Confused about my stroker specs

[mendelmax]

Hi guys. I'm new to the board, but I've been watching it for quite a bit of time. I did search, but still I'm not certain about few things.

I already made my stroker (actually the one for my friend) and it runs great (600 miles on it so far), but lately I'm more and more confused about some of it's parameters. The thing that concerns me most is the DCR (as usual...).

My basic specs are:
4.2 Crank (3.895 stroke)
4.2 Rods
4.0 .040 oversize pistons
16.5ccm dish

Stock 0.051" gasket
Deck clearance about 0.04 (we didn't have a chance to measure it precisely, that's a long story)
93 engine block, and 93-95 rebuild kit from EngineTech

Calculating static CR was easy, and it varies in a range about 9.8:1. The rig is currently fuelled with 98RON octane gas, and soon also 100+RON LPG, so the CR can be slightly high with no problems, despite the crappy 0.091 quench.

Now the problem is, that various calculators give me different DCR ratios. The camshaft we have is supposed to be a stock specs 93 camshaft, which then should have 75* closing point, that I was entering for my calculations. This would lead me to 7.2 DCR in most calculators (forum one included). How can it be, that I'm so low on DCR? It appears, that with 4.6 I'm not much above stock, post 96 4.0 DCR. Am I missing something, or did I make some really stupid mistake during my calculations?

What DCR's are you guys getting on your premium fuel without pinging? Because I was really hoping for nice efficiency and extra power from high CR and here's surprise...

Any input appreciated.

[Mgardiner1]

Maybe i'm missing something, but with the info provided i'm coming up with 9.3:1 SCR and 6.89:1 DCR with a 75 IVC.

Want better DCR? Choose a different cam OR advance the timing OR deck the block to zero (zero deck with the other numbers provided would give you 10.1 SCR and 7.45 DCR

[mendelmax]

I forgot to mention, that it appears that the head had 53ccm. I don't know how is that possible, since it was milled only 0.12mm. Maybe previous owner milled it more. It was measured in a simple, home way (siringe + oil), but there's no way it could exceed 55 (we were trying to be very precise).

The main problem was that we were on tight budget and therefore we had to use things that we already had.

[Mgardiner1]

Going for .040 pistons, you should have been able to locate some with a slight taller compression height, that would have kinda been a poor mans way of increase quench (cause you needed pistons already).

See the "Piston FAQ"

[mendelmax]

The bad thing was that my friend already had a full 4.0 rebuild kit, since at first he wanted just to rebuild. Therefore we had these pistons and we couldn't do anything about them. The only thing was to dish them to 16.5ccm and cover with ceramic coat (to prevent overheating from higher LPG combustion temperature).
The same thing with cam- it was already there and we simply had to use it. Maybe we will change it in the future if the dyno wouldn't be satisfying.

So you guys say, that it is possible that we have so poor DCR, and it's not just my calculation mistake?

[SilverXJ]

The problem with most calculators is that they assume too much.. they either assume the connecting rod length, the piston to cylinder clearance, the pin height or the deck height incorrectly. Then the cam number for the DCR could be off to. I know outs was off for the comp cam 68-231-4 for a while with respect to the advance. The best way is to do the math yourself.

As for what DCR runs what grade fuel its a crap shoot. Too many variables to determine what you can run safely.


Oh and LPG does throw another not in the equation.

[mendelmax]

Yeah, I know that there's no definite "safe DCR", but I assume, that if I'd know what range DRC can you guys run on your fuel, I would be pretty much certain it should work with LPG From what I have read here the still safe range varies between 7.5-8, am I correct?

It seems we are going to have to change cam though But first I'm going to dyno it and see how it all works out. I'm afraid that with this timing we are going to reach max torque very, very high....
Maybe a stupid question, but has anyone tried to change cam on the ZJ without engine removal? To me it seems possible, but maybe there is some catch that I'm not aware of?

[Mgardiner1]

There have been a few reports of people running on pump gas with no issue's that have a DCR from 8-8.25ish

There is one member of the board who had his engine assembled for him, and the calculated DCR is supposedly 8.50 w/ .04x quench and he's having pinging issues on pump gas, but can solve it by running like 110 octane. Myself and a few others are not completely convinced that this pistons were install in the correct direction either, apparently the KB944 installation sheet shows the pistons installed in the wrong direction. One other member caught that when installing pistons.