FAQ Regarding Cobra Drivetrain Setups

[Mac VABCH]

Scott,

Did you get your bell housing and reverse starter from quartermaster? I had problems with getting my starter to fit. Is your bell SFI?

[scottj]

Originally Posted by Mac VABCH

Scott,

Did you get your bell housing and reverse starter from quartermaster? I had problems with getting my starter to fit. Is your bell SFI?

Yes, I have the QM aluminum bellhousing (not SFI) & starter... no issues with starter fitment.

[blykins]

I get a lot of questions about whether or not Centerforce clutches are worthwhile.

I've never been a Centerforce clutch fan. Their major selling point is that they use a diaphragm pressure plate which has light pedal pressure. Centerforce basically buys stock replacement diaphragm pressure plates and then adds counterweights to them. The thinking here is that the counterweights will add extra clamping force at higher rpms. However, the counterweights cause a lot of issues with light cars and most Cobra owners/dealers end up removing the counterweights.

When you remove the counterweights, you're going right back to a stock replacement pressure plate with the performance of.........a stock replacement pressure plate. Even their "DF" kit (which is basically a gimicked name for dual friction) with the counterweights, will not support the horsepower and torque that most Cobra engines are making.

It is really hard to beat a good 11" Long style pressure plate and this is what I end up recommending for a lot of Cobra drivetrains. Customers often worry about pedal pressure, but with a correctly designed hydraulic clutch setup and an equally matched clutch disc, pedal pressure is normally not a problem.

A good 11" McLeod pressure plate with a dual friction (organic/bronze or organic/Kevlar) will support 550hp as will a RAM Powergrip clutch kit (with RAM being a lot less expensive than the McLeod). The RAM Powergrip HD will support in excess of 650hp. I would not trust a Centerforce kit for anything over 450-475hp and if you're there or a little under that rating, there are a lot less expensive choices.

[Sawdust]

Brent,
I'm currently looking at buying an SPF with a 351W stroked to 396 with 475hp. It has a Tremec 3550 5 Speed trans. Will I have any issues with this trans? Is this trans strong enough? Great thread!

[blykins]

No track time, no real sticky tires, it should live ok. The Cobra's weight helps a lot....so does a tire that will give up before it puts a lot of stress on the drivetrain.

[Sawdust]

Brent,
Thank you for your response! Tires are new Eagle GT's

[Dominik]

Brent,
while you are inquiring for hp values most engines on your side are still rated SAE, aren't they?

Example: my 496cui had 500Hp at the wheels, translating in 565Hp DIN at the flywheel with 12% losses in the drivetrain (in 4th, "direct" gear).

An US calculation resulted in 700Hp due to 20% losses and 80Hp for auxillary components and inefficient exhaust (assumed losses for exhaust).
700-80=620Hp
620x0,8=496Hp

With BfG the wheels were spinning endlessly in the 1/4-mile, with Hoosier AutoCrosser I was okay (in 2000). The cam was very torquey 0.560 lift mech roller, but using 1.8 ratio (Chev BB). The springs I had allowed for only 0.600" lift.

In a nutshell, you rely on rear wheel hp (rwhp) for your assessment?

[blykins]

No, I go by flywheel horsepower only. A Cobra has a terrible amount of parasitic losses through the driveline and through the exhaust. Some here have stated that they have removed the sidepipes and gained 100 rwhp. Some sidepipes are very restrictive.

It's easier for me to know what's going on when flywheel horsepower is mentioned. There are way too many variables when someone quotes rear wheel horsepower.

In my line of work, I get a lot of guys talking about 600hp small blocks and 700hp big blocks. I always ask to see the dyno sheets....and even then I take them with a grain of salt.

[Dominik]

Flywheel hp, makes sense...

After all, we ask you for advise for what comes right after the flywheel: the clutch!
And not what comes after the tires: the road ;-)

[blykins]

Yep....

And when customers come to me wanting an engine built, they always say something to the effect of, "I want about 500hp." It's almost a given standard that they're talking about flywheel horsepower.

[undy]

No insinuations here Brent, but big number sell engines for builders.

