Is it REALLY Imperative to run wide LSA's with boosted engines, I have had Nitrous set-ups for quit a while, and have never ran a "Nitrous cam" and have had some descent success, I have always put the heads/cam valve train package together be yied good N/A numbers and even better yet..et's. I have read several times from guys here saying that it was a good logic to follow..and it looks like most guys do similar things...with Nitrous set-ups.

But being Boosted has kinda always had the rule of thumb..."best to get a cam with a 114(ish) LSA..I dont necessarily agree with THAT number..even though it would seem that the LSA "should" be wide, BUT...over recent searches on the issue here, I have seen where guys are running 112 LSA's on boosted cars with great results, how much of an effect would a 110 LSA have on typical Blown and Stroked LTx application with typical LTx heads that flow 280ish/190ish, 3" exhaust and cam with high .590's-600's lift?

Thanks

 

the LSA doesnt matter, you want a tight overlap, i.e. -5 or so. The cam for my 5.3 is -11 overlap

216/215 .561/.559 113+2

 

Thanks man

 

LSA doesn't technically matter, but it's generally used as an indication of valve overlap, which is what leads to reversion problems.

 

{[(intake duration + ex duration) / 4 ] - lsa} *2
Soo....
Mine 218/224 on a 112
218+224 = 442
442/4 = 110.5
110.5- 112 = -1.5
-1.5 * 2 = -3 degrees overlap

I have always heard stick around between -8 and 0 for turbo
0 and +8 for blower

Some people say a tiny bit of overlap on a turbo can help push exhaust out when you have lots of backpressure. Need enough hair dryer to waste that extra air though.

Although all tests show more hp with more overlap - however at the cost of moving the powerband higher. Remember the whole thing has to work in harmony. I'm ditching my cam with the new turbo because its um bigger than the mp70 and I plan to spin higher than 5k this time around

My official answer is to cam it for how you want to drive it, and then add enough boost to get to the power levels you want.

 

I still wish I left my 236/242 111 cam in my car, now I have a 230/236 114lsa and they power band is shifted alot,I dont make any decent number until 4000+rpm....and it doesnt stop, they higher I spin the motor the hp goes up like crazy...from 6170rpm-6250 my car picks up 38RWHP!! thats alot for 80rpm.

Jay

 

So the 236/242 made power sooner than the 230/236??

 

im talking N/A yes...i made alot more power from 2000-4000 on my NA setup...but once the turbo spools i makes up for the lost lower end power lol

Jay

 

Whoa, man that is totally opposite of what I was thinking would happen, I would have thought the 230/236 grind would make the power sooner, dam..sometimes cam selection can feel like smoke and mirrors is a FI application...heck, with Nitrous..I would always set-up the car to make the power N/A and then add the giggle gas, seems to always work for me..My Impala wet 11.0's several times at with 90 degree Tx weather...just never could get the 10.9999999 I want, car has alwaya been full weight though 4445 with me in it.

I am doing a Booted plus Nitrous set-up this time, and they issue I am having is, the heads/cam/intake that is on the car seems like exactly what the car likes...fits the way i dirve it etc..

I am pulling out the JE flatops (-5cc) for some dished pistons and installing a intercooled R-ttim (OLD school T-trim predecessor) that set-up made about 17lbs on the car it came off of, would probable make 12lbs on my set-up due to the higher flowing heads and 398 cubes.

Thanks everyone for your input

 

I run a CC306 in my setup and it's a stump puller. I really don't see no affects as the pwr band is everywhere w/ the F1.

 

It's overlap that your looking for...LSA is just a small part of it. Here is a quick formula to help those interested in finding overlap:

intake duration + exhaust duration = total duation
total duration / 4
then subtract LSA
then multiply by 2

Example 218/218 112LSA versus 224/224 on 114LSA

218 + 218 = 436
436 / 4 = 109
109-112 = -3
-3 x 2 = -6 degrees of overlap

224 + 224 = 448
448 / 4 = 112
112 - 114 = -2
-2 x 2 = -4 degees of overlap

As you can see, the 224 cam has more overlap then the 218 cam despite the 224 cam's "blower friendly" Lobe separation.

I've heard of great results using a LSA around 112 deg. with the exhaust duration about 10-12 deg longer
than the intake duration when using a blower. A turbo will take a different style cam.

 

Thanks for that post man....thats good info.

So..with a blower, you want LESS overlap lap than the -6 value derived form the formula right, I guess form -1 to -3 would be optimum/

 

Well it would actually be more overlap but you have the right idea. 0 to +6 is what I have heard for a blower. A bit more pushing the exhaust out, where a turbo with a lot of backpressure might bleed back into the cyl with more overlap.

 

Overlap with a blower and overlap with a turbo have completely different effects. LSA is irrelevant.

On a blower, you're trying to prevent losing air/fuel out the exhaust during overlap. On a well-tuned NA setup, you could actually lose air/fuel out the exhaust, so I believe this to be a credible scenario since you now have high pressure aggravating the situation. How much overlap is too much is up for debate, though, as no one who has done extensive testing shares the results.

On a turbo, the exhaust pressure is higher than intake, so you don't lose air/fuel out the exhaust. The danger is pushing exhaust into the intake port. However, the positive benefits of overlap seem to outweigh the negatives. As far as I know, no one has gained power from decreasing overlap with a turbo. In fact, the opposite has been the case, up to about +5 deg @ .050" at 2/1 backpressure/boost. The actual optimum overlap will depend on the backpressure/boost ratio, head geometry, etc., but again, if anyone knows the correlation between backpressure and optimum overlap, they aren't sharing.

About 30 years ago, David Vizard published a book that stated that supercharged engines needed higher LSA's than NA. Everyone assumed he meant 114 and it applied to turbo's too and ran with it. He actually stated that 110-114 should be considered (basing that on 106 being good for NA). Keep in mind that the audience 30 years ago wasn't as knowledgable as the today's FI enthusiast, and didn't know to include factors such as backpressure, actual overlap, etc...

 

how about the gm847 or cc306??? would they be too much?