cam upgrade

[grumpyvette]

I'm sure we've all been at the point where we want more from our current builds and and start looking in all directions - inevitably we all look at the cam since it is the heart of the engine and everything else is based around it.

I'm currently looking for a bit more performance from the truck in my signature, I'm considering cam and rear gears; one at a time though...

I was a bit conservative when I built the motor that's in my truck and opted for a mild cam, a supposed clone of the famous "lazy ZZ4" cam according to the salesman at long-gone PAW. The cam and lifters came in at a whopping $109 so I figured I didn't have much to lose and I could always change it later.

Though I've long since lost the cam card I did manage to write down the specs, somewhere... can't find them now of course. The highlights are:
260/260 advertised duration, 214/224 duration, .443/.465 lift @50, 112 LSA

IIRC, the cam was "rated" at Idle-5500 RPM performance range. In my experience, my truck pulls well off-idle and up to about 2500-3000 RPM, then falls off. I am not sure how much of this is the cam or my gearing but I suspect most of this is the cam, which is why I feel a rear gear ratio change to lower gears won't net all of what I am looking for.

Off the line, in the 0-25% throttle range is where this truck seems to have the most power. Even with 3.42 gears and 31.5" tires I can spin the rear wheels through first and into second gear - so there is plenty of power, plus the rear end is light being a truck. Idle quality is pretty tame, no lope to it, but not quite a factory-steady idle too. With my current tune after installing EFI I have 18" Hg of vacuum at idle with a steady needle.

Being conservative, I originally backed down from 3.73 rear gears when I had a 3-speed trans in the truck, now with a 700R4 I regret that compromise but that's easily fixed (see my other thread on rear gear swap, what to get) - now I wonder if 3.73 is enough gear to make the swap with the coin with 31.5" tires.

Back to the cam, I am again leaning towards the conservative side and looking at Comp Cams XE268H (12-242-2) and hence the reason for this topic; how do we decide what cam to put in?

I've spoken to other folks with similar trucks, and folks with this exact cam in different vehicles.
I've called Comp Cams and been told "this will work for you," I've also used the CamQuest online, but it did not recommend this cam at all.

At this point it seems like I'm looking at a $400 gamble unless I can figure out how to make an intelligent, calculated decision so I decided to post this thread and perhaps this information (a real world cam selection criteria) can benefit everyone.

Any thoughts or wisdom? Grumpy?

Folks,

I thought that this would be of interest to some.

I have a mild 383 sbc with GMPP Bowtie Vortech 185cc heads (25534421) and a GMPP Hotcam in a 1972 Chevelle (TH350).

Static compression is calculated at 9.6:1. Cranking compression is 185 psi.

The dyno plots in blue- old exhaust system and stock convertor
stock iron exhaust manifolds, 2.5" true-dual pipes, Summit Turbo mufflers, tips to bumper

The dyno plots in red- new exhaust system and 2400 stall converter
Hooker sbc headers, 2.5" Pypes X-over pipe, 2.5: pipes, Pypes Race Pro MVR30 18" case mufflers, tips to bumper

I wish that I could take the converter off the comparison but it's too late.

Dave

video:

IF YOU LOOK CLOSELY THE VORTEC HEADS AND HOT CAM ON THAT 383 < OBVIOUSLY PEAKED IN THE 4000rpm-4500rpm RANGE, AS THE TORQUE CURVE DROPS NOTICABLY AFTER THAT RPM, WHICH WOULD BE EXPECTED WITH THOSE HEADS RESTRICTING A 383 , MUCH OVER 4500RPM




