What makes a good rally engine?

What makes a good rally engine?

Author
Discussion

camelotr

Original Poster:

570 posts

167 months

Tuesday 22nd November 2011
quotequote all
"Mechanics dream": I was asked to build a historic rally mini.

Happy and hurray and... well... I am lost in the wilderness.

I have some ideas but would like to hear Your opinion. What makes a good rally engine?

How to build a good 998 rally engine (post 70 mkIII mini)?

Greatefull on any comment.

davepoth

29,395 posts

198 months

Tuesday 22nd November 2011
quotequote all
You want good torque more than anything with a rally engine IMO, but it's going to depend on the owner - how good are they? A really, really top driver can probably work with a peaky engine to get the most out of it, but someone doing at as a hobby will likely want something a bit friendlier.

So the easiest thing to do would probably to plump for a 1275 block. smile

Other than that I guess balancing the bottom end would be good, perhaps lightening the flywheel a bit too, but not too much. Put the big effort into the head, get it to flow properly, match the ports, and make sure the carburettors are spot on, assuming you go for a pair.

DVandrews

1,315 posts

282 months

Tuesday 22nd November 2011
quotequote all
Obviously work on the head is the key to good airflow, on small bore engines you can use the 12g295 998 cooper casting with 1300 sized valves (you may need pockets in the block to clear the exhaust valves), you could persuade the long head in 12G940 form or 12g185(cooper S) form onto the small bore block. Cam choice will determine the nature of the power delivery, in a previous life I spent a lot of time building 'A' series engines and have used just about every combination of head/valves/cams. IMO the 731 cam is a good bet giving reasonable low speed torque and yet delivering good power, the next step up would be the old formula junior cam the 544 profile. 649 and above (sprint/supersprint) produce good power but at the cost of low down torque. A decent LCB manifold will help a lot with torque as will careful timing of the cam.

I am sure there are other profiles from Piper/Kent/Dave Newman that will do a fine job but do make sure you dont go for too much duration.

Dave

stevieturbo

17,229 posts

246 months

Tuesday 22nd November 2011
quotequote all
camelotr said:
"Mechanics dream": I was asked to build a historic rally mini.

Happy and hurray and... well... I am lost in the wilderness.

I have some ideas but would like to hear Your opinion. What makes a good rally engine?

How to build a good 998 rally engine (post 70 mkIII mini)?

Greatefull on any comment.
Find out what rules you have to abide by first !!

Huff

3,140 posts

190 months

Tuesday 22nd November 2011
quotequote all
Exactly!

Are you limited to building from the 998cc A+, allowed de-stroking a '1275' block for an 'A-series' screamer - or can you cram-in a Yamaha R1 5PW engine? (which at 998 cc may just agree with the notional cc limit, but makes significantly more power than either...)

GavinPearson

5,715 posts

250 months

Wednesday 23rd November 2011
quotequote all
You need to have reliability, power and a fairly wide power band.

Personally if I was building this engine I would be speaking to a Mini Sport or Minisprint type league expert in engines and just throw money at the problem. Get a proven solution from proven experts with a fantastic track record and avoid an unnecessary learning curve.

You also need to make sure that the gearing is matched to the engine's characteristics.

I'd also consider that it will cost just as much to build a 1275cc engine as a 998 so why not go for that?

camelotr

Original Poster:

570 posts

167 months

Wednesday 23rd November 2011
quotequote all
Why 998? The driver is a beginner, and easier to achieve any success in the under 1000 class than in the 1000-1400.

I have to keep the original engine (even A+ is prohibited).

I have the homologation sheet and studied the J and K appendixes soo I think I see my rules.



camelotr

Original Poster:

570 posts

167 months

Wednesday 23rd November 2011
quotequote all
I have some options for the gearbox, soo I can mach it with the engine.

I need to know what are the most important "rules" of the engine. Reliability? Power? Torque? Wide power band? What peak rpm to look for?

stevieturbo

17,229 posts

246 months

Wednesday 23rd November 2011
quotequote all
camelotr said:
I have some options for the gearbox, soo I can mach it with the engine.

