[mgarblik]

Nice write-up and pictures, John. I appreciate the way you were able to get in print some of the difficulties in going fast. Many who haven't done it think it's simply plug and play. (plug in more money and play faster).

[Jack Gifford]

Originally Posted by Half-Inch Stud

... I remember the GT2 being appropriate for "timing chain" use...

Good to see you researching the subject. After designing the timing belt drive for DOHC conversion of my hemi-four, I'm well aware of the frustration involved. Back-and-forth with a half-dozen belt/sprocket suppliers and with another half-dozen racers using cam belt drives, I was still left with some missing answers. One thing is for sure- none of the manufacturers/suppliers will provide any assurance of suitability for racing applications.

I'm hoping Langer will share more details on his belt incident- as you said, we need to know how this happens.

If you look at Gates info of the last couple of years, you'll see that GT-3 is now recommended for severe applications. It's a very small change to tooth profile compared with GT-2, but improves belt/tooth contact under high load.

Bottom line for me- I'll be nervous as hell when my engine goes on the dyno Wednesday......

[LiL Jack]

Originally Posted by Jack Gifford

Good to see you researching the subject. After designing the timing belt drive for DOHC conversion of my hemi-four, I'm well aware of the frustration involved. Back-and-forth with a half-dozen belt/sprocket suppliers and with another half-dozen racers using cam belt drives, I was still left with some missing answers. One thing is for sure- none of the manufacturers/suppliers will provide any assurance of suitability for racing applications.

I'm hoping Langer will share more details on his belt incident- as you said, we need to know how this happens.

If you look at Gates info of the last couple of years, you'll see that GT-3 is now recommended for severe applications. It's a very small change to tooth profile compared with GT-2, but improves belt/tooth contact under high load.

Bottom line for me- I'll be nervous as hell when my engine goes on the dyno Wednesday......

It said it right in the article, the valves hit the pistons which made the belt jump.

[Half-Inch Stud]

@Jack, thanks. I'm flat-footed on the GT3, but the tooth design difference for more severe duty is a tell-tale of a potentially better balt.

@ LiL Jack, Left a chicken-egg choice on this reader. Favoring a belt-jump. You have a point to pursue; does a hung-stuck exhaust valve load the cam train;no. but a closed-stuck exhaust valve does. The rocker popping off is a big clue.

[3fastgtos]

Originally Posted by LiL Jack

It said it right in the article, the valves hit the pistons which made the belt jump.

"The exhaust valve was touching the piston because the camshaft belt drive had slipped"

[LiL Jack]

I heard the heads were milled too much
Which caused the destruction

[RAIV-Z]

Let's get the facts from Langer.

[John Langer]

Originally Posted by LiL Jack

I heard the heads were milled too much
Which caused the destruction

Incorrect.

[John Langer]

When you kick off an exhaust pushrod it puts incredible stress on the intake valve since you have all of that pressure inside of the cylinder and it can't be released. This pressure can cause stress on the intake pushrod which causes stress on the lifter and subsequently the cam shaft. This "can" cause the belt to "jump" a few teeth. Now the million dollar question is why it kicked the exhaust pushrod initially.

[Jack Gifford]

That scenario makes sense, John. Thanks for posting. I'm confident you'll find the answers to the jumped pushrod.

I know about the extreme load on an intake when exhaust valve doesn't open. When I was fighting breakage of exhaust rockers in my hemi V8, a broken exhaust would almost always result in that intake rocker also breaking. [rockers were all fragile 392 Chrysler cast steel pieces back then]

[Aaron Quinton]

Are you sure that's right? Better look at in cylinder pressure when the valves are opening and against what pressure.

Congrats on the coverage for sure! PPTTFMF!



Langer;5862676]When you kick off an exhaust pushrod it puts incredible stress on the intake valve since you have all of that pressure inside of the cylinder and it can't be released. This pressure can cause stress on the intake pushrod which causes stress on the lifter and subsequently the cam shaft. This "can" cause the belt to "jump" a few teeth. Now the million dollar question is why it kicked the exhaust pushrod initially.[/QUOTE]

[Stan Weiss]

Originally Posted by Aaron Quinton

Are you sure that's right? Better look at in cylinder pressure when the valves are opening and against what pressure.

Congrats on the coverage for sure! PPTTFMF!

Aaron,

If the exhaust valve never opened after the power cycle wouldn't the intake valve see must higher than normal cylinder pressure than during a normal cycle? Not knowing when John's intake valve is set to open I would not be surprised to see anywhere between 250 and 325 psi at that point.

Stan

[John Langer]

With aluminum rockers you will most likely break the intake if you kick the exhaust. With steel jesels your not going to break the rocker. If the cylinder fires and the exhaust doesn't open, it puts incredible strain on the intake. Even at 1500rpm imagine how many times the valve opens and closes every second.

[mgarblik]

Originally Posted by John Langer

With aluminum rockers you will most likely break the intake if you kick the exhaust. With steel jesels your not going to break the rocker. If the cylinder fires and the exhaust doesn't open, it puts incredible strain on the intake. Even at 1500rpm imagine how many times the valve opens and closes every second.

That has been our experience also. We had aluminum intake/steel exhaust rockers. The exhaust had the issue but the aluminum intake was the one that snapped in two. Now we have all steel rockers and have repaired the exhaust issue. Ready to move on and break other stuff.!

