I had an interesting read on the turbo forums about how LSA affects affects a turbos ability to spool up. Having a healthy amount of over lap actually does help the turbo "spool" up much much more quickly. One thing I did not fully understand was how does more overlap allow for more horsepower to be made. I figured that due to the air being blown through the cylinder would mean that boost is being blead off; However, they mentioned about how that this over lap reduced the back pressure, keeping all of it low and I suppose it allowed for more power.

This makes me think of something like lowering compression (then making less power) to add more boost (which then increases power.) Is the reduction of back pressures making up for the bled boost ? Could it be for another reason ? Im really interested in how this works because I had typically believed the opposite when it cam to lopey cams and turbos. Maybe this only relates to lower boost applications.

Here is the turbo forums thread. https://www.theturboforums.com/threa....369240/page-2

 

Overlap can be an engine killer if the turbine side is too small. It’s basically reversion from having multiples of backpressure versus boost. Our T4 cams are limited to 2 degrees overlap. Our T6/twin cams allow a bit more. Spool is helped by earlier exhaust opening but not so early as to cause pushrod bending issues from pushing the exhaust valve open into prodigious cylinder pressure. Our (and Cam Motions) online cam timer tool is an excellent tool to visualize

 

Yea I was reading through that threat and they talked about it. As I read it it seemed like running the cam with that overlap sort of allowed them to use a larger turbine; but in the end it was needed to stop such high back pressure. I appreciate the cam recommendation however I have an lt1 but I could find something similar or go with a custom grind, I have a similar stage 3 t6 cam in mind but a tighter lsa. Ill check out that cam timer tool too. The summit page is such a great site for info and has other great tools as well.

 

What is considered a lot of cam overlap? I see lots of discussions around overlap in turbo engines and the statement "a little bit is okay, but too much will hurt performance" is often stated. But what is considered 'too much' or 'a lot' when talking overlap for boosted applications?

I get that it's application dependent and has a relationship to back pressure, but I often feel people panic when a cam for a turbo engine has any amount of overlap, like a few degrees, when in reality it probably has little to no effect. I guess I'm curious to put into context what is a little bit of overlap, versus a lot of overlap.

 

Id say too much over lap is when you’re losing a lot of boost pressure from the intercooler to the manifold. If you’re ten psi at the intercooler and your manifold is at 5psi then your bleeding of 50 percent if your boost.

 

If you're seeing 10psi at the intercooler and 5psi at the manifold you have a restriction at the intercooler or the pressures would be much closer than that and it has little to do with the cam choice.

 

Any cam is a turbo cam ! Here is a great video by Steve Morris. 10:00-20:00 he mentioned quite a bit about mild cams working just find with a turbo cam. I recommend checking out is hot her videos as well.


I feel running a mild cam would be nicer for moderate power, boosting up more quickly and making power out of boost.

 

How would you be able to tell how much boost your cam is bleeding off ? I assumed reading the map pressure would be simple enough.

 

It's late and I'm probably going to make mistakes trying to put my thoughts into words lol
LOL That's not Steve Morris, It's Richard Holdener, You have to consider the fact that the cam dictates where it make power in the rpm range but the video doesn't account for the ability to build boost while the car is stationary or at low RPM under load. So yes any cam is a turbo cam but again it's not that simple when trying to build boost. I'd like to see him setup a two step and see the effects of different grinds.


There is nothing simple about it. As I said pressure is just a measure of restriction, You'll never build boost without something restricting one end (Valves) while the other is compressing it (Turbo)
Anything the air encounters can cause a slight restriction in flow like the size of the tubing/ The length of the tubing,/ turns in the tubing/ Surface finish the air travels over/ Intercooler/ Butterfly and so on. With that being said from the turbo to the valves you'll see more or less of a pressure at any point depending on the volume of air you're trying to flow while taking a measurement at any point along the path depending on the restrictions along that path.

Look at it this way, You have a pump capable of pushing 1000 gallons a minute at 40 psi and you hook it to a 20 foot long 10" hose with a 10" valve at the end, The pressure would be nearly 0 at the outlet of the pump as it would be if measured just before the valve because there is almost no restriction, The only restriction would be the surface of the inside of the hose. So you have almost no pressure and nearly full flow. Take that same setup and put a 6" valve at the end and still the pressure difference would be nearly identical at both measuring points but flow would be restricted so pressure would rise.

Now take the same hose and put the 6" valve in the middle of the hose and nothing at the end, You'll see a higher pressure on the pump side and no pressure on the outlet end and reduced flow.

The point is if you see 10psi on the outlet of the turbo and 5psi in the intake something in between has to be causing the restriction and not something at the end of the line.

Calculating the effect of overlap on a turbo combination is above my paygrade, You'd have to figure how much flow is passing in and out of the combustion chamber without being trapped and the effect that would have on the backpressure of the turbo. Hell even retarding or advancing the camshaft would have an effect given the same amount of overlap. The same can be said for ignition timing.

 

Wow I feel terrible lol, I think I mixed him up with Steve Morris and the engines Richard was using at the Dynos.

I didn’t ever think it would be that complicated to figure out how much boost was being let out. I thought manifold pressure was just good enough to get you in a ball park. If you have your turbo set to pump 10 psi ect...

It would be really nice to see how different cams work with building boost but I suppose that’s too much work involved for some YouTube videos.

On the turbo forums thread he mentioned the wide lsa cams were for street and made for snappy boost but they ran smaller turbos. Yet the narrow lsa cams had to run a larger turbo to lower back pressure, but the overlapping air flow also spun up the turbine faster. I’d say a cam with more over lap would allow for a better flow of air and lowering pressures. Could this be why they would make more power even though boost is bled off ?

 

Are you going to race this car? Does it have a trans brake? Converter to match?

 

I will not be racing it. I may take it to the track once a year for the fun of it. I’d like it to be fast for the street. I don’t care for a trans break and I really do like the FTI 0086 converter.

I simply started this thread to learn more about cam overlap and turbos.

 

Ok, what’s fast for the street?

 

When I say fast for the street, I mean I’d like a nice combination where the car retains drivability; doesn’t fall on its face with a loose converter. I wouldn’t want to murder it’s enjoyments just to drop a couple tenths at the track.

Why do you ask ?

 

Just been around here awhile, the OG who race are all gone.
my car is simple small cam small turbo but runs 8’s and will kill you in two blocks on the street. I drive it with respect.
So just curious.

 

We at least have these forums to go back and see all of the knowledge that they have given. Its how I managed to figure out a lot more about converters and a couple of other things; reading text that was posted decades ago.

How do you manage to run eights with a small cam and turbo ? I am not looking for big power by the way. It keeps cost down, and streetability up.

 

Oh I just thought about this. Maybe a cam would be nice for power out of boost and the lowered dynamic compression would allow for safe boost compression ratio if the static compression was from 10-11:1

 

i reckon theres a reason nearly every successful turbo cam have IVC and EVO events all within a few degrees of each other, especially for cathedral port heads.

 

Shelf cams get boring Cathedral port mafia here, 30 degrees of overlap, spools fast, works great!

 

yep youre amazing bro. 30* of overlap, .030" sparkplug gap, 30* of timing. we all worship you