View Full Version : Battery and alternater question

keith w
27-09-01, 05:38 PM
Another question for all the technical wizards out there.
Im running (or will be when I've finnished) a s.b. chevy 350 bored to 355 which should give me 360bhp. The only part I need now is the alternater. I've been recomended to use a 'conventional' one along with battery both from a diesel because,
1.The diesel alternater and battery give a higher output
2.The parts will be easyer to replace offthe shelf.
3.It will be cheeper than specialist parts.

Can any one help
cheers from a frustrated cobra builder who has to wait till january for the body work to be builtx(

27-09-01, 06:37 PM
Hi there talk to Real Steel in Uxbridge re alternator there not that expensive also ask them about the required cranking current for your motor this will tell you what batt, to go for
Paul :-) :-)

27-09-01, 08:46 PM
Hi there.

I run a Chevy 350 and have run perfectly well since 94 with a Cortina 2.0 alternator. Battery is from a Ford Transit diesel. No issues whatsover on starting or durability.

Nigel B

27-09-01, 09:00 PM
I use a red cell, although they do cost around 120, they can be mounted any way up, and they also don't mind being subjected to the g forces on a track day.

Best Regards

Forum Admin

27-09-01, 09:27 PM
Forget the bit about a diesel battery. These are often mentioned because they are of a heavy duty type compared to the equivalent battery used on a petrol engine. Three things are important in choosing a suitable battery . 1) Reputable Brand. Ie will it last. Cobras are used generally at weekends and normally in summer only, difficult conditions for a battery to live with in charging terms. 2) CCA – cold cranking amps, these give you the power to turn your motor over from cold ( ie 0oC ). As a rule of thumb an 8 cylinder engine requires between 750 ad 850 CCA. Unless you have a very high compression motor, more CCA is uneccesary ang more costly. 3) Reserve Capacity ie No of mins a fully charged battery at approx 27oC can be discharged at 25 amps until voltage drops below 10.5oC. Alternatively Amp Hours is sometimes used. ( multiply RC by 0.4 to convert to A.H. ). In all cases, the more RC the better. Close to 120 minutes RC would be desirable.
Solution, in my opinion of course- Quality battery such as Fulmen or similar, around 800 – 850 CCA with 120 mins RC.
For your alternato think about what electrical equipment you intend to use in the future. I am using a 64 amp hour which was fine until I put some serious music in the boot. I think I need a 100 amp hour now. If you are still building, it may be worth considering a 1 wire application for a tidier engine bay. Only difference is you need to blip the throttle on start up to get it charging.
Think I’ll go for a beer now. All the best.
;-) ;-)

01-10-01, 11:09 PM
I use type 069 batteries never had any problems with either small or big block
as for the alternator ,before you go out and buy something big enough to light up a small town the chances are that your cobra wont have heated front and rear screens,heated electric adjustable seats,electric sun roof/soft top, mirrors,
windows (imagine that),it may not have a 4000 stereo or cigarette lighter so
i wouldn't worry too much about the output, and if funds are too tight for one
from real steel MK3 cavalier / astra alternators can be bought from a breakers for arround a tenner and need very little modification.

02-10-01, 01:30 PM
I have got a Standard alternator from real steel with internal regulator 63 amp (64 ish)and a Halfords 620 amp batt and starts fine with higher comp. pistons, perhaps would not want any lower rating than that though.


04-10-01, 10:10 PM
Hmmmmmmmmm - methinks I spot a fellow electrical engineer in that Mikey fellow. But he is right - you don't need more alternator output than 60 ish Amps for the relatively minor electrical demands of our Cobra's. Any more and all you are doing is wearing out your fan belt, and to stop it slipping, probably tightening it to the point where the water pump bearings are over-stressed. Plus paying more for the alternator than you need.

Battery?? Try Delco Freedom 30-66. Enough capacity to run my car from Warwick to Leics at night with headlamps on and no alternator charging at all (broke the regulator). (Didn't take too long tho' as M69 my fave place to put the pedal to the metal). Also has had plenty of abuse turning over my stroker 351W whilst setting rocker preload etc. with no probs.

I could understand that a realy (and I do mean sound-off type) big sound system could require more electrical oomph, BUT WHAT ARE YOU FITTING THIS FOR - SOUND OF A V8 NOT GOOD ENOUGH FOR YOU????

