Need somone to help me understand.

[blu3fan]

My dad and I are slowly building a 4.6 stroker into a 92 Cherokee.
We have everything test fitted after having the block bored +30. All the crank and rod bearings were perfect. When I checked the deck, the pistons are 1mm below the deck.Is this good?

1992 Block had 230k miles, so, as I said bored +30
Stock style pistons I think Speedpro, with a shallow dish...
Pic link:
http://s135.photobucket.com/albums/q143 ... C01357.jpg
http://s135.photobucket.com/albums/q143 ... C01358.jpg

Crank was new from SCAT, and the rods are new, but stock 4.2's.

Head was skimmed to true it up... I think it was 0.020".
The cam is a Comp Cams... either a 240H or a 252H, but I forgot which one, with the matching springs, and Yella Terra Valvetrain.
We are in California, so has to be smog complient, so were planning to use all the stock EFI, but just with larger injectors.


we posted this to the stroker yahoo groups and these were their reply's
first response:
That's right at 0.040", which is correct for a stock 4.0L. The head
gasket will be another 0.040-0.042", leaving a total quench height of
0.080-0.082". Like I said, correct for a stock 4.0L. If you are going to
be running EFI and stock 4.0L pistons you will have to run premium fuel
as the compression will be roughly 9.7:1. With a carb you can tune
around that. With EFI you either run premium or buy dished pistons to
lower compression. Another avenue that works is a high overlap cam to
bleed cylinder pressure. Many people deck the block so that the pistons
are flush with the top of the block (cut it that 1mm/0.040"). That
raises compression to around 10:1, but you can generally run mid grade
or regular because of the tighter quench height. The reason is the flame
front doesn't get as close to the edge of the pistons when the
combustion chamber is squeezed that little bit tighter. With the looser
quench height the edges of the piston will get hot enough to cause
preignition (detonation) at the high compression. Pistons made to keep
compression down to stock eliminate the problem also. I had the dish
increased in stock pistons.



second response:
Yes, it will work with the deck clearance as is. The only problem will be detonation IF you run regular gas. You might be able to get by with mid grade, but it will like premium better. Definitely fill up with premium for the smog test.

You will need 24# Ford injectors. You may get a better deal on 8 than on six. Used is fine -- check e-bay. They were stock on the 351 engines (I forget liters... 5.7?).

I ran my first stroker (late 1999, one of the first 20 or so built, and before we knew as much as now!) until 2003 built just the way you describe. Had a different cam, but use stock 4.2L rods, stock 4.0L crank and pistons. I could run mid grade gas but had to be careful with my driving to keep detonation down. The only thing you can do after it's built to run regular fuel is adjust the CPS to cut the timing a bit.

The replacement pistons are probably a bit further down than stock to make up for the slightly larger bore, that's typical. The longer stroke is what brings the compression up.


to be quite honest this is a little above our understanding. can somone help me.
if we leave the block as is and don't skim it what are the pro's and cons.
thanks in advance for any information.

[gradon]

I gave you the first response on yahoo, which I stick to and won't repeat. The one you say is the first response here is the 2nd response.

[blu3fan]

sorry gradon my dad posted that one up. dont have access to a yahoo account atm.
honestly im just starting to understand how all of this works and its still a little beyond me.
so i re read and re read and this is what im getting.
if i leave it as, is im going to have a quench of 0.080-0.082, and that's stock quench. is this OK for running the 4.2 crank and rods ? providing i run premium.
i was considering that to be a given anyways all the guides say the strokers love premium.

so as is the compression is 9.7-1 and if i deck it, its going to be 10-1
is 10-1 better ? still trying to wrap my head around a lot of this.
"With the looser quench height the edges of the piston will get hot enough to cause preignition (detonation) at the high compression."
so do i need to deck the block ? or with injectors (that i allready bought) will it work ?
thanks.

[Cheromaniac]

I think the effect of quench is being overstated. My stroker has a 9.25:1 CR, 0.088" quench, and will run on 87 octane fuel.
I don't know which pistons you used but I'll guess that they're either Speed Pro H802CP/H825CP or Silvolite 3241HC/3242HC which have a compression height of 1.592", so they'd leave the deck clearance at 0.038" and the quench at 0.081" (with 0.043" HG).
Since you had 0.020" shaved from the head, your SCR will be ~9.8:1. The grade of fuel that you'll need will depend on the cam choice, and it'll need to be longer duration to keep the DCR in check (CompCams 68-231-4 is OK, 68-235-4 would be better). In any event I think you'll be needing minimum 91 octane.