as always it really helps as great deal to do some research into what has proven to be successful, in the past, HERES WHAT I WOULD DO!.
Id suggest making 100% sure the current engines running correctly, verify the exhaust back pressure at wide open throttle remains under 3 psi and your ignition advance and fuel pressure remain where you intend them to be, Id verify the TDC and cam timing, do a compression test and get a fuel/air ratio meter and an IR temp gun, and vacuum/pressure gauge to verify the exhaust gases are showing the correct fuel ratio, and your not dealing with a restrictive or partly clogged exhaust
I'd be trying to duplicate the proven successful design vs randomly matching components and hoping the results will be successful., stock vortec heads heads will start to become flow restrictive on a 350 engine at about 4500rpm, and become very restrictive by 5700rpm, so your unlikely to get good power at over 5500rpm so cam it accordingly, do the tune so your running at about 14:1 fuel/air ratio at idle to about 2800rpm then slowly transition to a richer 12.7:1 ratio by 4500rpm, yes this might require larger injectors

a cam like this, LUNATI HYDRAULIC FLAT TAPPET LINKED BELOW, with a 2800rpm stall converter , and 1.6:1 ratio roller rockers will wake up that combo (obviously you need to verify your clearances and use the correct valve springs (stock heads have about a .470 lift clearance youll need springs and clearance for a .550 lift) and verify your exhaust back pressure is LESS than 2 psi at WOT at 5500rpm, set the ignition advance curve to start near 14 degrees advance at idle then steadily advance to about 36 total at about 3200rpm
LUNATI tech line
Tech Line: 662-892-1500 TALK TO THESE GUYS< ASK QUESTIONS GET THEIR INPUT,DISCUSS YOUR APPLICATION, BEFORE YOU BUY ANYTHING
http://www.lunatipower.com/Product.aspx?id=2929
YES ID USE rhoads lifters (yes they tick like solids at idle)
http://www.rhoadslifters.com/Pages/OriginalFT.html

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[grumpyvette]

I'm not running over 0.500" lift, IIRC my current lift is 0.443"/.465"

It does keep pulling past 3000, but it's not pulling strong at that point, it seems to make it's peak power around 2750 - which is around the same point that my timing stops advancing.

Since having only recently upgraded to EFI I should revisit my timing curve/strategy and fuel map.

You need to deal in verified FACTS not guess, your engine tune, which obviously will include both the ignition advance curve and fuel/air ratio must be verified, you might be needing a bit more ignition advance or your fuel pressure might be dropping or the injectors are maxing out, or it could be a dozen other things,youll need to check and verify each potential area, I recently had a guys corvette that ran good up to about 3400rpm where it just seemed to max out, it turned out to be a partially clogged catalytic converter that was restricting exhaust flow, strangling power as the volume of exhaust gases increased with rpms, obviously measuring the exhaust back pressure at each rpm level at about every 1000rpm from idle to maybe 5500rpm would help, and Id check the valve springs load rates


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[Indycars]

To get decent back pressure values, wouldn't the readings need to be
taken at wide open throttle while under load???

 

[grumpyvette]

To get decent back pressure values, wouldn't the readings need to be
taken at wide open throttle while under load???

thats a valid point, but you would be amazed at how restrictive many car exhaust systems really are, how few guys realize how vital having a low restriction exhaust behind the headers is to power potential, and how early or low in the rpm band many exhaust system restrictions start strangling power.
most guys don,t either understand how important it is to keep the exhaust flow restriction minimized or the catalytic converter slowly melting and restricting flow happens over months and they fail to realize the power robbing effects until it becomes very obvious somethings wrong, and about 99% OF THE TIME , they start thinking TUNE-UP or WIPED cam, rather than the real issue.
the test procedures listed in the link
no one can, make intelligent decisions without facts, and you get the facts by testing!
and it should be obvious that testing needs to be done under the current operational conditions at the rpm range that your concerned with.
A stock automotive exhaust might have over 5 psi OF BACK PRESSURE, AT PEAK RPMS WHERE EXHAUST FLOW IS AT MAX,or even more in a few cases, of back pressure in the exhaust system if measured at the collectors at max engine rpm, with a decent accurate pressure gauge, while a good aftermarket exhaust system will roughly cut that in half to the 3-5 psi range. BUT An excellent performance exhaust will get down in the 1-2 psi or LOWER restriction to flow range at max engine rpm. keep in mind that the efficiency of the headers scavenging the cylinders , and helping to draw in the following intake charge,is almost totally dependent on maintaining a very low flow restriction or back-pressure in the collectors ,especially at mid and upper rpm levels, any significant restriction to flow reduces the effectiveness of the headers ability to scavenge the cylinders by allowing the previous exhaust gas inertia mass to help drag in the next intake charge following it into that cylinder as it exits the cylinder thru the tuned headers primary's.

