scoop
|
| posted on 7/8/10 at 08:55 AM |
|
|
Skimming for compression...
When i rebuilt my pinto i had the head skimmed just to square everything up. I then had a few probs with geting it running right and had a compression
test done and all four cylinders came up at 8.5:1.
My question is how do you know how much to skim off to get a particular ratio. Would like to get 9.5/10:1. Is that possible with a pinto and is it
safe?
|
|
|
|
|
flibble
|
| posted on 7/8/10 at 09:06 AM |
|
|
8.5:1 sounds about perfect for fitting a turbo, go on, you know you want to 
|
|
|
snapper
|
| posted on 7/8/10 at 09:08 AM |
|
|
You can't work out static compression using a compression tester, however Burtons Pintos have 190 psi when fully built.
To find actual static compression you need to know.... Bore size, deck height ( distance the piston is down the bore, usually between 0.5 & 0.32
mm) the gasket crushed thickness, 1.65 standard, 1 mm Felpro and finally the combustion chamber volume, usually 48 cc on a Pinto.
Using a Felpro gasket on a standard pinto will raid compression from 9.2 to 9.7
[Edited on 7/8/10 by snapper]
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
|
|
|
snapper
|
| posted on 7/8/10 at 09:27 AM |
|
|
If you have the right fuel you can go a lot higher with compression but need forged pistons
As a rule of thumb upto 10.3 for standard unleaded, 10.7 for Super ,Pinto pistons safe to 10.5
just to add another variable, the more duration the cam has the less of the compression the engine sees due to a later closing moment for the inlet
valve
[Edited on 7/8/10 by snapper]
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
|
|
|
MikeRJ
|
| posted on 7/8/10 at 09:44 AM |
|
|
quote:
Originally posted by scoop
When i rebuilt my pinto i had the head skimmed just to square everything up. I then had a few probs with geting it running right and had a compression
test done and all four cylinders came up at 8.5:1.
A compression test tells you only how healthy the bores/rings/valves are, you can't determine the compression ratio from it with any semblance
of accuracy.
Just as proof of this 8.5:1 is lower than a standard Pinto and you've had the head skimmed. The cam profile will make a big difference to
compression results, as will the health of the starter and battery.
|
|
|
scoop
|
| posted on 7/8/10 at 11:48 AM |
|
|
I thought this might open a can of worms. So from what ive read i need a new cam. Ive had the head match ported and flowed and was told by the guy
that did that, that i would only see the real benefits of his work with a cam.
Anyone recommend one for the job?
I love to thrash it and am thinking of going hillclimbing/sprinting it if that helps with recommendations.
Thanks guys.
|
|
|
mookaloid
|
| posted on 7/8/10 at 12:18 PM |
|
|
I got very good results with a kent fr33 cam with my pinto.
"That thing you're thinking - it wont be that."
|
|
|
snapper
|
| posted on 7/8/10 at 12:33 PM |
|
|
I would second the FR33 as I ran an FR32 in a 2.1 , it was very good but I always felt it could take more cam.
How much more work do you want to do to the engine? With standard rods and pistons you will have a rev limit of 6800 to 7200 depending on the rods and
a compression limit of 10.5 to 1
I eat to survive
I drink to forget
I breath to pi55 my ex wife off (and now my ex partner)
|
|
|
scoop
|
| posted on 7/8/10 at 01:02 PM |
|
|
I think cam and new exhaust manifold will be my lot.
Im thinking of something japanese and turbod. 200sx is obviously fave but that wont be this winter probably next. I didnt do all the work and spend
all that money on the pinto not to get some use out of it.
|
|
|
grub
|
| posted on 7/8/10 at 02:16 PM |
|
|
i think i would be having the block decked but how much is another question
|
|
|
mookaloid
|
| posted on 7/8/10 at 04:57 PM |
|
|
quote:
Originally posted by grub
i think i would be having the block decked but how much is another question
Usually this means down to the top of the pistons at their max travel
"That thing you're thinking - it wont be that."
|
|
|
hiflow
|
| posted on 7/8/10 at 09:19 PM |
|
|
if the combustion chamber is an even profile, ie bath tub shape then you can take a know amount of the head, ie 0.020" skim, may give you 5cc.
Redo the CR figs. You should then be-able to work out the exact amount you need to remove for the head. as the combustion chamber is a uniform shape.
If you have slighly over cooked it, take a bit materail out of the head, near the valves & combustion chamber wall, this will reduce valve
shrouding & improve flow. Its not a easy task and it may take 2-3 skims to get it right, but worth it Can you plot out a graph that shows you
the correction stroke when using long duration cams. But as a guide i always deduct 1 point to my compression reading. Ie would skim a head to give
11.2:1 (static reading) this would give you an effective compression of around 10.2:1 when using a 285 duration cam. Please remember 95 ron will det
at anything above 10:1 , so you need to run 97 or 99 ron and we know what that means.
Andy
MD Hiflowheads.co.uk
|
|
|
scoop
|
| posted on 7/8/10 at 10:03 PM |
|
|
Andy i dont know if any of that was typo but im sorry i dont understand a lot of it
What does it mean if you use high octane fuel?
|
|
|
Angel Acevedo
|
| posted on 10/8/10 at 03:44 AM |
|
|
If you have made up your mind about skimming you need to:
-Remove Head,
-Clean Combustion chambers.
-Measure Bore
-Measure Stroke
-As stated above consider gasket Installed Thickness
- Measure Combustion Chamber Volume
- Measure Piston Dome Volume (-) or dish.
- put a disc of modeling clayon top of one piston to check Valve clearance at Full Lift, you will be looking for a healthy clearance if you will be
running very high revs and you may get valve float.
From measured values determine Total cylinder volume, and Combustion chamber volume.
That wil give you actual CR, then, substitute in the formula the desired CR and find out required Combustion chamber volume.
You will be looking for the difference in volumes.
Lets say you measured your Chamber to be 50 cc, and you require 45 to get desired CR, fill your Combustion chamber with 45 cc of liquid (Head level)
and estimate how much you need to skim, make sure you have clearance for your valves.
HTH:
Beware of what you wish.. for it may come true....
|
|
|
