Jim_H

Would the different grind on the GT cams affect the amount of boost that a system provides?
In other words, all things being equal will the same system on an S4 provide identical boost on a GT.
I could see the GT intake making a difference but not the cams.

Anyone?

Lagavulin

The GT cams will register slightly lower boost versus the S4 cams.

Jim_H

Thanks Lag. By slightly do you mean as much as 1 psi?

Lagavulin

Yes.

The intake-valves are open longer, allowing more pressure to 'escape' into the cylinders, thus showing up as lower pressure, or boost, on the intake-side of the intake-valves.

Vlocity

Lag:

Would the same be true for the 85 -86 model year car or will the slightly smaller valve size offset the more aggressive cam profile?

Thanks,

Ken

Lagavulin

That's a good question.

But firstly, I'd have to say that the 85-86 cams are the more aggressive versus the GT cams. The exhaust cams are the same, but although the intake lobes are identical, the 85-86 intake lobes are 8 degrees retarded, or in other words, opens 8 degrees later and closes 8 degrees later than the GT intake. This typically provides more top-end horsepower at the expense of bottom-end.

However...

Now that I wrote that and thought about it, what that means too is that the GT cams have more overlap (..but there is none at the factory rated lift) due to the intake lobes being 8 degrees advanced with respect to the 85-86 intake. Another way to think about it is that the nose of the intake is closer to the exhaust's nose, and the 'overlap' is the area below the intersection where the downside slope on the exhaust lobe intersects with the up-side slope of the intake lobe. With the greater overlap comes better cylinder scavenging and cylinder fill, which means more fuel to burn to push down on the piston and turn the crank.

So what we have now is an 85-86 cam which should provide more top-end, but suffers too because it doesn't get much help from overlap, or lack thereof, compared to the GT cams. Additionally, it's low-end will not be as strong as the GT cams.

On the other hand the GT intake opens 8 degrees sooner, supplementing low-end power production, but, it also closes 8 degrees sooner, hurting top-end power production. However, offsetting that, one must consider the additional overlap the GT cams enjoy over the 85-86's, and it's positive effect on cylinder scavenging and filling.

Alright, so upon reflection, maybe the GT cams are more aggressive. To answer your question then, I agree that there'll be reduced power at the top-end with the smaller valves. However, low to mid power production should favor the smaller valve heads due to increased port velocity providing more cylinder-fill.

BC

As it pertains to cams, I have decided to keep the S4 cams for my first 89 engine, which is stoke dispacement.

I will use my 85/86 cams in a worked head and a larger displacement motor with more boost in a third motor.

Thanks for the info Lag.

Vlocity

Thanks Lag.

I was curious how my particular engine may react to some mid level 7 PSI intercooled boost. I was not sure if lack of overlap on a forced induction motor would be a good thing or not?

The only other mods are my x pipe and dual 2.5 inch exhaust.

Regards,

Ken

Lagavulin

I just noticed your sig and see that you're going to put a positive displacement blower onto your car. Since that's the case, I would stick with the 85-86 cams since they hold the intake open longer while the piston moves up the bore, decreasing the dynamic compression ratio. This is very important, and something you should be vividly aware of.

This information is significant in light of what this dyno reveals and verifies:

https://rennlist.com/forums/928-forum/214527-supermodel-dyno.html

That dyno-run verifies what on-lookers observed/reported at a Devek-day where several positive displacement blower installs were attempted.

You'll notice that the dyno run does not start until 3500 RPM. The first obvious question that must be asked is, why are there no numbers showing-off the massive low-end power production a positive displacement blower is known for? (...once again, the dyno is 'speaking'; imploring us to ask hard-questions)

Here's the inside 'scoop' from several innocent by-standers why this is so:

Upon installation at Devek, the cars experienced idle problems.

Continuing and more interestingly, at the beginning of the dyno runs, the cars were detonating so badly, the observers/reporters were convinced that the engine was going to break the motor mounts from rocking so violently from the detonation. For those who don't know, the technical term is 'tip-in detonation', and probably the most servere case ever recorded (..there are several posts from others on this very topic) since seismologists were excitedly picking up faint tremors that day, but couldn't pin-point the exact location, but they were sure it was somewhere between Walnut St and Scott Ave in California.

So after considerable head-scratching and consultation on how to continue, it was Marc Thomas who suggested to bring the engine up gently to 3500 RPM to avoid the tip-in detonation, and then start the dyno run: thus why various 928 positive displacement blower dyno's show their run starting at 3500 RPM. Go look for them and you'll see more... I have links if you need them.

There are several plausible explanations for this un-civilized behavior:
#1. Lean condition
#2. Too much compression
#3. Intake charge too hot

According to the astonished by-standers, shortly thereafter, a freshly installed positive displacement blower was unceremoniously removed from an engine along with it's intake manifold. Once the intake manifold was removed and a cursorily inspection performed, it was literally torn apart by the bare, brawny hands of Marc Thomas himself since it had already partially self-destructed on it's own; jaws dropped, women and children giggled.

So the clues obviously point to 'Door #1', a severe lean condition due to a leaky manifold due to improper welding. In the manifold's defense, how could it possibly seal out un-metered air if the welding wasn't done competently? Under those brutal circumstances, there is NO WAY any engine with a leaking manifold can be expected to idle properly.

