Blowers overcome the shortcomings of overlap. Large LSA are not needed.

The only way valve events can be altered on an existing cam is by advancing or retarding the cam timing.

Please allow me to make an analogy.
If your child were sick and dying, would you ask a doctor to help,
or would you go down to the oil change place and ask a grease-monkey what to do?

Of course, you would go to a medical expert. :thumbup:

I would recommend that you do what the experienced blower manufacturer recommends and save yourself a lot of grief.

 

xntrik is more versed on cam stuff than I am, but I will say my 2-cents. One of the reasons for wide LSAs on forced induction engines is reducing overlap. Having them open together for too long just blows fresh intake charge right out the exhaust valve.

The proper overlap is therefore necessary. I'm not good with blowers, but I know that many turbo cams have ZERO overlap.

 

Those who say wide LSA is better on blown motors may have been talking about using fuels other than gasoline. Here is the recommendation copied from the BDS site:

"Choosing the proper camshaft would be the most important requirement for a blower motor. An improper cam will cause a variety of problems that can easily be avoided by following a few simple guidelines. Hydraulic cams are recommended if you intend to drive the vehicle frequently, requiring little or no maintenance, and the maximum engine RPM's are kept around 6500 or lower. Roller rocker arms are recommended. Flat tappet and roller cams are recommended for high performance applications especially where the engine will see high RPM's. Exact camshaft specifications vary depending on the performance level you wish to attain. BDS offers ten different types or stages of cam grinds specifically made for blower motors. Refer to camshaft specs listed in tech info for BDS' individual engine camshaft specifications and their intended uses.
If you wish to purchase your cam from one of the many fine camshaft manufacturers, we suggest using our camshaft specs as a guidline. Extremely high lift and long duration cams are recommended for high RPM, high performance racing only.
The lobe center of the cam will play an important role in determining the performance characteristics of an engine. Wide lobe centers (112 to 114 degrees etc.) will create higher cylinder pressure providing more horsepower with cooler burning fuel such as alcohol and methanol. We have found 110° lobe centers to produce the best overall power on gasoline.
Whatever cam you choose, make sure that it will operate and perform properly in the RPM range required for your application."

 

Blowers overcome the shortcomings of overlap. Large LSA are not needed.

The only way valve events can be altered on an existing cam is by advancing or retarding the cam timing.

What I meant to say, is if I have a cam I'm happy with now, and I want to modify the design for a blower, how would I go about calculating that so I could tell the manufacturer what to grind for the new cam?

And techinspector, thanks for the link. I already read that page and it didn't answer my questions, but thanks for the link nonetheless. I've emailed BDS but they have yet to reply.

One thing on my mind, for example, is that many guys run full length headers with a supercharger. That strikes me as a little inefficient - seems like if you correctly design the cam with just the right amount of overlap, you can scavenge the cylinder - probably more efficiently than you can with headers - and not have to deal with the expense and heat problems of full length headers. One of the reasons I'm going with the supercharger in the first place is so I won't have to deal with the heat problems generated by a turbo. That, and the fact that I think a lot of turbocharged cars sound like vacuum cleaners. (Is my age showing yet?)

Hey! It just dawned on me about what you meant about blowers overcoming the shortcomings of overlap. With a blower I'm going to get a better idle than I would have with natural induction. Cool!

 

Xntrik put me onto a book about blowers. Lee how about you post about this book. I strongly suggest you buy it and read it from cover to cover several times before you do anything.

In this book it says to run the exhaust as big as possible. Bigger is better. On the blown race cars they just run zoomies to get the hot exhaust away from the car. Don't need equal tuned or any of that just big.
I built my small block headers with 2" primarys and a 4" collector to a 4" Dynomax muffler, should be free flowing.

 

What I meant to say, is if I have a cam I'm happy with now, and I want to modify the design for a blower, how would I go about calculating that so I could tell the manufacturer what to grind for the new cam?

And techinspector, thanks for the link. I already read that page and it didn't answer my questions, but thanks for the link nonetheless. I've emailed BDS but they have yet to reply.


