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Old 10-10-2011, 01:57 PM   #1
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Question Designing cams for a medium strung N/A VVT

The goal is to make an engine with the highest possible efficiency in the range 5000-7500rpm. Revving higher means adding weight due to the regulations (which I wrote most of).
This is the first engine I get built and I try to gather as much info for the sources I know of.

The engine I'm in the process of building will most probably get
11:1 CR (maybe a bit more due to skimming) using ST pistons, forged rods (local supplier).
+1mm IN valves, +0mm EX valves, MS SUBs, ST Dual springs and Ti retainers

Porting have been made to get the following flow at 10" water, CFM wet air followed by dry air.
Code:
Intake Port Original   Intake 34.0mm
 1.0mm   16.2  16.4     19.7  20.0
 2.0mm   35.7  36.3     41.0  41.7
 3.0mm   51.4  52.2     59.3  60.2
 4.0mm   69.8  71.0     75.3  76.6
 5.0mm   84.8  86.2     91.1  92.6
 6.0mm   99.6 101.3    105.6 107.3
 7.0mm  108.8 110.6    117.1 119.1
 8.0mm  114.9 116.8    126.2 128.2
 9.0mm  118.1 120.1    133.2 135.4
10.0mm  120.7 122.7    138.0 140.3
11.0mm  122.3 124.3    141.7 144.0
12.0mm  121.4 123.4    145.0 147.4
13.0mm  121.5 123.5    145.6 148.0

Exhaust Port Original  Exhaust ported
 1.0mm   12.5  12.7     12.5  12.7
 2.0mm   27.0  27.5     29.3  29.8   
 3.0mm   44.3  45.0     48.0  48.8 
 4.0mm   60.5  61.5     65.1  66.2  
 5.0mm   77.8  79.1     82.5  83.9
 6.0mm   89.8  91.2     92.2  93.8   
 7.0mm   95.9  97.5     98.0  99.7   
 8.0mm   99.1 100.7    102.8 104.5 
 9.0mm  100.9 102.     105.7 107.5 
10.0mm  102.7 104.4    109.9 111.8  
11.0mm  104.1 105.8    113.3 115.2 
12.0mm  105.2 107.0    115.9 117.8 
13.0mm                 118.2 120.2
Intake will be Jenvey with 90mm trumpets but I'll get it running on a squaretop first. Header will be a crappy RB 4-1 until I get the time to do a full exhaust (it's daunting enough to get this running on ITBs with a custom airbox, the Squaretop is a fallback strategy).

The cams will be around "290-300 duration" (about 11mm lift) made from blanks, but the questions I have is where to place the lobe centers to be able to start and run if the VVT suddenly "feels like" going fully retard (fallback solution).
That would mean a balance between some overlap and some reversion (intake closing late into compression stroke.
How much overlap can be tolerated to be able to start and get going (standing starts at about 3000rpm currently), 20 degrees?

I will have to keep advance in check, since the Interference nature of this build will not be able to take full advance.
But if full retard is 20 degrees overlap, the sweep would probably not be more than 20 degrees (40 degrees overlap at max VE) I guess, maybe even just 15 (one tooth).

Am I bonkers to think that VVT will add anything to this build (I'm in too deep to stop now however)?
Should I stop worrying and aim for the stock lobe separation (and let the overlap grow from -5 to whatever it will be)?
How much would tuning the Exhaust cam timing do (compare to set the lobe center right at the grind)?
How tough springs could be sensible for relative mild ramps? Could the single springs from ST be enough?

The plan is to have this running by Easter, so it's no hurry, but it's not that far away either (especially when custom stuff is in the mix).
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Old 10-21-2011, 06:11 AM   #2
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I went with the tougher SuperTech dual springs, so now it's "only" the cams that needs designing (the springs will dictate the max ramps possible).
SPRK-MM8D, 13mm max lift, 12.1 lbs/mm rate, seat press [email protected] compression.

But it will take some time to get everything together (it's a low priority work to keep the labor cost low).

A more specific question; How would a engine run/(fast)idle with 290 cams set to zero overlap (exhaust opening 110 degrees before BDC and intake closing 110 degrees after BDC)?
I guess not that well.

The real tricky question is where the lobe center on the intake cams should be, as stock or shifted in any direction (10 degrees retarded maybe, advancing with the VVT even at idle)?

The exhaust cam can be centered anywhere (preferably as stock) since there is no sensors using it (a simple adjustable Cam wheel would make the day).
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Old 10-21-2011, 07:55 AM   #3
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That's a lot of seat pressure on those springs - that will def rob some horsepower due to the losses. Are you sure that you need that kind of a figure? I went with single-spring, 55lbs.
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Old 10-21-2011, 08:38 AM   #4
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Originally Posted by Reverant View Post
That's a lot of seat pressure on those springs - that will def rob some horsepower due to the losses. Are you sure that you need that kind of a figure? I went with single-spring, 55lbs.
Seat pressure can always be lowered (and then give me greater margin to coil-bind at max lift).
There will be machining all over the place anyway and I trust my builder to do the right guesswork (even if the margins might be a bit large sometimes).
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Old 10-21-2011, 10:17 AM   #5
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That is some advanced cam timing you are asking about. I have my doubts that many here have done enough cam timing development when it comes to vvt. I would think that the control of the vvt would be more important than where you place the centerline of the camshaft. Not my expertise.

Also, there is a reason that most vvt engines have small camshafts. There are some limitations for some reason. Also, a 290*-300* of duration seems like too much cam for a 7500 rpm motor. What do you think the camshaft duration will be at .050" lift (or metric equivilent)?

