I recently was reading some articles and books by David Vizzard.

He said in one of his books that a car with a very efficient exh system would not benefit from 1.6s on a split pattern cam , and it might actually hurt performance a few hp.

Allways being the sceptic I thought I would try this out so I grabbed the ol G tech and made 4 runs with 1.6s on both sides.

My set-up is a 383 shortblock (16cc dish pistons 5.7 rocs), AFR 190 CC heads, mildly ported, 53cc chamber, 52 mm t/b, Hooker 1 3/4 long tubes, mufflex 3 inch y, 3 inch hooker cat-back.

My times were like the y normally are on the G-tech (after cooling with ice)

11.84 @ 117.3

11.78 @ 117.8

12.24 @ 115 (spun a little)

11.96 @ 116.9

I then went home and added 1.5 roller rockers to the exhaust, tightend them to 1/8 a turn past "0" lash like I had the 1.6s drove it around to make sure they were all ok and let the car cool off again (with ice).

My 4 runs all were slightly faster..

11.79 @ 117.8

11.69 @ 118.2

11.73 @ 118.4

12.36 @ 114.5 (over reved in 2 to 3 shift)

What do you guys think, simple variance or actual gain?

 

Well, it appears that you do have an efficient exhuast system, but I would think that you would be better off with the 1.5s on the intake and the 1.6s on the exhaust rather than the other way around. Basicallly you are just modifiying the lifts. Something that could be built into a cam if desired.

But whatever works.

 

Interesting....

I wonder what the result would be if you did the 1,6s on the exhaust...

 

Looks like regular variation to me. 11.78-12.24 vs 11.69-12.36

115-117.8 vs 114.5 - 118.4

Lots of overlap in those results.

Dan

 

Well my first series had the 1.6s on the intake and exhaust , Got LT1.

Thats what I was wondering, Of course at an actual track I'm about a tenth and some change slower and my mph is down about 2, but I wonder how much is actual variation with the G-tech, I try to launch exactly the same as much as possible.

 

I had ment if you did 1.6s on JUST the exhaust...

 

The 1.6's modify ramp rates also, not just lift.

Vizard's tests that I've seen show that the 1.6/1.6 didn't lose hp from 1.6/1.5, but the curve wasn't as good. The 1.6/1.5 made more low/mid with the same top-end.

The theory I heard was that the reduced exhaust lift meters the exhaust out, so you have more exhaust velocity towards the end of the exhaust stroke to help reduce combustion chamber pressure and pull inlet air in.

There are a few people having sucess using cams with Extreme intake lobes and Magnum exhaust lobes. This has basically the same affect as running 1.6/1.5 rockers since the ramp rate and lift is lower on the Magnum lobes.

If you thow out the outliers, you get and average of 11.86 @ 117.33 for the 1.6/1.6 as compared to 11.77 @ 118.13 for the 1.6/1.5. Since Vizard's show a maximum of 8 ftlb difference, I think your results agree quite well.

Mike

 

You need to get the fuel and air in before you get the exhaust out.IMO the higher ratio should go on the intake.Chevy has a pretty good exhaust port and the exhaust is under more pressure and will get out quicker than the intake will get in.

 

Ummmm no.

The intake side of the motor on a NA setup will always like more lift than the exhaust. The good thing about lift is that it keeps virtually the same valve timing but allows more flow into the cylinder head in the same amount of time.

As mike says you are also making the valve action faster since the rocker arm multiplies ALL of the effects of the cam. The ramp doesn't actually change but what the valve is doing changes.

High lift on the exhaust really doesn't help since the highest pressure that the exhaust port sees is at low lifts. A lower lift exhaust lobe will also extend the exhaust pulse and create a longer exhaust scavenging pulse. Some motors can benefit from this, some can't. Depends on what the exhaust is designed to do.

Larry, wouldn't you say that a head with a shallower valve angle is going to help the exhaust more? Most heads with less valve angle flow much more on the exhaust than a SBC does, but a SBC for the most part is better than something like a stock Cleveland. Either way the pressure and manging that the correct way is the most important part.

Bret

 

Unlike the intake stroke, the exhaust stroke isn't really about "getting as much flow as possible".

You open the exhaust valve early enough on the power stroke so that cylinder pressure "blows down" and is low when the exhaust stroke begins. This reduces pumping losses by reducing the pressure the piston is rising agains.

The rest of the exhaust stroke is basically setting up for a better intake stroke. With a properly designed exhaust valve/port, header tube length/diameter, and valve timing events you can pull somewhat of a vacuum in the chamber that pulls intake air in when the intake valve opens.

Keeping this in mind might help explain why less exhaust lift could work better.

Mike

 

Bret,
You are right.I was speaking stock to stock.Ford has a good intake and so so exhaust,the reverse is true on a Chevy.
Now if we get into 12-15* or even18* stuff you can have your cake and eat it too.The flater the valve sits in relation to the piston,the better off everything is.

 

The information I read into this has mainly been on carbs...And they have been using a higher ratios on the four corners(1,2,7,&8) to compensate for the differences in intake manifold runner length....

Since our air source is from the front of the intake, wouldnt it be a benefit to raise the ratio on #7 and #8 corners??

Has anyone ever though of this???