jshuberg

I'm hoping that someone here can help explain my dyno results to me. This is an '84 US 928S that I've recently brought back from the dead. The Fuel injectors, plugs, wires, cap & rotor, belts, air & fuel filters, mass air sensor, fuel lines, etc. have all been recently replaced. I also had Euro cams installed, which was the reason for doing a dyno tune.

I'm using 92 octane non-oxygenated gas, and the ignition timing was set to 37 degrees BTDC @ 3000 RPM for the results below. I know the recommended max is 31, but the shop that did the tuning stated there was no knock or predetonation at 37, so it was safe to set it there. I'm guessing this is a result of the gas being used, as well as the fact she's running a little rich.

My question is, whats with my HP and torque between ~2600-3100 RPMs? HP levels off and torque actually falls off. I'm no expert by any means, but I don't believe this is normal, and is indicative of one or more issues that should be addressed. Does anyone have any suggestions for me?

Thanks much!!

SwayBar

I addition to the Euro cams, you will need the Euro spider, etc. which allows more air flow. The US spider chokes the engine.

James Bailey

Your dyno charts show a calculated 250 HP at the flywheel so a good increase over stock......what have you done for exhaust ?? Read some of Kibort's posts about his partial euro engines it details the gains from various mods but bottom line is the camshafts alone are being limited by the valve size , port size, intake runner size and the size of the throttle body and low compression ratio.....and the airflow meter to some extent.
When Porsche decided to make 300 HP they determined that ALL of those THINGS NEEDED to be changed and so designed and built the required parts.

DR

If you have an auto tranny the dyno looks typical/normal.

Cheers,

Dave

123quattro

No, you don't. The US spider and throttle are plenty big enough. You need the heads/cams and an exhaust. My 84 makes 272whp (320 at the crank) with a US intake and throttle.

It looks like your exhaust is choking off the top end power. A/F looks spot on.

atb

Torque numbers look good, your knocking on 32v numbers with close to 260 ft lbs. The US 4.7L is a deceivingly strong runner, pulls strong with stock cams. Those euro cams I bet really liven things up.

jshuberg

At this point she's got a stock exhaust, but I'm actually in the process of figuring out what I want to do as far as an upgrade.

Thanks much for all the input

mark kibort

that's awesome! I started out with 175rwhp hp bone stock. after headers and exhaust, I got it up to 200rwhp. I compared to others that had been to the dyno and a real strong one was 220rwhp. with the intake ( throttlebody, runners plenum) and euro cams, I got up to 240ish rwhp. with the euro cams, we lost peak torque and gained more HP and average HP.
fuel got dialed into 12.5 almost across the board, with a RRFR. we got it tuned from 235 to about 240 with fuel and spark. ignition timing is best around 22 degrees vs the stock 20 spec. not much of a change. any advance helps max torque, but hurts max HP , and you want to maximize hp.
doing the heads and putting a 5 liter under it, gave it 280rwhp, with the equal length headers bringing the LJet up to its max potential, of 290rwhp.

mark kibort

how can you say that!!!! really???? the US throttle body is 2.5" while the euro is 3.3. its basically a restrictor plate. plain and simple. its the narrowest part of the intake path. very restrictive.

Now, I want to know how you got S4 power , (270rwhp ) out of a 4.7 US with Ljet system. my engine was a little tired from the start at 200rwhp ,with headers and 3.5" exhaust, no cat,..... but it only should have run abour 220rwhp if it was not as tired. not 270rwhp!! that's crazy strong. the AFM and throttle body is one of the reasons that when my same 290rwhp on two cars, was put on a CIS system (5 liter, all the bolt ons the same) and it got over 310rwhp due to the restriction of ONLY the Ljet barn door, which is the same size as the throttle body stock US. that stock US is a restriction. if you got 270rwhp with it , you might get 280-285rwhp by changing the euro runners and TB to the euro version.

GlenL

Looking at the graph it's clear there's a flow limitation. The peak HP RPM is low at 4900 RPM. A full euro peaks nearer 6000. The torque peaks a bit low, too at 4250 vs. 4400. The way the torque tails off you can tell it's no breathing well. In or out? Hard to say but it's _through_ that matters.

Check this thread for a dyno graph

It shows my car mostly stock with a power peak at 6000 RPM and modified at 5700. Torque peaks at ~4400 for both.

People talk "power" but it's torque peak and trend that's more revealing.

jshuberg

I did a full intake refresh around 3 years ago, and replaced the air filter prior to the dyno run. The stock exhaust is now 30 years old, so I'm guessing that's where the problem lies. I'm currently planning out a custom exhaust, and am soliciting input here:

https://rennlist.com/forums/928-foru...n-exhaust.html

Could my ignition advance also be contributing to the RPM where max power and torque occur? I'll definitely have another dyno tune done after the new exhaust goes in.

Thanks again to all for your help

mark kibort

I don't know about that...... my torque was killed by the euro cams. it breathed better up top, gave more average HP in the operating range, therefore was faster all the way around. the things you care about, all happen at the higher rpm range. you can tell some things from that range of RPM, but the more telling info is up top.

I do think that the max hp of the 4.7L US, is around 5k, where this graph is. the euro cams changes that to near 6000 as it is with the euros. the restriction will usually show itself up top and not as much down low.

mark kibort

nah, the advance setting just raises and lowers hp and peak torque.

123quattro

I ran my car with all the US parts and made 190whp. Then I swapped in the heads/cams/full exhaust. The dyno curves were identical up until about 3000rpm. Beyond that the Euro parts kept making more torque and made about 80 more whp up top.

Just because the throttle is the smallest part in the intake system doesn't mean it's not big enough. It would probably make a little more power with Euro throttle for sure. I was just saying you don't need to find a Euro throttle and runners to make significantly more power. I'd bet the AFM is quite a bit more restrictive than the US throttle.

Based on the power my engine made with the same top end as he has I'm confident it's the exhaust choking off the power.

My LS powered truck was previously making 376whp at 5800rpm with a questionable exhaust. I put full dual 2.5" mandrel exhausts on it and power jumped to 436whp at 6400. That lined up a lot better with how the cam was ground. Poor exhaust just make engines lazy on the top end.

GlenL

"Then I swapped in the heads"

That's huge.

We should have a pool. I bet whatever exhaust mods are made the engine doesn't add more than 15 rwhp.