How do you calculate potential for valve lift?
Discussion
Lets say that i have a cam that i'd like to improve upon with more lift,but i'd like to know the safe limits of just how much more cam lift my current valvetrain will reliably accept.How do i go about finding this out?I'm thinking of piston to valve clearance,valve spring potential before binding(surely not as easy to measure as it first sounds?) and valve guide clearance.What else do i need to know in order to calculate maximum possible lift?
Apart from stripping everything down and measuring it then your best bet is to find a company who supply replacement cams for your engine.
The measurements will only give you the actual foul conditions but the potential for bounce will depend on the strength and frequency of your springs which will be somewhat variable if the springs are either not new or are not of good quality.
Steve
The measurements will only give you the actual foul conditions but the potential for bounce will depend on the strength and frequency of your springs which will be somewhat variable if the springs are either not new or are not of good quality.
Steve
Lot of maths, easy on a computer if you can master any programming language, but you must still have decent geometrical skill 
Measure crank, rods, piston height and make a rotational model. Add a valve face to piston clearance, its pretty straight forward if valve is parallel with bore.
Id still always check my calcs by dry assembling and using DTI and timing wheel You also need to take some measurements from your own engine to plug in to the calcs anyway so if it isn't stripped its a bit of a chicken & egg.
Generally the area that always seems to be most critical is the exhaust valve, there seems to be much more freedom with the inlet just because of the way most engines breathe.
I've got some of the math in my RV8 animation if you want to look at some hacked Perl
its the bit that shows the pistons going up/down (you saw it in sunlight over here a while ago IIRC?)
ETA: lifted this out for you
ETA: remove the \ from first 4 lines (escaping stuff on PH eludes me)
\#!/usr/bin/perl -w
\#
\# engine valve clearance quick calc...
\#
use Math::Trig;
my $Stroke = 77 ; #stroke
my $RodL = 130 ; #Con Rod length centre - centre
my $ValveClear = 7 ; #valve clearance to piston @TDC
my $PatTDC = $RodL + $Stroke / 2 ; #Y coord of little end at TDC
for ($deg = 0; $deg <360; $deg++ ) {
$CrankX = sin( deg2rad($deg) ) * $Stroke / 2 ;
$CrankY = cos( deg2rad($deg) ) * $Stroke / 2 ;
$dy = sqrt( ($RodL ** 2) - ($CrankX ** 2) ) ; #hypoth of rod
$PatY = $CrankY + $dy ; #height (Y) of little end to Mains
$PdownBore = $PatTDC - $PatY ; #piston down bore
$CalcValveClearance = $ValveClear + $PdownBore ;
printf "%3i degs - %6.2f mm\n", $deg, $CalcValveClearance ;
}
Measure crank, rods, piston height and make a rotational model. Add a valve face to piston clearance, its pretty straight forward if valve is parallel with bore.
Id still always check my calcs by dry assembling and using DTI and timing wheel
You also need to take some measurements from your own engine to plug in to the calcs anyway so if it isn't stripped its a bit of a chicken & egg.Generally the area that always seems to be most critical is the exhaust valve, there seems to be much more freedom with the inlet just because of the way most engines breathe.
I've got some of the math in my RV8 animation if you want to look at some hacked Perl
its the bit that shows the pistons going up/down (you saw it in sunlight over here a while ago IIRC?)ETA: lifted this out for you
ETA: remove the \ from first 4 lines (escaping stuff on PH eludes me
)\#!/usr/bin/perl -w
\#
\# engine valve clearance quick calc...
\#
use Math::Trig;
my $Stroke = 77 ; #stroke
my $RodL = 130 ; #Con Rod length centre - centre
my $ValveClear = 7 ; #valve clearance to piston @TDC
my $PatTDC = $RodL + $Stroke / 2 ; #Y coord of little end at TDC
for ($deg = 0; $deg <360; $deg++ ) {
$CrankX = sin( deg2rad($deg) ) * $Stroke / 2 ;
$CrankY = cos( deg2rad($deg) ) * $Stroke / 2 ;
$dy = sqrt( ($RodL ** 2) - ($CrankX ** 2) ) ; #hypoth of rod
$PatY = $CrankY + $dy ; #height (Y) of little end to Mains
$PdownBore = $PatTDC - $PatY ; #piston down bore
$CalcValveClearance = $ValveClear + $PdownBore ;
printf "%3i degs - %6.2f mm\n", $deg, $CalcValveClearance ;
}
Edited by spend on Friday 20th March 01:32
Edited by spend on Friday 20th March 11:37
For my BMW I built a model using GT valve train where I characterised the rocker arm, valve , cam etc etc and even the piston.
To double check I chopped up a cylinder head and did s dummy build to take a look.
