I've researched this one and can't find any real information out there on it. Does anyone know how the overlap affects spool speed?

I would tend to think that it would slow the spool rpm down a bit since it bleeds off cylinder pressure until you get in the upper rpm band, therefore making the turbo spool later then if it did not have any overlap.

Thoughts?

 

dont quote me in this but this is my understanding usually you want to go with about a 114 lobe separation or more to minimize overlap,, on some sbc 350's guys seem to have good luck with the summit 1104 cam

call mark with bullet cams,, he knows exactly how it works

 

LoL never knew they made a "F" cam for chevys (i know its not a F cam but it's damn close)

Sorry, no offense.. that was just too funny. I'll try and give mark a call.

 

This is wrong. Overlap increases backpressure, spooling sooner. The bigger the turbine, the more aggressive the cam needs to be. There is no black and white as far as cams go. A 114 lsa doesn't mean a cam is suitable for a turbo setup. It depends on the entire combo, with one of the most critical parts being the turbine. A cam with more overlap will generally spool faster than a cam with less overlap.

 

Hmm.. seems contradictory to everything I've ever seen written on turbo cams. Not that I'm arguing..

So how does a cam with overlap create more backpressure?

So you want more backpressure pre-turbo.. and very little post turbo?

 

I'm not sure if you came across this in your research, but it seems to address some of your questions, at least in part.

http://www.carcraft.com/techarticles/ctrp_1106_turbo_camshaft_guide/viewall.html

I too was always wondering about overlap on turbo applications and was also getting the "you don't want a lot of overlap on a turbo car" answer. It always bugged me because some of the most powerful turbo internal combustion engines on the planet have noticeable overlap.

 

As little as possible post turbo always. As far as pre-turbo it's a toss up between eye sucking horsepower or decent response time. If your just shooting for peak HP the lower the turbine pressure the better. Guys shooting for peak power that care little about response have a 1:1 or even better pressure ratio. (think salt flat guys) These guys usually will have large duration high overlap cams much like the high compression NA performance engines.

The typical "street" guys that need snappy response and quick spool times run as much as 3:1 pressure ratios. This is where the low duration/overlap cams come in. (think OEM turbo setups)

Then theres a grey middle ground area where most of us are. Thats why if you ask 10 people what cam you should run you'll get 10 different answers.

 

You pretty much answered your own question in your first post. Overlap bleeds off cylinder pressure. This pressure isn't lost, it's going directly into your exhaust used to spool the turbo. This is one reason larger cams (more overlap) are used with larger turbines, to improve spool. It's also why everyone thinks that a tiny cam with very little overlap is great for a turbo setup. The majority of people don't have enough turbine for their cubes, rpm... which is forcing them to run smaller cams to keep backpressure in check.

You don't really want more backpressure pre-turbine, as that's going to cost power up top. It just depends on you're combo, what your goals are, how you want it to perform... It's very combo dependant.

 

This is mostly true, but some of the reasons are wrong. Your first example, with the 1:1 ratio, the cam selection is very similar to a n/a cam but it's not because they're shooting for a 1:1 ratio. It's because they're using very large turbines and the additional overlap helps spool them while improving power.

The 3:1 example, with the OEM setup, the cam isn't chosen to increase backpressure, it's chosen to minimize backpressure and the other usual reasons OEM cams are so small (bottom end power, emissions, idle quality...). The cam isn't what's making the turbo spool quickly in an OEM application, it's the severely undersized turbos they use, forcing them to run smaller cams to minimize backpressure. It's the same as running a 70mm MP with a p-trim turbine on a stock 5.0. It spools quickly because the turbine is so small, not because the cam has no overlap. This is where the "turbo cams are 114lsa and have no overlap" thing comes from, being under-turbined.

 

I'll have to disagree with that. Exhaust reversion back into the intake port is not a good thing. And a a high overlap/duration cam isn't going to help you're spool up time or power output if your at 3:1 ratio.

According to Duttweiler anyway...

Great article, worth the read IMO.

Read more: http://www.carcraft.com/techarticles/ctrp_1106_turbo_camshaft_guide/viewall.html#ixzz2Xt8gaSyT

 

I never said you should run a cam with a lot of overlap when your backpressure is already 3:1. I simply said that overlap is directly related to turbine size. Small turbine = less overlap. Large turbine = more overlap.

Your Duttweiler quote is saying exactly the same thing that I've been saying.

 

Duttweiler also mentioned that attempting to build a turbocharged engine with a set LSA (such as 112 or 114 degrees) can lead you astray. He mentioned some work he did way back in the early Buick Turbo V6 days while racing these engines in NHRA Stock Eliminator. Stock class rules required the intake and exhaust lift and duration specs to remain stock, so to improve power, he tightened the LSA on these engines to 109 degrees to help the Buick's really small cam improve power. The engine responded by building boost much quicker. 'When you spread the lobe-separation angle way out, the engine gets lazy,' Duttweiler says.*Read more:*http://www.carcraft.com/techarticles/ctrp_1106_turbo_camshaft_guideviewall.html#ixzz2XtrYOX61

Straight out of the link you posted. Tightening the lsa (more overlap) reduced spool time.

