samnavy

iTrader: (7)

At face value, that looks like an decent deal... mild head-work, brand new OEM valvetrain, and a set of turbo-grind cams for $1500... plus another $100 for shipping. What am I missing?

TravisR

iTrader: (9)

The problem is no one is really ready to go to this level of tuning yet, or at least I don't think they are. When you really start looking at optimal grinds there are allot of issues you get into. Some of the biggest issues have to deal with, if you want really huge power you have to run a high overlap cam. How do you do that with a turbo, ever seen those pictures of drag cars with turbo headers so long the turbo serves double as a mounting place for the rear view mirror?

For instance, high overlap cams can be used on a turbo header whose primary has more than 50 inches of length, and medium over lap cams come into play somewhere in the middle, small over lap cams somewhere near 8-15 inch region and no overlap if your turbo is hooked to the exhaust valve. NASA actually figured that out in like 1931 I think? Funny how the automotive industry is always behind a bit.

So in order to produce a cam that truly does what it’s supposed to do, you almost have to offer a full kit. It’s also good to mention that if you are looking at the cams, the head, and intake manifold need to be on the chopping block in short order. The intake manifold on this car is terribly restrictive, and so is the head. The problem is it’s a well tuned system. So most people who change one part don't see increases, and that’s just because no system is going to operate well unless its setup together.

I've been working on an intake manifold and cam kit for about the last 2 years... It takes a while to manufacture and make perfect. Even after that gets done, I won't be satisfied because I'll have not fixed the head flow problem yet, or how I'm going to get exhaust gasses out of the head efficiently so that it works along with the camshaft.

samnavy

iTrader: (7)

I had no idea that cams and header length were that closely dependent on eachother.

My interest for my forthcoming '99 build is maximum power on minimal boost. I'd love to see 300whp@12psi. To get there is going to require a lot of "flow" help. I'm still doing the pro's/con's of cast vs. tubular vs. log manifolds and haven't picked a winner yet.

How about running the numbers of the "perfect" cam for the 2 most common manifolds. The FM and BEGi cast manifolds probably have almost identical dimensions, and the BEGi S4 tubular manifold is probably a good "average" for most of that type.

When the time comes to pull the head, there's no reason to cheap out on cams if we're only talking about $500 for the set if it means I can run 2psi less and get the same power. Swapping to a different intake manifold isn't really in the cards due to price.

I am following you wrt a "system" designed to work together... but when you talk about "big" power, are you talking about 300whp (cause that's big to me). Would a GT28R/S4/3"exhaust/stock intake manifold benefit significantly from a set of the Corksport cams in the grind they're selling... or would my money be better spent elsewhere?

TravisR

iTrader: (9)

Its all so relative. They offer about as much information about their package as they couldn't. The first cam spec they put up seems pretty mild to give you 12 hp on a lightly modified engine. I have no idea whose products are actually good out there, because all the Miata company’s seem to think everyone is going to steel their designs. Maybe rightfully so, prototyping can be expensive. I have never seen dynamometer graphs on a Miata camshaft set though and I think that’s because none of these companies really know how to tie everything together and make real power out of them.
If I were to build camshafts for turbo cars and standard manifold lengths available on the market I would do it this way.

I would break it into 3 sizes of turbo. Small Medium and Large.

Small turbos require minimum overlap for the most power because they create the most backpressure. Exhaust cam lift is not important because the restriction point is the turbine. You can probably run a stock exhaust camshaft here and be fine. The intake cam shaft should have longer duration and as much lift as possible without getting too noisy. The intake cam shaft duration should be centered so that IVO occurs at the same point, possibly a few degrees later and extends IVC farther down.

Medium Sized turbos are rather more tolerant of overlap because of the pressure ratio between exhaust and intake. Here you could probably also stand to run the stock exhaust cam. You’re certainly not going to pick up any flow by switching to a bigger exhaust cam. What I would recommend would be adjustable cam gears so that you can move your exhaust cam timing around a little bit. The intake cam shaft should be quite large and make sure that your lift matches the capabilities of your head. The Intake cam can now move the IVO mark farther back behind EVC because the pressure ratio is better. This gets the intake flow moving sooner and gets you more charge into the cylinders.

Large Size turbos require something entirely different. These turbos almost always create very low pressure ratios and with the proper turbo header can very nearly create 1:1 between exhaust header and intake plenum. What this means is that basically you run big fat aggressive naturally aspirated cams for best power. This is why turbo drag cars lope so hard. You do this because the exhaust gasses are about even with the intake gas pressure. This allows you to run long overlaps without resurgence of the exhaust gasses into the intake tract, and ensure each cylinder is filled to its maximum given the pressure available. This is also where pressure waves become really important.

There are a lot more parameters in there then just what I mentioned, but that’s the basics.

The devil is in the details. How much duration? How much ramp rate is reasonable? All those really small things require alot of money to figure out.

samnavy

iTrader: (7)

That's a fantastic education... thanks for taking the time to write up both of your posts.

When you speak of the different sized turbo's, are you speaking of them in terms of application to a sub-2litre motor? I would say that: Small=T25/14b. Medium=T28-T3/TD04. Large=T3/T4.

It looks like the 1.8 Miata exhaust cam with an adjustable gear would be ideal... but there's gotta be a "sweet-spot" intake cam-grind that would work equally well for a GT28/T3s60/20G/TD05/etc... maybe not perfect for any one turbo, but well-rounded overall, assuming the same exhaust manifold is used. Am I putting too much thought into this? I'll admit I probably don't have the requisite technical knowledge to even ask good questions here.

I know the 1.8's of any yearill do 300whp fairly easily... at that power level, would I see any worthwhile benefit to a "matched" intake cam to a GT28/S4mani, or is that not enough power/flow to make the $$$ worth spending.

How does selecting a grind change when you throw in VVT?

disturbedfan121

iTrader: (5)

sam, look at pauls set up. only thing he has different than yours is the absurdflow manifold, dp, and exhaust. for intake manifolds the begi manifold is the way to go. he pulled i think 30ish(give or take)hp out of it alone. on a 1.8 it really doesn't seem that hard to hit 300, just a decent ammount of money and time.

As for cam's i wanna find out from corksport if they can work with stock valve springs and lifters etc....hks claims their cams will work with a stock head.

I would love to hit 300 on my 1.6 but idk if thats possible w/o fully building the motor.

TravisR

iTrader: (9)

I think its probably easiest split up by turbo horsepower capacity. small is 150-210 medium up to 350 large unlimited.

As far as VVT It really won't change the intake cam grind that much? On the two lower turbo sizes your system can only stand so much duration until exhaust reversion and intake reversion happens. Once you have a duration that works it won't really matter which way that duration goes your still stuck at a specific value that works. Now VVT will dramatically increase bottom end torque so spool up will be better, and that’s by turning back the intake cam and creating less intake reversion by holding the valve open too long.

Savington

iTrader: (31)

Sam, look long and hard at the ETD shorty tubular manifold. If I were looking for 300whp, that's where I would be going.