I found out that on an M20 if I put a 45 mm inlet valve and a 37 mm exhaust valve and have a higher than 2mm valve lift at TDC both valves will hit one another
To double check I chopped up a cylinder head and did s dummy build to take a look.
I found out that on an M20 if I put a 45 mm inlet valve and a 37 mm exhaust valve and have a higher than 2mm valve lift at TDC both valves will hit one another
I think the old fashioned way of establishing max lift is to put some thin blu tac on the piston - where the valve is closest .. and build values with just the inner weak spring .. then take a know value lift and work upwards until there are marks on the blu tac.
You obviously would need to have a dial guage and a simple rig to push the valve down very accurately .. but easily do able.
Just from memory - I think people use a clearable like 20 thou inch for valve float .. co the clearance at the piston should reflect this.
You obviously would need to have a dial guage and a simple rig to push the valve down very accurately .. but easily do able.
Just from memory - I think people use a clearable like 20 thou inch for valve float .. co the clearance at the piston should reflect this.
Hmmm, I thought Mike wanted to work backwards so he could spec a cam? Easier for a lot of us to tweak with rocker ratios but his is an AJP.
Thats why I'd calculate max lifts available (during rotation), then the max mechanical lift you could achieve due to the springs / seats (maybe some room for innovation?) as base parameters for what you can put on the cam lobes?
AFIK you are not at max lift for very long - its how much lift you can get at critical points that is important. From what I've seen it is nearly always making the exhaust as tight as possible. I certainly think there is leeway for better breathing over areas of partial lift if you can map exactly your clearances and have a cam designed to be safe within your engine. Standard cams will have much greater safety factors built in, as well as erring on the 'soft / drivable' side.
Thats why I'd calculate max lifts available (during rotation), then the max mechanical lift you could achieve due to the springs / seats (maybe some room for innovation?) as base parameters for what you can put on the cam lobes?
AFIK you are not at max lift for very long - its how much lift you can get at critical points that is important. From what I've seen it is nearly always making the exhaust as tight as possible. I certainly think there is leeway for better breathing over areas of partial lift if you can map exactly your clearances and have a cam designed to be safe within your engine. Standard cams will have much greater safety factors built in, as well as erring on the 'soft / drivable' side.
I always mock up with plastiscene for piston to valve clearance and allow extra for the exhaust valve as it's opening when the piston's rushing up to greet it. Changing cam timing obviously determines when the collisions may take place so you may have a bit of scope there. Use solid components for mocking up and a soft spring as already mentioned. As for valve springs, they do float so I'd fit new items and not go much higher on lift.
You could do with a spare block and head or find peak lift on your engine and then use a dial guage to see how much further the valve will drop before contact?
You could do with a spare block and head or find peak lift on your engine and then use a dial guage to see how much further the valve will drop before contact?
rev-erend said:
I think the old fashioned way of establishing max lift is to put some thin blu tac on the piston - where the valve is closest .. and build values with just the inner weak spring .. then take a know value lift and work upwards until there are marks on the blu tac.
You obviously would need to have a dial guage and a simple rig to push the valve down very accurately .. but easily do able.
Just from memory - I think people use a clearable like 20 thou inch for valve float .. co the clearance at the piston should reflect this.
You obviously would need to have a dial guage and a simple rig to push the valve down very accurately .. but easily do able.
Just from memory - I think people use a clearable like 20 thou inch for valve float .. co the clearance at the piston should reflect this.
but i would make that nearer 60thou rods stretch quite a bit at full chat too 
interesting post AJP Griff 
Edited to say... i am referring to run of the mill rods by the way but good/strong ones cant comment on what your AJP lump will be fitted with 
Edited by That Daddy on Friday 20th March 20:26
Boosted LS1 said:
^ I think he means add an additional .020" for the valve float on top of what's already allowed for piston to valve clearance.
As an aside, rods don't stretch at .060", no way. Think about how much that really is.
No your right Boosted,but i would rather lean towards safe,lets face it if you add another 30thou to avoid valve piston contact its not going to affect your max valve lift after all rather this than finding out the hard way costing the motor:lets face it only a matter of having slightly deeper pockets on the piston ( see what you mean about the additional lift(sorry Rev) As an aside, rods don't stretch at .060", no way. Think about how much that really is.
only my cents worth really Edited by That Daddy on Saturday 21st March 08:05
How many any of these deformations, that you propose allowing 'safety factors' for, are occurring at potential kissing points?
I can't see the point of adding 'safety factors' for all components at all crank angles. I would have thought that when the piston or valves are in compression the tolerances should work in your favour, and to the detriment when the piston is in tension or the valve coming off cam?
I can't see the point of adding 'safety factors' for all components at all crank angles. I would have thought that when the piston or valves are in compression the tolerances should work in your favour, and to the detriment when the piston is in tension or the valve coming off cam?
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