I have have done some posts searches and reading on this topic. I am still learning, but am starting to put the pices together slowly on what works where.

First Question:
One thing that is not clear to me is if/when larger valves are going to produce a noteworthy gain for a smaller displacement street BBC, ie 396/402. Would a bone stock 396 realize the benefit of larger valves? If not, at what point does a street oriented car benefit or is it only for cars that see mostly track time?

Second Question:
I see some people go to the larger intake and exhaust valves, but others increase only the intake valve. If I understand correctly, the idea behind not increasing the exhaust valve size is to promote higher exhaust velocity which I presume promotes scavenging. Is this a personal preference thing or the generally preferred appraoch for a street 396/402 engine? At what point is it worth doing both valves or is this a track only thing too?

I appreciate all comments, pros/cons that will help this novice figure out the next stages of modifications. smile.gif

The problem you run into with the smaller bore of the 396 when switching to larger intake valves is that the cylinder wall actually starts to shroud the valve which hurts flow. 2.25" intake valves on a 396 is definitely way too much valve for the motor. 2.19" intake valves would be about as big as you want to go with a 396 motor. I remember reading in "How To Hotrod BBCs" that the intake valve size doesn't start to hurt your bottomend power until you go beyond 2.19" (when comparing numbers of the 2.06" and 2.19" intake valves).

As for why people keep the smaller exhaust valves...I don't know. Money maybe? 1.72 exhaust valves are really small in my opinion. All things being equal, smaller exhaust valves could really hurt your horsepower numbers. It doesn't matter how much air/fuel you can stuff into the motor if you can't get rid of it in a timely manor.

I believe some people keep the smaller exhaust valves because there is a danger in hitting a water jacket or getting very close when machining for hardened exhaust seats for 1.88 valves. Personally, for a tame to moderate street 396-402, I don't think there is enough gain to be made with the larger valves to be worth it. after I put them in mine with no other changes except .030 bigger bore, I could tell no improvement by seat of the pants. No time slips though.

I could tell no improvement by seat of the pants. No time slips though. Do you think your butt can feel .5 a second? 2 tenths? 3 tenths? My car is fairly mild mannered,it has a good rumpity rump sound but I imagine there are some 13 second cars with lower gears that you'd swear would out run mine. The butt dyno does not work. As for the valves, most every engine made will benefit from more valve,especially a big block chevy, hell they put 16 valves in a damn 4 banger. Breathing is what its all about,,,the most air in,the most air out, the more power you'll make...of course cam and compression come into play,,,but ya gotta have valves big enough to get the job done. minimum 2.19/1.88 graemlins/thumbsup.gif

I recently had my heads redone. I had already had the hardened exhaust seats done for 1.72' then decided to go to 2.19/1.88's. We could not open the exhaust up enough to put bigger seats in the accomodate the 1.88's. You could do 1.88's without installing hardened seats though. In my case, the exhuast valves had also already been replaced with a good aftermarket valve with reduced stem and back cuts, so I opted to do the intakes to 2.19's and use a reduced stem valve there also. I made up the difference in a longer exhaust duration and concentrated more energy on massaging the exhaust ports for flow. In reality, an increase on the intake side of a naturally aspirated motor is no where near as the mismatch in flow dynamics of a supercharged engine. The bigger exhaust mainly will show up in the upper RPM numbers. My feeling is if a 1.88 valve can exhaust a 600CI motor, a 1.72 should be plenty for a 400CI motor. Small block 400's get by fine with 1.66's. Go figure.

A good exhaust port doesn't need a real big valve. The latest high flowing Dart and Profiler Heads only have 1.8" exh. valves whereas they have intakes in the range of 2.47".

Harold,
True, but to put things in perspective we're talking about a stock exhuast port.

Pro Stocks use reletively small valves, especially for what one would think you'd see in cars that run 6.70's at over 207mph in good air.

Ok - thanks for all the comments guys.

Von, I would agree that cutting into the water jacket would be bummer - what kind of risk is this? 90% or 10%? THere seem to be a number of guys running these heads (say 063s or 215s) on larger motors with the bigger valves, maybe there is a way to be sure to avoid the problem?

But I am still not sure what the bottom line is :confused:

So maybe the bottom line amounts the percieved dollar value by the owner of the performance gain. Soooooo......
Assuming a 396, closed chambers, headers dual exhasut, intake, carb, GOOD STREET CAM FROM harold - just a good basic warm over of stock - how does this compare to the same engine with the heads worked for the bigger valves? Perhaps the idea of HP and/or ET gains would help anyone considering the head work to determine the value vs dollars spent...

