19 replies to this topic

[KTS]

cam follower failure means i need to replace the MD266 that was fitted.   

 

I could just get another MD266, but may as well take the opportunity of seeing what else is available that would work well with the compression ratio i have (..having said that, i'm not averse to skimming a little off the head if necessary..)

 

 

 

 

[Spider]

If it were me, I'd work from dynamic compression ratios and tyre and land with a more sportier profile that also leaves it friendly on the street. as a Dynamic Ratio, I've found 8.2:1 to be the magic number on 97 Octane fuels.

There's several methods for calculating this, however one of the simplest is to work our CRs as you normally would for Static Ratios (ie, how you arrived at your 10.2:1 SCR), but, when you input your stroke length, instead of using the full stroke of the crank, work from the stoke above that where the Inlet Valve closes. It also lets you play around a little with where you time the cam in, placing the power band closer to where you want it in the rev range.

eg, working from your 266 cam, the Inlet Closing Angle is 56⁰ ABDC or put another way 124⁰ BTDC (180⁰ - 56⁰ = 124⁰). At that Crankpin Angle, your effective stroke (for the purpose of this calculation) is 2.461", assuming you are running a standard 3,200" (full) stroke. So, you'd use that 2.461" instead of 3.2" for your CR calculation.

You'll also see from this that the wilder the cam profile, the higher the Static Ratio needs to be.

Tweaking that angle a few degrees either way can not only 'fine tune' the DCR but as mentioned, move the power band - hopefully to somewhere more favourable to what you are seeking. Advancing the Cam timing lowers the revs where the power band is while retarding it raises them, though I'd suggest not moving more than about 6 - 7 degrees from that recommended by the grinder.

[KTS]

That's great. Thanks Spider

I'll work the numbers with published cam specs and see what drops out...

[KTS]

after a lot of head scratching i eventually managed to get a dynamic compression formula to work in excel (i can't guarantee it's producing the right answers    ), so i could start playing with the numbers

 

if i take my current engine spec of 10.24:1 static compression with the MD266 gives a dynamic compression ratio of 8.65:1 - not sure if this would be considered a little high, but seemed to work well enough (80lbft+ from 2300rpm, max 96lbft @4600rpm)

 

If i've understood this correctly and using an MD286 with the intake valve closing at 66 degrees ABDC as an example, that would give a DCR of 8:04:1, which is possibly a little on the low side, but as an alternative to raising the static compression by skimming the head, if i timed the cam in at 103degrees instead of the 106degree spec figure, that would produce a DCR of 8.23:1 which should be OK, and also shifts the power band down the rev range.

 

 

 

 

Attached Files

•  dc_model.zip   9.66K   7 downloads

[Spider]

I've just now been able to look at this. Sorry, I can't get your .zip file to open, likely an issue at my end as my excel is an old version.

I came up with a figure of 2.738" effective stroke length working from 66⁰ ABDC closing angle.

 

I don't have your total combustion chamber volume, but your DCR figures look right.

 

The DCR of 8.65:1 for your 266 cam if running with optimal ignition timing is in hand grenade territory.

 

I'd be more inclined to run at 104⁰ just to leave some margin in there.

[KTS]

OK - i'm getting different numbers for the effective stroke length so I guess my method is wrong. 

 

the excel calculation is based on the figure3. formula on this page, and looks like this :

 

=SUM(((B3/2)+B4)-SUM(SQRT(((B4^2)-((B3/2)^2)*SIN(B6*PI()/180)^2))+SUM((B3/2)*because(B6*PI()/180))))

 

where

 

cell B3 is the crankshaft stroke

cell B4 is the conrod centre-to-centre length

cell B6 is crank angle (BTDC)

 

 

out of interest, what crank angle should give exactly half the stroke ?

 

EDIT:  autocorrect doesn't like the cosine notation in the formula and keeps changing to because instead !

Edited by KTS, 21 April 2024 - 09:55 AM.

[Spider]

OK - i'm getting different numbers for the effective stroke length so I guess my method is wrong. 

out of interest, what crank angle should give exactly half the stroke ?!

 

 

On a standard 3,2" stroke, I work it to be 81.9999⁰ ABDC.

Let's call it 82⁰
 

[KTS]

 

OK - i'm getting different numbers for the effective stroke length so I guess my method is wrong. 

out of interest, what crank angle should give exactly half the stroke ?!

 

 

On a standard 3,2" stroke, I work it to be 81.9999⁰ ABDC.

Let's call it 82⁰
 

 

 

thanks; i get the same

 

can i check one last number; and that's effective stroke at 90 degrees, for which i get 1.82709"

[Java_Green]

I have also done the trigonometry session.... The formula I came up with I believe is correct up to the errors you put into it.