Hey! It just dawned on me about what you meant about blowers overcoming the shortcomings of overlap. With a blower I'm going to get a better idle than I would have with natural induction. Cool!

Techinspector copied right off their website.

QUOTE= If you wish to purchase your cam from one of the many fine camshaft manufacturers, we suggest using our camshaft specs as a guidline.= QUOTE

Righto = With pressure blowing in, there is no reversion from the exhaust system during overlap. At idle you might not be making positive pressure but less vacuum, so the effect is still reduced. :thumbup:

 

I'm still waiting to hear back from BDS. They list their cam duration and LSA, but don't list the advance.

I'd still like to know what parameters are different when calculating for boost vs. NA.

 

I'm still waiting to hear back from BDS. They list their cam duration and LSA, but don't list the advance.

I'd still like to know what parameters are different when calculating for boost vs. NA.

I see, you are wondering if they recommend the cam timing be advanced or straight up.

That has nothing to do with the grinding of the lobes, only the degreeing-in.

Although cams that are recommended to be installed advanced 4* are often ground with 4* advance to simplify degreeing-in, in case a person doesn't have a multi-key sprocket.

If they give you the event timing the advance can be determined from that.

 

"They list their cam duration and LSA, but don't list the advance."

What advance are you talking about. The advance that's ground into the camshaft when it's manufactured or the advance/retard you might give it on installation?

If it's the former, you can figure that by subtracting the intake lobe centerline from the LSA. If it's the latter, install the cam per the centerline given on the cam card.

 

I see, you are wondering if they recommend the cam timing be advanced or straight up.

That has nothing to do with the grinding of the lobes, only the degreeing-in.

Although cams that are recommended to be installed advanced 4* are often ground with 4* advance to simplify degreeing-in, in case a person doesn't have a multi-key sprocket.

If they give you the event timing the advance can be determined from that.

OK, fine. Let me rephrase. "They list the duration and the lobe separation angle, but they don't list the valve opening and closing events so I don't know where to place the lobes on the cam blank." How's that?

 

OK, fine. Let me rephrase. "They list the duration and the lobe separation angle, but they don't list the valve opening and closing events so I don't know where to place the lobes on the cam blank." How's that?

I can't find any data on their site, except the price. No specs will come up at all, everything I click on gives me the price :smash: .

LSA is the cam timing....... straight-up 0* timing = LSA

Any advance is taken from that figure.
If they don't recommend an advance, then straight up is preferred.

Remember LSA is the only figure given in CAM degrees, all else is in CRANK degrees.

So if you install it at 110* (ATDC) Intake Lobe Centerline it will be at 0* straight-up.
(and the exhaust valve will be at 110* BTDC)

110 intake=110 exhaust, so there is zero offset(zero advance) straight up 0* timing.

Each lobe will be displaced 110* from the center.... which is the 0*.

If it were advanced 4*, the installed Intake Lobe Centerline would be 106* and the exhaust lobe would fall at 114*.

Both the intake and exhaust would open 4* sooner than straight-up. You build a little more cranking pressure but the exhaust blows out the ex valve sooner.

 

I can't find any data on their site, except the price. No specs will come up at all, everything I click on gives me the price :smash: .

LSA is the cam timing....... straight-up 0* timing = LSA

Any advance is taken from that figure.
If they don't recommend an advance, then straight up is preferred.

Remember LSA is the only figure given in CAM degrees, all else is in CRANK degrees.

So if you install it at 110* (ATDC) Intake Lobe Centerline it will be at 0* straight-up. (and the exhaust valve will be at 110* BTDC)
Each lobe will be displaced 110* from the center.... which is the 0*.

Yes, I understand. However, as an example, most Crane cams have 5 degrees of advance ground in - for example, crane part number 109821 has 214/222 duration, with the intake lobe centerline (LCA) at 107 degrees and the exhaust at 117 - in other words, a 112 deg LSA with 5 degrees of advance already ground into the cam. BDS just gives duration and LSA, without listing anything that will tell me where the lobe is centered on the cam. If I seem overly concerned, it's because the modern engines seem extremely sensitive to cam advance - we see 200-300 rpm difference in the power band between two identical engines with nothing more than a sloppy timing chain.