I run closed chamber #215's on my 454..no port work or big valves

Bow-Tied,
I run the EXACT setup you are talking about. I ahve a .40 over 402 (410 ci) 063 heads with 2.190" intake valves, and 1.720" exhaust valves. My engine was built by a pretty well known and respected race engine builder. The reason to keep the original exhaust valves is that they cause less shrouding. Going to larger exhaust valves will do nothing for a street engine that will never, or very rarely see 6000 or better rpm's. Also, going to larger exhaust valves goes pretty close to the water passages (depending on the head)even if you dont break trough the jacket you can easily find yourself fighting heat issues. Especailly in a street driven car. As far as performance, our best Engine stand dyno pull was 511 hp and 596 tq.

Regards,

Originally posted by USFATL:
Bow-Tied,
I run the EXACT setup you are talking about. I ahve a .40 over 402 (410 ci) 063 heads with 2.190" intake valves, and 1.720" exhaust valves. My engine was built by a pretty well known and respected race engine builder. The reason to keep the original exhaust valves is that they cause less shrouding. Going to larger exhaust valves will do nothing for a street engine that will never, or very rarely see 6000 or better rpm's. Also, going to larger exhaust valves goes pretty close to the water passages (depending on the head)even if you dont break trough the jacket you can easily find yourself fighting heat issues. Especailly in a street driven car. As far as performance, our best Engine stand dyno pull was 511 hp and 596 tq.

Regards, Wow, 500 ponies and 600lb.ft is pretty sweet! Would yo mind sharing your complete combo?

I am glad you replied, I recall past posts were you have mentioned that you have enlarged the intake only - do you have any comparisons from before the intake upgrade?

Thanks for any additional comments (Steve or anyone)...

The big block Chevy exhaust port is relatively inefficient and can always be improved by adding a hump in the floor about 1/4 high then tapering down to the floor where the short turn is as is done in on the factory ZL-1 exhausts ports. Some slide in exhaust plates that go into the exh. and are held in place by the exh. header flanges utilize this simple principle. I think there called (I love my exhaust plates).

1971 402 bored .040 over (410 CI)
1971 GM 063 Closed heads 2.19 Intake 1.72 exhaust
Flow #'s @ 600 Intake 298 exhaust 205
Static Compression Ratio 9.48:1
Combustion chamber: 100cc
6223 cross drilled crank
1.72 comp cams rockers
Comp Cams xe284H 110 lsa
matching springs retainers etc.
Eagle I-beam Forged rods
Speed Pro forged 10:1 pistons
Edelbrock RPM airgap
Holley 4160 750cfm carb.
1.75" long tube headers into 2.5" H-pipe exhaust
700-r4 transmission with 2700 stall
12 bolt rearend Auburn posi with 3.73 gears

This engine was put on the stand dyno twice. The first time with the Holley 4160 mentioned above. The best numbers on the first run were 498 Hp and 560tq. The second run was made with a new Holley 770 (4150). The first run was also pulled only to 5800 rpms the second runs were made up to 6300 rpms.

Regards,

I run basically the same engine setup as USFATL.
My truck heads have hardened seats and I opted to retain the 1.72" exhaust. The intakes are 2.19. I wish my motor made 511hp/596tq, more like 470hp/460tq ,did not have it on the dyno.
My et/mph below.

I have seen a similiar discussion. If you put in a larger intake the thought was you need to perform some chamber work to unshroud the valve. Other wise you don't seem to gain much. I would at least do some bowl work, even if you don't add larger valves. I had larger valves added to open chamber heads on a 454 with some port work and was happy with the performance. I made to many changes to say how much was added by the larger valve size.

Thank-you for all the great info everyone!!!!

Ron!
It's Mike, but from Sweden. ä ö and stuff.


My older motor had reworked (stock closed heads with larer valves 2.19\1.88). I had port work done and the cam was the same as it is now. 280\288 total.

I breached two intake seats at the same time and my motor hydro+locked. happy days those where!

Just de sure you go to a good shop. I say, for the money, you should get a set of the Merlin heads like I have. These worked better BONE stock than my chevy heads did all opened up.

Donät know how that hurts your show quality thought. most never notice my "Merlin" badging.

THanks for the comments Mike, maybe some day I will go with Merlins, but for now I have the heads and kinda want to learn the hard way with respect to working on the heads first. Your warnings are well observed, thank-you. I think I would need to upgrade pistons for use with the Merlins though.

Sweden?!?!? Cool - send us some pics of the local scenery! ;)