I need to know what are the most important "rules" of the engine. Reliability? Power? Torque? Wide power band? What peak rpm to look for?
I guess a lot will depend on the type of events. Although at 1000cc I guess it's never going to be a monster. So short gearing is a must.
For the engine, Vizards A-series book is always a fantastic read, and would cover most things. Even more important as I guess you'll be doing most of the work yourself. it will cover absolutely everything you need to know about the A-series tuning.

http://www.amazon.co.uk/Tuning-Engine-Definitive-P...

But the gearbox as you know isnt a simple swap if gearing needs changed, nor are diff ratios. So it may be worth asking other users about their gearing if you can, what speeds they achieve on different rallies etc and try and work something from there.
I dont think it will be difficult to make it reliable though. The A-series in n/a form is a pretty robust engine.


camelotr

Original Poster:

570 posts

167 months

Friday 25th November 2011
quotequote all
Thanks for the input soo far.

I have a "donor" engine I would like to alter to suit the rally needs.

It is a Mini Se7en engine done by a well-known specalist.

The spec of the engine as far as I seen now:
- Omega forged (?) pistons with not very high compression (not measured yet).
- MG Metro cam (252in/268ex)
- MED crankshaft lightened quite well
- lightened std conrods
- 12G940 head well modified with 31mm ex and 36mm in valves - done by a very good specialist
- bronze guides
- 1.3 forged rockers
- very light flywheel
- sintered clutch
- grey diapragm

The engine did 97Bhps. Redline at 8K.

I will put the engine on a SC/CR box with the BMC ST ratios and a 4.57:1 sc/crosspin diff.
I would like to modify it to suit a rally car. No matter if we sacrifise some of the peak power. I dont think we will use it at 8k on a rally. I plan to get the switclight set somewhere at 7300.

Things I was thinking about to do:
- replace the head with a more mundane item (rimflow 31mm, 35,6mm and only port it slightly - in order to get a decent idle and some low-down torque.
- replace the sintered clutch with a normal heavy duty item, and also replace the diapragm with a normal 1275S item (green?).
- Is it mandatory to replace the flywheel with a heavier one?
- any other ideas?

DVandrews

1,315 posts

282 months

Friday 25th November 2011
quotequote all
I'm surprised you dont have more low down torque with such a mild cam. Sounds like something else might be awry.

With thiose valve sizes the head is at risk of cracking between the inlet and exhaust valves, so a more modest set of valves is another casting might prove more reliable.

If it were me I would go with a little more cam to bring up the mid range (4000-6500) a bit more.

I'd use an orange diaphragm

Dave

Pumaracing

2,089 posts

206 months

Friday 25th November 2011
quotequote all
camelotr said:
Things I was thinking about to do:
- replace the head with a more mundane item (rimflow 31mm, 35,6mm and only port it slightly - in order to get a decent idle and some low-down torque.
- replace the sintered clutch with a normal heavy duty item, and also replace the diapragm with a normal 1275S item (green?).
- Is it mandatory to replace the flywheel with a heavier one?
- any other ideas?
I'm a little disappointed in you young Jedi. I would have thought the Force would have been stronger in you by now. Why do you think you need to change a perfectly good head to get idle and low down torque? The head spec has bugger all to do with that barring very small effects from port size and gas speed. Do 16v heads not idle just as well as 8v ones with much less valve area? Does the Ford Pinto engine with 42mm valves and 36mm ports not idle just as well as the Ford CVH one with the same 42mm valves but only 31mm ports? If port size or valve area had any great effect on idle or low down torque then most modern road engines wouldn't even run. It's cam duration that determines idle and low rpm behaviour - not the head!

Also why would you want to saddle any engine with a heavier flywheel than it already has just to add more inertia for it to lug around and consequently have less acceleration?

