How much more power with bigger valves?

[deputycrawford]

I have a 383 with 9.35 to 1 comp. I have 906 heads milled around .050 and have 75CC chambers, as well as mild port work. My performer RPM intake has been port matched also. My question is my heads have the 2.08/ 1.78 valves. (I think that is stock.) I have the Comp Cams high lift .545 cam. How much more power would I see if I went to 2.14/ 1.84 valves and 70 CC chambers? Can I run the big valves with a 383 at .040 over without hitting the cylinder walls?

[Chryco Psycho]

using the 1.81 or 1.84 exhaust wil help , don`t bother with the 2.14 intake though

[deputycrawford]

Ok, would the 2.14 valves kill any velocity of the in coming air charge, therefore killing low end? I am going to a 4.30 gear and I jetted the Demon way up. I finally have color on my plugs. Now I can shift at 6500. I was hoping the 2.14s would help in the quarter. I going for mid 12's.

[Ghoste]

It's like any of that stuff, if you have the cam and heads to support that kind of flow then sure.  Otherwise, it's like you said, you're just killing the flow velocity.

[Chryco Psycho]

2.14 on a 383 is overkill IMO

[deputycrawford]

Ok, thanks Chryco and Ghoste. If I do pull the heads, I will only do the exhaust valves and cut the deck. I have cut .050 out of the heads for 75cc chambers. Do I have enough left for 70cc? Im looking to get the comp up but don't really want to go to a steel shim gasket.

[Ghoste]

About the best example for valve sizes on a street engine can be gained by looking at the Ford and Chevy Trans-Am engines.  They were both 4 inch bore and 3 inch stroke and the Chevy had 2.02 intakes.  No torque and no power either below 3 or 4 thousand rpm.  The truck drivers hated the early Z28's because they didn't have enough torque to get them off the trailers.
The Ford was no better.  The Boss 302 had those enormous 2.23 intakes and there is a real good reason you had to get a four speed behind it.  In fact for the racing engine, the tunnel port, so named because the ports were so oversized they need tubes ("tunnels") inside the port to run the pushrod through, came with smaller (but still huge) 2.12 intakes.  Again, it was an engine with no low end torque but lots of high rev scream.
Not exactly a good setup for a street driven heavy car like a Charger.

[deputycrawford]

Those are good examples. I hear so many guys going with the Eddie RPM heads that have the big valves. I hear even the 383s are running descent. I want to keep the torque but need to flow up top too. I will find the medium by running the exhaust and mill alittle more. I just don't know how far I can go on the milling.

[Ghoste]

Well, there's more to it than induction size and I won't even pretend to know a fraction of what a lot of the guys here do but bolting on heads with bigger ports and bigger valaves means a bigger carb to support it, bigger fuel supply to support that, better exhaust, better cam and then gears and converter to keep it all in the rpm range.  It's a vicious cycle for sure.
To me, it seems like the 383 with it's shorter stroke and lighter reciprocating assembly could be built to take advantage of that and a bigger valve would help it in the higher rpm.  Then again, maybe the displacement just doesn't support that kind of flow?

[deputycrawford]

You figured me out. I have the 9 1/2 inch converter, the Mighty Demon carb, the 1/2 inch fuel system, the 4.10 gear, and all the other works stuff. My vicious cycle is nearing an end...lol  I hope. We'll see what Chryco or Firefighter says. Not to mention Chevy again but Their stroker small block motors usually have the 2.02 or 2.05 intakes for 90% of the builds. My 383 is really just a large small block.

[Ghoste]

Yeah, I'm not a Chevy expert by any means (hell, I'm not a Mopar expert ) but they do seem to like the bigger valves.  I suppose their light valves and pistons let them get to those higher rpm quickly enough to utilize it?  I don't like them but I have to admit that it seems to work for them.

[firefighter3931]

The key to making more power and having it come in where you want it is intake flow velocity. Going too big on the port volume will hurt the low end power and make the engine feel sluggish. A stock iron BB head has relatively small ports and that's why these engines make such good bottom end power. Doing the big valve upgrade can be worth as much as 50hp and sometimes more depending on the specific combo. You don't need to do an work on the ports for what you're wanting out of this combo. In fact it might end up being counterproductive.

The easiest way to maintain torque and increase horsepower within the parameters you've laid out is a bowl blend. You can either use the 2.08's or the 2.14's and both will work well. If you do go the 2.14 route, you need to open the bowls up all the way to the valveseats to realize the increased flow from the oversized valve. Simply slapping in the large valves w/o any bowl work won't show any increase in power, and in some cases will result in a loss of flow.

