Marti

Hi Thom, I am the PO, Marti.

I am talking with the Colin the cam supplier and will be relaying data to try and help understand what the issue might be.

There does seem to be a clear line of thought here that the cams are too big for the standard engine but I don’t see any theory why, nobody has asked what the specs are. Although I can’t confirm the duration right now as I am traveling they are not exactly race car spec and certainly didn’t give me cause for concern when looking to purchase.

There could well be some adjustment to the timing required although when checked after several hundred miles they were exactly as specified by Colin for standard installation.

I will get more answers soon when the exhaust system is swapped out for a more suitable version

Strosek Ultra

Theoretically optimum intake gas speed at 6500 rpm for a 5 liter 928 engine occurs for an intake port having an area of appr. 1120 mm2 which is equivalent to an 38mm circular port. The S4 stock intake port is appr. 1700 mm2 which is good for a 6,5 liter stroker at 7500 rpm. The intake valves should be 40mm to match the stock intake port.
Those figures are for an all out race engine having full race cams. It has been shown oversize ports work quite well on engines having very small cams like the 928 S4.
The stock intake manifold is a problem, how much power did Mr. Brown managed to extract from a 6,5 liter stroker using the stock intake manifold - 380 at the crank if I am not mistaken. For another stroker having the Simard ITBs I remember he had 600 at the crank. Do correct me if I am wrong.
Åke

ptuomov

I, of course, agree.

Many people are pointing out things about the intake manifold. I agree that if we'd run this engine with smaller diameter long (12" or so) straight tapered trumpets off the head, it would probably make more power. But let's think about what we need to do relative to that ideal engine if we replace the ideal intake with the S4 stock style intake.

The first thing we need to do is increase the compression ratio. If the curvy intake manifold doesn't fill as well at the peak torque rpm as the ideal intake manifold, then we are filling the cylinders less well. This allows us to increase the compression ratio.

The second thing we need to do is reduce the exhaust duration. There's less burn exhaust gas in the cylinder and it needs less time and/or area to get out from the cylinder at optimal velocity. For the same reason we could benefit from smaller header primaries which would maintain the exhaust port diameter for a fairly long period before the first step.

We should furthermore keep the EVC event and the valve overlap about the same. All of the exhaust duration (and lift) reduction should come from making the EVO later. The overlap is there to start the pull from the intake immediately at IVO, which makes it arguably more necessary with a restrictive intake manifold.

If you look at the SAE paper #962514 on restricted intakes, the paper prescribes a slightly smaller intake cam, a lot smaller exhaust cam, and no change in the valve overlap, relative to unrestricted-intake optimal cam.

Now, let's talk about magnitudes.

According to Colin, his cams have the following specs:
Intake: Lift .437" duration @.050" 230
Exhaust: lift .437" duration @.050" 228
LSA 114.
(source: https://rennlist.com/forums/928-foru...l#post13089746)

Per Colin, GT from the factory has the following specs:
Intake: lift .393" duration @.050" 211
Exhaust: lift .353" duration @.050" 200
LSA 110

That's 28 degrees more exhaust duration at 0.050", and likely more at the seat duration, compared to GT. In my opinion, that's a lot. Personally, I'd go a lot smaller with the exhaust cam for this engine. Keep the EVC as close to the current location and thus keep the overlap, but regrind the exhaust cam to something like 218 at 0.050" by moving the EVO later. Then advance both cams to deal with the very large intake cam also.

The GT exhaust duration is equal to about 95% of the intake duration. If you go to 218 degrees at 0.050" by regrinding Colin's exhaust cams, you'll get about the same ratio that GT cams have with 230/218.

As some background for comparison, here are Rob Edwards's measurements of 928 cams, including his race cams on the bottom for a custom-intake high-rpm stroker motor:



I'm not critical of Colin's or Colt's cams here. I'm critical of the idea that there's one-size-fits-all camshaft solution across rpms, displacements, and intake/exhaust configurations.

ptuomov

Here's an idea: Why not head to the dyno and simply drop the exhaust? That is, dyno the car with an exhaust system that terminates with the X-pipe and doesn't have any restriction beyond that. I'm not saying you drive it that way, but this will be a quick check whether there's a big restriction after the X-pipe that's holding the motor back.

I would do that test before going larger and custom on the X-pipe-back exhaust. There's a risk that it'll cost a bunch of money and just make the car louder but not make any more power.

ptuomov

I'm not following that statement.

