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My official L31DETT RHD thread.


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Ok, so look here for updates every couple days As Ill just keep adding to this thread for my updates of my progress on the L31DETT RHD car. I got the intercooler installed, as well as the alluminum r

There is ALOT of cost in time that has gone into this project. Over 500 Man hours Between my father and I for sure. We have been doing LOTS of calculations, trial and error, quality controll, etc.

I have absolutely no evidence of this, but have heard that top notch RB26 guys are getting close to 390CFM out of their heads. Not net-boy-racer bullshit to the best of my knowledge.

 

according to those numbers, you're outflowing even a ported RB26 head (and matching it's exhaust flow)

that's very impressive

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I have absolutely no evidence of this, but have heard that top notch RB26 guys are getting close to 390CFM out of their heads. Not net-boy-racer bullshit to the best of my knowledge.

 

 

 

It is possible, but thats one of those deals I dont belive it till I see it, ya know. Sure with big valves, TOTAL port reconstructin, etc.

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Ok, so here is some pictures of the re-ground cams. One thing I guess I dont understand/like is how peaked the new ground cams are. Sure we increased the duration about 12-15 degrees at .050" and the lift about a 1/16th of an inch, BUT at maximum lift and a few degrees of rotation on both sides of the max lift point, the lift starts to fall off quickly. GENERALLY this is a good torque grind, but for all out HP, I dont see the valve getting much flow at max lift. Does anyone have any insight on this, as this is the first DOHC cam griding I have done. Here are some pics.

 

 

REGROUND:

 

DSCN1374-600x450.jpg

 

 

STOCK:

 

DSCN1376-600x450.jpg

 

 

REGROUND:

 

DSCN1375-600x450.jpg

 

 

STOCK:

 

DSCN1377-600x450.jpg

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Would that be considered the duration? Seems like less duration now...?

 

I do know the looks of a cam grind can be deceiving, but that looks like less duration to me, too. Granted the base circle got smaller, so maybe the radius at the tip just got proportionately smaller. But offhand that sure looks "pointy" to me, like the radius at the tip got "more smaller" than the base circle did.

 

Wasn't your stock lift pretty low, like in the .300"s? seems like it would have been nice to get your lift up into the .500"s with the flow characteristics your showed.

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That looks like the ramp on that cam is a lot tighter, and would contribute to excessive valve-train wear. It looks like it would also make the valve open at a much steeper point than a nice rounded lobe that would allow it to open gradually.

 

But I believe a grind like this would allow extra pressure to build up at the valve head before full lift, and with it trying to squeeze passed the narrower opening it would, as you had mentioned, be a good torque grind.

 

But for CFM, I can see the highest point of the lobe being the only thing good about it. I'd actually trade off more radius for more ramp.

 

Save the valve train campaign... :mrgreen:

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Yea, BUT I know that the Cams stock comapred to a L series cam, has a much different profile, with about the same duration, It has to do with the direct acting cam, as compared to the ration lifter cams like L series. I will try them and see what and how it performs, its pretty simple to change the cams, it it performs like crap, then They will come out immidiatly.

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You guys are comparing and contrasting a cam grind that is direct acting to one that is run through a ratio-rocer setup.

See if you can find some of the grind profiles for VW Cams (or possibly Chevy Cams) where they are ground for 1.7 Ratio Rockers as compared to stock 1.25 or 1.5's.

 

The ramp angles on ratio rocker cams are MUCH less agressive as the ratio rocker will increase valve acceleration by a factor. The grind on a cam with 310 duration and 620 lift for a 1.7 rocker will look MUCH LESS AGRESSIVE than one that is ground to the same specification if using 1.25 ratio rocker arms.

 

When the cam is direct acting on a bucket, and the bucket on the valve the ram can 'look' agressive as hell, but in reality is very slow to open and close because you are not using a rocker to intensify both lift and duration.

 

To try to explain this, think of an L-Cam with the same duration and lift. The nose on a direct acting cam would be 1.5X NARROWER and the ramp rates less steep by roughly the same ammount to get the same action as you would have on the L-Cam Grind.

 

I have seen L-Cams with lobes that look like your finger bent over, big flat nose. Comparably specifications for a Toyota 18RG-R engine with similar .050" duration (around 304-310) looked positively 'peaked' by comparison.

 

I hope what I'm saying makes sense. I have seen the different grinds on VW's and some Chevy cams, and when they are sitting right in front of you it all makes sense.

 

One of the things in the VW world you never want to do is put the big rockers like 1.5 or 1.7 onto a HOT cam that was ground for 1.24 rockers. The acceleration on the valve and accompanying stresses will bend pushrods, make the valves float way early, etc.

 

When you are direct acting, the width of the bucket also comes into play, as how wide a swath the thing is rubbing will increase with a longer duration (maybe this makes more sense, explaining it this way)---when your stock cam is wiping the bucket, you will have a wipe pattern say, .250" wide. THIS is your actual 'at full lift duration'. On the peaked cam you will have, you will find that your wipe pattern likely will be considerably WIDER, meaning more lift because of the difference in base circle, but also more duration because of the steeper angles on the sides of the cam. Look at the arc the 'peak' will have to travel as it wipes the head of your bucket, and you will see the narrower peak and steeper ramps actually contact the bucket longer in the 'full lift' position that it does stock with that 'rounded nose'.

 

Additionally, when you indicate off the bucket with a dial gauge, you will be able to see this phenomenon directly, and it should make a lot more sense than my palaver here.

 

From my experience with TRD cams for the 18RG and 2TG's of the 70's with 12mm lift and 310 duration (at 1mm lift) your cam looks about right.

 

Hope this makes sense Bryan (all).

 

Let us know how it indicates with a degree wheel on it, that will be your final test of wether someone screwed it up.

 

Direct-Acting Cams are a different animal than Rocker-Actuated Systems, don't apply the looks of one to the other, it will get you in trouble.

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You guys are comparing and contrasting a cam grind that is direct acting to one that is run through a ratio-rocer setup.

See if you can find some of the grind profiles for VW Cams (or possibly Chevy Cams) where they are ground for 1.7 Ratio Rockers as compared to stock 1.25 or 1.5's.

 

Yep - that was why I mentioned that looks can be deceiving - without knowledge of the rest of the actuation scheme it's hard to tell what "right" should look like. As you mentioned, even if you do know the actuation scheme it's hard to tell due to the way the cam wipes across the bucket, etc. It's a much better idea to use a dial indicator than to try to eyeball things.

 

The L-series lobe shapes that we are used to seeing are also distorted by the fact that the lobe contacts the rocker at different points while it wipes across the pad (and at different points during the opening vs closing ramps), thereby changing the effective rocker ratio as it wipes. A direct acting cam lobe should see less (or none) of this effect.

 

I was trying to limit my comments based on "before vs after" rather than what I'm used to seeing, but now I kinda regret even doing that. :mrgreen:

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