FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: c9zx on September 14, 2011, 08:42:44 AM
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I'm looking for suggestions on a solid flat tappet cam. I'm particularly interested in overlap and LCA (LSA) as well as intake valve closing point. Engine is Genesis, 4.25 bore, 4.125 stroke, 6.700 rods, Edelbrock Stage X heads from Barry R, 10:1 CR, pump 91 octane gas, 2500 lb car (fun street car), no vacuum accessories, manual transmission. I've looked at numerous vendor listing and they seem to be all over the place on specs. All input appreciated. Thanks, Chuck
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Kind of depends on what you want. If you want to keep it fairly tame, and Comp 292S cam would idle nicely in an engine that size, and give you plenty of power up to 5500 RPM. Or are you going for a big horsepower number?
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What rear gear ratio are you running? What rear tire size? What manual tranny (ratios)? What max rpm will your valvetrain support and/or what max rpm are you comfortable running?
All that information will help with a suggestion.
paulie
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I'd like to see it make peak power about 5800-6000 RPM with max RPM being 6500 RPM. The rear gear is 3.55:1 (light street car) with a Tremec TKO 600, 2.87 1st, .82 OD. I'd like to see it idle at or below 1200 RPM. No particular HP number as a goal but, it seems that 510-520 should be doable. Thanks for the help. Chuck
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Given those constraints I think I'd go with a 306S solid cam from Comp, or something similar. That cam idled at about 1000 RPM in my 446" 390 stroker, so it should be a little tamer in a bigger cube motor like yours. You should easily get to the horsepower numbers you are talking about with that cam.
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Thanks for the input. Do you think the the 306S is a good match with the currently planned 10:1 CR? Do you feel the Edelbrock stage-X E/I ratios are good enough to run the straight duration and lift cam. Thanks, Chuck
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In my opinion I think shorter seat duration would work good with your compression ratio. Maybe 280-290 degrees. And I think you could peak at 6000 rpm with somewhere around 252-256 degrees @ 0.050 on the intake. I think you would want about 4-6 degrees more exhaust duration. I'd go with a 108 degree lobe separation angle.
I forgot, what intake manifold and carb (or FI) are you running?
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Blue Thunder single 4V, 850 or 950 Holley or Quick fuel carb. Chuck
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Hmmmm. I would suggest a 1" open or cloverleaf spacer under the carb. I think a Holley 850 would be better than a 950. You probably know this, but the 850 flows more than a 950. With your displacement, light weight, and low overall gearing you can use a lot of carb and induction flow.
One thing about the Compcams Magnum solid flat tappets is that the ramps are very slow. That is not good or bad necessarily. Slow ramps make it much easier to control the valvetrain without using very high valve spring pressures. The fact that that the ramps are so slow makes the duration figures a bit misleading compared to modern cam lobe designs. While a Compcam 306S (with 260 degrees @0.050") sounds huge, it's not as huge as it sounds. Don't get me wrong, that's a big-ass cam. But if you get a more more modern, more aggressive cam lobe, it'll have less duration as measured at advertised and at 0.050" lift, yet it will have a similar rpm range and make more power everywhere. The cost is it will need more spring pressure, and it's life will probably not be as long.
you have a cobra kit car?
paulie
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Paulie, The engine is going into a FFR "Cobra". Thanks for the discussion of the ramps. I'm dealing with a fast ramp cam in a 408C right now. Runs very well but, very low vacuum. Thanks, Chuck
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You won't get a lot of vacuum out of the 306S either, but I didn't think that vacuum was a concern with no power brakes. Paulie is also correct about the compression ratio; you should bump it to 11:1, or at least 10.5:1, if you are going with the kind of seat to seat duration that that 306S cam has. If you don't want to do that, you should back down on the duration somewhat. But with an engine that big, you will need that much duration to peak in power around 6000 RPM.
Life is full of choices :D
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I didn't mean to confuse things (vacuum) with reference to a different engine for a different car. Thanks for the input on CR needing to be increased to move the peak HP to 6000 or so. You guys are confirming that I'm trying to have a foot in two different worlds with this build, I need to make some adjustments to the "plan". Maybe shorten the stroke to lower the displacement and set CR at 10.5:1 to keep the usable RPM range. Modern pump gas is a real curse. Please keep the advice coming. Clearly I can use the help. Thanks, Chuck
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You can get the best of both worlds, performance-wise by ordering a custom ground cam with much more aggressive ramps. Less seat duration, similar duration @ 0.050", and more area under the curve.
For instance I have a solid flat tappet Compcam with 270/276 duration @ 0.020", 240/246 duration @ 0.050", and .598"/.608" lift. It peaked at 6200 rpm with 432 cubic inches. My heads are little more hogged out than yours, but you get the idea. The downside is the spring pressure required. I ended up with about 405 lbs of open pressure. I used EDM lifters to help lube the cam/lifter interface. It ran fine for about 5000 miles before I pulled the cam to put in a larger one. I ran up to 6500-6600 rpm with no problem. Titanium retainers were a big help avoiding valve float, too. What I'm getting at is you can have your cake and eat it, too. It just costs a bit more money and requires a bit more maintenance.
