Required Octane Rating?

I have a 302 that is made with a stock '88 5.0 block, GT40P heads, a custom Lunati solid flat-tappet cam ground to American Sedan rules, and ignition timing locked at 36°.  What fuel do I need to run in this? Do I really *need* 93 octane? Would it make more power if I ran 87? 

What sayeth the hive?

The BTU content of both those fuels is nearly identical, so less octane won't give you more power.  Proper octane that allows the correct ignition lead will make the best power.

Ignition locked to 36 pretty much guarantees you'll need higher octane.

But way too many factors to know... static compression ratio?  Flat tops, dishes, or domes? Cam intake duration and LSA?  EFI or carb?

My rule of thumb is to go one grade higher than when you no longer notice ping.  If it pings on 87, but not 89, run 91.  If it pings on 89 but not 91, use 93... etc.  Detonation doesn't have to be audible to be happening.  Hence the one extra step.

Have you run the calculations for your static and dynamic compression?  That should give you a much better idea of what you need.  If you have E85 readily available there is a lot of power to be mad with it if you are willing to deal with bigger injectors, bigger fuel pump, and less range.  I;m at 11.5:1 Static and 8.7 Dynamic tuned for 93 which is about the ceiling for pump gas from my understanding.

The calculations aren't hard if you have a cam card and you engine dimensions.  There are a number of good online calculators that I used having the same concern.

In reply to 84FSP :

Ok but what if you have a mystery motor that definitely isn't stock but you don't have any of the info on it? Asking for a friend.

84FSP said:

  I;m at 11.5:1 Static and 8.7 Dynamic tuned for 93 which is about the ceiling for pump gas from my understanding.

 

I'm at 11.3:1 with a 234/244 cam in an LS and I anticipate that will be stretching it on 93.  It entirely depends on a bunch of factors.  In his case, he has iron heads which raises octane  requirements significantly (but in general makes more power as it retains more heat in combustion instead of absorbing it like aluminum)

But if you think back over the years, a Ford Flathead had 6.5:1 and super high octane gas, and it had trouble not pinging.  Combustion chamber shape, dome/dish/flat pistons, quench, cooling system efficiency.... thousands of factors involved.

Daylan C said:

In reply to 84FSP :

Ok but what if you have a mystery motor that definitely isn't stock but you don't have any of the info on it? Asking for a friend.

Without tearing it apart, not much you can do.  ID the heads and see what you can find on them.  Peek in the plug hole and see if they are flat/dish/dome, and make a SWAG.

The real way is to pull the heads.  Measure stroke, bore, deck height, chamber volume, and take a peek at the cam to see if you can find any numbers.  That doesn't always work since many cam grinders use a casting number for the blank and then grind any number of profiles on it.

In reply to Curtis :

Ok I do know some info. And the heads are off right now. But the cam is definitely an unknown and I don't really want to pull it to look for numbers if I don't have to. I'll probably end up just doing 93 to be safe.

Degree the cam and measure the lift will tell you what the cam is. Not hard to do especially with the heads off. In fact with the heads off this is the perfect opertinity to do this. Knowing what the cam is by the numbers is good but actually measuring it and degreeing it is the only way to know exactly what you are dealing with. 

dean1484 said:

Degree the cam and measure the lift will tell you what the cam is. Not hard to do especially with the heads off. In fact with the heads off this is the perfect opertinity to do this. Knowing what the cam is by the numbers is good but actually measuring it and degreeing it is the only way to know exactly what you are dealing with. 

Solid thought. Blindly forging ahead and winging might work here but if it's not hard to actually figure out what I have I think I probably should.

Pardon my ignorance but how is the timing locked at 36 degrees?  The distributor won't rotate?  Has no built in advance capability? In the late 70's I had a 302 with a Mallory distributor and it had centrifugal advance which could be internally adjusted to limit how far past initial advance it would go. I still have the little tool for that in my tool box

If he locked his like I lock mine, he either dropped a #8 bolt in the hole on the weights or he just tack welded it in place.