I suspect we have a difference of vocabulary. We need to get together for a beer/wine/soda/coffee/pizza and discuss this in person.

Oh yeah - click on the tech info => blower cam specs on BDS's site to see the cam specs.

 

Al you need to look at is the camshaft cam card, which I believe most of the sites have a link into. Comp does for sure. That is the only place you will find the valve events.

 

Al you need to look at is the camshaft cam card, which I believe most of the sites have a link into. Comp does for sure. That is the only place you will find the valve events.

Except the BDS cams we are talking about don't have cam cards online, darnit.

 

To my knowledge BDS though doesn't make cams or, from what I read here, are they recommending a specific cam. ALL they said is they suggest a 110 LSA. So, it's not going to be too difficult to figure some options based on the fact cam choices will fall somewhere around 110 to 106 ICL. Then the point at which the camshaft will act on the valve is TOTALLY determined by the ramp on the lobe and the type of camshaft, again, which I don't think BDS makes. So you have to work directly with the camshaft companies on the lobes and match it up from there.

 

Clevite has cam specs in a PDF file. See (PDF) pp. 115 - 132.

I bought a 229-2305 cam for my 454 from them (the price was right ... I was working for NAPA, and they're a NAPA supplier. )

I'm running 224-2519 (+.030) forged flat-tops and (stock) 781 heads right now with 212-1325 valve springs and Crane roller rockers. I had the engine built with a street blower "uprade path" in mind. If/when I get there, I'll have the heads machined for bigger valves, and port & polish them.

I didn't have much of an idea exactly what I was looking for, but tried to find something with specs close to Comp's "Nitrous / Blower" grinds.

 

Yes, I understand. However, as an example, most Crane cams have 5 degrees of advance ground in -

If I seem overly concerned, it's because the modern engines seem extremely sensitive to cam advance - we see 200-300 rpm difference in the power band between two identical engines with nothing more than a sloppy timing chain.

I suspect we have a difference of vocabulary. We need to get together for a beer/wine/soda/coffee/pizza and discuss this in person.

Oh yeah - click on the tech info => blower cam specs on BDS's site to see the cam specs.

PIZZA AND A BEER........ :thumbup:

BDS just keeps looping me to the price. :smash:

Ground in, correct, but that only assists in degreeing with the chain. If they want more than the usual chain keyway at 4* they will grind it in for ease of degreeing. As I said, 4* ground in will work with a stock cam set that has 0 only timing.

Always remember, advancing or retarding is a "crutch" for an "almost-correct" grind. If you like the power curve of a cam when advanced, a cam lobe could have been ground that gave you that power curve when "straight up".

Advancing is only a "tool" to vary the power range My 350 and your 350 might be similar and with the same cam we can tailor our combination.

Most cam companies grind a cam with too wide of lobe centers and recommend advance because most hot rodders want too big of a cam and these two things help tame the cam down to be more driveable. That is, gives back some cranking pressure and tames the idle by lessening overlap.
****************
A 350 with 1.94 valves likes a 107* LSA (using Vizard's chart). So if you take that 112* cam you illustrated in post # 13 and advanced it 5*, the intake lobe would be timed at the ideal place for a 350 (107*) but the exhaust would be opening 14* early.

 

I have to mention that David Vizard did a lot of cam testing and development work for Crane Cams. So if the numbers you gave seem to ring a bell, that is why.

 

BDS just keeps looping me to the price. :smash:

OK, try this. Go the the menu and click Tools => Internet Options. Then click the buttons to delete all cookies and temp files and clear the history. Then maybe you won't get directed straight to price.


On the cam, so what you're saying is that instead of a 112 LSA with 5 deg advance ground in, the cam would have been optimal ground on a 107 deg LSA installed straight up?

 

Supposedly a 107 is ideal for a NON blown 350 engine with a single pattern cam and 1.5 rocker arms.

