GTL

I had the pistons of my 86-944-NA upgraded to the 2.7 engine sizes (89-944-NA). Nothing done to the top yet (valves, injectors, etc). The improvement was very significant, a lot more torque and thus faster acceleration. However, there was a noticible knocking only under high load conditions (going up a small hill, for example) and worse when the gas tank was at low levels. This lead my mechanic to suggest to use a non-lead octane booster. This solved the knocking problem. I had a test pipe and a Autothority chip. During my last smog test, I re-installed the cat and installed the stock chip. I was expecting a decrease in performance, but surprisingly, no degradation at all. What it was puzzling was that the knocking went away and no need for octane boosters anymore. However, the cars now runs a bit rough and no knowcking when using standard 95 octane fuel. With the octane booster, it runs extremelly smooth and well. How can I fix this problem permamently? Rise the fuel pressure? Larger fuel pump? Larger injectors? I know that the 2.7 89 car came with a higher pressure pump, regulator and injectors. Should I try this next? Anybody out there with a similar experience or ideas?
Thanks for your time.

David Ray

May want to see how severe the differences are between the '86 and '89 DME. Or try placing your 86 DME (designed for a 2.5) with a 89 DME(designed for a 2.7).

GTL

That's a good idea, thanks. That area is definitely a good candidate. The octane booster suggested that I needed to run richer, but when I went back to the 86 stock chip, the knocking went away! Supposedly the chip curves enrich the mixture a bit, so this doesn't make sense to me. For some reason the stock 86 chip was better. I think I will try first the 89 chip instead of switging all the DME. Will that be sufficient or are there other things in the DME box that are needed?

xsboost90

do you still have a 2.5 head? or did you get a 2.7 head? I would say your gonna need some 2.7l injectors and pump, or you could put in a bigger pump.

GTL

xsboost, I still have the 2.5 head, nothing done there. As you suggest, maybe I should try the 89 fuel pump first and maybe later replace the injectors. I guess I should also replace the pressure regulator to 89 right? And how about the chip, go 89 too? Oh man, so many possibilities. Just need to proceed in the most logical way. Any ideas for a step by step road map without spending a bunch of dough in one pop?

Steve Lavigne

By retaining the 2.5 head, you increased the compression ratio. You likely need to retard the timing somewhat and increase fueling. You could start by trying the FQS switch. Looks like position 5 will get you 3.1% more fuel and -2.77 deg of ignition retard. Position 7 will get you 6.3% more fuel and -2.77 deg of ignition retard. Also, you could try a 3.0 bar FPR and position 6 (3.1% less fuel and -2.77 deg of retard).

http://members.rennlist.org/951_racerx/FQS-adjust.html

Maybe try these different combinations, then take what seems to work best to a dyno with wideband O2, then send the charts to a chip tuner and have them work up a new chip for your engine.

Tom M'Guinn

I'm confused about what you changed? The bore (and hence piston diameter) is different on the 2.5 and 2.7 motors -- did you enlarge the bores on an 86 to fit 89 pistons somehow? If you did, you'd also need to modify the 86 head to match the bores and get the compression right. (Or get an 89 head.) If your compression is too high and/or if you have hot spots (from a non-chamfered 2.5 head for example), then this may be more of a mechanical issue than a tuning issue. Just food for thought.

Also, where do you get 95 octane gas in Sunnyvale?

Matt H

No need for the later pump IMHO. Steve is right, you will need to retard the timing and fuel. The turbo pump is capable of almost 425HP (so it seems) and I bet the NA one is just fine for this application as well.
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GTL

Thanks a lot guys for the excellent advice. I knew about that DME switch but apparently had a mental fart and forgot all about it. I will try the switch settings first, hopefully that will do it. It will be great if I can keep the standard pump, FPR and injectors and just need to re-tune the chip using the dyno and O2 charts (great idea!). Sorry to ask so much, but will the chip remapping guys will also compensate for the switch setting so I can bring it back to the stock setting? Is that how it works or the chip and the DME setting will have to go hand in hand?

Tom, yes, we enlarged the bores to 104mm and machined the head to accommodate, but other than that, I kept the 2.5L head. The compresion should be the same as in the 2.7L, I think, which is higher than in the 2.5L. Sorry about my typo, I meant 91 octane gas instead of 95. Once I heard that there was 76 station somewhere in San Jose selling higher octane gas for racing but I forgot the address and I never used it.

GTL

Going to try the DME switches soon. Any last minute advice from anyone? Thanks a lot.

ewainwright

I would call Autothority first before changing the FQS. I read several post where that chip does nothing in anything but the first position. Only the stock and Guru chips (do not consider this an endorsement of his product) does a change in the FQA affect timing and fuel delivery. When Autothority burns their chip it is very specifically done for a stock motor. If you change any parameters the chips need to be reburned.