120 replies to this topic

[AndyMiniMad.]

Mmmm....Im also a bit confused as to why a 1380 cant be reved beyond 6000 without the risk of a big bang!!! My cam is designed to come "on" at 2500revs and pull strongly up to 7250. Peak power at 6250. The head is sorted, the bottom end has a strap. The crank was from an MG metro and was perfect so didnt need any grinding. As I mentioned earlier the flywheel is lightened (Mini Spares), I didnt think It needed a balance being a non Race engine, But am dissapointed to hear I cant rev beyond 6000. My mates 1380 engine is used in his Rally clubman and he has not had any problems even though it is up around the 8000 rev mark on a regular basis. His spec is very similar to mine!!! Although his cam is much wilder.

[1977 Loud_Mini]

I used to sometimes take my A series 998 engine past 6000rpm and that didn't even have a centre main strap so i can't see why a 1380 (105cc more than 1275) would break if run over that mark and everything is in good condition. Also driving a car with a high lift cam would surely only mean you need a little more revs and slip the clutch for longer to pull away, as i will be using this engine everyday in my mini.
I seen numerous videos of someone actually pulling away in traffic several times with hardly any revs and that was on either a 300 or 310 lift cam. So i'll see how mine is when it's built and add some comments about drivability.

Evan.

[bmcecosse]

You CAN rev a 1380 beyond 6000 - but there is little point because the head (I'm assuming a 5 porter) can't flow enough air to make the power! So it's better to think of it as a high torque engine - fit a cam to suit (I would use the 544) - and then you will have a torquey/powerful engine that will LAST!
And yes - the gearbox can be modified to run 'dry sump' - but I'm not sure how long they last like that! A windage tray may be worth having - although it's really there to cut the massive upwards spray from the gears into the engine - rather than to cut 'windage'.

[Cooperman]

You will be OK so long as you have a straight-cut close-ratio gearbox so that when you change up the cam will still be working. I guess the change-up revs will normally be around 6500 to 7000 if driving quickly. Of course, you'll need a low ratio final drive, possibly a 3.9:1 would work, although even then 1st gear will be a bit high, but the engine will stay 'on-cam' quite well with that ratio. Cruising speed would then be around 70 mph at 5000 rpm (or 84 at 6000 rpm, which would be right on cam).
A 998 revs better than a 1275 (or 1380) as it's a much shorter stroke and nearer-to-square engine.
Presumably with the 296 cam, this will mainly be a track car as it will struggle a bit on the road with the need for those revs all the time.My own 1310 cc Cooper 'S' with a 286 cam is not nice on the road (horrible, in fact and not as nice as my standard 1973 Innocenti Cooper 1330), but fantastic on rallies where the thing can be kept 'on cam' and over about 5000 rpm.

[mini93]

You CAN rev a 1380 beyond 6000 - but there is little point because the head (I'm assuming a 5 porter) can't flow enough air to make the power!

you really just cant make a comment like that what a load of tosh! im sure you know as well as everyone else it depends what spec you have everything at!

[Cooperman]

For a 1380 to work at high revs, the critical thing is the head. As 'bmc...' says, a 5-port head will struggle. It will need 37.5 mm inlet valves and 31 mm exhausts and really radical gas flowing. The average commercial moidified head may not be able to work well enough. Remember, we are talking about the Kent 296 cam here which is really a racing cam with high-overlap and at 1380 to get the right amount of mixture in at, say, 6700 rpm, which is the quoted max power point of that cam, will require a very free-flowing head.
I think the point 'bmc...' was making was that with 1380 cc unless you have that sort of head to go with the race cam, you just won't get the breathing right and the cylinders may not fill properly. I've seen that with softer cams and not very good heads. They just don't make the expected power.
However, as you rightly say, to make the definitive remark, the total and full engine spec would be necessary. From what has been said already we don't know what the head spec is. To get sufficient mixture in the big valves, probably not with unleaded seats will be needed, maybe offset valves to get the necessary size (leaded or unleaded). If the valves are too small and the cam is the 296, then the engine will struggle to run at low revs because of the cam overlap and struggle to breathe at the top end because of the head restriction- the worst of all worlds in fact. It could end up with less 'under the graph' than a 1275 with the right bits on. Capacity is only any use if the mixture can get in efficiently.
Building a race 1380 is not as easy as might be thought.