Extending the LSA to 110 decreases the overlap 6 degrees, to 112 decreases it 10 degrees, with the same duration cam.

When duration is added to either lobe, the overlap increases and each valve on that changed lobe is opening sooner and closing later.

When the blower begins producing enough airflow to reduce vacuum, the overlap event effect changes. Under boost, the intake air is blown out the exhaust valve of the overlapping cylinder, and the sequential effect of the common (single-plane) plenum is negated.

Each company has a recommendation based on their experience, and there are differences of opinion. Nice part about it, you can just dial in more boost.

 

OK, I'm beginning to understand. But you brought up another point - single vs. dual pattern cams. Is the lobe center angle on the exhaust lobe as important as the intake LCA? Or should one be more concerned with the amount of overlap?

 

OK, I'm beginning to understand. But you brought up another point - single vs. dual pattern cams. Is the lobe center angle on the exhaust lobe as important as the intake LCA? Or should one be more concerned with the amount of overlap?

http://www.iskycams.com/techtips.php read em all but see # 2003

Isky says the long duration exhaust lobe is to reduce low end torque by blowing the power stroke out the headers.

They might be wrong but ED invented the cam market and they have been grinding cams 50+ years.

 

http://www.iskycams.com/techtips.php read em all but see # 2003

Isky says the long duration exhaust lobe is to reduce low end torque by blowing the power stroke out the headers.

They might be wrong but ED invented the cam market and they have been grinding cams 50+ years.

I don't buy it. I think he' If the heads are perfectly optimized, then yeah, I can see only needing a single pattern cam; but a lot of times you don't have the magic 75% flow ratio between intake and exhaust, and you have to compensate with the cam. The vortech heads, for example, only have about a 68% flow ratio, and we've seen our best ET's AND top speeds with 8-12 degrees more on the exhaust duration. The LS1 guys, on the other hand, have a 76% flow ratio right out of the box, and you actually see quite a few cams with a single pattern or maybe only 2-4 degrees difference on them. On the other hand, the LS7 heads, while they have ridiculous flow numbers (360 cfm on the intake side as cast), have a rather poor 62% flow ratio. The stock LS7 cam is 211/230 duration as a result.

 

I don't buy it. I think he' If the heads are perfectly optimized, then yeah, I can see only needing a single pattern cam; but a lot of times you don't have the magic 75% flow ratio between intake and exhaust, and you have to compensate with the cam. The vortech heads, for example, only have about a 68% flow ratio, and we've seen our best ET's AND top speeds with 8-12 degrees more on the exhaust duration. The LS1 guys, on the other hand, have a 76% flow ratio right out of the box, and you actually see quite a few cams with a single pattern or maybe only 2-4 degrees difference on them. On the other hand, the LS7 heads, while they have ridiculous flow numbers (360 cfm on the intake side as cast), have a rather poor 62% flow ratio. The stock LS7 cam is 211/230 duration as a result.

Who says that 75% is a MAGIC number?

All that varies with the combustion chamber efficiency, exhaust system efficiency, etc, etc....

Also if you read carefully, ISKY does say that it removes lower rpm torque and often ADDS high rpm power.

QUOTE from Isky# 2003 http://www.iskycams.com/techtips.php

Example #2 (Drag Racing) At the drag strip it's a little different and I feel more honest. Here, racers have long enjoyed longer exhaust and longer durations across the board (If I may add specifically for the purpose of "killing" low-end torque) to keep the tires from too easily breaking lose. This has been successful and sometimes actually results in a slight increase in top end power - something you can actually use in drag racing since it is a full throttle endeavor through the lights. Keep in mind here though, it's quite possible that a longer duration cam overall would have done just as well or better. In other words if you needed that longer exhaust for top end, perhaps the intake could have benefited from such a lengthening as well


And, David Vizard does/did testing and development for CRANE CAMS and he says the same thing.

That's 2 fairly substantial authorities that agree.

 

Who says that 75% is a MAGIC number?

Well, Vizard does, for one - page 42 of his cam and valvetrain book.