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[grumpyvette]

I am putting together my notes to post, but the best calls were with Mike Jones, Lunati and Crane Cams, I felt these companies really listened.

David at Lunati was great. We talked about two of their cams, 10120702 and 10120703. Specifically the 10120702
219/227 duration at .050
262/268 Advertised Duration
112 LSA, 108 ICL is 4 degrees adv
.468/.489 lift at valve
INT opens at 1.5, closes at 37.5
EXT opens at 49.5, closes at -2.5
RPM Range: 1400-5800
Good with 3.42 - 3.73
Good with 2200 stall
Better with a 1.6 rockers (.499/.521)
100/300 # springs

Lunati felt their 10120703 (268/276) was "on the border" for me cause they would want to see more converter and 3.90 gears to match the power band of the cam. They would also like this on a 1.6 rocker vs. 1.5, however .537 lift is questionable for me without more head work. So this is an option if I want to go deeper into the build and budget and have more of a hod rod than a cruiser.
IMO the Lunati 10120703 is the same class of cam as the Comp XE268H but a better version according to many.

Mike Jones and Crane Cams both gave me the same advice:
They feel the cam I have in my truck now is pretty good if not perfect for my application and driving style. Neither of them though the 112° LSA was reason enough to change the cam.
Both felt I'd do better with 1.6 ratio rockers and a lower rear gear. Both recommend 3.73 gears with my current cam or anything similar to it. Mike Jones was pretty emphatic that 4.10 gear was too much gear for my setup and if I wanna run 4.10s I should put more cam and converter into the truck, but he didn't recommend it. Nor did he think 3.90s would be a good compromise, he was set on 3.73 for me.

I was very impressed that neither of these companies felt compelled to sell me something. I really respect that, and it was great to speak with Mike Jones himself - it was like talking to a distant uncle that had my best interests in mind.

The consensus from almost ALL the cam guys seems to be:
-keep my cam or buy one really similar to it
-3.73 rear gears
-1.6:1 rockers (would take me to .472/.496 lift on my current cam)

I am tempted to do these two upgrades before buying a cam. First the gears, then maybe rockers, and fine-tune my setup before going into the engine to swap cams.
Seems reasonable to me.

 

[grumpyvette]

as stated earlier, in the thread, Id first verify the listed factors, like exhaust back pressure and potentially limits on fuel delivery and ignition timing, mentioned above are not causing the power drop off issues that you described ,BEFORE swapping cams, and I'd point out that changing valve springs and adding the required valve train clearance will usually NOT require removing the heads or machine work,on those heads, as there are off set retainers and valve locks and shims and beehive springs and a whole list of options that can be used to increase the clearance , but even if the heads require machine work to gain the necessary extra clearance its hardly a major expense or difficulty to remove then re-install heads.
Id also point out that adding the 1.6:1 roller rockers and new cam suggested required as stated the higher stall converter and rhoads lifters, both serve a purpose, in making the engine much more responsive on BOTH ends of the rpm or power band,the rhoads lifters will allow the cam at lower rpms to act as if its a smaller cam, yet as the rpms pass about 3400rpm the full breathing potential is available,partly compensating for the restrictive head flow, and the higher stall converter allows you to instantly access the more efficient torque,produced a bit higher in the rpm range, with the longer duration cam, and that longer duration partially compensated for the restrictive vortec heads in the higher rpm band.

heres a few links , with sub-linked info that might help

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https://www.youtube.com/watch?v=rQhqn9MJNyg

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https://www.youtube.com/watch?v=8rl-_q09Jsg

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[octanejunkie]

Thanks for all the info and links, Grumpy... I didn't realize there was a thread here being populated with my posts!

