The truth about cam LSA with a street 6-71?

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70Duster440

70Duster440

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Been doing the research. Starting to gather parts. I'm interested in real world experience, not someone's "I heard that...".

Everything I've read and found suggests using a cam with 112-114* lobe separation - with 114* being the target. A little more lift and duration on the exhaust side is recommended but lobe separation is the biggy. BDS Blowers, on the other hand, recommends 110* across the board for a gasoline engine.

Something as simple as a Summit 6401 cam fits the bill for blower use but the MP Purple Shaft 280*/.474 with 110* separation that I've been running in a NA engine appears pretty close to what BDS suggests. It just doesn't have the split lift and duration.

I don't know how critical this is for a street blown engine that'll never see a track.
 
70Duster440 said:
BDS Blowers, on the other hand, recommends 110* across the board for a gasoline engine. I don't know how critical this is for a street blown engine that'll never see a track.
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Some overlap is ok for street use as this cools the combustion chamber.
 
Supercharged engines like different valve timing than do N/A engines. They prefer a little less overlap because the intake charge is forced in rather than drawn in so you dont want the forced A/F charge coming in the intake & going right out the exhaust because of too much overlap. If I were to be going to the expense & trouble to put a blower on a car I'd look to the cam manufacturers & see what they recommend or talk to an engine builder who has done a lot of supercharged engines before.

Gotta love the effect of a big ol' blower sticking outta the hood & making that whine as you're comin' down the road! All polished & shiny, yeah!
 
112-114 for max efficiency/power,the 110 as mentioned will give some "blow down" or a little extra cooling due to the overlap which is nice for the street.With what your doing I would not hesitate to use a 110 since it will still make a ton of power.
 
lead69 said:
112-114 for max efficiency/power,the 110 as mentioned will give some "blow down" or a little extra cooling due to the overlap which is nice for the street.With what your doing I would not hesitate to use a 110 since it will still make a ton of power.
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Thanks for the replies, all. I'll certainly save some time and money re-using the cam. The downside - other than blowing unburned fuel out the tail pipe?
 
70Duster440 said:
Thanks for the replies, all. I'll certainly save some time and money re-using the cam. The downside - other than blowing unburned fuel out the tail pipe?
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Other than losing some efficeincy i.e. power none really,makes for a cooler running street car.As a buddy of mine puts it-its a giant furnace sitting on top of the engine!.
 
ONe other side effect of a wider lsa cam profile is that the exhaust event tends of begine sooner before BDC. This is done to utilise some of the energy of the still expanding combustion gasses, to help get the spent charge to evacuate the cylinder. Since you're forcing the charge in, it helps the engine to get the spent stuff out. The tradeoff with doing this is that by using the thermal expansion to push the gas out the EX valve, means that (For those degrees of crank roation BBDC that the EX valve is opening early) that energy isn't pushing the piston down the bore. And is in effect wasted energy.
This isn't as big of a dealio in a race motor. As typically the increases in performance are so dramatic that the losses aren't missed. (And the increases in charge etc are also so great that the extra time to evacuate the spent gasses are moreso needed)
The same caan be said for nitrous cams where a big hp shot is being administered. Which is why up to 150 shot (Give or take) the cam profile is less critical.
 
Valvebounce said:
This isn't as big of a dealio in a race motor. As typically the increases in performance are so dramatic that the losses aren't missed. (And the increases in charge etc are also so great that the extra time to evacuate the spent gasses are moreso needed)
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So, is it a bigger deal in a non-race street engine? Will I be giving up noticeable performance?

The Mopar 993 cam that i have is 280*/.474. That's 238* @ .050" with 110* LSA.

The garden variety Summit cam they show for blower use has 282*/.465 int. and 292*/.488 exh. or 224* int/234* exh. @ .050. with 114* LSA. (higher lift, longer duration on the exhaust side).
 
I don't think it'll be too much of an issue. Since (Short of removing the blower drive belt) there's no real way or "turing the blower off".
Whereas with say a nitrous engine, the engine would have a bit less off the bottle.
If you were really worried about it, maybe you could run the tighter lsa, and run with a higher ratio rocker arm on the exhaust side. A bloke I know who used to race a bottle fed chev used to do that.
 
Turbo, Supercharger, and Nitrous cams are all very very similar. Wide (112+ degree) lobe separation, and bigger exhaust lift and duration. Truth be told, max effort race cams do this too.

Basically what it comes down to is that the wide lobe separation (and the lack of overlap) keep you from blowing your fuel and air mixture right out the exhaust port. (look up "overscavenging" for the Naturally aspirated equivalent of this phenomenon)

And the big exhaust lift and duration help move out your spent gasses better, since although the intake side is getting helped by your power adder, your exhaust side isn't. (and, thanks to boost, there's now MORE exhaust that has to be moved. so it needs all the help it can get.)

Using the purple cam isn't ideal. But it's not BAD either. it's not like the racier 108-106 lsa cams like you'd find in dirt track cars and the like. Stock cams tend to respond well to supercharging and turbocharging since they share a lot of the same traits with blower cams.
 
I've heard everywhere that it needs to be close to 114 for turbos or superchargers but, i found this article the other day. Just did a quick read on it but, it seemed like it was saying that only old turbos needed 114. http://www.carcraft.com/techarticles/ctrp_1106_turbo_camshaft_guide/viewall.html

Not sure what aspects would differ in turbo vs blower.

i figured id present it as data to be considered as it went against the norm.
 
As a follow-up, I reused the 110* cam and while it may not be ideal, I'm not disappointed. :)

This IS a street car.
 
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