JRider

iTrader: (1)

It would be nice to know how much exhaust system plays into this, I believe I have 651 cams, CMI big tubes and shotgun inserts in the tails. Is the duration accounting for wet header/tail/muffler? inefficiencies beyond the exhaust port?

MILD THUNDER

iTrader: (3)

I did believe that additional exhaust duration aided when there is a fairly restrictive exhaust system. I don't know if that is in fact true. Only way to know this, would be to test it back to back.

What I do know about exhaust duration, is a certain amount is needed to sustain a specific rpm . As well as, the proper exhaust opening and closing points. That can have a substantial impact on low speed torque , and overall power production. Too much isn't good, not enough isn't good.

From some experts, what they have found during testing , was that when exhaust duration was increased, the engine did not nose over as hard at peak, but was down a bit on average power. Example would be, and engine that maybe makes peak HP of 700 at 6100rpm, but maybe only falls to 690hp at 6400. Vs shorter exhaust duration, where the engine may have peaked at 690 at 6100, but by 6400, was making 660hp.

When you play with programs like pipemax, and plug in say, the 651 cam specs and what not, you might find the intake side is good for 5900rpm, and the exhaust side is good for 6400rpm. Just throwing those numbers out there as I'm not in front of my computer, but just making an example . So that may be what you get with the 244/256 cam with 109 icl. Now, let's say you switched to a 244/250 , the intake side would still be good for 5900rpm, while the exhaust drops to maybe 6100rpm .

Works the same way with things like valve diameter. While the intake side with that cam may be good for 5900rpm with a 2.3 valve, if you stick a 2.19 valve in it, it may be good for only 5700rpm.

These modern programs are pretty interesting. Certainly a better way of picking a cam for the diy guy, than the old days of basically throwing darts based on something we heard at the bar over a few Budweisers

getrdunn

iTrader: (7)

The talk over a few beers ar the bar usually involves the guys sitting next to you drinking Budweiser and smoking Marlboro reds saying how kick azz their 3/4 race cam is. Hah.. I've read how the 3/4 race cam came about but never actually confronted any individual that said they had one. Probably because I wouldn't understand them cause most their jibs (teeth) are missing.

-JRider your post makes good sense and would imagine comes into play more than most think.

-MT good post also...

MILD THUNDER

iTrader: (3)

Ok, so playing around right now. Lets look at the 651 cam. 244/256 @ .050 , .632/.632 lift. 114 LSA , 109 ICL . With a 2.30 intake valve, and 1.88 exhaust valve.

Intake = 5913 RPM
Exhaust=6343 RPM

Now, I'll change to a 2.19 Intake valve
intake=5824 RPM.

Now Ill increase lift from .632, to .700 on the intake side, with the 2.3 valve
Intake=6131 RPM

As you can see, increasing lift, also increased RPM potential. Increasing valve diameter, also increased RPM potential.

compedgemarine

I had a 3/4 cam once but it ran a lot better with the whole cam. that or it was a v6 cam, either way it didnt make the power they claim.

getrdunn

iTrader: (7)

i have to admit I'd expect to see more gains. Does your software show flow difference when inputting changes in valve size?

MILD THUNDER

iTrader: (3)

This particular program, does not. I do have a more advanced program, that can sort of simulate changes in velocities based on valve diameter, throat diameter, CSA, and so on. Thats really for head porters. Some things I have learned in regards to that.

Sticking a big valve in a head, doesnt necessarily equate to gains in airflow.

Enlarging a port, doesnt always mean more flow.

Reducing a ports volume, doesnt always mean more velocity.

Adding valve lift, can amplify low speed reversion. Adding valve diameter, can amplify low speed reversion. Think about it. We know intake reversion, is from the intake valve not being fully closed , and some charge is pushed back into the intake. How much of that gets pushed back, can vary not only when the valve actually closes, but how much of that charge can be put back into the intake due to the valves position during that period, and , the size of the valve. A .700 lift setup, is going to have the valve open further at this timeframe, than a .550 lift setup. This is why GM RV engines dont like big valves. They are a low rpm engine, and need to be efficient at low rpm. A good head porter, will focus on developing not only good flow from the port at higher lifts, but also design it to flow bad at very low lifts due to this. You dont want a setup that flows big air at .100 valve lift. That usually means big reversion as well.

Lots of guys go to a bigger head, and many times this bigger head, also includes a larger valve. They blame the head's port volume, when the engine is lacking low speed power. But, it may not really be the ports volume thats causing it directly, it likely is the larger valve that is hurting at low speeds. You go sticking a 2.4 intake valve, on a 502 with .720 lift, and some duration, you're not gonna wanna go pulling tubers around. That thing is gonna run like a bag of chit until very high rpm.

getrdunn

iTrader: (7)

Pretty basic info. We all learn from and have our favorite head guy. There was a time I learned from mine many years ago and I will say it was several trips back to the flow bench to make sure I was going in the right direction and not all trips were gains but learned a lot. I now only do very basic bowl and chamber work. When I want to see real gains on bench and dyno as well as on the water or at the strip they get dropped off to my head guy. I leave all the "doesn't means" up to him.

MILD THUNDER

iTrader: (3)

Alot of this stuff, you can get super deep. Some of it applies to guys building pro stock stuff, or max effort competition engines where 5hp means the world.

In the marine world, I've seen guys spend buko bucks on trick headwork, freak over a few degrees of cam duration, or .030 worth of valve lift, intake manifold mods, and then go throw on an exhaust setup that is totally not what the engine needs/requires.

MILD THUNDER

iTrader: (3)

Played around with the dyno simulator earlier tonight. Did a 540ci, are 315 flow numbers, 9.6:1 compression, 1050 dominator with large tube headers. 244/256 .632 lift "651" crane cam.

Initial simulation showed a peak HP of 687 at 6000rpm. I started shortening exhaust duration 2 degrees at a time. 254 deg of exhaust was the only time peak power increased. It went up 1 single hp to 688. I kept decreasing. Each time I decreased it, there was a small drop in peak HP, and a small gain in low speed torque. Really wasn't much power to be lost or found from simply changing the exhaust duration .

What was drastic, was how fast power dropped off after peak HP, when shortening the split. For example, at 6500 , the 651 cam was still making 670hp. Only down 18hp from its peak. Running it as a 244/244, did make more low speed torque, and only gave up about 8hp at peak, but by 6500, it was down 49hp!! The engine basically nosed over after peak HP. This is exactly what I had expected based on what I stated earlier.

Decided to see what happens, if I took that 651 cam, and ground it with a 110lsa, and 106icl. Engine now made 712hp at 6000, and torque was up slightly as well from 3000-6000. By 6500, it dropped 25hp, to 687hp.