chipenstein

<Rookie mistake, I posted this originally over in Internal Engine by mistake. >

I'm buying a 6 speed, LS6 car (curb weight 3,850 lbs.) out of state, and it's been modded professionally. AFR 205 heads, cam, lt headers, free flowing cats and exhaust, underdrive pulleys, tune. Here's the dyno run with the stock setup (340.6 rwhp, 338.6 rwtq) compared to the "after the mods with a custom grind cam" (420.6 rwhp, 399.5 rwtq), then another larger custom grind cam and 1.8 roller rockers was put in the car (437 rwhp, 395 rwtq) and I've inexpertly copied over that larger cam's dyno graph onto the original one for direct comparison sake. I used a ball point pen and it should be relatively obvious which one is the bigger cam.

This car will be my DD but I'll "enjoy it" occasionally. My question is regarding the difference in the character of the car with the larger cam vs. the original custom grind (not the stock LS6 grind). It seems to me that the larger cam isn't doing a whole heck of a lot for the car. It makes more horses after 5500 rpm, but gives up a boat load of low-end torque in the process. I hear talk about "area under the curve" and it seems like the 1st custom grind just puts up overall better numbers, but maybe I'm wrong? Somebody enlighten me here. Maybe the larger cam is ok because I'll stay in the powerband most of the time I'm accelerating?

Another question I have is the car has a ported ls6 intake and tb. Will going to a fast 90 intake and tb give me back some of the torque that was lost as well as provide some nice hp gains? Opinions?

sidewayz28

iTrader: (24)

i can see a FAST 90/90 setup making some solid numbers especially with the heads you got on right now

Slowhawk

iTrader: (12)

The smaller cam has a better average power.The big cam will pick up more up top with a 90mm setup but the small cam will still have a better midrange. This is pretty common.It is the same reason alot of shops push smaller cams because like us,average power is more important than peak #'s.

dynocar

Good info, all newbies read that at least three times!

Wish I had a dollar for every person I've told that to but still go with the larger cam. I've even tried the stratagy of recommending the larger cam hopeing they would choose the smaller cam, but that doesn't work either.

chipenstein

What if I switched from 1.8 roller rockers to 1.7 roller rockers? That seems less expensive than swapping cams. I'm not exactly sure what the effect will be on the engine torque and horsepower curves by swapping rockers though. Anyone?

dynocar

You are on the right track, like your thinking. Yes, doing that rocker switch will increase power low to mid and decrease up high concerning your RPM Vs power. How much, varies from engine and cam. It is also much easier on the valve train.

Just a quick story, a friend of mine had to rebuild his LT4 engine and got talked into a slightly larger cam by his buddys. He is a weekend race track road racer with this Vette and knows how to drive. He could not turn the times he used to and told me that it felt too doggy getting out of the turns. Fortunately he had a baseline from his old combo on our dyno, we redynoed it and sure enough, lost power to 5500, gained above that. Now he is considering the alternatives such as rockers too.

The quickest street/roadrace combo I ever had was a SBC engine with the Accel SuperRam intake setup on it. It made lousy peak HP numbers but it did'nt care if I stomped it at 2000 or 6000 because the power curve was long and flat, it was a well built engine with heads and a mild cam. I've had some mega HP cars but if someone snuck up on me on the street and caught me in the wrong gear, I was the laughing stock of my car gang. Cruising around at 5000 RPM was not the answer either. Yes, if everything was just right, I was very FAST ONCE IN A WHILE. What I liked about the previous combo was that I was much QUICKER MOST OF THE TIME and the car could double for pulling train cars. I think this is what you are onto.

vettenuts

iTrader: (4)

I am struggling to understand your curves. You state a boat load of torque is being given up (and I know you put the curves in by pen) but at 3,500 RPM you have the same hp but the torque is vastly different. Conversly, at 6,250 the torqu is the same but the hp is vastly different. Since the hp is merely computed from the torque curve, how accurate is the depiction above?

What kind of rockers are on the car now?

Do you have the original curves?

chipenstein

Vettenuts - you're absolutely right! What a bonehead I am! I've got the second dyno with the last, bigger cam on it but it's kinda poor resolution and I couldn't read the numbers on the side of the graph. Unfortunately I don't have anyone to ask about it for clarification, as it was done a few months ago and the tuner is out of town. So I made the mistake of starting at 5250 (where I know the curves cross) and I worked backwards down each line, using the same 50 increments on the torque side as I did on the hp side. After you made that comment, I pulled it up again and now I see that there are a lot more #'s on the tq side than on the hp side, so I was wrong.

Here it is:



There are 11 #'s on the tq side, and they must work down in 25 ft. lb. increments. That makes a lot more sense and actually makes me feel a hell of a lot better about the cam that's in there.

THANK YOU for pointing that out to me. What a dope I was.

chipenstein

Alright, I'm an idiot.

Here's the updated dyno - looks a lot better now, doesn't it? With the heads I have, I think I'll spring for the FAST 90 setup, try to fatten the hp and torque curve up and down the rpm range.