Torque or Power for a Race engine?

[Trevor Phillips]

Looking to build a new 2.0 litre race engine for next season and would appreciate the technical arguments as to whether you should specify:

1. Tuned for outright power with narrow power band [6500 to 9000rpm] and hairy cams or

2. 30 bhp less but peak power approx 750 rpm lower and stronger torque throughout [approx. 12% over the rev range], max revs 500 lower

 

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[Richard Ince]

Sounds like a Duratec to me.

[DanB]

Presumably the answer will depend on the gearing - if the gearing of the car is such that you'd be able to keep it in the 6.5k-9k power band at all times then I can't see why you shouldn't go for the lairier engine. After all, 2.5k rpm isn't that narrow a power band.

 

Dan

[Highlander]

I've jsut had an attack of Deja-Vu

[Peter Carmichael]

Have you got the torque/power curves? I can plot these to give you a normalised comparison.

 

FWIW, my (championship winning) big engine was properly on song from 6500 to 9300. It had no nasties in the torque delivery so that from 4000rpm it was pulling hard. Dropping it into the 4000-6500 range allowed you to have a less frenetic torque delivery exiting corners, but these were circumstances where I was feathering the throttle anyway.

 

You can probably find an argument to advise both routes. The normalised plot will show you how low down the rev range you theoretically have to drop before the torquier engine generates an advantage. This might be lower than you think.

[Graham Perry]

Or even Deja vu, all over again 😬

[Trevor Phillips]

Peter,

No power curves available. Just based on plausible [but not proven] figures being quoted at me by reputable (unless your name is ) engine suppliers.

IMHO it really isn't until you see the engine on a dyno that you get the true picture of how it all went together. Then all these myths about outputs are dispelled.

My current 255 bhp n/a Cosworth engine from my Sylva has a good blend of power and torque. I haven't the time to engineer a transplant into the Caterham [currently Vauxhall on TB's at 205 bhp] so need to sell both and get something that drops in easily with good driveability and a target of 550 bhp/ton[ 🤔ne.

Duratech? or all out BDG?

 

Racing pics and items for sale here

[Peter Carmichael]

Hmmm. I don't like the sound of the 12% more torque claim across the rev range cos you're not going to see any more than 96lbft/litre whatever you try.

 

What you have to work out is whether this is because the "high power" engine is dropping torque or whether the "high torque" engine is making bogus claims. Buy whichever engine makes 190lbft of torque and sustains it to high rpm. If the high power engine is making just 171lbft (add 12% to this and you get 192) then it is a rubbish engine. If the high torque engine claims 200+lbft of torque then it is an outright lie, so go with the high power variant.

 

The high power engine should produce almost the same torque peak value, but over a narrower range. If it has the same porting work, then the VE should be very close.

 

Peak power at 9000rpm on a balls out 2.0 litre should be 290bhp.

 

Peak power of 260bhp (30 bhp less) at 8250rpm would be ~185 lbft peak using the normal rules of thumb - this is less torque than used in the 290bhp calculation, which makes the 290bhp one seem like an overambitious claim.

 

I reckon a 290bhp engine is rather more than many notable competitors have achieved for sprint use - sprint use and drag races are one thing, but race use would make me worry about the thing grenading.

 

On this basis I would advise not aiming to go much beyond 260bhp... See if that helps.

[EFA]

"On this basis I would advise not aiming to go much beyond 260bhp... See if that helps."

 

You're worrying him Trevor..... After all Peter very nearly got beaten by someone as useless at driving as me in what everyone on this forum knows is an engine well down on power against my claims

 

 

 

Regards

 

Arnie Webb

Trip Organiser

 

To book for this years Le Mans Trip see The Le Mans Trip Website

[Smokin Joe]

IMHO

 

Torque means instant steam in any gear without having to wait for the revs to build. For a faster street car go with the torque. How often on the street can you be balls to the wall in every gear all the time. I dyno at 300lbs + from 3500-6500 rpm with the V6 whilst the HP peaks at 352 @7000 (Flywheel measurements)

 

with my T5 gearbox and 3.42 posi rear end It is just a great ride in any gear all the time.

 

Cheers Joe

[EFA]

Peter,

 

Does this 96lb/ft/litre rule apply to engines regardless of cylinder count?

 

Is there a link between this figure and the extent to which the gasflow rate through the engine can be maximised???

 

Fat Arn

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See the Lotus Seven Club 4 Counties Area Website here

 

[Peter Carmichael]

The 96lbft per litre is determined by the combustion chamber type and is for pent roof four valve per cylinder naturally aspirated designs. Consider what a single cylinder in your four cylinder engine achieves:

 

one quarter of the torque, from one quarter of the capacity. i.e. still 96 lbft/litre at best.

 

This is a good guide whatever the cylinder count.

 

Please remember that this is a rule of thumb, so is not the last word in definitive laws. It is based on the difficulty of filling a cylinder effectively. You can probably arrange for all your exhaust and inlet resonances to constructively peak at a marginally higher figure, but this will give a very sharp torque peak with a sharp fall off of torque - a peaky engine; nasty.

 

The reason for not going above 260 bhp is because it is more embarrassing when you lose. Next year I will be competing with my revived Supersport engine on throttle bodies: ~150bhp and deliciously driveable. I reckon that by paying attention to the chassis setup I will still be chasing sub-60 second double lappers at Curborough. I will also be venturing further afield than the club events and if I get on the hills I reckon that 150bhp will be plenty to keep me busy.

[Graham Perry]

Sub 60 with 150 BHP........ Jeeeezzz. I think its time I retired !!

 

Graham (high 63 sec best ) Perry

[Peter Carmichael]

Well Powder Puff is pushing 61.5 seconds with a standard Supersport on ACB10s and I have all the stops pulled out on a chassis that was running 58.1 in test with the big engine. I reckon on balance that 60 seconds is possible if I remember how to change gear...