Marti

The engine is a fairly new install, less than 2000 miles and the clamp being used is a ritech super clamp so there should be no movement to be concerned about.

I am still gunning for the exhaust system, here is why

The previous engine did feel way down on power, I put this down to a non operating flappy, worn MAF and maybe malfunction knock sensor although I have no evidence on the knock sensor.

I gave the car a high speed run one evening when the roads were clear and it literally topped out way under the factory number, we are talking 40mph. While performing this test visible hot embers or debris started to fly out the exhaust causing me some alarm as I thought I might have had a fire on board causing me to stop pretty quickly and check everything over.

Moving onto the current engine which was a rebuilt unit due to a very unfortunate engine failure (due to injector failure > hydraulic lock > snapped con rod) felt much the same power wise.

My conclusion based on that high speed test and the replacement engine is that something external to the engine is sapping power rather than the engine it’s self. When you get a collapsed CAT it can seriously affect power output so I am guessing that I have this going on.

Either way the exhaust system needs replaced with a higher performance version so this will test the theory hopefully in the next week or so. I am going for a twin 2.5” three box system, probably using Mangnaflow boxes as they get a good rep along with the obligatory cross.

Then I will be itching for dyno run number 2 to see the results

FredR

Marti,

Are you running with cats or are you on an open loop system like mine?

Funny you should mention embers flying out of the exhaust- exact same thing happened to a local friend of mine as we were driving up to Dubai for Porshe track day experience at the Dubai Autodrome that was nearing completion and we were the first to use the new circuit. My friend had a bit of friendly 928 rivalry with my 90 S4 and tried to demonstrate what it could do- as I charged past him I could see all kinds of crap coming out of the exhaust- after we stopped to check things over I concluded he was in effect doing a good old fashioned "Italian tune up". After we crossed the border into the Emirates there is a lovely section of road with fast sweeping bends- I then gave his 928 and other club members a demonstration of what my 928 could do with its handling mods - never saw them again!

Just remember if you have a faster exhaust [as I have] you need less advance- that you were able to dial in so much advance- particularly in mid range may be symptomatic of a slow [i.e. partially blocked] exhaust and if so, you may find your engine will not like your current mapping big time if indeed this is the case.

GregBBRD

In order to get any kind of intelligent help, you need to make sure your details are accurate.

You said you had headers and an X-pipe, when tested.

When you say it has an X-pipe, we automatically assume that you have an X-pipe that replaced the converters (which is what all "X-pipes" do, in the US.)


Yes, pretty obviously, a plugged cat/exhaust will result in poor performance.....and could ruin that new engine, really quickly. (If you can't get the exhaust out of the engine, the combustion chamber temperatures are going to go very high, very quickly.).

I'd suggest you quit running that poor engine at WOT and make sure you do not have a restricted exhaust system.

Marti

What I said is accurate, it has an x pipe and no converters, my mentioning CATs was because I recalled some dyno before and after results where CATs were causing a blockage.

Its actually pretty difficult to diagnose a blocked exhaust, the dyno test nails it for me.

As I said before the exhaust was always going to get changed, it’s now become top priority

ptuomov

Hello!

if I recall correctly, your engine doesn't have a high enough compression ratio for these cams and intake. At the next opportunity, I’d shave the heads a bunch.

The 928 S4 heads flow so much straight out of the box that in my opinion you should either spin the engine to much higher rpms or install smaller cams, or both.

With the stock style intake, the exhaust cam in particular would in my opinion have to be smaller. You could have your very long duration exhaust cams reground such that EVO event is later but EVC stays at about the same spot relative to IVO.

A shorter duration exhaust cam with the same valve overlap and much higher compression would in my opinion help pull the air thru the stock style intake during overlap. Once you get the intake stroke started right with that pull, then the relatively late IVC doesn’t really hurt that much.

Just my opinions.

Marti

Hi, if you see the rest of the thread I confirmed that I cocked up the CR test last time giving an erroneous low number. I did a random check on 2 cylinders cold and got 171 and 178. So I need to retest the CR warm and post back.

Current evidence points to some form of exhaust blockage which would be unusual (car has no cats) and this would tie in with being able to chuck loads of advance at the engine with the ST.

The exhaust system was on the list to be changed at some point but has now become top priority. I am currently in the process of gathering the mufflers and have spoken to a local fabricator who will perform the work. Then hopefully in about 2 weeks I will know where I stand.

Not withstanding the change in exhaust I absolutely agree that an additional increase in CR would be of benefit for the S4 engine. When the opportunity presents itself I plan on skimming the heads

ptuomov

What are your estimates of the static compression ratio and effective compression ratio computed from the IVC point? If I recall, you started with an S4 engine with early-style pistons, is this correct?

I agree with you that it is very unlikely that you have an exhaust blockage. First, what's causing the blockage if you have no cats? Second, you'd expect exhaust blockage to cause the power curve to flatten at certain rpm and the torque curve to nosedive. Instead, you see torque curve holding up and the power curve just climbing even at high rpms. This is not a situation where I'd diagnose an exhaust restriction.

I think that if you send your sharktuner 2 logs to JDS, they can guesstimate how much air your engine is ingesting. Figuring out things like the volumetric efficiency curve would be a step towards better understanding what's going on.

My instinct says that the problems are the low compression and the big cams, especially exhaust duration, that makes the engine not want to make power in the usual rpm range. With the 928 S4 heads on a five-liter engine, it's going to be hard to get the gas velocities up to the required levels with big cams.

