Over here in the States, we've been testing a prototype motor design dubbed Murray,which is based on a big-stroke crank from the ALH diesel engine. Others have used this crank in stroker conversions as well, but nobody (that I know of) had taken the approach of fitting shortened connecting rods to compensate for the changed throw of the crank. The thinking behind this unusual approach was purely marketing: I wanted to bring to market a "drop-in displacement" parts kit. Customers can keep their original pistons and entirely skip the engine pull and machine shop work associated with displacement increases. The new crank -- combined with the specially-made connecting rods -- married right to the other components, resulting in a 1968cc displacement while the engine never leaves its mounts.
Test fitment of this engine design was done over a year ago. It was performed on our own FrankenTT test car in March of last year, and in the time since we've been putting it through its paces. This post is to explain some of the data we collected.
Firstly, I should say that we will not be marketing a Murray-type engine. After hundreds of hours testing and troubleshooting, I have decided that the concept doesn't meet its critical requirement: value. The problem is that despite the lowered installation costs, this design has quite a few issues that need to be worked around by the customer. The piston skirts drop so low at BDC that they hit the oil squirters. So those have to come out. Then they hit the crank speed sensor trigger wheel, so that has to be relocated to the outside of the engine. But most importantly, the engine just doesn't make a whole lot of power. And that's simply a function of the weird crank angle that results from such a short rod attached to a long-stroke crank. The energy produced by the motor is just wasted on the inefficient geometries. All these things considered, I have closed the book on the Murray prototype, and given up on the idea of marketing a "FrankenStroker" product.
But that's not to say this project was a waste of time. Not in the slightest. Because the body of information and data collected during the protracted testing is really impressive. I have learned a ton. And not just about stroked engines, but also about the challenges of running an admittedly small turbocharger at the limits. One such area of interest involves exhaust back-pressures. Here is a bit of info on what we encountered and how we resolved it.
Almost immediately after installation, the car was having a strange issue in the top end. Power would mysteriously fall off, despite strong boost and good airflow values. And in extreme cases, the car would hit a virtual wall at around 6000rpms, causing it to run poorly and misfire. So while down-low torque was terrific, the upper ranges were dismal. Here is a comparative graph for the new motor versus the car in its stock form:
The logs also showed an issue. The boost levels would weirdly spike at the same point:
So what was causing this? Nothing like this happened on the old motor. It could boost merrily at 25psi all the way through the rev range. it was seeing 280g/s at the MAF. But with the new motor, things would go to hell at no more than 260. In an attempt to solve this we tried altering the wmi mixture, re-gapping the plugs & installing TSI coil packs. No improvement. So then we took more extreme steps: we removed the AEB cylinder head and restored the small-port BEA one. We tried bigger, naturally-aspirated type, camshafts. We tried a number of manifolds and even tinkered with the turbo itself. Nothing worked. Finally, after some prompting by BadgerBill, I decided to attach a data logger to the exhaust manifold collector. Was there something screwy happening pre-turbine? Here's what we uncovered:
So we had a direct correlation of the anomaly between back-pressures and boost. But which of the two was the troublemaker? Or was it simply a case of contamination from the exhaust getting into the intake? To answer that I had some cams re-degreed, virtually eliminating the overlap between the intake and exhaust. Again, no improvement. But that was when I finally caught a break: FFE's Ed Woolsey, looking at the data logs, had an insight: maybe all that exhaust back-pressure was "floating the valves". Well, that concept was a new one on me, but he quickly explained that since the valves open inward towards the cylinders, that insufficiently sprung valves can be pushed open by the exhaust back-pressures. So if his theory was correct, 50psi at the exhaust manifold would appear to be the breaking point.
I contacted SuperTech Performance to ask them if Ed's theory made any sense. Sure enough, they said. The valves in this cylinder head might not have enough seat pressure to resist the riotously high exhaust back-pressures caused by this oddball motor. They suggested a higher-rated set, one which was good for 50% more. And here they are, next to the ones they replaced:
On the right is the upgraded exhaust valve spring assembly. Though there's not much difference visibly from this angle, from overhead you can see one:
The inner, secondary spring in the uprated ones is full-height. The standard SuperTech variants do not have this. Here's a look at the differences with the spring assemblies separated:
The uprated types are again at the right. Any you'll also note the shim they provided, just to add a bit of insurance against back-pressure reversion.
So in they went:
With that done, were we finally going to see the engine tolerating back-pressures in excess of 50psi? Cautiously we raised boost until the manifold reached that troublesome level:
Things so far look very nominal. And here's how that compares to behavior on the original springs:
Well, so far, it's keeping it together. but what if we push things a bit:
Alright! Still holding. So what about...
Yes!! Still looking good. But let's not stop there! What about...SEVENTY PSI?
Fantastic! Absolutely no sign of valve float, all the way up to a hellacious 70psi of back-pressures. And the corresponding performance parameters for everything else look great as well:
So EGTs are nominal at even this level of boost and back-pressures. And at THIS level of airflow:
...and with this kind of timing advance:
So has anybody here seen that trifecta before? That much air, with that kind of timing advance, at such EGTs? I myself have not. Which makes me think that all the tribulations of the Murray testing might have uncovered a twenty-five buck mod that completely changes the rules for hybrid turbocharger tuning: beef up your exhaust springs.
...or maybe I'm just tilting at windmills again. Murray, Martindales. Who knows. Maybe someone here will.