18 replies to this topic

[grizzler73]

Hello all

I have recently joined the forum as I have just brought a 1971 GTM (Mini based kit car) and intend to build it for racing in the HSCC 70's Roadsports Championship. Now I know it is not a mini but pretty much all the moving bits are from a mini so if anyone can help me I think they will be here!

 My question is engine related, now I could go out and get myself a full race engine from Swifttune for 20k but I don't have the money, also I am restricted by the regs to a 1275 (max overbore +60 thou) with standard bore and stroke, it has to run twin HS2 (1 1/4") SU's and no SC gears or LSD. Do any of you guys have a good idea of what kind of spec engine I should go for? I would like it to be relatively tractable as it will be driven to/from the track but with good torque and revs. Oh and easily last a season!

Thanks in advance for any advice.

Grizzler

 

[nicklouse]

First things first. Welcome.

Are you mad?

Important things read the regs. Read the regs.

On to the engine. Balanced. Strap the main centre bearing as a minimum. Flow the oil pump. Light flywheel and backplate. Grey spring and suitable friction plate.

Power comes from everything above the Pistons. The head needs to suit the cam. What limits are there on the Cam. I would go for a rally speced one as you will not have mad revs but torque is good.
Carbs. Do the Vizard mods to them good trumpets and filters.

And a four pin diff for strength.

Good LCB medium bore to help keep the gas speeds up.

My old '67

Edited by nicklouse, 22 November 2015 - 11:46 PM.

[grizzler73]

thank you for the welcome!

Not sure why you ask if I am mad, there are many reasons I suppose, choice of car being just one...

Engine wise I had a feeing that would be the reply, I think I will talk to one of the mini engine builders and see what they recommend as well.

There is no limit on the cam profile, or any other engine internals. I am torn between buying all the bits and doing it all myself and going out and buying a complete unit. I think I may end up compromising and buying a stage 3 or 4 short engine and doing the head and other bits myself.

Nice GTM by the way, is it still around? Mine is about 70% rust at the moment so have a fair way to go!

[nicklouse]

Mad in that they are not the nicest of adding cars when pushed. The front does go light. Well the early ones. Did. Hence the later had the revised front. The rear steering lock off does flex so look at that or you get some fun cornering when pushing on a bit.

Otherwise great fun.

70% rust nice mine was more than that. I was the first to put an engine in it. After replacing most of the metal. I believe it is about. I was spotted at the odd hill climb after I sold it.

Without skin.

[Cooperman]

The requirement for no SC gears does restrict the cam which will work.

So first of all look at what you can do in terms of gearing. It should be possible to use the Cooper 'S' helical gear ratios, which are closer than normal gears, plus a lower final drive ratio (FDR). The old 'S' ratios work well with a 3.76:1 FDR and in the '60's many Minis were raced with this set-up.

Once you have the gear ratios you can select a cam which has a profile allowing you to use the torque/power curves with those gears.

In your case the choice of cam will be the critical thing on which your performance will be determined. If you 'over-cam' it you will struggle to keep it 'on the cam' when you change up due to the relatively big gaps between gears. A 276 cam or equivalent should be considered, but review the curves against the allowable gearing.

Of course, a very well-flowed head with the best valve sizes (see the regs again), a high compression ratio, top quality build with great accuracy and correct cam timing will give best results. Match/balance all components and use a lightened flywheel.

Being limited to twin 1.25" carbs will mean there is a need to go for excellent torque rather than heaps of top-end power, as the carbs will be strangled at over about 6200 rpm as sustained revs.

Bore to +0.060" which gives 1330 cc, but use the increased capacity to maximise the torque.

It's a good idea to use 'Total Seal' piston rings which have a gapless 2nd ring.

[nicklouse]

Do you have to run those carbs?

[grizzler73]

Thanks for that Cooper man, I can use the S gearbox ratios and valve sizes are free, it means I have more chance to get the torque in the right rev range if I get it all right. i am leaning towards buying a short block already lightened and balanced and doing the head myself, can you expand on the Total seal rings? I've not heard that before, are they on a specific piston type?

Nicklouse

I have a later Howard Heerey car, he added a reversed wishbone to the rear with a rose joint where the lower arm attaches to the subframe which improved matters but I think I will have to improve the location slightly. but you are right they have a reputation of being a bit snappy, should make it fun!

[grizzler73]

Nicklouse

yes regs are very specific HS2 only, but it means all the other A series have to too! Also I have to use 7.4" discs, but it only weight 550kg so not too bad.

[nicklouse]

The problem is actually the inner track rodend used to lock the steering out. It flexes too much. Also the steering arm flexes like mad.

Even the S ones.

So I had some alternatives made up after a long chat with Alan Staniforth. He did a bit with front Mini suspension in the back of race cars. Aldo rose jointed it both ends.

I still have the template will see if I can find a picture.

Here we are

Edited by nicklouse, 23 November 2015 - 11:31 PM.

[nicklouse]

Oh yes that A frame thing. I remember seeing one once. Can't remember it though. But I was better than the originals. Shame they went in a house move. A brazed tube on the bottom of the hub the a rubber mounted blog through it with two adjustable links back to track rod ends into the subframe. Standard top arm. Mmmmmm flexy.

[grizzler73]

I think I will follow the Terrapin route and flip the hub and use the steering arm bolts to secure a machined bloc to take the joints.

Powerflex at the hub side and rose joint to the subframe, then the adjustable tie rods back to the subframe. should be adjustable and steer free. just have to draw it up now....

[nicklouse]

might be an idea to look at some of the aftermarket hub that you can get with the three steering arm fixing bolt holes.

 

will give a better join but might not work directly with genuine S brakes but could work with reduced diameter 8.4 diamter discs.

 

eg KAD can supply such hubs if requested but problems with the discs...

http://www.kentautod...-hub-assemblies

[grizzler73]

I have looked at these and the minisport ones, trouble is I have to use drums on the back as it is in the regs and also the general consensus between GTM peeps that it is better for circuit racing. so it may not be worth the expense. I think I will get in contact with KAD and see if you can easily modify them to drum brakes, also means I can use beefier driveshafts.

[nicklouse]

you can use the big drive shafts any way but you are limited to the drum CV joints.

 

if you must run drums you are limited to a degree.

 

most of "road" ones run discs all round with the handbrake set up from the later GTMs.

 

good luck.

[fwdracer]

Another advocate of the close ratio GT/'S spec ratio gearbox. Shortening the final drive? My brother ran 3.44 on 12's with his GTM. There is less weight and frontal area and given the carburation restrictions, these will limit engine rpm potential - so I'd major on torque, no point on having a diff that allow the engine to wind to 7K if there is no performance there.

 

If you are allowed to use high lift rockers, consider them with any available cam spec. Look at SW8 - like a 286 but with more torque.