[samsfern]

as i say, i use mine everyday, mines fine in traffic and pulling away, hardly have to slip the clutch, can pootle aroud off the cam as well quite happily, fair enough theres not much power off the cam, but itll happily cruise along at 40 in 4th. But as said, you need the head to match the cam spec, otherwise theres no point having a hot cam. You need to get the compression right for that cam to work properly, im guessing itll be around the 12:1 region.

[1977 Loud_Mini]

I will be getting a cylinder head with porting work and 36/31 valves for this engine when funds allow. I was aiming at 11.5 CR but suppose it wouldn't hurt for 12:1 but i didn't think it would make that much difference as i'm not racing the car around a track.

[Cooperman]

I will be getting a cylinder head with porting work and 36/31 valves for this engine when funds allow. I was aiming at 11.5 CR but suppose it wouldn't hurt for 12:1 but i didn't think it would make that much difference as i'm not racing the car around a track.

The valves may be a bit small at 36 mm, which are probably actually 35.6 mm. At higher than 11:1 you will need premium petrol with octane booster or it is likely to 'pink' and bust the piston rings. The ported head needs to be fitted at build time or you might struggle as the head must match the cam.
I must say I'm surprised you are looking at a race cam for a road car. Remember, at 1380 the block will probably not be able to take another bore and using the sort of revs you'll need to release the full power will cause the bores to wear quickly, thus scrapping the block after a relatively short time. I'm finding it hard to understand why you want such an unsuitable cam for a road car as at, shall we say, normal revs, it will be very down on power, as 'samsfern' says. I think you will find it 'unfriendly' in normal traffic and to keep up with modern cars will mean using mega revs to keep it on-the-cam.
Whatever you do, don't try to run a 1380 with a 296 cam and a standard head. You will need a head professionally done by one of the top head modifiers which will cost, at a guess, around £750. An ordinary modified head from Mini-Sport or Mini Spares will probably not be flowed well enough. Remember, it's a racing engine you are building, not a road engine or even a rally engine.

[MRA]

1380, you still have 2 more rebores ..... 74mm & 74.7mm these will certainly NOT take the revs, but a properely built up 1380 will be fine with a 296 cam as the bigger engine capacity will help to overcome some of the inefficiencies of a race cam that you would typically see in a 1275cc engine...... a 5 port head on a 1380 will still work well, I have had quite a few over the years, however I would tend to suggest a 276 as a better option.

My question would also be what pistons are you using ?

1380 296 cam a fairly decent head based on the Cooper S or MG but mildly ported, good single box 1.875" to 2" exhausdt system and a IDA weber if funds allow...SCCR gearkit on 155 70 12" (165 65 12") 3.44:1 final drive will work well if built and dyno'ed correctly to achieve the desired results.

Edited by mra-minis.co.uk, 03 November 2010 - 09:51 PM.

[Cooperman]

1380, you still have 2 more rebores ..... 74mm & 74.7mm these will certainly NOT take the revs, but a properely built up 1380 will be fine with a 296 cam as the bigger engine capacity will help to overcome some of the inefficiencies of a race cam that you would typically see in a 1275cc engine...... a 5 port head on a 1380 will still work well, I have had quite a few over the years, however I would tend to suggest a 276 as a better option.

My question would also be what pistons are you using ?

Martin, I've seen a 1380 which someone tried to bore to 74 mm and it went through the cylinder wall. Now maybe that was just bad luck, but anything over 1380 is not guaranteed to go if you have an adverse tolerance on the block casting. I guess if you are racing or hill-climbing then a 1380 with a 296 and all the other transmission mods would be great with the very best head and running up to 7500 rpm+. For a road going car with, presumably, standard gearing and possibly a 3.1:1 FD plus a 296 cam, etc, would seem a bit, shall we say, optimistic. Pistons could also be an issue if the intention is to run at between 1:11 and 12:1 at up to 7000 rpm as you so rightly say.