I would have to agree with Reverent on your valve springs. Do not put the cart before the horse. Choose your camshaft first, and then match the correct spring rate and height.
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Old 10-21-2011, 11:34 AM   #6
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I think the reason the Mazda VVT intake cam is short is that they can meet the target torque curve (same as 99), with a shorter duration cam.

VVT will broaden your torque curve even with a long-duration cam. Being able to reduce overlap to near-zero will make it idle sweetly despite aggressive overlap in the midrange. It is overlap that makes idle lumpy with aggressive cams.

Take note that a longer-duration cam will tend to require a smaller optimal sweep range thru the RPM range (max advance minus max retard).
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Old 10-22-2011, 06:19 AM   #7
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Quote:
Originally Posted by JasonC SBB View Post
Take note that a longer-duration cam will tend to require a smaller optimal sweep range thru the RPM range (max advance minus max retard).
Sure, and in this case the max advance will not be in the "mid-range", but max VE would ideally be around 6.5-7kprm.
Intake and exhaust must also be tuned for that (and will be within 5 years or so, the Jenveys will take some time to get in place).

My though in placing the lobe center of the intake cam a bit retarded is to change the VVT sweep from 0 - 47 degrees advance to maybe -10 - 37 (looking at the lobe, the cam sensor will still be 0-47).
This would require smaller valve pockets, and would make running safer when/if the VVT control go mad.
I would feel safer if I have margins to interference over the whole sweep (and maybe use 15-25 advance), instead of needing to stay away for some of the sweep (e.g. bend the valves if the VVT slip past 25 advance at the rev limit). Modding the VVT to mechanically limit the sweep to 20 degrees would be another way (but nothing I would try unless it was very easy and verified to work safely).

I'm taking "some" shortcuts in getting near where I want to be, using too many OTS parts to be perfect. But if I'm lucky this engine will run 8 hours next year before it's time for the next winter with teardown and corrections.
Custom pistons/springs/retainers would be better but would also require more effort than I have available (I try to make the most of what I can manage, so I know there will be room for improvement).

What the real seat pressure will be will hopefully be known before the end of the year. If it's too high we'll lower the spring seats to get where we want. The head has yet to be welded up after the last mishap.

Thanks for giving feedback, it's not easy to get comments when trying this "dumb" stuff.
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Old 10-22-2011, 11:43 AM   #8
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The VVT sweep the stock engine wants is half of the available range. Like 24* out of 47*.

With the stock piston's valve pockets and the stock lift the engine is non-interference.
And IIRC the stock VVT cam lift is greater than 99/00 cams.

An extra mm or 2 of lift might turn it into an interference motor *when the timing belt breaks* but not over the VVT sweep range. Max intake lift won't occur til like 90* ATDC and advancing that point by 47* won't make the piston strike the valve at the end of the exhaust stroke. There simply won't be enough lift at that point.
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Old 10-22-2011, 01:07 PM   #9
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why did you only flow to 10" of h20, can you convert those number to standard 28"?
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Old 10-22-2011, 02:14 PM   #10
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Quote:
Originally Posted by JasonC SBB View Post
The VVT sweep the stock engine wants is half of the available range. Like 24* out of 47*.

With the stock piston's valve pockets and the stock lift the engine is non-interference.
And IIRC the stock VVT cam lift is greater than 99/00 cams.

An extra mm or 2 of lift might turn it into an interference motor *when the timing belt breaks* but not over the VVT sweep range. Max intake lift won't occur til like 90* ATDC and advancing that point by 47* won't make the piston strike the valve at the end of the exhaust stroke. There simply won't be enough lift at that point.
Shave a bit on the block to make the pistons flush (8krpm stretch margin in the head gasket),
skim the head to maker the chamber back to 50cc,
"11:1" pistons with the hope to reach 11.5+:1 (pistons with decent pockets),
11mm lift (about 0.5mm overlift to keep the nose milder).
All these things together I don't know how interference it will be until we get all parts together and clay it.
Quote:
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why did you only flow to 10" of h20, can you convert those number to standard 28"?
My builder use 10" in his flow bench, no other reason. Conversion would be really tricky since turbulence etc can be different (so it may level out at a different lift).
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Old 10-22-2011, 02:24 PM   #11
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superflow has conversion factors to convert inches of h20, my head was flowed at 25" because that was what our flowbench was calibrated to, and then converted to 28" because that is industry standard. what was done for head work? how extensive was the porting. my 99 head race ported is flowing 247cfm on the intake side @ 28" ([email protected]")
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Old 10-22-2011, 02:56 PM   #12
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Quote:
Originally Posted by ctdrftna View Post
superflow has conversion factors to convert inches of h20, my head was flowed at 25" because that was what our flowbench was calibrated to, and then converted to 28" because that is industry standard. what was done for head work? how extensive was the porting. my 99 head race ported is flowing 247cfm on the intake side @ 28" ([email protected]")
The porting is not a quick fix and work is done all the way out to the port (which is not enlarged in any way), but it's hard to judge how much more there would be in there.

Using this table
http://www.angelfire.com/fl4/pontiac...onversion.html
11mm intake -> 140cmf @ 10" seems to point to about 233 @ 28".
But I would not use three digits precision in a conversion of that large range.
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Old 10-22-2011, 05:52 PM   #13
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Quote:
Originally Posted by NiklasFalk View Post
Sure, and in this case the max advance will not be in the "mid-range", but max VE would ideally be around 6.5-7kprm.
You want max VE (aka max torque) to be 20-25% below your revlimit. You don't want it closer than that. This way you maximize the *average power* in the top 20-25% of the rev range, which yields the fastest car.

If you set peak VE higher, you will have higher peak power but lower *average power* in your powerband (rpm range you're using when driving at full boil). This will yield a slower car.
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