I personally think he'd be better off leaving about 10 degrees of advance, but that isn't as easy as dropping a tack weld.  Too hard to know without knowing the cam specs, but it's unlikely that timing locked at 36 is a beneficial thing in this case.  The only time I ever do it is if there is a large mismatch in cam/compression to get something on the road until I sort it out.  Big cam and small compression can be a good situation where locking the timing helps.

Snailmont... what RPM ranges do you see on the track?  Manual or auto?  With timing locked at 36, octane requirements would change if you drop into the lower ranges.  Obviously as the piston moves faster, less octane is often needed.  If you drop below a certain RPM a lot, you would need more octane than if you constantly keep it humming at 4000 and up.

I agree with Dean.  You know it is a Lunati cam.  Measure the lobe lift and compare it to the Lunati catalog.  That should narrow it down to a small number of profiles.

Curtis said:

Snailmont... what RPM ranges do you see on the track?  Manual or auto?  With timing locked at 36, octane requirements would change if you drop into the lower ranges.  Obviously as the piston moves faster, less octane is often needed.  If you drop below a certain RPM a lot, you would need more octane than if you constantly keep it humming at 4000 and up.

I agree with Dean.  You know it is a Lunati cam.  Measure the lobe lift and compare it to the Lunati catalog.  That should narrow it down to a small number of profiles.

 

It does drop down to 2k or so a couple times a lap. It's either a Mallory or MSD distributor. Here's everything I know about the engine. 

Good to see the GT40 "p" designation.  Those had a couple fewer CCs in the chambers (59-61 instead of 61-63cc for stock 88 HO heads).  So you're probably in the 9.2:1 compression range.  With that much cam and that little LSA, you're probably sorta OK with the ignition locked out, but you'll likely need 91 or better octane for the lower RPM pulls, especially if it's a manual.  If it's an automatic, high loads will cause a downshift anyway and you won't spend a lot of time lugging it below 2000 rpms.... At any rate, with that much cam and that little compression, you probably need to keep it singing anyway to make best power.

I would say play with compression like I had mentioned above (3-4 octane points above audible detonation).  The octane content won't make any difference in power, so you'd be choosing octane based on keeping the engine happy.  If you want to get even more scientific, (and you really want to save 5 cents a gallon), try creeping down the octane and keep pulling the plugs to check for aluminum specks.  Detonation blasts little flecks of aluminum off the pistons and they melt onto the insulation around the tip of the plug.

But, no, lower octane won't give you more power.

Do you have some dyno sim software?  Most of it is crappy at predicting actual output, but where they do excel is with plotting accurate torque and HP peaks.  If you want to be faster on the track, it is quite possible that dropping to a better-matched cam will gain lap times.  It might drop a little peak HP, but getting those cylinder pressures up in the mid range will make mountains more torque.  It is possible that your average TQ will increase far more than the loss of 6300-rpm HP.

... but then we'd be talking about putting some mechanical advance in that dizzy. :)

What sort of cam do you recommend?  I have a Motorsport E-303, and a Comp XE270HR from previous builds. Would either of those end up with more area under the curve? I realize that I would be losing AS eligibility by going to a roller, but I only track it. 

On edit: I'm also not above letting the distributor do its job. 

Going to a roller will almost always gain area under the curve.  Generally more lift and more aggressive lift rates mean more power for a given duration (or less duration and better manners / wider powerband for the same peak power).  Higher ratio rockers help with this as well.  Of course, going too far in that direction can be pretty hard on valvetrain parts. 

I remember Hot Rod did an article where they got 400 hp (flywheel) with a stock cam, headers, and rocker arms.  Unfortunately, they used AFR 165 heads which would likely be a disqualifier.  380 tq at 4200 rpm.  The amazing benefit there is all of the lower-end torque of a stocker with added breathing up top.  Good heads will really make a win-win.

