3900 cylinder pressure through the roof

[Guest]

I'm in the process of returning my wiring harness back to the Turbo Grand Prix specs for running 8F code after failing at my attempt to run the engine with code59 and DIS ignition. I decided I would check cylinder pressure since the engine is very new for a reference as the engine wears. I have allowed the camshaft to start in it's stock position of fully advanced, to be retarded at a set rpm:

1__235 psi
2__220
3__225
4__220
5__213
6__213

I wish I had checked it with the cam fully retarded for comparison. If it runs properly after restoring the harness back to where I had it previously when the engine last ran, the efficiency with that kind of pressure should produce excellent fuel economy.

The last it was driven was just before the upgrades and I noticed an intermittent power drop as if timing was being pulled. I have theorized that it was the old fuel and reduced octane from aging. I powered up the pump to remove the near 2.5 gal and added new fuel. Hopefully it runs without trouble.

I couldn't get anywhere with code59, that's ashame because it's nice code with lots of extras. It just doesn't like DIS systems.

[TiredGXP]

snip

I couldn't get anywhere with code59, that's ashame because it's nice code with lots of extras. It just doesn't like DIS systems.

Well that sucks...I just picked up a 7749 ecm to give it a shot myself.

[Joseph Upson]

Well that sucks...I just picked up a 7749 ecm to give it a shot myself.

You can try it anyway unless your engine is as complicated as mine. I don't know how advancing the cam has affected the engine and I'm also not sure about the MAP sensor wiring as I'm in the process of going back to the 8F harness configuration and the 91 Beretta wiring diagram has the MAP sensor and TPS sensor pin assignments different than the TGP, Sy/Ty and 92 Beretta pin assignments and I don't recall which diagram I wired the first harness according to but I'm siding with the other diagrams instead of the 91 Beretta.

You maybe able to figure the timing parametes out, the engine did run for about 5 seconds on my first attempts with it. There is also the possibility I was getting to much fuel as the plugs were black. I just don't have the time to work out the kinks. I want to drive my car.

[TiredGXP]

I have a pretty simple set-up compared to what you have:

Iron head 3.1 bored .75mm over, using DIS, ported lower intake and heads
GT2871 56 trim .6A/R compressor, .64 A/R turbine
SyTy W2A intercooler
expecting to run 10-12 PSI boost
Custom intake to replace the upper and mid Fiero intake pieces and dual 48mm LT1 throttle body.

It's amazing how the scope of a simple project keeps increasing.....

I wanted to explore using the $59 code mostly because of the high resolution VE table. Other than that, I suppose $58 offers all the other capabilities that seem relevant to this application (control I/C pump and boost control solenoid), but probably the same issue with DIS.

Um, went back and read your original post - how the heck are your compression numbers that high? What kind of compressionn are you running?

Cheers

[Joseph Upson]

I have a pretty simple set-up compared to what you have:

Iron head 3.1 bored .75mm over, using DIS, ported lower intake and heads
GT2871 56 trim .6A/R compressor, .64 A/R turbine
SyTy W2A intercooler
expecting to run 10-12 PSI boost
Custom intake to replace the upper and mid Fiero intake pieces and dual 48mm LT1 throttle body.

It's amazing how the scope of a simple project keeps increasing.....

I wanted to explore using the $59 code mostly because of the high resolution VE table. Other than that, I suppose $58 offers all the other capabilities that seem relevant to this application (control I/C pump and boost control solenoid), but probably the same issue with DIS.

Um, went back and read your original post - how the heck are your compression numbers that high? What kind of compressionn are you running?

Cheers

If you have room to reinstall the distributor code59 is a much better option. It is very impressive, WBO2 sensing, and knock gauge interface among other things that help make tuning easier and power making greater.

After reviewing my old 3500 thread abandoned for the 3900, the cylinder pressures are normal as both engines have a 9.8:1 stock compression ratio and the highest cylinder pressure I recorded for the 3500 is 244 psi. These engines are more efficient cylinderhead wise I guess to have a low grade fuel rating with such high compression.

[IsaacHayes]

On the topic of compression, is it possible to pop in some LS1 slugs to raise the 3900 compression, and if so, what needs to be modified?

I really want to build a 3400/3500 cammed hybrid with 3.4L RWD pistons... I wonder if it could run on 91 octane though as that's the highest we can get here.

