LSA Question

[Joske2]

I've run 2. 111 and 112. While not world beaters. They both made reliable power and are low maintenance on parts. Both a little north of 900hp. You would be amazed at a Pro Stock cam. 118 or more.
Doug

Yes, but with 14/1 CR or more.

 

[Dodge Bros]

This is where I’m at with my stuff. It’s hard for me to imagine putting in a camshaft with 22 degrees less duration on a tight LSA would make my car faster. But I have not tested it.

We went that direction with a lot of our cars, still have some set up with tighter LSA’s, some single patterns. We got to the point on some cars that all more mid range push was going to do was make them hard to hook up, especially on the street. We went to bigger cams, bigger E/I splits, 8 to 10, widened the LSA, basically keeping the same amount of overlap as we were running before with the narrower LSA, but raised the compression, and went to more modern stall converters that drive like stock converters but stall to 4-5 thousand. We also went from race gas to pump gas on many of them. Even though the new combo actually has higher compression.

 

[Gunner1]

I’m thinking street mostly. Sound tough at cruise night and not need to shift beyond maybe 6k if raced.

 

[Dodge Bros]

I’m thinking street mostly. Sound tough at cruise night and not need to shift beyond maybe 6k if raced

I think I recall you saying planning for 3000 stall. What gears and tires do you plan on running? And what weight do you think your car will be?

Keep in mind if your basing this new 440 combo from the experience of the previous SBC Elgins combos there are differences in how cams are ground also. Some cams are symmetrical, others are varies levels of Assymtrical (less acceleration on the closing side of the cam). The symmetrical profile have choppier idle than the Assymtrical with the same LSA, and scavenging is moved a little higher into the RPM band. I don’t know what those Elgin cams were, I suspect more symmetrical. Also if your looking at .904” designs SFT cams they hold the valve open more when the piston speeds are the highest versus the .842” lifter designs. You can run bigger E/I splits and add extra overlap and it won’t pollute the intake charge with exhaust as much, almost as good as roller cams.

I don’t know if it was mentioned, but Vizard’s 128 equation for LSA’s with SBC engines is for single pattern cams. When you add more exhaust duration to the cam and keep overlap the same the LSA always becomes wider. So with 8* extra exhaust duration like those Elgin grinds that equation should really use 130 instead of 128. But then all the other overlap, compression and CID charts get thrown off and the equation is starting to shoot in the dark. On a BBC instead of 128 the number is 132. So with 8* more on the exhaust that gets it up to 134* as the starting number. Not narrow at all. People often claim Vizard only does narrow LSA cams, but they mostly only look narrow because he likes single pattern cams. He usually adds 4* at .050” switching from a dual pattern to a single pattern. His cams are not necessarily very small.

 

[Geoff 2]

Vizard points out in his book that there are correction factors for the 128 rule, & NEVER recommended tight LSAs for mountain motors & high comp engines. This seems to have been lost in the translation....

 

[Dodge Bros]

Some cams also have different acceleration rates between the intake and exhaust, and that changes the way the engine scavenges too and is another thing to consider when considering the LSA. When you compare the cams with less acceleration on the exhaust side to a single pattern cam, the single pattern often has the same profile on both intake and exhaust. If you try to get a similar overlap triangle with a split duration cam that has less acceleration for it’s exhaust the LSA number changes when you compare the seat timing or the .050” timing. An example is a Comp cams retro fit thumper cam. Those thumper cams the LSA is 107*, but it has a 14* E/I split at .050”, it has the equivalent LSA @.050” of a 103.5* LSA single pattern cam @.050”. It has less acceleration on the exhaust though (20* more @.006” on the exh), at the seat timings to compare overlap to a single pattern the cam runs 102* LSA. Tons and ton of split pattern cams put less acceleration on the exhaust side, the BB Magnum HP cams did the same thing, it helps them scavenge like a narrower LSA cam but keeps the main part area of the cam where the piston speed is faster. If you pull it too it’s knees a lot it will probably discolor the exhaust manifolds from all the heat, but it is another way to get more scavenging done with wider LSA cams.

 

[gtxrt]

For the street a good compromise would be 4*- 6* intake exhaust split. most comp cams have a 6* split. a big split will keep your power up after peak horsepower at a cost to low end torque. for the street I always favorite torque over horsepower. Mr isky cams would say if you need a lot more exhaust duration then your intake duration is too small.

 

[Dodge Bros]

For pump gas if the compression is on the high end for the octane we often make the split 8 or more. I think it depends a lot on how well the exhaust flows. It is different also with a full exhaust on a street car. When you add more duration on the exhaust it allows the engine to pump more compression, it raises the tq peak a little higher and broadens the power band out, and makes the power band smoother. When you pull the exhaust duration back, it does the opposite, then the intake profile might need extra duration to keep the engine happy.

The biggest mid range power is going to be reverse splits. With that combo you start with the overlap you need for the peak RPMs, then maximize the compression and knock the pumping compression back with a bigger intake profile. Power drops like a rock when you hit the peak RPMs, but the mid range power becomes really wide. For pump gas the pumping compression has to be lower than the other cams because the engine has to work more to force the exhuast out. Usually the LSA is pretty narrow and has a more symmetrical profile on the exhaust side. It is more common for trucks or towing that do not see a lot of high RPMs. Tall geared cars can fall in that category also though.

 

[Gunner1]

Yup I’m thinking a 2800 to 3000 stall that has low slip above lockup. My car weighs 3400 empty with 2 gal of fuel. And I’m running 3.73s with 275/60/15 tires. Right now Cooper cobras but some sort of street drag radial would be appropriate. My thoughts are that a lot of this is dependent upon head flows and I/E flow ratios. I wouldn’t want to over scavenge. For me this is just an old hot Rodgers final flight of fancy. I’m not building a world beater street cam, you all are savvy enough to know just how much one component can change cam choices. Thanks to Eric Winegarten I have my heads flow rates. He did a set of the new CNC Stealths. The old ones would allow a lot more rpm with flows into the low 300’s. Mine flow. About 280 at 600. He does his exhausts without a pipe. So I’m guessing add 15 to 30 CFM on top. Not on par with a fully ported set of race heads but sufficient for a 6000 rpm street 440.