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Valve Overlap Rb26 Tomei 260 Pon Cam

jet.26r

By jet.26r,
in Engines & Forced Induction

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jet.26r Explorer

jet.26r

Posted
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Hey guys just wondering how well you know your 260pon cams rb26.... To start off all shims are adjusted correctly.

OK well my cams have an aggressive over lap causing a serve low compression reading for example... the piston travels down, inlet valve opens, piston reaches bottom of cylinder and begins to come up and the inlet is still open.

it causes a low comp reading of about 100 and i drop in stock cams and it goes all the way to a strong 150. can anyone shed some light on their overlap?

Gerg_R31 Mentor

Gerg_R31

Posted
Posted

All cams will cause loss of compression with valves being open during the compression stroke

I.e the cams i got are 252 & 262 @ 0.050" at 10.75:1 static compression ratio dynamic compression ratio is roughly around 8.5:1

Valve over Lap is when the exhaust valve is closing and the intake is opening at the end of the exhaust stroke/ beginning of the intake stroke

Adjusting cam timing via dialing in which should be done with all new cams or on a dyno / series of road tests etc and this will change the valve overlap and can add or take away a percentage of the compression stroke

jet.26r Explorer

jet.26r

Posted
  • Author
Posted

just had a quick look on the just jap website and they say the pon cams are 260.... yeh i have the adjustable gears does anyone know how many degrease is safe to turn before valves can actually hit piston? and should i turn them clockwise or anti do i turn them bothe in the same direction...?

The Mafia Grand Master

The Mafia

Posted
Posted

I had 256 cams and I know people running bigger and they don't seem to have this issue. These cams are setup to be in the optimal position. You shouldn't need to f**k around with cam gears straight off the bat.

I'm fairly sure with the issues you are having you've got one or both cams timed incorrectly, ie. cam pulley is out a tooth.

Check it properly. No offense but you don't seem to have a clue what you are doing so it might pay to have someone that does to check it.

You definitely shouldn't be losing that much compression on such mild cams.

djr81 Grand Master

djr81

Posted
Posted (edited)

Hey guys just wondering how well you know your 260pon cams rb26.... To start off all shims are adjusted correctly.

OK well my cams have an aggressive over lap causing a serve low compression reading for example... the piston travels down, inlet valve opens, piston reaches bottom of cylinder and begins to come up and the inlet is still open.

it causes a low comp reading of about 100 and i drop in stock cams and it goes all the way to a strong 150. can anyone shed some light on their overlap?

Numbers shown are open, closed and cam centreline.

ATDC = after top.

BTDC = before top.

BBDC = before bottom.

ABDC = After bottom.

ACDC = Shite band much loved by bogans.

Stock cams

Intake 240 degrees 7 BTDC 53 ABDC 113 degrees

Exhaust 236 degrees 63 BBDC 7 BTDC 118 degrees

Tomei Poncam A

Intake 260 degrees 20 BTDC 60 ABDC 110 degrees

Exhaust 252 degrees 61 BBDC 11 ATDC 115 degrees

Tomei Poncam B

Intake 260 degrees 20 BTDC 60 ABDC 110 degrees

Exhaust 260 degrees 65 BBDC 15 ATDC 115 degrees

You can work out the overlap from there - basically the stockers dont overlap and the B's have 35 degrees of it.

Having had them installed then removed I can confirm the Poncam B's take a massive hit on cyl;inder filling at idle.

Edited by djr81
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jet.26r Explorer

jet.26r

Posted
  • Author
Posted

hey thanks for your interest in trying to help, but timing is spot on im tending to lead towards the wrong cams were given to me. might get some other cams as i put standard ones in compression comes back up.

I had 256 cams and I know people running bigger and they don't seem to have this issue. These cams are setup to be in the optimal position. You shouldn't need to f**k around with cam gears straight off the bat.

I'm fairly sure with the issues you are having you've got one or both cams timed incorrectly, ie. cam pulley is out a tooth.

Check it properly. No offense but you don't seem to have a clue what you are doing so it might pay to have someone that does to check it.

You definitely shouldn't be losing that much compression on such mild cams.

Gerg_R31 Mentor

Gerg_R31

Posted
Posted

I have 10.8mm lift cams 10vc dome piston and 0.130" intake and 0.180" exhaust piston to valve clearance. but i also have +1mm valves and 0.095" ground off the head and the pistons are sitting above the deck 0.005". Minimum piston to valve clearance is 0.080"

And ac/dc were are good band in the day

jet.26r Explorer

jet.26r

Posted
  • Author
Posted

Thanks mate most useful bit of information good to see some one who really knows what their talking about thumbsup.gif

Numbers shown are open, closed and cam centreline.

ATDC = after top.

BTDC = before top.

BBDC = before bottom.

ABDC = After bottom.

ACDC = Shite band much loved by bogans.

Stock cams

Intake 240 degrees 7 BTDC 53 ABDC 113 degrees

Exhaust 236 degrees 63 BBDC 7 BTDC 118 degrees

Tomei Poncam A

Intake 260 degrees 20 BTDC 60 ABDC 110 degrees

Exhaust 252 degrees 61 BBDC 11 ATDC 115 degrees

Tomei Poncam B

Intake 260 degrees 20 BTDC 60 ABDC 110 degrees

Exhaust 260 degrees 65 BBDC 15 ATDC 115 degrees

You can work out the overlap from there - basically the stockers dont overlap and the B's have 35 degrees of it.

Having had them installed then removed I can confirm the Poncam B's take a massive hit on cyl;inder filling at idle.

jet.26r Explorer

jet.26r

Posted
  • Author
Posted

i better get out the clay and measure my valve to piston clearance in the cylinder whistling.gif hehe.

I have 10.8mm lift cams 10vc dome piston and 0.130" intake and 0.180" exhaust piston to valve clearance. but i also have +1mm valves and 0.095" ground off the head and the pistons are sitting above the deck 0.005". Minimum piston to valve clearance is 0.080"

And ac/dc were are good band in the day

discopotato03 Grand Master

discopotato03

Posted
Posted

I think the thing to realise is that opening and closing , valve timing , is related to how the cams are timed when fitted . You can have a pair of std cams set to have a lot of overlap or long cams with SFA overlap .

Its how the cams are set in relation to each other and where the pistons are in the cycle .

If someone with the same cams as you have can tell you where to set their timing and you can degree the crank with the cams their should not be any dramas , always assuming your engine is in good nick .

A .

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