In general, the 928 S4 seems to be pretty insensitive to advancing cams. I've come to the conclusion that this is because the heads flow so well. In 2-valve engines that don't have the head flow capacity at low valve lifts, the big problem is to get the valve open enough by the peak piston speed crank angle which is usually at about 75 degrees ATDC. Advancing the cam starts opening the intake earlier and gets the valve open more by 75 degrees ATDC, compared to straight up cams. That's usually beneficial in 2-valve engines. For the 928 S4 4-valve heads on a 5.0L bottom end and stock redline, the heads flow so much at any reasoanble cam advance or retard by 75 degrees ATDC that the engine isn't really that sensitive to advance/retard of the cams and factory spec for the cams is usually pretty retarded (no pun intended). Porken has done a lot of experiments on cam timing and can confirm or deny my claim about insensitivity.

ptuomov

Here's one more comment relevant to the exhaust camshaft duration.

The 928 exhaust port is about 40mm (1.575") diameter round at the header flange for stock 320 hp or so.

The latest Ford Coyote race engine that makes 750 hp has 37.5mm (1.480") round exhaust port. The first five inches off the head are 1.625" OD header tube. (Some details here: http://speedtalk.com/forum/viewtopic.php?f=15&t=55767)

A modern race engine will push 55 hp worth of exhaust gas thru a square inch of exhaust port diameter. The 928 S4/GT will push about 20.5 hp per square inch.

Ford Coyote is one of the better designed engines out there on the market. The stock Gen 1 intake valve is stock 928 size at 37mm. The stock Gen 1 exhaust valve is 31mm. The intake valve area to exhaust valve area ratio is 1.13+, so 928 has 13% more exhaust valve area. For Coyote has the same intake and exhaust camshaft duration, I think it may in fact be identical profile. The race cams also have approximately identical duration between intake and exhaust (typically of the order of 230/230 @ 0.050). If you just crudely subtract the 13% from the intake duration of Colin's intake cams or Coyote race intake cams, you'll be looking at something like 230/1.13 = 204. The area going down with max safe lift going down with duration might suggest 230/sqrt(1.13) = 216. I'm not saying that this is the correct exhaust camshaft duration, just food for thought.

To summarize with two points:

First, I can see how keeping exhaust pumping losses low, camshaft overlap small, and pipes big can help with fuel economy and emissions. It's probably not the way to make most power from given displacement.

Second, does it make sense to stick a Coyote race exhaust cam into a 5.0L low compression 928 with a restrictive intake?

Strosek Ultra

This has been up for discussion before, it seems that the GTS intake specifications in the Workshop Manual (page 15-124a) are incorrect. Intake duration at 1mm is specified to 228 degrees like in Tuomo´s chart above. I did measure a GTS intake cam once and came up with a duration of 212,5 degrees. Think Rob Edwards also did measure the GTS intake cam and found the same.
Åke

Marti

The exhaust system I will have fabricated will be from the manifold backwards, so will include a new x-pipe which is currently at the standard exhaust diameter size.

I am going to need to go to twin 2.5" to ensure the system supports the rest of the engine tuning that has been done. It will probably be a little noisier than the standard system but I am using the biggest 3 boxes I can fit in to keep it as quiet as possible.

I have seen 2 different places to put the x section on the centre pipe, near the front of the centre section just after he manifolds (most common) or near the back of the centre section. I need to try and gather information on the difference in approach

All things being equal I should be able to replicate similar spec engines with the same cams, which should put my engine at about 370-380 hp. I need to ensure there are no issues with the engine prior to starting testing of my new inlet manifold as measurements could be flawed otherwise.

SwayBar

Colin, can you post the 350 rwhp dyno which uses your cams?

Of course, the owner of the car can remain anonymous

Or, is this story I've heard simply not true? If so, I apologize.

This the first time that I'm aware of that results of your cams have been posted in a thread, and we need some help and assurance that these cams will work with an engine.