 

Used variables:

Piston dish (V_dish) [cc]

Combustion volume (V_head) [cc]

Gasket volume (V_gasket) [cc]

Ringland volume (V_ring) [cc]

Deck height (h_deck) [thou]

Bore diameter (d_bore) [mm]

Stroke (L_stroke) [mm]

Con rod length (L_conrod) [mm]

Intake valve closing, ABDC (alpha_int) [deg]

 

Formulas:

Lost stroke length (L_lost) = SQRT(L_conrod²-(0.5*L_stroke*sin(alpha_int*pi/180)*(0.5*L_stroke*sin(alpha_int*pi/180)))-0.5*L_stroke*because(alpha*pi/180)-(L_conrod-L_stroke/2)

 

Theoretical compression volume (V_theory) = pi*d_bore²*(1/4)*L_stroke/1000

Real compression volume (V_real) = pi*d_bore²*(1/4)*(L_stroke-L_lost)/1000

Unswept volume (V_static) = V_dish+((h_deck*25.4/1000)*pi*d_bore²/4)/1000+V_ring+V_gasket+V_head

 

Theoretical CR (CR_stat) = (V_theory+V_static)/V_static

Dynamic CR (CR_dyn) = (V_real+V_static)/V_static

 

Hope it is of use for someone.....

[imack]

Is it actually possible to accurately calculate dynamic compression using cam manufacturers valve event data? I was under the impression that the valve opening and closing numbers are not actually the point that the valve contacts or lifts off the seat. I was under the impression that valve event data figures were given at something like 50 thou valve lift, and this figure varies from cam grider to cam grinder, and that most grinders don't disclose how their data is calculated, making cam comparison very difficult.
I ended up giving up on calculating dynamic compression in this manner, partly because I'm not clever enough to do the calculations and partly because I don't know the valve event data for my cam.
I ended up physically measuring piston position when the valve contacted the seat during dummy build to calculate the dynamic compression. But as previously mentioned, advancing or retarding the cam, or altering valve clearances raises or lowers the dynamic compression ratio.

[KTS]

Is it actually possible to accurately calculate dynamic compression using cam manufacturers valve event data? I was under the impression that the valve opening and closing numbers are not actually the point that the valve contacts or lifts off the seat. I was under the impression that valve event data figures were given at something like 50 thou valve lift, and this figure varies from cam grider to cam grinder, and that most grinders don't disclose how their data is calculated, making cam comparison very difficult.
I ended up giving up on calculating dynamic compression in this manner, partly because I'm not clever enough to do the calculations and partly because I don't know the valve event data for my cam.
I ended up physically measuring piston position when the valve contacted the seat during dummy build to calculate the dynamic compression. But as previously mentioned, advancing or retarding the cam, or altering valve clearances raises or lowers the dynamic compression ratio.

 

i've no idea, but i'm coming to the conclusion that i'm not clever enough to do the calculations either !

[Spider]

Is it actually possible to accurately calculate dynamic compression using cam manufacturers valve event data? I was under the impression that the valve opening and closing numbers are not actually the point that the valve contacts or lifts off the seat. I was under the impression that valve event data figures were given at something like 50 thou valve lift, and this figure varies from cam grider to cam grinder, and that most grinders don't disclose how their data is calculated, making cam comparison very difficult.
I ended up giving up on calculating dynamic compression in this manner, partly because I'm not clever enough to do the calculations and partly because I don't know the valve event data for my cam.
I ended up physically measuring piston position when the valve contacted the seat during dummy build to calculate the dynamic compression. But as previously mentioned, advancing or retarding the cam, or altering valve clearances raises or lowers the dynamic compression ratio.

 

What you have done here is what I do as a check on assembly.

I haven't looked in recent times, but when I last did look at most of the bigger grinders in the UK, the published closing angles were at a checking clearance of 0.016" or 0.019". Granted, this does introduce some small errors on paper, but the flow at these figures is so small that compression within the cylinder is occurring. When I look at the quoted figures to see if they make 'sense', I can see if they are at a checking clearance or at 0.050".

 

 

can i check one last number; and that's effective stroke at 90 degrees, for which i get 1.82709"

 

I get 1.8271", so yes, I think your figures here are valid.

[Earwax]

Hi KTS - totally off track from your question, but possibly worth a thought before you rebuild.  ( forum rule 101, don't answer the question being asked }}}} )

 

 

 

You may have all these sorted to, but ....

 

 

 

What contributed to the cam followers failure?

 

Is a pushrod bent?

 

Did the lifters have proper ground radii

 

Were you running hi lift rockers. Was the geometry correct.... was a pushrod shaft just touching the outer wall at full lift?

 

Not relevant to the MD266 (that i know of) -  but if you can get hold of a Cam Doctor report for a cam - you will see quite a few quick rise camshafts make it hard on the follower ( or even recommend wider followers)

 

I have no direct experience but have heard of good reports of the AC Dodd cams, and I am sure he could assist with suggested matching CR, valve clearances, running in procedure, recommendation for followers etc 

 

I can say I highly recommend APTFast hard followers as not being made of cheese and coming to you with correct radii

 

Good luck on your build - 

[Spider]

I can say I highly recommend APTFast hard followers as not being made of cheese and coming to you with correct radii

 

Theirs and Isky's are the only followers I'll fit these days.
 

[timmy850]

There are a few helpful dynamic compression calculators on google

Unfortunately none are helping me get a decent result for my 1100 engine at 11.5 compression ratio!