[Scott Roberts]

Originally Posted by John Langer

With aluminum rockers you will most likely break the intake if you kick the exhaust. With steel jesels your not going to break the rocker. If the cylinder fires and the exhaust doesn't open, it puts incredible strain on the intake. Even at 1500rpm imagine how many times the valve opens and closes every second.

About 12...

[Jim Robertson]

John, Sorry for your troubles but many thanks for sharing the story and the diagnosis.

[Aaron Quinton]

Hi Stan,

I don't disagree on the pressure estimate. I would think (haven't looked at the data) that given the constraints on Pontiac head and a max effort NA engine, the intake valve size would take priority (2.35+). Driving the exh valve smaller (1.70ish or less) even with a 300 + cfm exhaust port, driving a 320+ seat to seat exhaust event. I would think the cyl pressure would be high (600 psi plus) at exh opening normally but need to look at it. If so, the load on the exh at opening is likely similar or higher than an intake load with extra cylinder pressure and larger valve area. If we assume 100% mass is trapped due to exh push rod kicking and an estimate on heat rejection would get close on a PV calculation. Brother you are WAY better at this than me so if you rough calc I'm in!

I have no idea what margin is in the belt drive but I'm wondering if it's worth the effort. I'm beginning to think chain drive, LS coils, cam trigger and a crank tone wheel with the Holley Dominator system is the answer? Do you do that or run the belt drive, belt driven distributor, etc? I'm thinking coil on plug.

Thanks for the value added post sir.

Aaron

Originally Posted by Stan Weiss

Aaron,

If the exhaust valve never opened after the power cycle wouldn't the intake valve see must higher than normal cylinder pressure than during a normal cycle? Not knowing when John's intake valve is set to open I would not be surprised to see anywhere between 250 and 325 psi at that point.

Stan

[Aaron Quinton]

I appreciate the feedback and the engine gets the final vote for sure. Good info on the rockers for sure. If you ran it long enough with no exhaust and continually building cylinder pressure cycle after cycle with no exhaust, I could see how it could happen.

Man if it was me I'd look at why the exhaust kicked a 1/2" push rod. I wonder if that isn't the root cause of the problem, mechanical lock up as it kicked the push rod out drove the load to jump timing? That's where my heads is without looking at the parts. But I'm only looking at part of the story. Would a grossly loose lash jump out of the cup causing the valve to hit the piston? I would think the pistons would help tell the story.

Originally Posted by John Langer

With aluminum rockers you will most likely break the intake if you kick the exhaust. With steel jesels your not going to break the rocker. If the cylinder fires and the exhaust doesn't open, it puts incredible strain on the intake. Even at 1500rpm imagine how many times the valve opens and closes every second.

[Stan Weiss]

Originally Posted by Aaron Quinton

Hi Stan,

I don't disagree on the pressure estimate. I would think (haven't looked at the data) that given the constraints on Pontiac head and a max effort NA engine, the intake valve size would take priority (2.35+). Driving the exh valve smaller (1.70ish or less) even with a 300 + cfm exhaust port, driving a 320+ seat to seat exhaust event. I would think the cyl pressure would be high (600 psi plus) at exh opening normally but need to look at it. If so, the load on the exh at opening is likely similar or higher than an intake load with extra cylinder pressure and larger valve area. If we assume 100% mass is trapped due to exh push rod kicking and an estimate on heat rejection would get close on a PV calculation. Brother you are WAY better at this than me so if you rough calc I'm in!

I have no idea what margin is in the belt drive but I'm wondering if it's worth the effort. I'm beginning to think chain drive, LS coils, cam trigger and a crank tone wheel with the Holley Dominator system is the answer? Do you do that or run the belt drive, belt driven distributor, etc? I'm thinking coil on plug.

Thanks for the value added post sir.

Aaron

Aaron,
Using a 572 ci engine, Bore = 4.375" Stroke = 4.75" Rod = 6.75 and CR = 15:1. If we say that peak cylinder press is @ 12.5 ATDC and is 1650 psi. This decay graph is based solely on PV changes no heat added or removed. The green line is done using a K- Exponent of 1. Most of the Paper I have seen say it is around 1.38 so the red line is done using K-Exponent of 1.4. I do not know where John's exhaust valve is set open, but a 320 degree exhaust in @ 110 ecl evo 90 ATDC, @ 118 ecl evo 98 ATDC. The Rod and Crank are 90 Degrees to each other @ 70.62 ATDC.

Stan

[Aaron Quinton]

Dang brother, I need some time to have a look!

Unless that engine is a tall deck I'm not sure how he's keeping piston skirts on the pistons. I would think it would be closer to 4.40" bore but skirts look to be a problem? If pistons every 40 runs is no object then rock on?

Originally Posted by Stan Weiss

Aaron,
Using a 572 ci engine, Bore = 4.375" Stroke = 4.75" Rod = 6.75 and CR = 15:1. If we say that peak cylinder press is @ 12.5 ATDC and is 1650 psi. This decay graph is based solely on PV changes no heat added or removed. The green line is done using a K- Exponent of 1. Most of the Paper I have seen say it is around 1.38 so the red line is done using K-Exponent of 1.4. I do not know where John's exhaust valve is set open, but a 320 degree exhaust in @ 110 ecl evo 90 ATDC, @ 118 ecl evo 98 ATDC. The Rod and Crank are 90 Degrees to each other @ 70.62 ATDC.

Stan