This effect on performance is directly linked to engine rpm. Higher engine speed causes greater exhaust-gas velocity, which relates to greater momentum of the exiting exhaust gases. This is why a longer-duration camshaft produces power higher in the rpm range. It also causes the loss of low rpm power and economy that larger cams experience.
But at higher engine speeds these conditions are minimized due to the slight lag time it takes to get the intake charge moving into the cylinder.
The cam's centerline is used to correlate valve timing to the crankshaft's rotation. This is again measured in crankshaft degrees. Centerline is explained as the number of degrees that the crankshaft must rotate from top dead center until the cam has rotated to the peak or centerline of a given intake or exhaust lobe. For the engine to run at peak performance, the valves must open and close at the correct time in relation to the piston's position and the crankshaft's speed.
The intake centerline is the point of highest lift on the intake lobe. The exhaust centerline is the point of highest lift on the exhaust lobe. The cam centerline is the point halfway between the intake and exhaust centerlines. Close Ad. Join MotorTrend. Degreeing a Camshaft - Bang, Suck, Blow Sooner Or Later Is degreeing a camshaft worth the time?
Randy Bolig writer. Effects of Changing the Cam Timing. Advancing a camshaft from its original position causes all of these valve events to happen earlier in the cycle. A camshaft advance of 4 degrees will cause each opening and closing event to occur four degrees sooner than before, changing the ability of the cylinder to build pressure. For example, if the intake closing event is designed to happen at 55 degrees after bottom dead center ABDC it will now close at 51 degrees ABDC, or 4 degrees earlier.
The same is true of the exhaust events--they will happen 4 degrees earlier, even though they don't happen until after top dead center ATDC of piston travel. Although all valve events occur earlier, the greatest impact of an advanced cam is to close the intake valve sooner in the compression stroke. This means that a greater volume of air and fuel gets trapped and compressed before being ignited and has the effect of creating more torque and power.
Contrary to what a lot of people think, cam "timing" and ignition "timing" have nothing to do with each other anymore than a timing "belt" and a fan "belt" have anything to do with each other. They just happen to share the common word of "timing", but that word is in a completely different context from each other. Cam timing is a relationship between where the cam is indexed and phased with the crank. When it's referred-to as "straight-up" or at "zero", it means it is in line with the TDC index of the crank, where most stock cam manufacturers wanted the cam to be indexed at.
They base this measurement on the 1 intake valve's centerline. It isn't advanced, and it isn't retarded in relationship with the crank's TDC position. Many performance and race cam manufacturers will index their cam a few degrees one way or the other from the crank's TDC position. Advancing a cam opens the intake valve sooner, which brings the power curve down to a little lower RPM. In other words, advancing the cam 2, 3 or 4 degrees will make the cam's power curve come-on sooner.
More sooner for each degree more you advance it more or less anyway , and later for each degree you retard it. DOHC allowed me to play with cam positions in relation to piston position and the numbers are almost identical to what a SBC likes. One of the Ferrari engineers really like Duntovs piston speed and rod to stroke ration so they copied it Works pretty good cause a stock Fiat engine can turn from the factory and with aftermarket cams and some decent springs 10, From that work I was able to move the cams to where I wanted on the engine dyno and in a couple days I hit a sweet spot I have been working with ever since.
In my way of thinking piston speed and peak head flow determine the valve events, and they all revolve around how efficient the head is at accelerating the flow off the seat and getting the port up and moving air. Isky has some of the best towing cams in the biz I would think You will make plenty of cylinder pressure with a small cam, efficient ports and big inches from the stroke portion of the engine. Same heads as on a GM crate engine.
They are better than almost any steel head out there and will outperform them in the range you are talking about hands down. They make more power than the old GM Bowtie head up to about rpm's.
Post by Kdub » Fri Apr 27, am I never seen anyone have any luck advancing a small or smallish cam beyond what is ground into them. Any I have seen pinged under load and power seemed to be down. Rather than an open spacer, use an insulating stock 4-hole spacer. Build a cold air intake system and book some time on a chassis dyno. Post by johnretired » Fri Apr 27, pm If you degree that stock cam in that , I bet you find it is retarded a bit.
I did what you are doing many years ago. It was a in a 76 work truck. Stock chain was tired. Replaced it then a month later decided to advance it. It was 8 degrees retarded, advance it to 4 advanced. Really helped THAT motor. Started better, mileage did improve too. John- I was debating on swapping the cam out for aftermarket but it just wasn't in the budget right now.
Evently it will get swapped out for something that's a little more power friendly. Something along the lines of a Isky mega cam lol I figured it was probably retarded atleast 4 degrees due to the engine is a 71 and that's when all the emmission crap started.
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