That's hardly a fire breather with only a standard MG Metro cam in it. In fact I'm sure it will make a perfectly good rally engine left well alone - or even a road one come to that. I doubt if it has quite as much as 97 bhp, closer to 90 bhp I think, but assuming low 70s ft lbs per litre at peak torque it should produce peak power at about 7000 - 7250, rev out to 8000 if required by gearing but not need to of course, pull from next to nothing with such a mild cam, certainly under 2000 rpm and be as happy going to Tescos for the shopping as blatting round a race track.

In fact it sounds to have been excellently designed using exactly the same rules as I use myself for any engine whether road, rally or race. Get the biggest valves you can physically fit inside the bores in there and then use as little cam duration as you can get away with to meet your target horsepower and reliability constrained rev limit. I suspect if the original budget had allowed for a stronger bottom end and stiffer valve springs he'd have used more cam and it would have well over 100 bhp.

Stick it in a car and drive it first to see how it goes before you bugger about with it and make it a lot worse. And read my website again!

camelotr

Original Poster:

570 posts

167 months

Friday 25th November 2011
quotequote all
Okay Master!

You are maybe one of a dozen species on this bare planet whom I take every lesson as rockhard fact.
But may I try to tell You my "whys"? And You will tell me why I think wrong smile.

Soo.

The ports of the head is huge. Realy. It is much bigger then I ever seen. Even on well modified 1380 engines.
I was told that theese race engines idle around 2k. The cam is mild - I accept. Soo I thought that the gas speed in the ports at low prm is low, soo the fuel atomisation would suffer. This engine was built for track racing (Mini Se7en), soo no real problem with idle/low torque.

The same applies to the flywheel: it was built to rev high. But idling speed may suffer, and also the negine may be hesitant to pick up reves from low rpm. But I would be happy to use the light flywheel, just would like to make this engine idle at around 1000.

Padavan is waiting for the force-transition biggrin.

camelotr

Original Poster:

570 posts

167 months

Friday 25th November 2011
quotequote all
The original cam choice was due to class limits (use of this cam is mandatory).

Pumaracing

2,089 posts

206 months

Friday 25th November 2011
quotequote all
The engine will idle at the same rpm as any other engine with the same cam in it. Stop listening to what know-nothings tell you and try the engine out first.

Edited by Pumaracing on Friday 25th November 11:13

camelotr

Original Poster:

570 posts

167 months

Friday 25th November 2011
quotequote all
I must rebuild the engine as
a, this block does not meet the homologation of our class
b, on one cylinder bore I found a nasty casting error, which I would never accept in a race engine
c, the tranfer gear failed apperently on its first race, soo it needs sorting out.

Thus if I have to rebuild the negine, I would like to make the changes which looks promissing.

camelotr

Original Poster:

570 posts

167 months

Saturday 26th November 2011
quotequote all
As I told before, I intend to move the engine containment into a homologed block. It is mostly straight forevard. I think the best would be to copy the clearances of the original engine.

Though some controll may not hurt.

Soo I measured the clearances.

Piston-bore clearance - 0.09-0.1mm. The piston is forged. Do I realy need such a huge clearance?
Piston ring end gaps: 0.6-0.8mm. Same question here.
Crankshaft main bearing clearance 0.03mm.
Big end bearing clearance 0.04mm.
Valve-deck clearance 0.15-0.2mm, plus head gasket (app. 0.8mm).
Valve stem-guide clearance with bronze guides 0.7-0.9mm.

The honing pattern is "shallow", maybe 25-30degree and quite fine. Is it ok?

Were do I need to take care if I am building a high reving engine?

duncancallum

839 posts

177 months

Saturday 26th November 2011
quotequote all
are you allowed to put a mainstrap on on a 4bolt centrecap?

With it being historic do you have any parts limitations like roller rockers, cam belt conversions and the like?

camelotr

Original Poster:

570 posts

167 months

Saturday 26th November 2011
quotequote all
Mainstrap only the 2 bolt type, using the original main bearing housing.
No belt conversion allowed. No roller rockers. Only "S" forged rockers homologed with 1.3 ratio.

Pumaracing

2,089 posts

206 months

Wednesday 30th November 2011
quotequote all
We've been through engine clearances in great detail in previous threads. I also wrote at length about forged piston design and clearances recently which you should look at.