Thom

I noticed a significant change in engine behaviour trying different cam timing adjustments on the whole range allowed by the cam gear on my 944 S2. It took me many fine adjustments (less than 0.5°, really) around the optimum point where the engine felt finally "correctly" balanced, and that engine runs a cylinder head very similar to the 928 S4 heads, obviously though with a little less shrouding issues due to the bigger bore.
If varying cam timing on a 928 S4 causes little to no variation then it sounds to me like everything before the inlet ports and after the exhaust ports is actually remarkably restrictive and the heads actually do not flow dynamically as much as they do statically on a flow bench, unless perhaps adding some boost to help them reach their actual dynamic flow potential. Yes the inlet ports on S4 heads are smaller to help with flow velocity as there is not enough room in a 928 engine bay for runners as long as on a 944 S2/968. While there does not seem to be any real world intake alternative for a 928 S4, 928 headers seem to be the worst design ever, while all 944 headers are proper 4-2-1 units, and this is probably where anyone should first consider searching gains on their 928 S4 even perhaps before trying aftermarket camshafts, along with a free-er breathing exhaust. As the 4V heads flow perhaps 30% more air on a flow bench than the 2V heads for a similar peak valve lift, there has to be some serious physical restrictions before and after the heads for a stock S4 engine to make only 10hp more than an 928 S2 engine, while the S3's rated 288hp further illustrates my point. In fact the low durations camshafts on the S4 were probably the designer's solution to keep the engine make enough torque when the head were not granted with suitable intake and exhaust designs to flow anywhere they can, certainly also in a marketing context limiting reaching the full potential.
In any case fitting longer duration cams with "too much" overlap without opening up the bottlenecks before and after the heads seems to be counter-productive as shown by the results here. Just my 0.02€ anyway as I don't even have a 928 (yet).

ptuomov

Let me apologize in advance for my persistence with my suggestions and questions. I don't mean to imply that I know the answers or that I am expert. I'm just thinking out loud.

Your early post says that the car has the following:

• 928 motorsport merged headers
• X pipe

Could you please post the approximate diameters of those components? Currently, 928 MS appears to be offering headers that have 3" collectors, do you have those? I am assuming you have versions with smaller collectors. How does your x-pipe marry the standard exhaust to the 928 MS headers?

Fabricating an exhaust is a time consuming and expensive proposition. I am wondering if there's a reason why you wouldn't first dyno the car with the x-back exhaust dropped?

Is 380 engine hp your goal for this build? If it's 400hp or under, I think that a single 3.5" or dual 2.5" pipes after the x-pipe should be enough. Although I'd probably invest in the PipeMax program before investing more on exhaust fabrication.

Marti

No problem at all, happy to hear well thought through theory. Also good that you have Colin’s cams duration/ lift. I don’t consider those durations as excessive for a road engine which can breathe properly, breathing is the key here. My ultimate aim was always to replace the stock inlet manifold with my own to radically optimise airflow with nice long inlet tracts to help pull torque down the rev range compared to other fabricated examples. This is maybe key to the exhaust design? Peak power for the manifold is expected to be around the 6k mark.

Right now exhaust wise I have the newer 928 Motorsport (MSMD) headers with merged collectors which terminate in a 3” outlet. This is mated to a reitech X pipe which is designed to replace the standard header connector - so is the standard S4 size. 928 MS supply a graduated outlet from the headers which go down to the standard S4 exhaust size so these were simply mated to the Reitech X pipe. The remainder of the system is standard factory.

My final HP aim is somewhere between 410 and 450 hp at the crank with the new manifold, without the manifold I am expecting something like 370 - 380 at the crank.

ptuomov

In my opinion, it's going to be very hard to get 450 hp from a 5.0L 928 S4 engine at only 6000 rpm. You'd be looking at 123% or so volumetric efficiency which is simply very difficult to do, custom intake or no custom intake. In my opinion, and this is just an opinion, you'll either need to go higher with the rpm and cut down the power goal. Perhaps you meant that your power goal is 380 hp at 6000 rpm and the 450 hp goal is at much higher rpms, like 7000 rpm? Then the next question is how will the rotating assembly and oil control handle that 7000 rpm peak power rpm.

For what it's worth, for an engine with 6300 peak power rpm, 11:1 compression, and 10 degrees smaller exhaust cam, PipeMax gives the following preferred locations for the X-pipe and/or H-pipe:

• H-Pipe= 16.4 X-Pipe= 65.5 distance behind end of Primary Tube ends
• Best HP/TQ Tuned Collector Lengths= 16.4 , 32.7 , 65.5 , 130.9 inches long
• Worst HP/TQ Loss Collector Lengths= 24.5 , 49.1 , 98.2 , 196.4 inches long
  

Marti

• H-Pipe= 16.4 X-Pipe= 65.5 distance behind end of Primary Tube ends

Are these sizes also in inches and does the term “Primary Tube end” refer to the end of the header or something else?

What this formula in Pipemax is suggesting is that it would be better to have an H up front and an X approximately 49” behind that

Strosek Ultra

Right, here you go H up front and X behind.
Åke