[Joseph Upson]

On the topic of compression, is it possible to pop in some LS1 slugs to raise the 3900 compression, and if so, what needs to be modified?

I really want to build a 3400/3500 cammed hybrid with 3.4L RWD pistons... I wonder if it could run on 91 octane though as that's the highest we can get here.

Yes, but the engine would need to be balanced. There are two pin diameters, one of which is about ~.0015 larger than the 3900s wrist pin which the 3900 connecting rod can be honed to accept provided the piston is floating, or you can take the approach I did due to the deal I got on the pistons and have custom bushings made for the .927 pin to fit the 3900 rods. Getting pistons with the larger pin would be more cost effective because you would only need to have the bushings worked.

You may also need to have notches cut into the pistons to clear the valves also

[IsaacHayes]

I guess what I'm asking as well, is what do stock LS1 pistons bring the 3900 compression to? Or are there higher compression off the shelf LS1 pistons that you would use?

[ForcedFirebird]

I'm still not understanding the problem with DIS and $59. Perhaps the re-pin? The $88 in my stock '7730 is able to control DIS, and they are both looking for the same 5v ref signal for advance.

[Canyonero]

I'm still not understanding the problem with DIS and $59. Perhaps the re-pin? The $88 in my stock '7730 is able to control DIS, and they are both looking for the same 5v ref signal for advance.

Oh really? Will that be near enough to run my 3.4L w/ DIS? I put in some hard work on my 1227727 swap harness last night. I need to get that done.

[ForcedFirebird]

Oh really? Will that be near enough to run my 3.4L w/ DIS? I put in some hard work on my 1227727 swap harness last night. I need to get that done.

Yep. Just have to change the offset to ~60*.

[Joseph Upson]

I guess what I'm asking as well, is what do stock LS1 pistons bring the 3900 compression to? Or are there higher compression off the shelf LS1 pistons that you would use?

You have to find the right piston for your application, the LS1 piston comes in many different compression ratios and compression heights. 3900, .304 CH, 3.898 piston dia, 18 cc dish.

I'm still not understanding the problem with DIS and $59. Perhaps the re-pin? The $88 in my stock '7730 is able to control DIS, and they are both looking for the same 5v ref signal for advance.

It apparently isn't that simple for all applications particularly considering the likely difference in spark logic across different module types for example and crank throw spacing in a 90 deg engine vs a 60 deg engine. I have read of others having problems with distributor code attempting to run DIS ignition as well. All the 5 volt reference does is turn off the bypass circuit so the ECM can control timing through the IC terminal. If the spark logic computations do not come out correctly the timing goes to full retard whatever that is I believe. Once proper cranking speed is reached that's when the 5 volts is turned on and if the logic isn't correct you can hit timing so low that the engine never seems like it will run, which I suspect is what happened to me.

I'm about to go make a start attempt now after having restored the harness back to 8F specs. I hope it works.

[ForcedFirebird]

But we are talking about using a code written for a even-fire 4.3 and even fire 60 degree. They both fire very 120 degrees. I'm about to use a TCE external DIS wheel on my BMW so I can use a '7730 on it as well since it also fires every 120* crank rotation.


EDIT: I'm sure there is an option for odd fire as well. I know a lot of guys use the odd fire crank since it has shared rod pins and is said to be stronger than the .1" split-pin design.

[Joseph Upson]

But we are talking about using a code written for a even-fire 4.3 and even fire 60 degree. They both fire very 120 degrees. I'm about to use a TCE external DIS wheel on my BMW so I can use a '7730 on it as well since it also fires every 120* crank rotation.


EDIT: I'm sure there is an option for odd fire as well. I know a lot of guys use the odd fire crank since it has shared rod pins and is said to be stronger than the .1" split-pin design.

I don't know what the problem is with code59, but right now code 8F is my friend because I just heard one of the sweetest sounds coming from my engine a moment ago. Not only does it have vacuum up around where it should be compared to the previous 13 inches at idle when the cam was retarded, but it idles effortlessly without having to give it gas. The mufflers sound good also and I can quiet them down a little more if I want by adding silencers I found to fit them.

Now I've gotta take a break and then get down to getting it ready to hit the road at some point tomorrow for tuning. I can only imagine how the headers and cam advanced startup is going to improve bottom end performance. I haven't tested the VVT module yet to see if it is going to retard the cam when engergized.

[IsaacHayes]

Sweeeeeeet.