Get the BIGGEST heads ya can and build the motor around them.

 

You can do anything then... the AFR's will not be as much of a problem with compression but I'm certain that the more valve you put in it the better off you are. If you had dish pistons I would have said the LT4's.

I still like the LT4's but they need intake seats etc... to get the big valves in there.

Bret

 

Well, that is sort of my question I guess...Do I really need the larger valves? I was talking with a guy that is runnin 2.0/1.56 valves on ported LT1 castings and with a moderate SR cam is making 11.15 @ 120 with it, again in a 4200lb car...He swares by the smaller valves to keep the velocity up through the head...

This is where I am stuck.

Troy

 

About 300CFM is all ya can flow through a 2.0 valve and ya need way more air than that to make a 396 REALLY run. Ya need to get as close to 350CFM as ya can and the 210's will put ya on that path. Mine flow 330+ with a 2.080 valve so I can't see any use for a 2.125 which will flow way more than your ports and disrupt the flow and increase valve shrouding. The flow ya get on the bench isn't the flow the engine will see all other things considered.

As an example I had a set as while back that had 2.180 intakes and they would flow 410 on a 4.030 bore and 425 on a 4.125 bore so you be the judge.

 

The valve size all has something to do with curtain area.... you can have the same flow with a larger valve, which means lower discharge coefficient but the car will go faster down the track. 23 deg heads really seem to like this.

http://speedtalk.com/forum/viewtopic.php?t=217
http://speedtalk.com/forum/viewtopic.php?t=1880

Interesting reading

Bret

 

That depends on how much the bigger valve is shrouded in a 4.030 bore.

 

For what its worth, I too have ran heavier cars with larger cid sbc.(406) Each time I made changes I switched to larger valves. I found that 2.08/1.60 work great with 200-215cc intake runners, ~<77% IE efficiency and 1 3/4 or 1 7/8 primary tubes. I did try to run 1 /58 primaries but it was hitting a wall at 5800. The throttle response was snappy still, even with 230/246 @.050 and 109 lc. Just one man's opinion, but Brett, Lloyd and others on this board have offered advice that has mirrored my experiences very closely.

Generic-ly, it is hard to over valve or cam a large small block if you have deep pockets! Good luck, be sure to post a vid or 2 when youre done.

 

This is what the pro had to say about large valves in a medium RPM range.


Darin Morgan

PostPosted: Mon May 17, 2004 7:16 pm Post subject: Lower RPM range Reply with quote
The individual from Meaux Racing Heads put it into perspective and he is most definitely correct in his statements. When I stated that the valve should be around 51-53.5% of the bore I was referring to engines designed to achieve max power ( VE ) at very high engine speeds. If your tuning an engine to work in a lower RPM range you need to match your valve area and the corresponding cross sectional areas ( air speed ) to the RPM range you will be working in. Its a little easier to shrink the valve area and the areas above the valve in order to lower the point at which peak VE will occur but you can still use a big valve and perfect a port design with smaller area ( air speed) to achieve the same result which is what the gentlemen from Meaux Racing Heads was referring to. If it where me and I was designing an engine to run in an RPM range of say, 6000 to 7500 RPM I would put a 2.050 valve in the head with a 1.600 exhaust and pay damn close attention to the air speed ( cross sectional area) in the port and manifold in conjunction with the camshaft design. Most people do not realize the fact that if the air speeds in the induction and exhaust systems are correct the engine will not only accelerate out of the corner better, it will make more power and have a broad power band as well. I would still tune the area ( Throat ) under the seat to 90% of the valve. That would put the area at the apex of the throat at 1.845 diameter. The 90% rule will never hurt you. Shocked Its when you try to exceed that point that things get really dicey. You really have to know what your doing to push the throat past that mark. It can be done but your much better off not trying to do it unless your the one that actually designed the entire induction system and know the engine your dealing with inside and out. If you have any more questions do not hesitate to ask. Laughing

 

That right there is more what I was looking for. I have read a few articles and talked to a few people that said similar things, just not with that much theory and numbers behind it.

Maybe if Lloyd pokes in here he can give his input since the heads will most likely be sent to him...I would like to see if he agrees and if this is something that he can watch out for...I agree that the 396 will eat up all the valve, but the rpm range is where I want to make sure I don't get into trouble...

Troy