Cam numbers and a little more info on flat tappet

[trotterlg]

Below are the revised numbers on the re-grind of the SSI performance cam, also, if anyone else will be doing this, you will need a different cam sprocket set, the roller lifter cam is retained in both directions, the flat tappet cam is only retained for movement to the rear, movement to the front is done by the slope of the cam lobes which force the cam to the rear of the engine, it is not mechanically restrained for front movement. The cam sprocket is held on with 3 bolts on the flat tappet cam and a single center bolt with the roller cam, Other than that, it is a drop in fit. Larry

[geoffinbc]

Running those numbers shows an overlap of 71* at a checking point on .006 which is the norm for hydraulic cam. That is endurance racing territory. Did you ask for this?

I can also see they have a very tight LSA of 99. 45 which is very odd. If your running a 3.4 L with 1.72 intake valve then lobe separation should be 108* to 110*. So I really do no know why they decided to make the cam this way.

If you wanted a street cam the overlap should be much less (around 30* to 50*) which means you could keep the same duration but widen the LSA to a more optimal 109* but that would not be possible if the original cam you ground it from was made with a tight LSA.

If the cam grinder was trying to achieve what you were after but started with a shitty blank with a really tight LSA then I would not run it. You wont be happy.

[trotterlg]

It started life like this:

398/420 LIFT . 260/270 DURATION. (194/204@50).104 LOBE CENTER.

Now it is like this:



The .050 overlap is 19* with a fairly slow lift rate. I talked to them at length about the valve to piston problems with the 12 to ones in it and they seem to think going with the lower lift and longer duration and overlap would produce some good higher RPM power. This is going into a Fiero that is not a daily driver. Guess we will see how it works out. Delta Camshaft is probably the largest re-grinder in the country, not that big is better, but they seem to know at least a little about things, guess they may be really wrong. What do you think the engine power curves will be with the high compression pistons, aluminum heads, carburetor intake and this cam?

[geoffinbc]

If you wanted more piston to valve clearance you should get a cam with a wider LSA which in this case will significantly improve performance. Minimizing lift is going to kill torque. You should always get the valves as wide open as possible on any 2 valve engine because they are typically under valved and need the lift to get the most possible flow. I don't think it was wise for them to just hang the valves open in order to try and band aid a poor cam.

It always worth a try but I don't think you going to like it and I don't think the carb is going to like it either. But hey if it is not a daily driver give it a try and see what happens. There is clearly power that will be left on the table so maybe in the future give something else a try.

[trotterlg]

When I read the numbers it looks like the intake lobe is advanced 99.5 degrees and the exaust is retarded 8.5 degrees, doesn't that indicate a lobe seperation of 108 degrees? Larry

[dskopek]

What do you think the engine power curves will be with the high compression pistons, aluminum heads, carburetor intake and this cam?[/QUOTE]

Post what displacement the motor is, the true static compression,are heads stock or ported,what carburetor intake you are using to fit the aluminum heads and carb size and you might get some good feedback on power and rpm range.

[trotterlg]

The engine is a 3.1 bored .020 over. Static compression looks like it will be about 12.1 to one. Stock aluminum heads. LS6 springs and roller tip rockers. When I work out the amount of airflow it appears that at even 7,000 RPM and 100% cylinder fill, the engine cannot flow more than 360 CFM, so I think one of the 350 CFM Holley two barrels should be a good fit. The intake manifold will be fabricated to fit the stock lower. Ignition will be with a mechanical advance only distributator. Larry

[geoffinbc]

No

I calculated lobe seperation and overlap from the raw numbers on the bottom of the card. So the lobe seperation shown is correct. The 8.5* advance built into the cam

[trotterlg]

This is a good education, what does the paper mean when it says intake advance 99.5 degrees, exaust retard 8.5 degrees overlap 19.3 degrees? Larry

[geoffinbc]

It says LbSep/Adv

The top number is the Lobe Separation Angle. Also called Lobe Centerline Angle or Lobe Displacement Angle. In your case it is 99.5* This is the number of degrees separating the intake lobe center from the exhaust lobe centerline. This is the second most important cam spec in power production, after overlap.

The lower number is the intake centerline angle. This is the position of the intake centerline in relation to the piston at TDC. In a cam ground with no advance or retard it is the same as the lobe separation angle. In your case the cam is ground with 8.5* retard which puts it the intake center 108*. Which is the number you see on the very left under "CLine"

The overlap says 19.5* but if you check above you will see the checking point is at .050" and not .006" which is where you calculate your total overlap. You can calculate overlap at any lift point by adding the intake opening even to the exhaust closing event at any lift point.

[trotterlg]

Thankyou, things are becoming much clearer now. What type of power curve charistics will this thing likely posess? I take it the over lap will be considerably more than the 19.3 specified. Will it idle? Larry

[geoffinbc]

Not likely. But you do have a carb so it will run and possibly could get down to an idle below 2000RPM. Lots of overlap and a narrow lsa, especially that narrow, both make for a poor idle. If the heads were shitty and the engine was big with more pistons then it would be a better scenario.

I ran a quick simulation using your info and it shows. 179LBS of torque at 4500 RPM and 184 HP at 6000 RPM

Compare that to a normal high performance cam with similar duration and lift (Crane 272) and we get 200lbs of torque at at 4500 RPM and 201hp at 6000 RPM nothing else was changed.

The Crane cam will idle better and make more vacuum. The low end was was up also so drivability will improve.