Compression ratio

[940_Turbo]

Has anyone messed with the compression ration on a gas converted 940 turbo? Mine is now on 220K miles, and I know it has fairly serious bore wear. I've never really liked the engine that is in the car. I've had it since it had done 55 or 58K miles, can't remember and it has always had one of the roughest idles I've known on a redblock. I think the bore wear is largely down to problems with the gas conversion early on. Themileage doesn't help of course. I've used this car hard. The evaporator was fitted upside down and the nozzles for the injectors were fitted badly.

It ran chronically rich until I finally got the thing sorted. It runs well now, but you can tell it is getting tired. The idle is getting worse. I know the bore wear is bad as I had the head off at about 150K miles and it was fairly sapectacular then. Strangly the oil consumption is OK still.

I'm thinking of building it a new engine. I have a decent very low mileage bottom end. I have a set of Pauter forged rods, just because. I also have a 531 head and I'll probably re use the IPD turbo camshaft it runs now. My intention is to get the crank, rods and flywheel balanced.

The question is is there any point in raising the compression ratio, bearing in mind it still has to be capable of running on petrol.

I believe ?? that the Prins VSI system the car has can't realistically be pushed beyond 240BHP. I want a bit of a safety margin, the car ought to be at 200 or so with 12psi/1 bar of boost.

If I control the boost electronically I should be able to drop that easily to 7psi on petrol to compensate for the lower octane rating. A 15G turbo should easily suffice.

So what is a meaningful compression ratio? I want to retain reasonable fuel economy. If I wanted to build a really powerful 940 I'd go a different route.

[classicswede]

Personlay I would leave the CR as close to original as possible especialy if retaining std engine managment. You will be fine with a typical skim off the head but much more is going to cause issues.

The limit of the Prins system is not only with the vapouriser/injector combination but with the 940 also the ecu. The issue is getting little enough fuel at idle as the petrol injectors fire twice per power stoke. Most other LPG systems have a meas of filtering out the second squirt to help magange fueling at low load but prins have a bit of a comprimise on this one.

With the altertaion you have set out I'm sure you will find the LPG system will need remapping even through the petrol side will cope ok.

Keeping the CR down would also allow you to run full boost on both fuels. If you are going to use mapping engine managment then that does open more options for you. You could run two seperate maps, one for each fuel.

[940_Turbo]

Interesting, I don't really want to spend money on an after market ECU at the moment, though it probably is ultimately the way to go.

I wasn't awate of the idle issue,, that is interesting. Have you tried chipping the ECU and EZK on an LPG car?

[classicswede]

I have converted a number of chipped 7/9 series turbo cars with no problems. What would be interesting is chip specialy set out to get the most from LPG.

The idle problem can simply be overcome by dropping the LPG flow down but this then leads to switching back to petrol at full load. With Prins cafully choosing the injectors and fine tuning the gas pressure will balance out both ends of the range but I'm not sure how you will come off on a tuned engine. Using the manifold pressure balance pipe on the vapouriser does help a lot.

Compression does improve MPG with LPG so I understand why you want to go as high as possible by the ignition curve will need altering to cater for the change in burn pattern. A mild increase in CR will not cause a problem (typical heavy 30 thou skim or even double that could still be ok)

[940_Turbo]

Originally Posted by classicswede

I have converted a number of chipped 7/9 series turbo cars with no problems. What would be interesting is chip specialy set out to get the most from LPG.

The idle problem can simply be overcome by dropping the LPG flow down but this then leads to switching back to petrol at full load. With Prins cafully choosing the injectors and fine tuning the gas pressure will balance out both ends of the range but I'm not sure how you will come off on a tuned engine. Using the manifold pressure balance pipe on the vapouriser does help a lot.

Compression does improve MPG with LPG so I understand why you want to go as high as possible by the ignition curve will need altering to cater for the change in burn pattern. A mild increase in CR will not cause a problem (typical heavy 30 thou skim or even double that could still be ok)

The dropping back to petrol is what the car used to do when it was first converted, especially when driven hard. I used it in Germany a lot and typicaly you'd fill the car up on the Autobahn, boot it hard on the slip to rejoin the traffic flowing nicely at 100 mph and it'd beep and switch onto petrol.

Eventually I figured out the vaporiser was on upside down, after I drove it to Prins and got them to have a look at it. I think that is probably one of the reasons it has so much bore wear.

I think one of the Dutch guys has an LPG specific LH 2.4 chip set. I got told about it but lost the info. I'll try to track him down.

Have you got the gear to re set the Prins system? What is a ball park figure? It'll be a few months before I get the new engine in.

I did buy some bigger valves for it, but it went a but Turbobricks, paid the money, never got the valves; bloody annoying.

[classicswede]

A basic retune I normaly charge £30 or a bit more for more extensive setting up. I don't have a prins lead at the moment but I can get one in to do it with.

The LPG sounds like a good idea for your project. Shame about the valves as they do make a worthy improvemnt.