Hmmm... cam.  At 9.2:1, I don't like to go more than the 220s duration, but you could stretch it a bit.  On stock Ford heads, you probably want a split profile (more duration on the exhaust).  Shoot for 8-10 more degrees of duration at .050 on the exhaust side.  The 108 LSA isn't awful, but it does make it a bit peaky.  A 110 LSA might pick up some fun torques.  I might shoot for something like a 228/236 roller on a 110 LSA?  You'll give up a little top end, but not much.  And you'll likely add gobs more torque between 3000-5000.

You probably already know this, but the whole thing should match.  The head flow you have is adequate, the compression is a bit low, and what is happening is that the extra duration is trying to chase high end HP, but you don't have quite the flow and compression to support it.  You can keep adding duration and get a few more ponies way up top, but they all work together to make the magic happen.

Curtis has a very good point about good heads helping to make more power without needing as much cam. 

Curtis,  do you think this would be a net improvement?

I like the numbers on that cam.  It should make decent power with the GT40P heads and is a decent fit for your compression ratio.  Better flowing heads would yield more power with that cam (especially at high RPM), but I see no issue using it with what you've got. 

I agree.  It is a very well-matched cam for the compression.  The 114 LSA is a bit broad for race.

You will drop power from your current 241 cam and I think you'll find yourself shifting at 5700 instead of 6300, but the torque should be impressive.  Since we're working with a fixed compression, the question becomes if you would rather have a well-matched milder setup, or push it a little bit to give up low end in favor of a few more ponies.  

Let me put it this way... that cam will be a great match for the heads and compression.  It may not be as fast as what you have now on the track.  I would consider maybe shooting for one small step up; 224/230 on a 110 or 112 LSA?  That should perk things up but not be excessive for the compression.

Pro tip:  (not that I'm a pro, but a tip that a Pro gave me)...  There are basically 4 manufacturers of cam blanks, then the individual companies grind their own profiles.  That means the quality of the blank is likely not going to differ much between brands.  If you were going flat-tappet I would suggest one of the bigger names, but for rollers, don't be afraid to shop cheap because roller.  Melling has an impressive catalog of lobes, many of which you can mix and match to get exactly what you want.  Their cams have served me very well with the builds I've done, and they're often less than half the cost of Lunati, Comp, or Crane.  Melling's ramp speeds are not quite as aggressive as Lunati, but the few torques you give up with that will certainly add to valvetrain life.

I should also ask... what is the race weight?  Gearing?  Transmission?  A light weight car can take more cam than a heavy car.

Curtis said:

 

I should also ask... what is the race weight?  Gearing?  Transmission?  A light weight car can take more cam than a heavy car.

3400 pounds with driver, currently a 4:10:1 gearset, with a '94 T5. I'll be changing either the rear or the overdrive ratio at some point to help with top speed.

What do you guys think of this cam for the application?

This cam currently resides in the Victor Jr.-headed 347 (10:1 compression?) in my Fairmont. I've been told that I need .600" lift to take full advantage of the heads, so if this is the ideal cam for the Mustang, maybe I get to multi-purpose an improvement for both cars, which I love to do.  

I had the Mustang at an autocross on Saturday, and it seems pretty pitiful at anything but the top end, even though it has 4:10s. Everything was too fast for second, and the engine didn't have the torque for third. 

Can you stuff a little more rocker ratio in it to get your lift?

I really like the looks of that cam.  Should be a good setup... stretches the HP a bit up high without sacrificing too much torque down low.

Looks like it's ground 5 degrees advanced which is probably perfect.  I might consider investing in an adjustable cam timing set.  Actually, I would run it as-is and see if you like it, but an adjustable set could be added.  Backing that off to 3 or so might give you some high end for bigger straights, and bumping it up to 6 (check valve clearance) might boost the low end for tight/technical tracks.

In reply to Curtis :

Do you like it for the 302, or the 347, or both?