What EXACTLY can I expect with increased cam overlap in a Centrifugal supercharger application? I realize this is a multi faceted question, which is why I specified "centrifugal" supercharger. Even more specifically; Can I use cam overlap as a "tool" to bleed off overboost with a higher compression engine like an LS3? Will the increased scavenging/blowthrough effect of the overlap, coupled with the boost, cool the combustion chamber enough to reduce tendancy of detonation in this application?

 

My understanding is that you will bleed off boost which costs you some efficiency from the blower. However, it does bleed off compression by decreasing your dynamic compression ratio. The overlap probably won't net you any more power (since that's really meant to "draw" in the intake charge on an N/A motor, which isn't necessary when it's force fed). However, the increase duration will probably net you more power. Whether that power outweighs the decrease in efficiency of the blower is beyond me.

I'm using a 230/236 on a 112 LSA with my D1SC on a 10:1 CR motor with a 3.70 blower pulley. We'll see if that's enough to keep detonation under control!

 

More overlap will soften the power in the lower rev's, but then again a centrifugal supercharger does not make much power at lower rev's anyway. End result, more power at high rev's, but again that is where the centrifugal supercharger makes the most boost, at max rpm's. So I guess what I am saying is this: you will kill what low rpm power you were getting from the blower, but more than make it up on the top end.

 

I don't believe this to be correct.
Here is 1 example, 742RWHP 722RWTQ Forged 347ci LS1. Compression 9.5 to 1, CNC ported 241 Heads nearly 300cfm of flow, Polished Weiand Intake to match heads, small cam - GM HotCam (218/227 .525 .525 112lsa) w/other supporting Internal mods.

Procharger D1SC w/ 14 to 15 max psi.
Snow Meth Injection w/1000ml spray added in early 2007.

My car,

751 rwhp 639 rwtq Stock short block 346ci. cnc'd ls6 heads 11.4:1 Fast 90mm intake 220/224 .576/.581 116 lsa
Procharger D1SC @ 12#,s


I make less boost but have more HP. The first car used a 3.4 in. pulley I use a 4.0 pulley. He does have a 12 bolt w/3.73 and my rear is a stock 10 bolt, but that would make only a small difference.

 

LS2/402/EPP blower cam/AFR225 heads with F-1A at 14.4 psi of boost.
http://www.exoticperformanceplus.com...Car.php?car=29 Here is a link to this car. Bob

 

My car is 427 stock crank ,stock heads ported, forged pistons, 9: 5-1 COMP YSI MY FIRST cam had no overlap made 804 rwhp fran at RPM made a custom cam dont want to lie I think 20 ish degrees of over lap car picked up over 100 rwhp now mybe the 1st cam suck but either way the cam with over lap made more power 954 rwhp on dyno jet uncorrected 1005 it was 30 degrees and dyno jets over correct

 

Considering the altitude we live at, these are the numbers I'm looking for. The rest of the equation is; 427cid LS, LS3 heads, GMPP LS7 Stage 3 cam, LS3 intake, FAST 90mm TB, 42#-60# injectors, LT 1 7/8" headers, MSD coils, and a Paxton Novi 2000RR blower. The 72cc heads and JE pistons produce about 10.8 compression with .050 head gaskets (which may be pushing it too). This cam is the real issue though. The LS7 heads do not have a breathing problem. They have a rather high inequality in flow numbers between the intake and exhaust sides. Ergo the the wierd (233/276 .595 lift W/1.7 or .630 W/1.8 rockers) specs of the LS7 stage 3 cam. I can see why they don't put these cams in production cars. With as much unburnt hydrocarbons the scavenging effect would put through the exhaust, the car would never pass an emmissions test. lol But long story short, the cam was obviously designed to take advantage of the killer intake flow of this head and boost the numbers of the exhaust to help it keep up. ..42 on the overlap, I believe. The LS7 produces 327 f.lbs. & 118 hp at 1900rpm with this cam. Numbers top out at 543f.lbs. and 645hp. I can't see how a blower could hurt these numbers any at either end of the spectrum.
My question would be: Does anyone know what would happen if a substantial amount of boost were added to this engine/cam equation? "Substantial" = I'm running an 8" Innovations West crank pulley, 2.7" Riechard Racing blower pulley, and a 3.54 step up ratio. The Novi2000 is NOT a small unit. I have already spoken to Paxton and they said that the blower is capable of spinning much higher than the 55,000rpm recommended max. 55,000 is simply its max efficiency. It isn't really doing anything more for the engine past that point. The way the pulleys are set up, I'll be seeing max efficiency by 5600rpm or so. The engine probably won't see the high side of 6000.
Its cool to lose some boost to the cam if it is actually helping other factors such as cooling the cylinders, reducing detonation, increasing volumetric efficiency, reducing the amount I have to enrichen the fuel supply, etc.
Let me know what you guys think... or we could just pool bets on how long the engine lasts. hahaha

 

Im having an issue like that on my car to. I have stock heads 241 heads 90/90 setup and a forged short block GM head gaskets on a 347. the cam is .588/.601, 228/236, on a 114 lobe sep. it has only 4 degrees of overlap. Also a D1SC with a 3.4 Riechard racing pulley. at bout 5500 in stops building boost at 10 psi. tomorrow im gonna pressure check for boost leaks. Not sure if the race valve is lose letting out all my bost or what. Im stumped folks say different things from the wrong cam, boost leak, MAF problems, or delete the MAF and get a 3-bar MAP sensor and tune it with it. I considered goin with the EPP forced induction cam .595/.608, 232/240, on a 115 lobe sep. but thats just more money to spend.