 

Comparing apples to oranges here. As I said originally typical “street” setups similar to the OP’s aren’t going to benefit from high duration/overlap cam’s I still stick by that and see nothing to convince me otherwise.

Duttweiler doesn’t mention anything about the drive pressure on the buick engines. Pressure ratios are still in the grey area. Pretty sure the turbo buick classes he’s talking about didn’t limit the turbo size. So those guys weren’t’ seeing high back pressure readings. I recall seeing the stock class engines with 70 and 80mm chargers on them at the buick events.

I believe Duttweiler is talking about modern race engines with modern turbo chargers where they are seeing 1:1 or better ratios. Then I agree using the higher duration/overlap cams benefit in spool and power. (which is what I said originally) He also specifically says in my original quote that if the exhaust pressure is greater than the intake pressure the engine will drop power while in overlap. And the easiest way to fix this is to reduce overlap with wider LSA.

I still say on a typical 2:1 or greater engine you won’t see improved spool or performance running a high duration/overlap cam. How could you? With both valves open and a large amount of spent exhaust gas under pressure sitting in the hotside , burnt exhaust gasses will be forced into the cylinder/intake. How is this going to increase power or spool? How will this not reduce power, and make the eng. more susceptible to knock?

Also wouldn’t tightening the LSA and increasing overlap shift the powerband higher in the rev range. This is another reason OEM uses smaller low overlap/duration cams. I’m sure the OP doesn’t want to rev his JY engine to the moon. I’d want power to come in at extremely low RPM to avoid additional wear and tear on stock bottom end components.

 

I think what he's getting at is you don't have to have 2:1 or 3:1 ratio for a quick spooling street engine. If the OEMs didn't have to worry about emissions they would be doing things VERY differently you can bet that. There is no reason to run a restrictive turbine just so you then have to run a small cam and fight reversion. Put a properly sized turbo on it from the start and stop putting bandaids on a small turbo.

 

I get that. And agree somewhat, but for some it may not be the most economical/reliable option. IMO For a cheap JY dog turbo motor its easier and more reliable to run high boost and low rpm engine with a small cam/low overlap, and a smallish turbine. I think this is a more reliable way to build an engine that’s limited by RPM and bottom end strength. If it gets you to your goal, I don’t see it as a ”bandaid”. It’s just a different way to skin the cat.

 

Good info guys.. the real reason I asked the question is b/c I was thinking I could possibly swap cams to improve (lower) the rpm my turbo starts to spool at.

I'm not really after a custom cam, nor do I have a stock cam to stab in it. I realize I can lower the spool rpm a bit with wrapping, and with running things a little leaner out of boost, and possibly with more low RPM timing.

Just don't know, and was curious to find out if swapping to something without as much overlap as my E303 would lower the rpm that boost comes in at.

 

I'd look for a 12cm or 14cm housing if you want quicker spool at lower rpm. I doubt you'll get the spool up you want with a cam change. With a manual trans I think you'll like it much better.

 

On a 302 with a turbo that size I'm sure a cam could get your spool up. It will be a tighter LSA cam with more overlap most likely for more exhaust volume to help it spool. A good starting point would be to measure your back pressure so you know how the hotside is working.

 

It absolutely is not apples to oranges. Having a properly sized turbine doesn't somehow make it a race engine. You act like a typical engine is over 2:1 backpressure and that's flat-out wrong. With a properly sized turbine, backpressure is easily less than 2:1. This whole small cam mentality is 100% the result of people running too small of turbines. With a tiny turbine, you have to have less overlap, as backpressure is already high and there's nothing you can do about it, forcing you to have less overlap. The little cam is not what is making spool fast, it's the turbine. Again, I never said that more overlap will make more power when you're already at 2:1 or greater backpressure so I have no idea why you're acting like I did. With high backpressure from a small turbine, you have to have less overlap. Backpressure hurts power, therefore a cam with excessive overlap will hurt power, but it depends on turbine size. A larger turbine will have lower backpressure allowing you to have more overlap.

I'm sorry but you're just wrong. More overlap will spool faster. Larger turbines can tolerate more overlap without hurting power. A street engine does not have to be at greater than 2:1 backpressure ratio to spool quickly and a bigger turbine does not make it a race engine. 3:1 backpressure is way too high, even for a street engine. And just to clarify, for the 87th time, I never said more overlap is a good thing in every situation. IT DEPENDS ON THE TURBINE, among other things.

 

Nope, clearly this is wrong. You have to run a tiny turbine with tons of backpressure and a cam with no overlap to spool quickly. The oem's do this, it must be right.