I have a 3.2 (3.1 crank and rods with .030 overbore). With this stroker kit, I ended up with something along the lines of 8.5:1 compression. I would like to boost compression up to about 9.0:1 to 9.2:1. I have several sets of spare Fiero cyl heads, what will I need to tell the machine shop when I take my cyl heads and lower intake manifold to be planed? Do I tell him .010? or .020? I really have no idea what I'm talking about.
FYI, I know that a lot of the brackets won't properly line up anymore, but I can always slightly elongate the openings on some spare brackets I have too. I'm going with the H272 cam, and want to make the most benefit from it that I can.
The first thing to do would be to install the crankshaft, rods, and pistons into the block and measure the piston-to-deck clearance. This is the distance from the top of the piston to the top of the cylinder. You can measure this by rotating the crank until the piston is at top dead center, puttnig a straight edge across the top of the cylinder, and sliding a feeler gauge under it. You should measure at least 2 pistons, preferably on opposite corners of the engine. But measuring them all would be better. The ideal clearance (assuming your head gasket is a stock replacement) would be about 0.005 - 0.010". If the clearance is too great, have the block shaved. Then re-compute your compression ratio and go from there.
I've found that each 0.005" you shave off the heads yields about 0.1:1 more compression. This is an approximation, but is pretty close. Also keep in mind that if you shave more than 0.015" total off the block and heads, you will probably need to have the lower intake shaved to match. Otherwise, the intake manifold will sit too high on the heads.
For example: when I built my 3.2 stroker engine, I ended up shaving a total of 0.025" off the heads and block. As a result, I needed to shave 0.015" off the intake to make everything fit. But even after all this shaving, I had no problems with accessory bracket mounting. Maybe I was just lucky. :shrug:
I had .010 shaved off of the heads when I built my 3.4L. They shaved about .010" off the intake as well, But this was the Edelbrock aftermarket intake so I don't know what the clearence is with the stock lower intake. The shop that did the work on the heads also had the intake so they could verify everything.
The .010" shave gave me about a .4 increase in compression with the stock bore. Removing .010" off the heads reduces the head chamber volume from 51cc's to about 49 cc's. So if you look up a compression calculator, change the head volume from 51cc's to 49cc's with your new bore and it should give you a good idea what your increase will be. A shave of .020" should reduce head volume to a little less than 47cc's.
My question is why did your compression go down so much? Your displacement went from 524.85cc's per cylinder to 533.86cc's per cylinder with the overbore. So if the stock compression was originally 8.9:1, You were taking 524.85cc's and compressing it into 65.61cc's of space (stock), Now you are taking 533.86cc's and compressing it into 66.73cc's. Your compression should only vary by less than 0.05 change because you are adding 9cc's more of volume for a 1.1cc increase of compression space. When you changed pistons on the stroker kit, was the dish larger on the new ones? I don't know the difference in pistons between stock and the stroker kit, Does the stroker kit lower the compression to 8.5:1?
(Note: the above numbers were taken from a stock 3.1L.)
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07:12 AM
82-T/A [At Work] Member
Posts: 25716 From: Florida USA Registered: Aug 2002
My question is why did your compression go down so much? Your displacement went from 524.85cc's per cylinder to 533.86cc's per cylinder with the overbore. So if the stock compression was originally 8.9:1, You were taking 524.85cc's and compressing it into 65.61cc's of space (stock), Now you are taking 533.86cc's and compressing it into 66.73cc's. Your compression should only vary by less than 0.05 change because you are adding 9cc's more of volume for a 1.1cc increase of compression space. When you changed pistons on the stroker kit, was the dish larger on the new ones? I don't know the difference in pistons between stock and the stroker kit, Does the stroker kit lower the compression to 8.5:1?
(Note: the above numbers were taken from a stock 3.1L.)
