Cylinder Head Upgrade Advice for UK Cobra
 

Hi

I have a Magnum Cobra here in the UK which is currently running a 418 Stroker with AFR 205 heads, Edelbrock Performer RPM Air Gap and Pro Systems Holley Carb, MSD 6AL and Tremec TKO 600 trans with independant suspension.

Car runs well but doesnt make as much power as I would have expected form this combination (run on the street with few track days).

I am wondering weather an upgrade to the AFR's with TFS HiPo 225 heads would give an improvement?

Questions: Is this an easy swap? would current pistons be OK? Will current headers be OK? Current setup means rear plug access is already tight would this be worse with the TFS heads?

Hood clearance is also tight so dont want to change out the intake but could change carb and cam.

All in all guessing fair bit of work to change and would like to understand form those with more knowledge than me if it is worthwhile or what alternatives may be better and still provide more power.

All helpful suggestions and advice much appreciated.

225 hp
 

I run trick flow 225 high port head, super victor intake,950 quick fuel carb and a mechanical roller cam in a 408w stroker and make 615 horse power. I have about 1 inch of clearance between the hood and the low profile air filter.

First off, have you had the car on the dyno?

There's a few reasons for this. Is the engine performing up to it's potential? You may find an underlying problem that needs to be addressed. Or you may find that it's performing as designed, but you just need more power.

Second, have you done a compression test? If your cylinder pressures are all low, then it could be you have not enough compression. Or you might be down one cylinder.

IMO, you shouldn't make any changesyet

In the UK, my first finger point might be the side pipes. If they can't flow, you can't make power. If you can't roll a tennis ball to the collector there's room for improvement.

Here's some stuff that'll work your noodle a bit but also give you some insights into possible areas for improvement.

To make 100 HP it takes 10 lbs of air per minute or 600 lbs of air per hour assuming you are fueled correctly with maximum brake torque timing (optimized advance) good gas and no detonation.

There are any number of calculators on the net that will do the mental calisthenics for you and come back with a lbs of air per unit time that your engine processes at a given rpm. I did a calculation for a 418cubic inch engine with 98% volumetric efficiency. I am likely high for your engine so you may want to factor the volumetric efficiency to a number you feel good about.

At 98% volumetric efficiency a 418 cubic inch engine at 6500 rpm will process 54 lbs of air per minute. Divide 54 by 10 and multiply by 100 and you will get 540 hp at 6500 rpm. That translates into about 435 ft/lbs of TQ at 6500 rpm and probably around 480 maybe 500 at a lower rpm.

Unless your heads are particularly bad, improving port flow is not going to give you bottom end, it will give you top end. My bet is your complaint is bottom end related not 6500 rpm related. If that is true then increasing upper end breathing does nothing for and might hurt bottom end performance.

I suspect that any additional torque you will find will be related to cam phasing, compression ratio, gasoline octane rating and ignition timing. If you run poor quality (low octane) gas and therefore low ignition timing, to prevent knock, you are going to be disappointed by the engine torque output. Torque is what your seat of the pants dyno measures.

The solution is either to get better fuel, better timing and better cam phasing or get more inches. Don't forget, 10 lbs of air per minute is 100HP. The only way to get more air is more inches or supercharging.

If you want 20% more torque then you need 20% more displacement at the same rpm. 418 inches multiplied by 1.2 is 501 inches. There is a reason that big inch engines are popular in these cars. You can spend a lot of money to improve your heads and discover your bottom end did not step up at all and even worse stepped down — this would be a sad discovery.

The cheapest fix is to advance the cam for low speed and midrange torque, go to a dyno and get your fuel delivery system brought into focus for your engine's air appetite and while you are there take advantage of the dyno time to optimize your ignition timing and ignition curve.

You will spend about the same amount of money, the big difference is the visit to the dyno will actually make a difference. If you still want more then it is time for more displacement.



Ed


p.s. Nothing gives the seat of the pants dyno a blip like a positive displacement supercharger. My daily driver is a supercharged 4.4L (268 inch) v8 that produces 480 or 490 ft/lbs of torque at 1500 rpm, I don't recall anymore. When you touch the gas pedal, to say the car is responsive is an understatement.

My Cobra produces right at 650 ft/lbs of torque at 1500 rpm and is over a ton lighter than my daily driver. At 7000 rpm the engine still produces 642 ft/lbs of torque. This is the beauty and the bane of supercharging.

You don't know fear until you roll into your throttle at 100 mph and start to smoke the rear tires, in fourth gear! Until I turned on the traction control in the MS3Pro, the car was dangerous in the extreme at any speed and in any gear — so be careful what you wish for.

Keith,

Ed has some very good points above.

Now my queries are:

What are your camshaft specs, part number etc?

What diff ratio, and rear tyre rolling diameter?

Gary

AFR 220 SBF Heads
 

Hi Guys and thanks for the comments.

Car has been on the dyno some time ago but didnt make great power 347 BHP/349 lbs.ft

The Cam is from Flowtech as follows:

Lobe Lift .380 (in) .375 (ex)
Gross Lift (1.6 ratio) .608 .600

Duration@.050 lift 236 248

Lobe separation 110

Diff ration is 3.54:1

Car runs 17" rims with Toyo 888 tyres 315/35

Have adjusted ignition to give total of 34 degrees total timing and about 16 initial.

Agreed. Not what you would expect from a 427. What is your static and dynamic compression? What did your compression and leak down test show?

Don't just guess and throw parts at it. That's an expensive and time consuming effort in frustration.

I think Luce is on the right track, make sure you have free flowing exhaust, engines are air pumps. Might want to make sure ignition timing is really 34 degrees, check true TDC to the timing marker on your balancer.

I would sceptical of the dyno.
Coincidence the torque/hp peaks are so close numerically?

Something is wrong, exhaust, carb etc.

Any more engine details Keith?

Gary

It's not your heads keeping the HP that low, although a 215 head would produce more HP something else is holding you back. Both the heads and intake can support HP higher than what would be about 440HP at the FW according your RWHP number. You could use more duration on the cam. Gaz64 is on to something. Listen to him. Small exhaust will kill that 418. I tried to different style dyno's and they were 30HP apart. The Dynojet gave the higher number