Unlike the major auto mfrs the small guys typically dyno their engines under ideal conditions, perfect equal length headers, ideal mufflers (if any), electric water pumps, no restrictive (Cobra typical) air cleaners, etc, etc. Take the LS7 for instance... In the Z06 it's rated @ 505 hp (SAE "J" I believe). I've seen the LS7 "crate motors" dyno'd locally and they typically dyno 570 to 580 fwhp, SAE) They're running identical motors as the Z06 is running too. A lot of people don't understand all the variances in dynoing the engines and assume apples to apples. I don't entirely buy or subscribe 100% to the "inherent" Cobra dyno inadequacies. IMHO, the engines would also dyno lower in a Moosetang or another "typical" car, not just the Cobra. My 505hp Z06 develops more rwhp than my 600hp 482 Cobra. I honestly believe, given an "optimal" installation it would still fall waaaaay short of it's 600hp rating.

[blykins]

I agree Dave.

Engine builders have to sell engines and the numbers tell the story.

I do have a few of my engines that go to the drag strip though and there are rough calculations to see if the engine is really making the power it's supposed to make based on ET's and trap speeds. So if I provide a dyno sheet that says 600hp and the car runs a 9 second 1/8th mile.....something's wrong........

I do believe that a Cobra eats a little more horsepower than the usual application. A Cobra exhaust (with collector-less headers that have sharp 90° bends at the end going into golfball sized mufflers) consumes a LOT of horsepower.

[undy]

Originally Posted by blykins

I agree Dave.

Engine builders have to sell engines and the numbers tell the story.

I do have a few of my engines that go to the drag strip though and there are rough calculations to see if the engine is really making the power it's supposed to make based on ET's and trap speeds. So if I provide a dyno sheet that says 600hp and the car runs a 9 second 1/8th mile.....something's wrong........

I do believe that a Cobra eats a little more horsepower than the usual application. A Cobra exhaust (with collector-less headers that have sharp 90° bends at the end going into golfball sized mufflers) consumes a LOT of horsepower.

Understand, but "my" Cobra's is running perfect merge collectors, 2" primaries, true 3" flow path and well radiused bends, although not equal length primaries. It was also chassis dyno'd with no air filter and still achieved a pathetic 474 rwhp, in spite of it's "600" Hp rating.

Just to let everyone know, you didn't build it and truth be known, I'm not upset with who did. I further understand that if you (or anyone else) wants to be competitive in a market you have to use the same "standards" as the competition does or you'll shrivel and die. Kudos to you (unlike the other forum builders) for being willing to openly discuss these issues that have positive and negative conentations...

[Three Peaks]

Originally Posted by undy

Understand, but "my" Cobra's is running perfect merge collectors, 2" primaries, true 3" flow path and well radiused bends, although not equal length primaries. It was also chassis dyno'd with no air filter and still achieved a pathetic 474 rwhp, in spite of it's "600" Hp rating.

Just to let everyone know, you didn't build it and truth be known, I'm not upset with who did. I further understand that if you (or anyone else) wants to be competitive in a market you have to use the same "standards" as the competition does or you'll shrivel and die. Kudos to you (unlike the other forum builders) for being willing to openly discuss these issues that have positive and negative conentations...

Actually, if you were quoted flywheel hp, the 474 rwhp is just about right on the money. If you subscribe to the roughly 20% losses through the Cobra drivetrain, 474 rwhp equates to 592.5 fwhp.

Of course, if you were quoted 600 rwhp, ignore all of this senseless typing....

Bob

[Excaliber]

.....something's wrong........

Waves hand frantically, I KNOW this one, call on me!!!

Drag strip calculators make several assumptions which generally only apply to a seriously drag race prepped car. Weight transfer, drag slicks (warmed up), excellent traction on all accounts, perfect shifts, perfect weather, perfect driver.

For starters, Cobra's got no weight transfer to speak off, even with drag radials "we" are seriously traction limited. 500 or so horse in a light weight car isn't going to put that horse power down until somewhere around 80 to 90 mph, in third gear. This just throws a big old monkey wrench into the drag calculator numbers!

The real world numbers, like we see here on Club Cobra on occasion, I find rather interesting. You typically see the big blocks with a higher trap speed than a small block, even when the ET is almost the same. I can almost tell you, from the trap speed alone, if it's a big block or a small block. Both of which may in fact be running similiar horse power and even similiar ET's.

Where the numbers start to get fuzzy is with something like a 351W punched out to 427 cubes. Big or small block at that point? Another thing I like to see is the 60 ft time, that is a STRONG indicator of available traction and how the car was launched.

If you look at the numbers for the original Cobras, they both ran in the 12 second range, STOCK. But the 427 had a considerably higher trap speed, and a better ET by some degree, than the 289 cars. Bear in mind there were few, IF ANY, cars off the show room floor that could get into the 14 second range in 1965-1966. Making the Cobra mind blowingly quick at the time.