You are running the biggest unknown for our engines, Colin's cams, correct? There is a HUGE difference between running an MAF vs ITBs. A big cam can affect an MAF thru reversion, but is not an issue with ITBs. Reversion is a known issue with 928's and big cams.

Now, you are doing all of this work to test the effectiveness of your prototype intake. To do that, you need to have factory cams installed.

 

My current objective is to optimise the running of the engine without the prototype manifold as I need to start from a solid base. I had assumed that with the sum of the parts I have (which are known hardware - or so I thought) that I should be in a ballpark figures 320 - 330 rwhp - but I am not and I have been through the engine to ensure it's healthy as can be.

Having just looked up reversion I don't know whether reversion could be an issue but why would this not have effected other engines running these cams with stock internals. I could imagine perhaps a big CR engine perhaps acting differently which can also rev higher to get to the peak of the power curve.

 

In my opinion, MAF back flow and pulsing issue is not a serious concern and to the extent that it ever manifests itself it’s at idle rpm. At high rpm and WOT, I don’t see it being an issue.

 

Marti,

Here is the spec for mine:

JE Pistons - total rotating mass saved 1,25kg
12.5:1 Static Compression
Colin's cams
968 Intake Valves - 39mm
Ported heads
E39 M5 ITBs with their OEM Trumpets
33lbs/hr Ford Motorsport 4-hole injectors
928 Intl Equal Length stepped headers - I am lucky my car is LHD
Custom X-pipe 2.5"
Custom Exhaust 2.5"
Custom dual disc flywheel
Dry sump

Comparing my map to Dr Nick of Orange Car fame shows how rubbish we were when we tortured the car on the dyno. There is more to come for sure... One thing we noticed - the combo of ITBs and Cams - even GT cams - definitiely shifts the torque curve up the rev range. My car makes maximum torque of 520Nm at the wheels around 5000rpm. Mark Chilton - BIG GT2 on here can confirm. And his map was produced by one of the best mappers in the World - Wayne Schofeld of Chipwizzards....

 

928 Intl Equal Length stepped headers are among the best headers you can find.
Åke

 

A while back Greg Brown stated that he could not get Colin's cams to work/dyno correctly. Up in the rev-range they encountered an inexplicable dip in power, and then a rise afterwards, and the dip could not be tuned-out on the MAF equipped engine. He did not use those cams after that. I feel it is reasonable/plausible to speculate that reversion caused the power fluctuation at higher RPM.

Interestingly, a 5.0L with those same cams and ITB's puts out more horsepower than a typcial 6.5L stroker, so the cams are very, very good in the proper environment.

Now that I wrote that, Alex and John have shown that by installing ITB's on a 5.0, it becomes an attractive alternative to buying/building a 6.5L stroker for more power - and a lot easier on the wallet. Of course, tuning will be the big challenge with them.

 

I've used these on loads of cars with MAF/LH/EZK and have not experienced the dip that you are referring to. I also did not see Greg post that and he has purchased loads of my cams and even put down some really impressive numbers on a stock GTS rebuild.
I am really curious as to what is happening with Marti's car but something is off as I've never put them in a car and not had an increase in power. Even a stock car with stock exhaust saw 320-330 rwhp.

 

Now that I wrote that, Alex and John have shown that by installing ITB's on a 5.0, it becomes an attractive alternative to buying/building a 6.5L stroker for more power - and a lot easier on the wallet. Of course, tuning will be the big challenge with them.[/QUOTE]

I can not understand why someone builds a 6.5L or 7.0L stroker without ITBs. It's part of the package.
Åke

 

I don't think that "MAF reversion" is a plausible story for any problems that happen at high rpms. I don't see how the air flow could reverse at high rpms and to the extent that it is pulsing the pulsing is going to be minimal at high load and rpm compared to the average mass air flow rate. If there's any pulsing issues, they'd be an idle issue.

Three things worth noting. First, the LH 2.3 does not have a throttle position sensor and therefore transient tuning will always be inferior to what later ECUs have. There's a bit of a lag from the throttle opening to MAF signal and rpm rising and that means one needs to compromise a bit on the transient. Second, with exhaust manifolds (instead of headers) the cross-plane V8 is always going to have a lower torque spot at higher rpms when the 90-degree blowdown interference cylinders are compromised during the overlap. The greater the camshaft overlap, the greater the effect. Third, in some conditions the 928 MAF element simply runs out of measurement range and the voltage flatlines. This is usually not a problem with with normally aspirated cars, though.

 

Regarding reversion pulses that become very prominent with camshafts having more overlap, my personal experience is that the idle will be a bit rough but most prominent when driving at low speed and low load (eg 30mph on a flat or slight downhill road) whereby the engine will run jerkily. At slightly higher speeds when the engine is under more load, the phenomenon disappears. I had this problem on a BMW inline six once upon time. After installing three Dellorto twin choke carburetors, the problem disappeared completely and it also resulted in a good increase of engine power.
Åke

 

In my opinion, the problem you describe isn’t really about load measurement or fueling, it is about unstable cylinder filling. Even if the AFR is exactly rock solid and air flow is measured to the gram, a plenum manifold with long overlap cams will cruise in an unstable way. The way to cure this, as you suggest, is individual throttle bodies or carburetors that have the minimum volume between the intake valve(s) and the throttle blade. At high rpm and high load when the throttle or throttles are open significantly, there’s no difference.

 

Many thanks for confirming your set up, that compression really gets gets the gasses moving which I think helps make a big difference, possibly as much as the ITBs. If you have a flatter power curve than myself then longer runners might change the power curve. Certainly the shape of the power curve I am seeing at present is not flat enough to really manipulate peak power using runner length. I am however working on the timing of the cams which for a more standard engine set up does seem to be a sensitive area.

Can I ask where you got the JE pistons from and what rods you are running?

 

While I think that normally aspirated 928 S4 engines _love_ more compression, I don't think it's going to explain a large power difference. From the thermal efficiency formula, you get numbers like 1 or 2% from an extra point of compression with very high octane fuel. You may also get something more from the increased cylinder filling if the headers generate the right kind of a wave. On the other side of the ledger is the fact that pump gas engines are knock limited. The higher compression will force one to retard ignition with pump gas which may mean a net reduction in power. Even in the best case scenario, the gains from higher compression can't explain the types of numbers we're seeing (or, rather, not seeing) in this thread.

I would say that straight runner ITB intake, headers that eliminate any exhaust blowdown interference, and cams designed to match the aforementioned two are, together as a combination, where the normally aspirated high-rpm torque comes from.

 

Unfortunately I cannot find it either, apparently it was deleted. I remember the post like it was yesterday.

His post is why I never bought a set of the cams. And apparently the post was an inaccurate representation of the cams and why it was deleted.

I should have taken a screenshot of the post at the time, but that's hindsight for you.

 

Marti,

I am running the standard late GTS 2R rods. I got the JE Pistons from one of their two authorised UK agents - Performance Unlimited - I am using 100.5mm pistons and Westwood ductile iron sleeves.