Personally, if i was in your shoes; the MP porting templates would be my next purchase. Get yourself a die grinder and go to work. Finish it off with a good 3 angle valvejob and a 30* back cut to the intake valves and you will be pleasantly surprised with the results. You don't need to spend a ton of money to get a lot more out of these heads. The same rules apply to the exhaust side; bowl porting (with templates) on the area beneath the seat and 3 angle valvejob. No need for a backcut on the exhaust valves. I would also stay with the 2.08/1.74's at this point. The big valves don't really start to make the large flow #'s until you get into runner mods to the short turn and pushrod pinch. At that point you're increasing port volume, resulting in decreased port velocity. It's not the way to go with a short stroke and 383 cubes with 9.3:1 compression.

Bowl porting with a good valvejob will really fatten up the low and mid lift flow #'s to the tune of ~25 cfm across the board up to the low .500 lift area w/o sacrificing port volume and velocity. Profiling the guide bosses is also worth the time spent.

Ron

[deputycrawford]

Thanks for the very informative reply Firefighter. I had the bowls done and three angle valve job when the motor was built. Everything was done except going bigger on the ports and the valves themselves. The ports were cleaned and smoothed as well as matched. The intake was smoothed and matched also. I guess Im looking for a 440 to build next. Am I asking too much of a 383? Is a 12 second, 4000 lb Charger fast enough for a 9 to 1 383? Oh, I have the .545 lift 241-247 at .050 dur Comp high lift cam. Have I maximized the combo I have?

[Ghoste]

Cudaken and Runner have built a couple of very strong 383 combo's.

[RallyeMike]

I'm also building a 383 and have upgraded my 906's to 2.14/1.88 valves. The heads have been worked over and I think (as well as the race shop) that with the head prep, the valve size upgrade is going to be worth a reasonable amount of ponies. I did have access to a flow bench when I ported the heads, so feel pretty good about it working out. As oppossed to what you are doing, my motor is for road course and we will see all high rpm use. I'd let you know how it turns out, but the motor will never see a dyno.

[deputycrawford]

Thanks for the reply. I am probably going with a 4.56 gear soon. I have a complete 3.23. Sure Grip pig for trips out of town so I will most likely do it. I'm also putting the stock sway bar back on so I can run the Competition Engineering adjustable 90/10 shocks and going back to .920 torsions from the .960s I have on it. I am still gonna drive it to work but will spend a lot more time on the drag strip. I am thinking about doing the bigger valves. I might as well go all the way. Still thinking about it. Do you know if I can get to 70cc chambers with the 906's?

[Chryco Psycho]

I have no idea how you plan to lose 18 cc from a 906 head , can`t mill them that far
do you have a welder , nyrod & an oven

[Ghoste]

Do you mean 80cc Deputy?

[deputycrawford]

Sorry for the late reply. I just noticed your question Ghoste. I have had the heads cut .050 so far. I think that puts them at 75cc. I was hoping to get 70cc out of them. Chryco said I can't cut them that far so I guess if I go bigger valves I'll just go intake and hope I gain a lot of power every where. I won't hog the ports, just bowl blending and un shrouding the valves some more. I have to think about it for awhile.

[Mike DC]

The Eddy heads have more than just bigger valves working for them.  They're claiming that the slight spark-plug relocation makes something like 10-20 pounds of extra torque that offsets whatever low-end is lost with the bigger valves & passages.  I don't know if it's true or not, but Edelbrock seems to have a history of being pretty reliable about those kinds of claims.  And so far I haven't heard any complaints about E-heads killing low-end torque yet.

I also remember a magazine doing some flow-bench numbers and saying the E-heads are almost bowl-ported right out of the box, just by the shape of the casting in that area.  That's probably where a lot of the power gains come from.

.

[deputycrawford]

Thanks for the reply MikeDC. I will fix a couple of other small problems. Then think about the heads. I should be in the 12's already and Im not even done with the combo I have so far. I will keep them in mind though. I on ly have 75CC chambers now and still have 9.35ish to 1 comp. I will have to cut the eddy's big time. I wonder how small I can go with the eddy's.

[Lightning]

what about going for a higher compression ratio? would'nt 10.5:1 or so work better?

[deputycrawford]

I would love more compression. That is why I want to cut more from my 906 heads. I might just go backwards and use a shim gasket. That would take .020 from my deck height. With the calculations of my combo, that would put my in the 10.1 area. My quench would come up too; I think. I know Chryco and Firefighter would both frown on the shim gasket but I might be looking for the most from what I have.