 

Sounds like you are making a lot of power for 12PSI at 6000' elevation. What is your best ET, and MPH up here in Co?

 

Kevin's (kp) experience with cam swaps was the best comparo I've seen.

Bigger cam = more power but more duration can mean soggier low end and have to spin the engine/blower higher to make that power.

So would you run a 224/224//114 or run a 236/244//112? It.... depends.

These conversations should always start with... "My goal is to..."

 

I made 680rwhp and went 9.40@143mph with my YSi with some belt slippage, 236/246 ish cam. That was 18 psi with a very simple, super low compression 348ci. I shifted it at 7 grand. It made 754rwhp with 21.5 psi. Keep in mind that's thru a T400 so add another 70-100rwhp for an MN6 car. Went 5.9/116 so it could have gone 9.3@147 without belt slippage IMO.

 

ok. My goal is to break into 800+ streetable RWHP with the setup I'm intending to use here. My questions still remain: Can I use the cam overlap to my benefit? I'm hoping the increased cubic inches will offset the loss of low end power loss of my chosen cam. I'm hoping the overlap will increase the volumetric efficiency of the engine even with the blower. I'm actually hoping to get the results Ricky Bobby did.

 

I only recently learned to drive at the track, so not so good. I also learned that my spec stage 3 can't hold up to the power. It was slipping a bit and on the stock hydraulics, I couldn't shift out of first. So, My best run to date is a 12.222 @ 120.78 1.927 60'. My car is a pig at 3920 lbs. I did manage a 1.86 60', so I'm getting better, but the harder I launch the longer I have to wait to shift from 1st to 2nd, which really kills the E.T.

Also, the dyno was a Dynojet, so the #'s are inflatted, and they are corrected to sea level @ 20%, because he couldn't get the uncorrected #'s.

 

From what I have seen on the board, it went from little overlap is best, a few years ago, too a fair amount is better.

A bigger cam makes more power n/a, but boosted you would have to use a smaller pulley to push more air to get the same amount of air/fuel to stay in the cylinder as a cam with less overlap. This is why people believe less overlap is more efficient.

On the other hand, If my understanding of overlap is correct, the added overlap is for volocity. I would think that even if the air is pressurised, if it can move faster, than you would see the same effect of the higher volocity forcing in just that little bit more. Thus increasing volumetric efficency. The draw back to doing this is, you creat more parasitic loss to spin the blower faster.

The only way to truely know what overlap is best, is too try several cams until you find the one that makes the most power.

I know there are others that know way more than me, but since they are not here, I gave you what I know.

Sorry if it's not alot of help but, either way, you should easily make you goal of 800 hp. A 427 is a big motor and your cam doesn't appear to be so big that it would hinder the potential of your motor.

For myself, having to deal with emissions had alot to do with my selection. I passed last year, so I got what I needed.

Good Luck, John

 

All that AND you passed emissions. Thats impressive. I seriously doubt my car would pass and I haven't even set up the blower yet. hahaha I went from a stock LS1 cam to an LS6 cam and heads, to a GMPP Hot Cam and Trick Flow 225s. We don't have to pass emissions testing down here so I have a bit more flexibility with my setup.
My car is pig heavy too.. I love my stereo gear.
But I really appreciate the feedback. You are dealing with the same altitude issues Stock48 and I are (if not higher in Denver). So that makes your experience even more valuable in my situation. This altitude thing does change up the rules a bit.
This is where I agree with Lemons12 and his point of view; In this instance at least, my tuning is going to make or break me. Fuel mapping and timing are going to be critical issues and no "cookie cutter" flash pack is going to cut it here.
I have also read the back issue threads here on the overlap issue. The veteran LS1Tech guys all make valid points and know thier ****. I was just a little taken by the fact that noone seems to agree on what to expect with the overlap issue, especially as it pertains to forced induction applications. So I started this thread to see if anyone has tried or experimented with it already.

 

I was thinking about trying to use the cam in my sig for FI, but everyone says I'm crazy. I'm probably going to restall to 5000 whenever I go FI, so I don't really care about loss of down low power. Currently it's right at 23 degrees of overlap. My other thought was going to Trickflows 550hp package(since I want heads also) which comes with a 238/242 .595/.595 on a 112 cam, which is at 16 degrees of overlap.

 

I say give it a try and let us know how it works.

I've seen some crazy turbo cams run...and set records. If I told you the specs, you'd say I was FOS..lol.

 

Josh at KYTP is doing a couple of kits on 06 GTO's that he is building jigs for. So hopefully I'm not to far away from a TC78 kit. He said they would be available with a TC76 or TC78. Then it's time to get buck nutty.

 

TurboAV...

242/248 .621/.615 on a 111

The question really is if you had less overlap would the car make more power and be quicker at the track?

I think that this 242/248 .621/.615 on a 111 cam would be big in a stock cube engine, but would perform decent with a blower.

I ran a 240/244 cam in a 348 cid and it wanted to be shifted over 7100, if I had added a blower it would have moved my shift points even higher I think.