Some DOHC measurements, valves, springs, etc

[Corpse]

GM stated the 2.8 series rods to be balanced up to 7000 RPMs. The 3.1 got a different rod so, it could be balanced higher and so the 3.4 DOHC could aswell. But, GM is not making their engines to run at 7000+RPMs stock so, cation should be taken.

Get your rotating assembly balanced and get everything blueprinted. Have the block bored with the boring plates and have the latest honing technology used as most cylinders will not be consistant when using conventional honing methods.

You will pay a lot of money but, your bottom end will be setup to run smoothly at high RPMs. Then you must just setup the rest of the engine and you will have one beast of an engine.

Gm, and other car manufacturers, do not always machine their engines to the highest of tolerances. This is just a flaw of mass-production and high cost of manufacturing. Unfortunately, due to variances between casting to casting, you really cannot safely use un-machine castings for performance usage. Even excess materials left on the casting (flash) and such will cause material fatigue and could cause a failure.

Anyhow, realistically, i think you are crazy if you trust 4 cams spinning at 8000 RPMs, being driven by a giant rubber-band that must be replaced every couple of oil changes. Now that's a little unsafe. Also, there are certain situations you may put your motor into, take shifting for example, where shifting at 8000RPMs might make the rubber band (timing belt) go bye-bye.

Also, about the valve float. We could refer to both conditions as valve float but, nonetheless they are both a non-ideal situation where the valve get's away from the valvetrain.

The fact of the matter is though. Not only does valve float allow the possibility of a piston/valve colliding, or multiple valves colliding, you are also doing extreme wear to the valvetrain and making it undergo abnormal working conditions.

You must remember, first, your lifter is not designed to be used for extreme RPM's more than likely. Which means, at 8000 RPMs, the required amount of lubrication to inhibit wear may not be present. Then, you have serious valve float. What happens? If the valve floats past the cam's lift and also floats back to the seated position, as opposed to riding the cam, the cam may no longer have that lobe out at all. Which means, that .550 lift is how far the valve spring is goign to come back. Slam back. 400LBs open pressure springs may be underated in some crazy situation and cause valve float, right? What happens when that spring with 400LBs of fource is left with nothing to hold it? It slams, REALLY hard, either against the valve seat or, if the cam has come back around in position, which would be extreme, the cam is going to get slammed on the lobe. Cams don't usually like to be slammed by the valves/springs. As durable as a cam seems, if it repeatedly gets slammed by lifters at high pressures you may split your cam.

Actually, a buddy of mine with a Ford 351 just did that a few months back. Really bad valve float and then... We pulled the cam out in pieces.

Also, back to the rods. I know from experience, GM's rods are not always what they seem. Many of them have offset small ends and look at the balancing weights, some of them are really messed up. The 660's seem a little better off than SBC Rods though (Small and large journal, I've compared all)

Oh yeah, anyone interested in a set of small journal SBC rods? they were from a 60's 327 Vette. Can be modded for the 660's.

-Dave

[jmgtp]

Also, there are certain situations you may put your motor into, take shifting for example, where shifting at 8000RPMs might make the rubber band (timing belt) go bye-bye.

I've thought about that too. Even shifting at 7k is harsh if the clutch grabs hard and the revs drop nearly instantaneously 2k rpm. Then again, I'd much rather have a belt in there than a chain. At least the belt has the ability to 'give' some where the chain doesnt. Anyways, I think belts are more common for high rpm ohc engines.

[Corpse]

Yes, the belt is the way to go. A chain would require lots of lube and would weigh a lot. Also, a loose chain would slop around a lot. It's just that the belt must be designed for the intended usage along with the whole setup.

Look at superchargers. Many all out drag machines will lose the blower belt from time to time. That is just how it works. But, at least when that happens you don't have the piston hit valves.

It is all just a matter of situation. Just think of it this way, if the engine is at 8 grand and you shift, if it goes to 4 grand, the crank is trying to slow down the cams which were spinning at 4K RPMs and it is trying to bring them to half of that. How well do you think the belt is setup to "brake" the cam setup?

-Dave

[jmgtp]

You also have to consider that the cams are slowing down on their own as well: friction, valve spring loads, etc -- although no where near as quickly as the belt would be so youre right the belt would be 'braking' the cams, but I dont think it would be enough to shear a tooth. In perspective, 8k is not that much more than 7k. My belt tensioner broke at 40k miles at about 3k rpm and I regularly rev the motor to 7k. So, what I'm trying to get at is that the belt the dohc uses should be ok for 8k rpm, although a shorter belt change interval would be wise.