If your combo is the same as mine, but you have 36 more cubic inches, I would guess you'd need about 7-8 more degrees of cam duration get the same peak. However, since my heads are more heavily ported, I think you'd have to add a few degrees more on top of that. That's why I say around 252-256 degrees on the intake side could get you there, if you're using an aggressive cam lobe.
If you used the same cam lobes I did, XTQ on the intake and TL on the exhaust, then you might have a XTQ286 on the intake and a TL292 on the exhaust. That would be 256/262 degrees @ 0.050". Even though the duration is less than a Magnum 306S it has more area under the curve and will peak higher. At least I think so.
Or you could move the intensity down a step and usea TL284 on the intake and maybe a HT295on the exhaust. Thoe are 254/260 degrees @0.50 and would be easier to contol, valvetrain-wise, but still way more agressive than a magnum cam lobe.
The possiblities are almost endless. Just food for thought.
JMO,
paulie
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I had assumed your compression was pretty much set, as if you had already bought your rotating assembly including pistons. If not that's good. Though 10:1 compression with a 4.25 bore and 4.125 stroke would require a fairly big dish, I think.
I don't think I'd lower the displacement just to keep the rpm range where you want it. It'd be better to find a way to bump the rpm's back up with the larger displacement engine. Up to the point where rpms become problematic.
On the other hand if you're trying to get a Cobra to hook up, more displacement is not necessarily good.
Here's a thought. Assuming you're still around 468 cubes. Bump the compression up to 10.5:1. Mill the heads a few cc. Use a thinner head gasket if necessary. zero deck the block if it hasn't been. Then you could use an off the shelf cam like this Lunati
#30330914
gross lift 616"/.616"
285/295 advertised duration
252/261 degrees @0.050"
110 LSA
Lash.026"/.026"
The exhaust duration is possibly a little longer than necessary, but since your car is so light, your overall gearing quite low, and the cubic inches pretty big, it wouldn't hurt a bit. And the extra exhaust duration should extend the rpm peak a little. It might not peak at 6000 but it'll be up there fairly close.
Or bump the compression up to 11:1 and use the Magnum 306S.
Maybe the best question at this point is what is the intended use for your Cobra? Street? Street/strip? Road race? And what parts do you already have?
paulie
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The heads, intake, and block are bought, the rest not yet decided on. Thanks for everyones input and help. Chuck
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Please let us know what you end doing.
paulie
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After a lot of re-thinking almost everything, let me approach this a second time. 2500 lb. Cobra, no vacuum accessories, TKO 600 trans., 3.55 gear, Genesis 4.250 bore, 4.125 stroke, 6.700 rods.The Diamond pistons PN 42510 (468 CID) yield 10.87 with the piston .005 in the hole. The first priority is that the engine must run on 91 octane pump premium with no high speed detonation. The heads are still Edelbrock 72cc, Stage-X (Barry R.), intake is still BT dual plane, 850 HP or "true" 950. Please look at the following cam specs on a solid flat tappet and give me your opinions especially regarding the detonation issue. I'd rather leave power on the table than fail the cam and/or the engine. All input is appreciated. 294/300 Adv. Dur., 248/252 Dur. @ .050, 109 LCA, install on 105 .620/.630 lift. Is there something to be gained by using slower ramps on the exhaust lobe? Thanks, Chuck
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I come up with a dynamic compression ratio of 8.4:1 with that cam installed at a 105 ICL with 10.87 static compression. I think that would work fine with your vehicle weight, cubic inches , overall gearing, and 91 octane pump gas. Even so, given the little need for low end power with such a combination, I would play it even a bit safer. I would install the cam at a 107 or 108 degree intake centerline. That would drop the DCR to 8.25:1 or less. That will help just in case you happen to have two 200 pound guys in the car, 100 pounds of luggage in the back, and are going uphill in 5th gear at 30 mph in 105 degree heat, and you romp on the throttle, you should still be okay.
Your ignition timing curve will play a factor as well.
JMO,
paulie
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I think I made a mistake in my previous post. Sorry. I recalculated the DCR and came up with 8.16:1. That's with 10.87 static compression and the 294/300 cam installed at a 105 degree intake centerline. That should be fine with 91 octane gas with your combo. Bring the ICL back to 107 degrees and the DCR falls to 8.01:1.
It is pretty common for the exhaust lobe on a cam to have milder/slower ramps than the intake lobe. I'm not positive about the details, but I don't think it's as critical for the exhaust lobe to open and close so quickly. The exhaust has residual cylinder pressure and a rising piston to help force it out. The intake side is a different story. It relys mostly on the pressure drop caused by the piston dropping and the cylinder volume increasing.? So in general you want to get the intake valve open wide, where it flows more, as fast as you can, then shut it fast when the intake flow stops, so as not to lose any intake charge back out the manifold.
paulie
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+1 on what Paulie said, except that in my opinion you can get any engine to knock if you lug it in a low enough gear or put a high enough load on it. But you should certainly be safe at higher RPM under normal driving conditions with that combination.