Truth be known, I'm not married to buying a new cam, I just want the performance I want. If I can get that from my current combo with tuning then great! If I need to upgrade rear gears, stall, rockers - I'm ok with that too.

I'm wondering if it makes sense to take this on a more linear and organic journey. Upgrade rear gear, tune then adjust, etc.

FWIW, my rear end shop advises against 3.90s, they said go 3.73 or 4.10.

I think I can easily upgrade to 3.73 without requiring any other upgrades (like a higher stall converter) it was my original plan all along. Once I've tuned everything (confirm TDC, optimize timing curve, optimize fuel curve) I can upgrade to 1.6:1 rockers and re-tune.

If I am still dissatisfied with performance I can look at a new cam, rhoades lifters, higher stall converter, etc.

I don't want to explode my brain, or my budget, chasing what seems like it should be addressed with fine tuning first.

 

[grumpyvette]

As always , drop to the basics , get out the meters and shop manual and realize that step one, in making changes, is to adjust test and maximize all the current existing components as required and verify they are ALL adjusted to maximize the current combos potential, if your not starting from a known and easily repeatable base and power curve you can hardly make changes and expect to see or verify those changes are making a positive improvement in the cars performance.
Id then install a shift kit and modify the current transmissions shift rpm point so the cars engine can shift into or reach about 5700rpm-6000rpm range under wide open throttle conditions, and go thru the basic testing to allow your current combo to function and provide power up into that 6000rpm range where the transmission shifts (notice I did not suggest you'll still be expecting to make peak power at 6000rpm, only that the transmission matched to the current engine should be adjusted with a shift kit and other mods to allow the engine to reach about 5700rpm-6000rpm before it shifts gears. you should ideally select a cam , tire diam. converter stall speed and differential gearing that put the engine rpm range in your power curve about 90% of the time

heres the resulting power band, of my corvettes 383 with an extensively ported custom stealthram intake and other mods like ported trickflow twisted wedge heads, erson roller rockers and a crane 119661 roller cam, with a 3200 stall converter stall and shifting at about 6350rpm N/A

notice changing the stall speed and shift points , by about 1000rpm, on both ends of the power band on the identical engine , being used on the transmission allows you to access a great deal more power from the engine far faster , the drive train change can easily result in 30% plus more power reaching the rear wheels, but it also allows you to select a longer duration cam, and is usually further enhanced with a matching change in rear gear ratio, from lets say a stock 2.87:1-3.08:1 to a more power transfer friendly 3.54:1-4.11:1 rear gear.
If you look at the graph the first stock converter power ranges from about a low of near 175 hp to near 500 hp, average near 338hp
If you look at the graph the second aftermarket,higher stall converter power ranges from about a low of near 300 hp to near 475 hp, average near 388hp, close to 80% more initial and easily 50 more average HP, yes that dyno with the higher stall converter and larger cam,a bit mis-leading because in reality you can,t use the more effective cam with the lower stall speed converter so in reality the difference is FAR GREATER once the matched components are used, as the power curve on the stock engine would much lower

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[grumpyvette]

3.73 seems too low due to your tire size, even if the cam manufacturers are recommending it. That'd be a good ratio with about 27-28" tall tires.

If you ratio 31"/28" x 3.73 you get 4.12. If 3.73's works well with 28" tall tires you need 4.10's for the 31" tall tires.

I certainly wouldn't do the 3.73's with your tire diameter. But if you're worried that 4.10's are too much then go with the 3.90 gear.

65_Impala, thanks for making me want to think about this more by standing your ground, so to speak. The rear-end shop I use agrees with you.

