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Discussion Starter · #1 ·
ok, i did a little checking in some valve head and stem diameters and this is what i came up with

mopar intake valve head is 2.02 inches = 51.308 mm
audi intake valve head is 1.56 inches = 40 mm

mopar valve stem is .375 inches = 9.52 mm
audi valve stem is .312 inches = 8 mm

mopar valve length is 5.05 inches = 128.27 mm
audi valve length is 3.84 inches = 98.5 mm

i give these numbers with two things in mind.

# 1 javad said that he recommends staying with the 8mm stem over the 7mm for strength, i don't think in any case that what happened to his head was by any stretch of the imagination "normal". the mopar valve has considerably more weight on the end of the stem for difference in stem diameter between the two. so i really doubt strength there is much of an issue.

# b maybe there is the possibilty of taking the existing mopar valves and just turning them down and making new keeper grooves. this could save material/time/money. plus the valves are fairly cheap in the first place. it would just require minimal machine/lathe time vs. setting up for a brand new valves out of stock material.

lemme know what you guys think :)
 

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Valves are usually not totally formed via machining. I wouldn't just turn one down. Besides, at hopefully $15-25/each new custom valves aren't bad. Calling today to confirm the price.

I don't think that a 7mm stem would be a problem, expect it'd have to have a bit more design time in it (at least for the exhaust, for the intake we could probably use the entire 12v V6 setup, valve, upper retainer, cotters, spring, guide, seal). All 5cyl 20v's use 7mm stems. All VAG 5v heads I think use 6mm stems. Some Hondas use 5.5mm stems. Just some food for thought...
 

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Derek, you're comparing a very low stressed Mopar engine to an incredibly stressed turbo Audi engine, so keep that in mind. What's a Mopar engine, 50hp/l? My I5 was more in the range of 170hp/l. It also is quite common do snap sodium exh. valves in the 10v, I know of 4 other occurences in the last 2-3 years, mostly track driven cars.

Its not an issue with "valve strength" or as you imply with the structural integrity of the valve, its a heat issue. In a turbo application, heat is the valves biggest enemy, the more heat you can remove from it, the longer and "stronger" the valve will be.

Ways to increase a valve's heat capacity include different alloys, wider valve seating area, larger stems, sodium filling the stem, longer valve stems and valve guides, etc.

The questions isn't whether or not a 7mm stem will work or snap off the first time you drive it, the question is "what can you do to maximize the longevity of the engine system you're building for the application you intend it for".

You'll need to make that decision, you can definitely get better flow with a 7mm stem, and I've considered using them, but with the right porting work and maximizing the 8mm valve sizes you can get 160cfm on the intake, a 20v posts about 190. If you go Inconnell then 7mm will still be stronger than steel in 8mm, so its not a 7mm or 8mm issue, its a design issue and you need to take all the factors into consideration.
 

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Discussion Starter · #5 ·
$15-25 ea. is a bit cheaper than i was thinking for custom valves. so i guess i could live with that.

as far as the sodium filled valves go, I had not heard of one snapping off before, in that case then, would a straight stainless valve last longer than a sodium filled? or does the heat handling trade off compromise the stainless in the long run?

as to the mopar power output, i've seen plenty of N/A,strokers,blown,turboed 5.2-6.0+ liters, making anywhere from 300-650+ hp which, for those v8s is quite a lot to handle for their original design and require a lot of beefing up and regular tear down, typically bottom end issues. i have to hand it to audi for putting forged cranks, heavy rods, forged pistons and such an awesome block/head bolt design that will stand up to sooo much abuse and not warp and generally disintegrate over a short period of time after what we put em' through.

everybody who puts in your ideas and knowledge into this pool of info i thank very much, and this forum rules, i haven't seen so much useful info anywhere else. i was think about how there isn't much support in the 5cyl world for performance parts and technology, and guys, we're the center of it, so keep up the good work and i'll keep up my incessant questioning to make sure that we have plenty to ponder :wink:

next thing on the agenda is valve stem seals, what is the best thing going in that area today, i know of teflon and viton, but don't really know too much about either.
 

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While we are on the topic of valves/valve strength, what do you think caused this?

This happened on a substained, high speed run last year...

Timing belt intact, on time and no piston damage.

Engine is a 10vt MC.

Steve
 

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Hey Steve, I've seen this a couple times, one Urq I worked on had a bad miss, one cyl had about 0 compression, when we pulled the head we found a pie shaped wedge missing out of the exh. valve.

I'd say its likely a combo of fatigue and high temps, on a high speed, sustained run - if things go lean and start heating up your EGT's, valve failure can occur. I snapped the head off an MC exh. valve, they're just not as strong as they could be, and with the age and abuse we dish at these things, it surprising they hold up as well as they do.
 
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