Marti

Hi, the engine is a mid 88 engine with standard pistons. The engine has been rebuilt with new head gaskets etc so should really have no problem replicating similar engine spec power outputs.

The cams have been run in other S4 engines with good power increases so I am not looking at these other than possibly timing.

Either way a standard system matted to a tuned engine does not add up.

ptuomov

Do you have or know of a clean dyno data set that compares an otherwise stock S4 with stock S4 cams vs the aftermarket cams you’re running? If you do, I’d be very interested in seeing that. My opinion (not a fact) is that those cams have way too much duration, especially exhaust duration, for S4 heads run on a 5.0L bottom end and stock redline rpm. But it’s just an opinion, and I’d like to know if that opinion is incorrect.

If it were just a flow restriction outside the part of the system where pulses matter, then you’d see the engine hitting a wall at some power level and torque curve nosediving. You don’t see that with your engine. Because of this, I am guessing (and it’s just a guess) that the problem is between the throttle body and the X-pipe. I am guessing that the throttle body or upstream components are not a restriction and, similarly, nothing downstream of the X-pipe is a restriction either. Dual 2” pipes would flow your current hp, in my opinion.

Jim Corenman has (had?) a 5.0L engine that has more compression and less camshaft duration that makes good power. Maybe comparing what you have to that engine might give you some leads? Even that engine would like to run into much higher rpms than Jim’s dyno charts ever go.

ptuomov

What do you mean by “fast” exhaust? High gas velocity or low pressure drop? And are you talking about the part where pulses matter to power (upstream of the x-pipe) or the part where pulses don’t matter to power (downstream of the c-pipe)?

FredR

Less restrictive. On my S4 when I was doing high load high rpm tuning I found that I had to pull a little advance rather than add to it. I was a bit taken aback by this fearing something might be wrong with my motor not that I could measure anything. I then noticed that the stock timing for the cat equipped S4 seemed to run more advance across the board and I was advised that when the exhaust is more restrictive it needs a bit more advance to compensate.

When Marti produced his timing figures I was somewhat surprised at the amount of advance he was able to dial in without knocking which made me wonder whether the cylinders were filling as they should. That the dyno suggests he is making around the same as a stock S4 and his power curve is climbing at 6500 suggested that something is not quite working as intended. Given the GT cam drops off at about 6200 rpms one wonders what increasing the redline to 7k rpm would throw up if it is safe to do so. Maybe a partially blocked exhaust could explain the dyno curve but whereas catalytic converters can and do block I have never seen that happen to a conventional exhaust other than get a bit sooty.

Now, I wonder if a less restrictive inlet would help the cam achieve the gains it was intended to produce or whether something else is going on. I have never read a bad review of Colin's cams so one just has to wonder what is not quite working as it should. Marti has eliminated the compression test concerns [I think].

ptuomov

My understanding is that you can and should run a lot more advance when cylinder filling is poor. For example, at part throttle the ignition timing has to be advanced a lot relative WOT.

Since this is a stock displacement engine with stock redline rpm, the stock intake is probably not holding it down on power relative to a completely stock engine. Sure, ITB system with long and relatively fast (small diameter) runners might make more power. Nevertheless, I don’t see intake being responsible for lower than stock torque at many rpms.

My guess is that exhaust gas isn’t moving fast enough between the exhaust valve and the x-pipe. This could be because the port and the header primary have a too large of a diameter. Smaller header primaries could help a lot. It could be because the exhaust camshaft has too much duration (228 at 0.05” is a lot for the S4 heads on 5L engine and 6200 rpm!). Later EVO could help a lot. It could also be that the too low compression ratio dilutes any overlap suction pulse from the exhaust and doesn’t allow the induction to start quickly enough.

SwayBar

Ptuomov, as a data-point, I've heard there's a guy with a stock-block S4, headers, x, and with Colin's cams and sharktuning, which supposedly put down 350 rwhp.

I would love to see the dyno-graph for it though. Those cams have been out for a while now, but there are hardly any dyno-graphs to consult/compare against.

Assuming the numbers are true, the cams Marti are running are good for 25-30 rwhp over GT cams - but let's see the dyno for it!

Thom

I have the same opinion and outlook as ptuomov but instead of speculating any further I think the PO should just advance cam timing settings and adjust the ignition profile accordingly. Decreasing dynamic compression ratio by increasing valve overlap must be compensated with earlier IVC. Only fiddling with the ignition profile will not be sufficient. Trying to tune the programming around a physically wrongly-adjusted engine does not work, in my experience.

If I were a cam supplier I would always recommend starting with a safe adjustment (~slightly retarded) as not to have bad surprises if a headgasket blows up, e.g. because of heads skimmed beyond the stock spec.

FredR

The stock S4 motor seems to clearly benefit from more advance mid range/full load but the amount Marti was dialling in struck me as somewhat odd and given the dyno curve presumably this was because the cylinders are not filling as well as we might have expected lower down the rev range- why that should be I have no clue. All I can say is that I was expecting to see 350 ish peak power and a shed load of additional torque mid range. The top end is not there at the moment not so sure about the mid range without seeing curves over laid as it were.

The only other thought that struck me was that the other "variable" was the work Marti did on the inlet trumpets/spacers- is it possible this did not work as Marti anticipated?

Now it seems Marti has a stockish power curve displaced some 600 rpms up the rev range and maybe more as we cannot really interpolate at what rpms it is going to crest at thus why it would be interesting to know what would happen performance wise if the motor were redlined at 7k rpms. The GT is redlined at 6700 and it maxxes out around 6200 rpm, or whatever compared to the S4's 6500 that maxxes at 6k rpms,.