Performance Cams

[lamchop77]

Thanks Hiro

Bearing what you have said in mind i might upgrade to a 254 Kent cam for the moment then (cheap option!!!!!). The 244 Kent cam was giving around 180 bhp with the same head and std pistons. Now that i will have a larger capacity/comp ratio i can probably justify moving up a cam. The 2.2 with the 244 fitted had loads of low down grunt and revved clean to 7000, so the new 2.3 with a lighter flywheel, better rod ratio should be pretty safe to go to 7500.

Out of interest, at what revs have you guys run into problems with the rocker arms? The 2.5 will be built to rev to 8000-8500 so i didn't want to risk using the std rockers on that, but i've got to wait for Harri's set up to be completed and tested before that can continue.

I've now got a contact for getting shot peening done and i'm sure that somewhere on here it was recommended to improve the strength of the std rockers. if they're done, what sort of rev limit would i have then?

Thanks again

Chris

[opelnut10]

Thanks Hiro

Bearing what you have said in mind i might upgrade to a 254 Kent cam for the moment then (cheap option!!!!!). The 244 Kent cam was giving around 180 bhp with the same head and std pistons. Now that i will have a larger capacity/comp ratio i can probably justify moving up a cam. The 2.2 with the 244 fitted had loads of low down grunt and revved clean to 7000, so the new 2.3 with a lighter flywheel, better rod ratio should be pretty safe to go to 7500.

Out of interest, at what revs have you guys run into problems with the rocker arms? The 2.5 will be built to rev to 8000-8500 so i didn't want to risk using the std rockers on that, but i've got to wait for Harri's set up to be completed and tested before that can continue.

I've now got a contact for getting shot peening done and i'm sure that somewhere on here it was recommended to improve the strength of the std rockers. if there done what sort of rev limit would i have then?

Thanks again

Chris

Turning a stock rocker above 7500 is asking for trouble, I lightened a set by milling down the side rails and shot peened them. I only turned my motor 7000-7200 but the pedal never left the floor until after the win light. The biggest draw back with stock rockers is the ratio, 1:44, and lifter length comes into play with a high lift cam. I never had the availability of roller rockers (they were still in the design stage, when I quit running the car) but ratios at 1:5 are what most cam grinders base their numbers on.

[Hiro]

Out of interest, at what revs have you guys run into problems with the rocker arms? The 2.5 will be built to rev to 8000-8500 so i didn't want to risk using the std rockers on that, but i've got to wait for Harri's set up to be completed and tested before that can continue.

no problem up to 7000rpm,
above this limit it much depends upon spring rate/cam lift/rocker wear.
the funny thing is I've seen more broken rockers on overreved stock old CIH than on full race ones,
might as well be that valve float is the key in this case?
I think our German friends are now working on using very light valve trains & relatively soft springs,
this should put less stress on the steel rockers for sure.
Hiro

[jeff denton]

Wasn't there going to be new rocker arm available soon? I wonder where they are with that. I'm not getting excited about building any Opel engine until something other than stock rockers are available.
However, I too can say I had no problems with stock rockers, .425 lift and stock springs at 7100 RPM pedal to the metal.

[RallyBob]

Out of interest, at what revs have you guys run into problems with the rocker arms?

Everyone here will tell you something different...not that they're full of BS, but because there are so many variables that come into effect. That said, I prefer to keep stock rocker arms under 7500 rpms. BUT, I will say that I ran a racing engine up to 8800 rpms reliably for about two years with a set of stock rockers (polished and shotpeened however). And I've had stock rockers break at 6500 rpms as well. To me, the biggest factor seems to be spring rates....open and closed pressures. If the closed pressure (on the seat) is too high, they break. If the open pressure is too high OR too low, they break. Too much open pressure simply overwhelms them, especially with aggressive cam profiles. Too little pressure means that the cam lobe 'throws' the valve open, then it comes crashing back down to earth, shattering the rocker arms. Keep in mind they are extremely hardened steel, with very little ductility. Think almost 'ceramic' and you get the idea. It's strong as heck until you shock it.

My thoughts anyway!
Bob

[wrench459]

Too little pressure means that the cam lobe 'throws' the valve open, then it comes crashing back down to earth, shattering the rocker arms. It's strong as heck until you shock it.
My thoughts anyway!
Bob

I don't have a big cam.
Is the valve bouncing off the seat with the low spring pressure and slamming back into the rocker there by breaking? Not that I would even think about using low spring pressures.

[hrcollinsjr]

We ran in the 7500rpm range more than I wanted to and only had trouble with rockers with one cam. The fulcrum balls would get a groove in them and shortly after that the rocker or the stud would snap. We switched to an Isky OR-99 and fixed the problem. I understand someone else wound up with the cam and had similar experiences. It was a little too much cam anyway. Just when it started pulling hard we were past the midway point on the straightaways and were still accelerating when the driver had to back out of it.

The rockers were shotpeened but not polished.

Harold

[RallyBob]

We ran in the 7500rpm range more than I wanted to and only had trouble with rockers with one cam. The fulcrum balls would get a groove in them and shortly after that the rocker or the stud would snap. We switched to an Isky OR-99 and fixed the problem. I understand someone else wound up with the cam and had similar experiences. It was a little too much cam anyway. Just when it started pulling hard we were past the midway point on the straightaways and were still accelerating when the driver had to back out of it.

The rockers were shotpeened but not polished.

Harold

Harold, it's *possible* the rockers were close to binding on the center slots. Depending on the base circle of the cam, the valve heights, the valve lengths, etc...the rocker arms may bind on the rocker studs. Then all heck breaks loose!