'Fix' this, and 'fix' that, but until one gets to the root of the problem like Marc did, a leaky intake manifold at the welds themselves, the problem will persist forever! Unless of course, one get's fed up and 'simply' remove it and shred it with your bare hands like Marc did. (Marc I'm sure the chicks are diggin' that part of the story! )

So in one fell swoop, an idle problem, and a most unsavory servere detonation problem were diagnosed as one leaky, manly ripped-apart manifold; good job trouble-shooting and wrenching Mark! (..pun intended)


Now to 'Door #2': too much compression. Let's assume that the manifold is not leaking too badly (..as a work-around, you can always richen it up to compensate within reason. Disclaimer: not recommended; implement at one's own risk. Get to the root of the problem like Mark did!).

It is well known that grass-roots-style effort is well under way as we speak to minimize low-end power production of the positive displacement blower wielding 928's (..there's posts on that as well); ironically, that's the very thing a positive displacement blower is supposed to do, and does best, and is it's main selling point. That sounds a little backwards, doesn't it? What a waste of time and money on hardware. That's no different than buying a submarine, and then trying to convert it into an airplane!

The moral of that story is, investigate, talk to other customers, and buy the right hardware the first time around; it is available. Don't get stuck with a submarine when you want to be flyin' high above the clouds like the Blue Angels are doing outside my window right now! (..say it warily like Sean Connery, 'Give me a ping, ..NOW!'; ..name that movie)

So instead of attempting the impossible of converting a submarine into a high-flying jet airplane, one can always go to Customer Service and return that submarine (..make sure to bring your receipt!), and then go buy that airplane you wanted/expected all along.

But getting back on-topic:

Now to achieve those ends, one can lower the compression ratio which provides several benefits to a boosted engine, especially one using a positive displacement blower (..but all bets are off if the intake is leaking; a chronically bad idle is the first clue). The hard way is to dismantle the engine and put in the appropriate pistons to lower the static compression ratio. However, the easy way is to install a cam that hangs the intake open longer, thus effectively lowering the dynamic compression ratio. Unbeknownst to most, the dynamic compression ratio is by far the more important of the two compression listings as it 'tells the tale' of what the cylinder-pressure really is, all-else being equal.

By lowering the dynamic compression ratio, you may be able to escape tip-in detonation like Tony did by lowering his static compression ratio with modified stock pistons.

<-- That was Tony successfully dishing his pistons...

The best 32v 928 camshaft by far for lowering the dynamic compression ratio is the GTS camshafts (..I have a set for $1100). Next best is the 85-86 camshafts, followed by the GT camshafts. The worst by far, are the S4 camshafts as they produce the highest dynamic compression ratio since they close the intake valve very early ABDC while the piston is way down near the bottom of the bore.

The recommendation for you then is to go with the 85-86 camshafts versus the GT camshafts if you plan on running a positive displacement blower. Much better yet is the GTS camshafts. And above all, get to the root of the problem like Marc did!

And be prepared to not operate your car below 3500 RPM for fear of tip-in detonation, the very RPM range where positive displacement blowers typically excel; look again at the tell-all dyno sheets.

Summing up, there is nothing better than to go into a situation with one's eyes wide-open knowing what's really going on behind the scenes, just like knowing what dynamic compression ratio is all about, and it's implications on engine operation. Given the hardware mix, you can have a lurking submarine hiding way below the surface, or a high-flying fighter jet taking on the big-boys!

Choose your hardware CAREFULLY!

BC

What cams are you running lag? What about the red car in WI and Tims? All 85/86 cams or GT cams?

I was concerned a bit about using the 85/86 with the CS on the new 89 motor (pistons somewhat machined at the top, but coated with swain stuff) - I wanted to make sure I didn't give up too much low end with the 85/86 cams. I was planning on the S4s, and was going to use the 85/86 when I had the GTS crank in a block.

Jim_H

Lag,

That was my GT that was a rock'n. It was the very first run after installation and yes it probably read on the richter scale. Though it only lasted a second it freaked me out big time, to say the least.
We got it tuned so there was very little tip in problems above 3K and I was ok with that. The linear power was incredible and I am worried about using a lot of power below 3K. It would seem to me that would produce alot of compression? I don't want to blow a head gasket.
As many know I didn't get to do too much tuning before...

Anyway, I still have the SC that turned 464 rwhp with less than 7 psi. I am also having an MX 424 built.
So...

Thank you for sharing Marc results. I know of no one that has been told of this. There are many SC's that are having idling problems and this is good info to know. Why everything has to be so secret pisses me off. Part of it has to do with Andy feeling like there are some people who look for the first opportunity to give his system a hard time. I have seen this done from both sides. The problem with this seems to be affecting 'us' the guys who are buying these systems.

What you shared above is HUGE to us guys who own these systems, yet I, for one, just found out here.

Me thinks there is some huge paranoia going on.

Jim_H

FWIW,

The system may still have some flaws but I love it.

Tony

So, what is the latest on Seths car from the Devek installation marathon?
Ive asked before about the device they added to "bleed" off boost?..any answers?
I havent seen any finished pictures of the polish set up under the hood

Tony

Agreed. Mine works.

Jim_H

I believe Seth 'hacked up a lung'.