Lastly - to keep your battery (any battery) in good shape with the kind of infrequent use we give our cars ('specially in the winter), invest 35 or so in an "Airflow battery conditioner" - basically an intelligent float charger. Nothing bu**ers up batteries more than sulphating due to sitting around doing nothing. Will more than repay it's cost by prolonging the life of your battery.

My 2c's


06-10-01, 12:08 PM
Hi Wilf
I'm not an Auto Electrician I'm an Arhitect. Hope my general knowledge was reasonably accurate. I agree fully on the alternator size, no reason to go too big which is why I initially bought a 63 amp unit. However they put out significantly less than this at low cruise speeds and as such it is why I think I need a bigger one, alternator that is! Its a 600 watt amp and the reason I got it was that I was feeling out of it with my 2 cv motor in my Cobra, so I taped a V8 and play that as I drive. Cheap on petrol too. No, real reason is it keeps the woman quiet, at least I think so cos I cant hear her :-) If you play the right track it gets her giddy too.:-) :-) I left a message on the forum - nice shiny bits which nobody has visited:-( :-( have I been put into Coventry:-( :-( I think its worth looking at for chromed parts as hes really cheap and they do alternators, water pumps etc etc.
Speak to you soon.

06-10-01, 12:24 PM

They have visited it, it just won't update the views until someone responds to it.

All the best

Forum Admin

Like I keep saying to my son:
"Play nice, please."

07-10-01, 11:01 AM
Mikey - if your amp is being given any "stick", it's gonna draw well over 50Amps! I sure hope youv'e fitted proper sized feed cables to that brute. Fit a tow bar and tow a generator!

I did look at your shiny bits post, noted the web address for future reference. Ta.


07-10-01, 11:53 AM
Thanks Wilf, I got a little put me right message from Robert. Fat Juicy cables to amplifyer I have, but as you say if its drawing 50 amps then it leaves very little for the rest.
Cheers Wilf
By the way if your listening Robert I cant seem to suss the procedure for getting a juicy aventar in my post. Can you head me in the right direction.

07-10-01, 12:10 PM
>Thanks Wilf, I got a little put me right message from Robert. Fat
>Juicy cables to amplifyer I have, but as you say if its drawing 50
>amps then it leaves very little for the rest.
>Cheers Wilf

Mike - it will only draw that kind of amperage when it is actually supplying 600 watts! How is that wattage measured - is it "both channels driven, RMS"? manufacturers have a smoke and mirrors way of describing their amplifier outputs - something called "music power watts" can usually be divided by at least two to get at RMS output, which is the "proper" measure.

I think it unlikely that your amp would draw 50 amps for any appreciable length of time - you would probably be deaf first! From my own experience, I once had 170 watts (RMS) of music installed in a car. I left it running up high with the engine off for several hours before the battery got a bit tired (party in a field!!!). If the engine had been running, I suspect there would not have been any problem.

In summary - try it before you go spending money upgrading the alternator.


07-10-01, 06:14 PM
I would guess that most bang for the buck for your 350 should be:

[li]Stock HEI with an Accel "Blue Print" rebuild unit
[li]Stock coil
[li]Nology module
[li]Accel's "Super Stock" plug cables

That would make up a cost effective yet well balanced ignition system. It is what I will use as soon as I get the chance to rebuild my engine (the Swedish law won’t accept more than 1kW per 10kg when I register the car for road use, so no use to rebuild it until I got all the papers in place).

/ Rob

07-10-01, 06:23 PM
>I would guess that most bang for the buck for your 350 should be:
>That would make up a cost effective yet well balanced ignition system.

Sorry I am answering before I read the question... I was thinking ignition system and you where asking about the alternator and battery... Sorry no idea at the momen... I still think my post about the ignition system would make up a good one thou :7

Technology is the answer, what was the question?

/ Rob :+

09-10-01, 11:11 AM
I run a 300W (thats RMS across both channels) Amp in my 1.05L Polo with a factory alternator. It works fine with a direct feed from the battery in 6sq mm cable. Peak current should be about 30A (which is what it is fused at). I run it hard most of the time (as my sub is quite power-hungry). I have never had any problems with this configuration, and it has mad no significant reduction in my max speed or fuel economy. :) The only problem I did have was with cooling, but I sorted this by bolting the amp to the floor rather that the back of the seats.