On the aforementioned 8800 rpm engine, I did enlarge the rocker arm slots to cope with the high valve lifts and smaller base circle. .545" intake and .528" exhaust IIRC.

[P.J. Romano]

Another question Bob, if you do not mind:

Based on the information I have, standard 1.9 cam has 0.257" (6.52 mm) lift. I am building 2.0 street engine with Isky "Combination" cam which has 0.430" (10.92 mm) lift. That is already +4.40 mm of extra lift. In addition, my 2.0 cylinder head is angle milled approx. 2 mm on the spark plug side (we discussed that in one of threads few months ago).

Should I be worried about possible interference between valves and pistons?

[wrench459]

Another question Bob, if you do not mind:

Based on the information I have, standard 1.9 cam has 0.257" (6.52 mm) lift. I am building 2.0 street engine with Isky "Combination" cam which has 0.430" (10.92 mm) lift. That is already +4.40 mm of extra lift. In addition, my 2.0 cylinder head is angle milled approx. 2 mm on the spark plug side (we discussed that in one of threads few months ago).

Should I be worried about possible interference between valves and pistons?

I would be worried!

[P.J. Romano]

I would be worried!

Need more facts. Let me explain: I understand that the "Combination" cam can be used on high compression (flat pistons) engine without deepening the valve pockets on the pistons. Yes, angle milling brought valves about 1-1.5 mm closer to the pistons.

How much should I increase the depth of the valve pockets? 2 mm? Any experience or suggestion here?

[wrench459]

Anytime whenever you build an non stock engine double check all clearances.
Clay the pistons and check..
My rule of thumb for piston to valve is .100in on the intake .080in on exhaust.

[GTJIM]

When ever you depart from 'standard' known components and machining everything has to be carefully checked ... and re-checked.

Angle milling of the head not only moves the valves closer to the piston top during overlap it also changes the direction in which they open which could move the piston/valve closest approach position away from the existing valve notches.

You simply have to go through the procedure for checking valve/piston clearance (or lack of it!).

'Guessing' what the clearance will be with custom engine builds (or hoping that someone has done exactly the same before) can become a recipe for disaster. The individual engine combination MUST be checked.

For a proceedure see: http://www.opelgt.com/forums/179994-post8.html

BTW: Your figure for the standard cam is lift at the lobe - not at the valve. At the valve it is closer to 0.370/0.390" lift so the .430" lift of the combination cam is not that much greater- it is the angle milling that is the wild card in the build.

[P.J. Romano]

My rule of thumb for piston to valve is .100in on the intake .080in on exhaust.

Dan, would that be with the head gasket or without?

Angle milling of the head not only moves the valves closer to the piston top during overlap it also changes the direction in which they open which could move the piston/valve closest approach position away from the existing valve notches.

Good point Jim, never thought about that!

[wrench459]

Dan, would that be with the head gasket or without?

With a gasket.

[GTJIM]

A used gasket is best as you don't ruin a new one - and the old one is already compressed to the thickness it will be when the motor is eventually buttoned up.

Count on having the head on and off at least twice and probably a few times more - check the procedure I linked to.

Also in the next post is mentioned using light springs on the valves - not the real valve spring so it makes it easier to turn the motor over and also push down on the valve to check for clearance. With light valves and knowing where in the revolution the closes point is you can use a dial gauge on the spring retainer or rocker above the valve tip to measure clearance directly (without play dough) for final checking.

[wrench459]

A used gasket is best as you don't ruin a new one - and the old one is already compressed to the thickness it will be when the motor is eventually buttoned up.

Count on having the head on and off at least twice and probably a few times more - check the procedure I linked to.

You're no fun Jimbo
I used a new one without crushing the fire ring all they way down.
Math is a little tricky but it works.

[GTJIM]

Your no fun Jimbo

... Abidonian Scots descent ... might be more like it!

[P.J. Romano]

A simple visual test (see pics) shows that the "zero point" (valves just touching pistons) should be with the valve pockets approx 1 mm deeper. With the neccessary 0.1" (2.5 mm) required tolerance this would mean that I will have to cut valve pockets another 3.5 mm (0.1378"). That would not leave much "meat" on the top of pistons. Any thoughts?

[RallyBob]

Usually the point of greatest interference is about 15-20° BTDC and 15-20° ATDC. As the intake valves are opening the pistons are going down the bore, as the exhaust valves are closing the pistons are coming up the bore. When both valves are open the same amount the piston is usually nowhere nearby.
So with any new combination I'm running, I always fit test springs and check the clearances at 10°-15°-20°-25° ATDC on the exhaust side and 25°-20°-15°-10° BTDC on the intake side, just to be sure.

That said, I once did a bit of diagnosis on an engine when I had C & R...the customer had bought some flat-top pistons from me, and a cam (CR-149), and a big valve head (1.72"/1.50" valves). The engine had a slight noise at idle that increased with rpms and as the engine heated up. It ran fine, just made a little knocking noise. Turns out the pistons were installed backwards...the valve notches were on the wrong side! The noise was the intake valves *just* touching the piston tops. Left a slight ding mark, but did not bend any valves, nor damage the pistons. The head was milled .030" (.75 mm), and the cam had .425" lift and 212° @ .050". If the head had stock valves, they would not have hit at all!

Regarding the piston top thickness. If they are OEM Mahle pistons, AE pistons, or KolbenSchmidt pistons, the piston tops are .500" thick! I've actually notched those brands of pistons .375" deep before with no issues. I can't comment on other brands of pistons however. I like to maintain a minimum of .080" (2 mm) intake side and .100" (2.5 mm) exhaust side clearance...the exhaust valves grow/expand more with heat so they need a bit more room.

HTH,
Bob