Compression ratios

EB Pete · 14-04-2006, 11:56 PM

Hi all, well i'm at the stage of me rebuild to get work done to the auto. i need to know the compression ratio to get the stall made but i am yet to get this figure and i can still bump the ratio up if it is to low.

So question 1 is:
What compression ratios is everyone running?

Question 2:
What is a good value to get some big rwkw's N/A for these motors and what fuel would i have to use?

Question 3:
Is there a web site or equation to work out the ratio that i can get for free? (found one called smokemup.com but have to pay to use it)

thanks for the help.
Pete

**ebxr8240** · 15-04-2006, 01:02 AM

You need to know what chamber size is etc...
http://www.rosspistons.com/calculator.php

BI6TIM · 15-04-2006, 10:33 AM

If you measure your chamber volume and also know the value of the dish in your pistons at tdc you can work it out using the calculator above. About 9.5-10.0 is pretty good for N/A in my opinion and if tuned right you could use BP Ultimate or Shell Optimax, no need for racing fuel or anything like that.

EB Pete · 03-05-2006, 06:45 PM

i have the dish value and head cc value but i got 4 thou taken off the block, .5mm oversize pistons so what does that change the values to? and where do i find the measurments for the gasket?

EF_Frmnt · 03-05-2006, 08:05 PM

off the block or the head?? takin stuff off the block seems strange unless it was just gettin decked and squared. im tryin to get in the 8s for compression as turbos dont like big numbers

EB Pete · 03-05-2006, 08:10 PM

yeah block was decked. i went the N/A way and i'm hoping for at least 10:1 comp ratio. the head was shaved 11 thou but i sent it back and i think 29 thou more will do it.

Walkinshaw · 04-05-2006, 10:15 AM

with 4 thou off the block deck, make sure you mill 4thou off the top of the piston crowns. These engines run a 0 deck height.

trick_xd · 04-05-2006, 11:03 AM

Yes i am lead to beleave that you can not take anything off the deck hight as i wanted to take just 1thow and was told i cannot do it ..

Perana · 04-05-2006, 11:13 AM

Quote:

Originally Posted by trick_xd

Yes i am lead to beleave that you can not take anything off the deck hight as i wanted to take just 1thow and was told i cannot do it ..

It can be done (A mates EF needed it done since the block surface wasnt too good).. but you have to take the same amount of the pistons.

EB Pete · 04-05-2006, 05:52 PM

are you serious? so do i just take the pistons to the place that did it and tell em to fix it?

EB Pete · 04-05-2006, 06:17 PM

this is what it says on the tax invoice:

CYLINDER BLOCK
Bore & Finish cylinders to size (6)
Surface grind head 0.004''

And i didn't have me head there either. so what the do i do?

EB Pete · 05-05-2006, 06:23 PM

i went and had a talk to them today and they said 4 thou won't bother it at all. i'm still a little worried but he said they have done that to many 4.0l blocks and they haven't had a problem. he said he very interested to find out where you got this info from. and he said to have a look in the book and it says under cylinder block 'surface machining limit 0.25mm' which i'm pretty sure is 10 thou. but it also says that for the head and i've gone to about 30 thou. so any comments on that and where abouts did you find out about not being aloud to take any off the block?

EB Pete · 26-05-2006, 07:07 PM

Quote:

Originally Posted by Walkinshaw

with 4 thou off the block deck, make sure you mill 4thou off the top of the piston crowns. These engines run a 0 deck height.

sorry to bring up this old thread but i thought a few ppl might be interested in what i found.

well over the last few weeks i've been trying to find info about this supposed 0 deck height and have come up with this.

1. been told by alot of engine builders that 4 thou is 2 parts of all and the head gasket will take that up.

2. went back to the place that did it and they cliam that they have done many 4.0l blocks and had no problems at all. and 4 thou wouldn't matter anyway as they have a 8 thou tolerence from the piston to the conrod (somthing like that).

3. gave a few performance shops a ring and one place said that the ford 4.0l normally runs at about 30 thou under the deck.

well i still don't want to take a chance so as a few places said, tomorrow i'm going to dummy assemble the block (just the crank and 1 piston) throw the dial gauge on it, get top dead centre then put a straight edge on the block and use a feeler gauge and see what i got.

and with a guesstimated deck height of 20 thou below my comp ration is 9.8:1.

Walkinshaw · 26-05-2006, 08:12 PM

4.0L run a 0 deck height.

EB Pete · 26-05-2006, 09:33 PM

Quote:

Originally Posted by Walkinshaw

4.0L run a 0 deck height.

yeah i'm not sure either way. i had a look in the workshop manual and it said 'surface machining maximum - 0.25'. so yeah if i check it all tomorrow i'll tell ya the results.

EB Pete · 27-05-2006, 07:07 PM

well after alot of cleaning and learning today i got my deck height! with me putting the crank in and one piston then the old man showing me how to get top dead centre with the dial gauge, we got an answer. with my block decked 4 thou i have 2-3 thou spare. now i know there will be ppl who disagree with me but this is what my old man got and he knows how to do it. with that all said i now need to know how to put 2 thou below deck hieght in the above web site link.