Hi Oreif,
Thanks for the info. When I originally built my motor back in... what, like 2001 or something, several people were telling me that by going with the 3.1 stroker kit, that I would end up losing compression. They stated that there were TWO kinds of stock pistons, one that were heavily dished, and ones that were slightly dished. I know you probably love my "technical" description of them... hahah... In any case, people were telling me that by going with the heavily dished pistons, my compression would drop to something like 8.0:1, but if I went with the small dished pistons I would only drop to 8.5:1. As I understood it, it only did this if you went with a stroker kit off the 2.8, otherwise an original 3.1 wouldn't do this. I'm not sure I understand the reasoning behind this...
quote
Originally posted by Blacktree:
The first thing to do would be to install the crankshaft, rods, and pistons into the block and measure the piston-to-deck clearance. This is the distance from the top of the piston to the top of the cylinder. You can measure this by rotating the crank until the piston is at top dead center, puttnig a straight edge across the top of the cylinder, and sliding a feeler gauge under it. You should measure at least 2 pistons, preferably on opposite corners of the engine. But measuring them all would be better. The ideal clearance (assuming your head gasket is a stock replacement) would be about 0.005 - 0.010". If the clearance is too great, have the block shaved. Then re-compute your compression ratio and go from there.
I've found that each 0.005" you shave off the heads yields about 0.1:1 more compression. This is an approximation, but is pretty close. Also keep in mind that if you shave more than 0.015" total off the block and heads, you will probably need to have the lower intake shaved to match. Otherwise, the intake manifold will sit too high on the heads.
For example: when I built my 3.2 stroker engine, I ended up shaving a total of 0.025" off the heads and block. As a result, I needed to shave 0.015" off the intake to make everything fit. But even after all this shaving, I had no problems with accessory bracket mounting. Maybe I was just lucky. :shrug:
Thanks Black Tree, the motor is already put together and has several thousand miles on it. I'm going to be swapping out the cam though, so I'll probably remove the cyl heads too while I'm at it. While it's out. I'll probably go ahead and perform the measurements like you suggested. Do you know what your compression is right now with the 0.025 shaved off?
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Originally posted by black88fiero:
i have an equation to find out how muc has to be milled to achive compreesion wanted i will post it tommorw if interested
Definitely! If you have time, I would really appreciate it!
The 3.1/3.2 motors were 8.5:1 compression, so when you converted your 2.8 to a 3.1/3.2 with the crank and pistons (both motors share the same block and connecting rods) you did lower your compression from the L44 (Fiero 2.8) number of 8.9:1. The other pistons you looked at were probably for the aluminum head 3.1 which has a radically different cylinder head design so the pistons aren't interchangeable.
JazzMan
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08:55 AM
Pyrthian Member
Posts: 29569 From: Detroit, MI Registered: Jul 2002
Thanks for the info. When I originally built my motor back in... what, like 2001 or something, several people were telling me that by going with the 3.1 stroker kit, that I would end up losing compression. They stated that there were TWO kinds of stock pistons, one that were heavily dished, and ones that were slightly dished. I know you probably love my "technical" description of them... hahah... In any case, people were telling me that by going with the heavily dished pistons, my compression would drop to something like 8.0:1, but if I went with the small dished pistons I would only drop to 8.5:1. As I understood it, it only did this if you went with a stroker kit off the 2.8, otherwise an original 3.1 wouldn't do this. I'm not sure I understand the reasoning behind this...
yes - thats the "iron head" pistons & the "alum head" piston. the Iron head version is the shallow dish, the alum head is the deep dish. if you seen the alum head & the iron heads side by side - you'd see exactly why they have different pistons. so - the choice depends on what heads you are using - stock Fiero Iron - or newer aluminum heads. and - it never has been quite made clear if the compression was 8.5 or 8.9 :1. most people say it is 8.9:1 - but not sure if that is just wishful thinking.
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09:02 AM
Blacktree Member
Posts: 20770 From: Central Florida Registered: Dec 2001
Originally posted by Oreif: My question is why did your compression go down so much? I don't know the difference in pistons between stock and the stroker kit, Does the stroker kit lower the compression to 8.5:1?
The stock 2.8 pistons are flat-top, whereas the iron-head 3.1 pistons have a 12cc dish top. That's where the reduced compression comes from.
My 2.8 block had an unusually high deck surface. When I installed the crank and pistons, I found the piston-to-deck clearance was about 0.030". The compression calculations (both by calculator and Desktop Dyno) came in at 8.3:1. So I had the block and heads shaved, to bring the compression back up to 8.9:1.