[Corpse]

Well, if you were doing some kind of all out drag engine, I would plan on changing the belt every few runs if you are extremely pushing it.

Also, you must remember that the belt's effectiveness has other variables aside from strength of the rubber. You have to remember the fact that the more you spin the belt and the harder you push it the more it will stretch. Also, if the belt is loose or under too much pressure, chances are that it will stretch and offset the cam timing.


You also must account for the fact that, the sprocket that drives the belt is throwing the belt off of it by centrifugal force, which helps the belt to skip a tooth. Although, the tensioner helps the whole situation. But, if the belt is too worn or something it could be loose. Also, at higher rpm's and such, you must wonder how well the tensioner is working. Enough stress could cause the tensioner to loosen up the belt excessively.

The fact of the matter is just that a belt isn't the strongest method or most relieable. But, it does have weight benefits. Realistically, the best way to go with an engine would be direct drive gears like you can use on OHV desings, timing gears.

I wonder realistically how well the 3.4's setup is, with the chain driven belt setup. If the chain was loose, and the belt was wearing or stretching the cam timing could be off by a bit.

Unfotunately for OHC/DOHC/T-DOHC setups that would be a strange one, although possible. But, nonetheless you take what you can get. I would much prefer the TDOHC 3.4 to an OHV 660.

-Dave

[pocket-rocket]

Another thing to think of is if the crank is at 8k, the cams are at 4k. I do agree where running the engine at 8k can be a bit extreme when the engine slows down between shifts very much. After all, look at the weight of each cam. But then again, since it all is running at half the speed of the engine, that should make it a little easier on the belt. BTW, I try to change mine at 20-30k right now. Mostly daily driving, some runs to 7k, and only a few weekends at the strip a season. I know I am not the easiest person on my cars here, but then again, I also know I am not the worst on them either.

[SappySE107]

Its not a rubber band, its some pretty strong materials. I wouldn't worry about 8000 as long as the bottom end can handle it. The valve springs probably wont like it stock, but either lighter valvetrain compenents or stiffer springs will work.

[Juglenaut]

What is the best Lifter to choose, I am looking to replace all my lifters, and springs when I get around to doing a top-engine on my 3.4L

[SappySE107]

Im looking into lifters now. I found some that have a coating on the face, and weigh 18% less than stock. Found some others that are even lighter but they cost a lot more. looking at 12.50 each vs 25 each (unless I can get a group going to buy the ones that are about 37% lighter than stock). Otherwise, quad 4 lifters work, but aren't cheap either and are only 2 grams lighter than our stockers.

[4spdz34]

where did u find those lifters? maybe i can get u a discount, i got alot of hookups here in detroit. if someone i know sells them i can get them at cost

[SappySE107]

Im selling them through my company.

[4spdz34]

ok thats good too hear.

[Juglenaut]

SappySE107 wrote:

Im looking into lifters now. I found some that have a coating on the face, and weigh 18% less than stock. Found some others that are even lighter but they cost a lot more. looking at 12.50 each vs 25 each (unless I can get a group going to buy the ones that are about 37% lighter than stock). Otherwise, quad 4 lifters work, but aren't cheap either and are only 2 grams lighter than our stockers.

Count Me In on that group deal !!!!!!!!!!!!!!!!!

Oh and BTW Awesome work on the valves!!!!!!!!!!!!!!!

I will take 48 lifters no joke!!!!

[SappySE107]

you do realize 48 lifters is gonna cost you about 1100 right, even with a bulk buy of 100. It would take 500 to get the cost per lifter into a sane range for most people. If I can get 100, I would sell them at 20 each. 50 is 23 each. 500 would be 16 each.

[Juglenaut]

I got the green well some mind you. NOT the other kind of green!

They don't have to be Ultra light, but might be easier on the poket to get mid range. 1100?? is that for the lightest ones??

Can you or do you know of any titanium keepers??

I am sorry but I am in the grind of finding the lightest valve train equip I can find.

Still ported heads are at the bottom.

I needs to keep up with those GP Comps..and I rather pay the doe to keep mine than buy a new car, plus I love the DOHC always have.

1100 is kinda pricy, but GM cost are 40 or abouts each crazy expensive for valves....Come on people buy a lifter or 2 for ME to get on deal of the century!!!!!!! I needs to do some tinkering narffff...My new desk job is depriving me!!!!!!

I will hold 48, but unless others come forth to buy some I might just get 24 for my GP My Z will just have to wait.