09-10-01, 01:40 PM
Thanks for responding. I definitely don’t want to spend unless I have to. Reason I suspect is that twice on short runs after stopping and trying to restart the starter motor seems reluctant to turn the engine. Once it starts turning the motor it is reasonably sluggish. Battery new & heavy duty with 780 cranking amps. I’ve only experienced this since amplifyer fitted. New amp whilst advertised at 600 watts is true 120 watts per channel so I assume around 15 to 20 amps drawn, but take into account the following.
2 No Cooling Fans ? amps; Lights-10 to 15 amps; MSD Multi spark Ignition-?; Holley competition fuel pump- ?;
I would value your opinion? Unfortunately the literature that comes with the equipment dosen’t give current draw so I can’t work out the overall requirement.
:-) :-)

10-10-01, 01:20 PM
Mike - I think you will need to find out if your battery has indeed discharged, or whether your sluggish re-start problem is down to another cause, such as a bad earth, or heat soak into the starter. The only way to do this would be to replicate the situation where you get the slugguish starting, and measure the battery voltage (across the actual battery terminals) with an accurate voltmeter (not a halfords 5 tester!). I am at work right now, and can't remember the exact voltages to indicate charge state on a battery, but will post them if I can find them at home. I think that a battery voltage below about 13.5V would indicate a badly discharged unit, but I will check on this.

Your cooling fans will draw in excess of 20 amps. From memory I think an MSD ignition box draws around 10 amps peak, again I will look it up at home and post it, The fuel pump should draw around 4-5 Amps.

Remember that only the ignition and fuel pump are constant loads, the rest is transient.

I can recommend you buy an airflow battery conditioner to leave on the battery whilst not using the car - it will maintain the battery at peak charge and prolong it's life. Also on these units there is a series of LED's which show the charge state when you first connect the unit, so you could see if your battery is becomining flattened during use. There is a web site www.calamander.co.uk/batteryconditioners/airflow.htm where you can see this unit - should be buyable at most motor accessory stores.


10-10-01, 01:35 PM
Your a star. I will visit site you suggest now. It sounds as if its worth getting one of these Airflow thingymijigs whatever my problem is.
Thanks for help.;-)

10-10-01, 09:34 PM
Mike - now home and have looked up the voltage you should see on your battery. Fully charged, and having been unused/uncharged for 3 hours, a fully charged battery should show 12.7Volts. If you go to www.traceengineering.com/technical/tech_notes/tn5.html

you will find a table of battery voltage vs charge %tge. At 50% charge, the battery would be down to 12.1 Volts. Note that this is a pretty low state of charge for a lead acid car battery and would indicate that the battery is being seriously over - discharged. Car batteries are not "deep discharge" batteries like the special ones used for fork lift trucks etc, and they don't like being discharged by more than about 80%, believe it or not.

You could also check your earth from the engine thus:

using a good quality voltmeter, set on, say 3 Volts full scale deflection (sorry, getting a bit jargon ish here - use the 3Volts DC or similar range), hold the red lead onto bare metal on the engine block(inlet manifold bolt should do), the black lead onto a good chassis earth, disconnect your main coil HT lead (so the engine won't fire), and have your radio-loving assistant crank the engine over on the starter for long enough for you to see the reading on the voltmeter - say 5 seconds max.
If you read more than about 0.5V, your earth return path from the engine to the chassis is not good enough. Try this with engine cold and after a run to see if heat is making any difference.

The MSD ignition should draw around 12Amps.

My old car had poor cranking speed problems at one time - I shelled out for a new battery before I realised it was actually poor earth connections, so check that first.



11-10-01, 08:44 AM
Thanks again Wilf
With your help, I will be able to sit an Electrical Engineering exam soon.


11-10-01, 01:11 PM
Just like to add that if you regularly expose your ears to music at what may to you appear to be even fairly modest sound levels, coupled with the constant road noise you get driving an open-top car - you greatly increase your chances of permanent hearing damage sooner or later . You don't even need side pipes for this !

My Tinnitus kicked in suddenly one day about 3 or so years ago (when I was 29), and I'm stuck with a high-pitched whine that's there all the time. Not much fun initially, but you get to put up with it. Sometimes its so bad it actually wakes me up in the night. A friend with a fairly big bike gets a nice low frequency droning Tinnitus - this drove him to desparation , but after psychological councelling he's now learned to deal with it. Two mechanic friends of mine also suffer moderate to severe Tinnitus.

We all take our hearing for granted. There's no way that I can go out in my Cobra now without wearing ear plugs - every single time. Otherwise I end aggrivating the Tinnitus and will get a headache that lasts me usually for the rest of the week. Takes the fun away a bit !

Be Careful - you only get one set of ears !


11-10-01, 01:34 PM
Pardon?:-) :-) :-)