Grumpy, thanks for the all links and formulas. I think I've stumbled onto something that begs consideration. Using the chart on your page
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I see a targeted RPM ranges based on tire sizes and rear gear ratios sorted by economy, best overall and peak power. Since I want best overall performance, I follow my tire size across from the left and low and behold the RPM range for best overall performance is 2755-3008; and 4.1 falls right in the middle of that range.

So I go to an online gear calculator at and run a few calculations using the three ratios 3.42, 3.73, 4.1. Using the RPM values from these calculations I can again follow the chart above to see where these results track on the chart. All 3 calculations concur with the chart.

So I then use the calculator Gear-Speed Calculator http://www.wallaceracing.com/gear-speed.php plugging in the three ratios 3.42, 3.73, 4.1, I get the following table.

I've used 1:1 as the final ratio and 65 MPH along with 31" as constants in all these calculations cause the chart at the top has a 31" row so no extrapolation to 31.5 is required, but given the mathematics of the evidence here,

I have to take another look at my gear selection and give 4.1 some more serious consideration.

According to the chart at http://www.wallaceracing.com/reargear.htm we see the % of change in rear wheel torque from my current 3.42 gear to either 3.73 or 4.11 is 9% and 20% respectively. Reading all the test on the page this calculator is on, I get the impression that almost any upgrade in rear end ratio that puts the vehicle into it's power band sooner is a good upgrade, assuming the rest of the combo supports it.

And this brings be back to looking at stall speed again... If 4.1 is the mathematical best ratio for my tire size and power range, is my current 2200 rpm stall (and my current cam) too small?

But Grumpy, here is what I don't understand...
Using the formula "rear gear ratio multiplied by trans first gear" using my current rear gear I get 3.42 x 3.06 = 10.47 which is right in the magic range you mention 10.1-10.5
if I do the same with 3.73 rear gear ratio I get 11.41 and with 4.1 we get 12.54

I'm not sure what these numbers mean, but if 10.1-10.5 is the place to be, 3.42 puts me there.
How does this work, or not work, using 4.1 as the rear gear ratio in the formula?

 

[grumpyvette]

as stated earlier, Id suggest getting all the factors like fuel pressure timing and exhaust back pressure tested and the engine correctly tuned , and do a compression check and verify the current valve train geometry and clearances, TDC on the damper and timing tabs match reality, and check the cam install (degree it in if it was not originally done, look for mechanical issues like binding valve train or springs ,before making changes.
most of the early muscle car tires used 15" rims and tire diameters in the 24"-27" tall range the 10:1-10.5:1 rear gear times first gear in the transmission is based on cars with those tire diameter ranges, once you start increasing the tire height , you need more gear ratio torque multiplication to keep the same ratio of engine rpm to leverage, because the distance from the center of the axle to the pavement has increased drastically with the much taller tire diameters.
keep in mind the whole concept of gearing is to provide leverage so the engine can effectively apply the torque it produces to the pavement surface, thru the tires surface.
modern over drive transmissions make this a decent choice, a transmission like a 700r4,4l80e or similar transmission drops the final drive ratio to something in the .70-.75 ratio thus cruising speed and mileage in the top gear need not have the engine spinning nearly as high as the original muscle cars would have required

tire surface circumference jumps from about 81" for the 26" tire to about 97.5" on a 31" tall tire
81/97.5= roughly 20%
so your going to need about 20% higher mechanical gear ratio to provide the same leverage, on the pavement, thus a 3.42:1 ratio with a 26" tire might be correct but increasing the tire diameter to 31" will require a 20% increase, 3.42x 1.20=4.10

 

[grumpyvette]

The 4.10 gear with the 700R4 OD ratio of 0.7 nets me 1824 RPM @ 70 MPH with 31" tires, which seems pretty darn reasonable to me.

yes and thats the point! ID agree!
the 4,10:1 ratio with the 31" tires seems like it matches the intended application rather well, but until you decide on the intended use and get the engine in its current config sorted out and tuned ,swapping to a different cam, rear gear ratio, or higher stall speed torque converter will not be necessarily the best idea