[MRA]

Well actually I can show you a 73.5mm bored block that has a hole in it customer brough tone along the other day and asked if anything could be done, this i sdown to carefull selection of the casting.

3.1:1 FD and a 296 would not work that well, a 3.44 was on it's limit, and needed a fair amount of revs to get it going........

I have used some of the best race heads on the market ..... Longman GT17 etc, these are almost useless on the road, as any temperature fluctuation, ie thrash up to the lights wait for them to change to green and it will crack between the valves

A fairly reasonable head not the ultimate will work well on the road, giving good reliability and fair power..... and still rev to 7500 easily enough. I would be very wary of the pistons as there are a lot of cheap imitaions..... ie they are NOT really pistons at all..... just door stops or talking pieces "wow what a big hole in that piston how did that happen"

Yes for certain, compression ratio & revs will kill a cheap inferior piston such as an AE..... or as they are now called Nural piston.

Also ignition timing will kill almost any piston if set incorrectly.

What BMC is saying above is that you will get better results from a lower duration cam shaft....... I guess what you want is to rev the nuts off it

ANy engine is a sum of its parts...... built right using the correct parts and it will give you good service.... 200bhp for 5 seconds deosn't even get you down the 1/4 mile 100 bhp with manners and reliability will still be giving power in 20 years.... IF th ecorrect parts and build are adhered to.

Edited by mra-minis.co.uk, 03 November 2010 - 10:12 PM.

[danrock101]

It works out cheaper for me to buy a new block and re-bore that, than to buy new pistons, omegas are £300 for cast ones, how long is going to be before you need to re-bore the block? My engine in my mini is 40 years old and hasn't had a re-bore

my long stroke engine with 73.5mm bore should rev to 8k with a 296 (though still thinking if sticking my 286 in) and MED rally head. I've seen a few decent second hand heads go for less than £300. You will certainly need a good head if you're using a 296, as for your crank, for the sake of £100 you could get it wedged and cross drilled it should be fine to rev to 8k, I think most places charge £50 for each 'upgrade' £50 to have it back drilled, £50 to have it cross drilled (or is it £50 for both) £50 to have it wedged, £50 to have it knife edged

[Cooperman]

Well actually I can show you a 73.5mm bored block that has a hole in it customer brough tone along the other day and asked if anything could be done, this i sdown to carefull selection of the casting.

3.1:1 FD and a 296 would not work that well, a 3.44 was on it's limit, and needed a fair amount of revs to get it going........

I have used some of the best race heads on the market ..... Longman GT17 etc, these are almost useless on the road, as any temperature fluctuation, ie thrash up to the lights wait for them to change to green and it will crack between the valves

A fairly reasonable head not the ultimate will work well on the road, giving good reliability and fair power..... and still rev to 7500 easily enough. I would be very wary of the pistons as there are a lot of cheap imitaions..... ie they are NOT really pistons at all..... just door stops or talking pieces "wow what a big hole in that piston how did that happen"

Yes for certain, compression ratio & revs will kill a cheap inferior piston such as an AE..... or as they are now called Nural piston.

Also ignition timing will kill almost any piston if set incorrectly.

What BMC is saying above is that you will get better results from a lower duration cam shaft....... I guess what you want is to rev the nuts off it

ANy engine is a sum of its parts...... built right using the correct parts and it will give you good service.... 200bhp for 5 seconds deosn't even get you down the 1/4 mile 100 bhp with manners and reliability will still be giving power in 20 years.... IF th ecorrect parts and build are adhered to.

Absolutely, it's all down to quality build and suitability for the application.
I think a 3.76:1 final drive is about as high as is reasonable for a 296 if it is to be used in traffic, but that will mean quite a lot of revs to keep up with modern cars. Personally I wouldn't dream of using a 296 in a road car. In fact I would not go beyond a 276.
I am also thinking that a cross-pin diff would be strongly advised.
What are your thoughts on the current Hepolite 21253 piston? The oil holes are now replaced by slots, I notice, and it looks to be of lower quality then the older ones. I run Karl Schmidt in my rallly 'S' and they have always been fantastic, but I paid around £500 for the last set which I got from Germany a while back.

Edited by Cooperman, 03 November 2010 - 10:27 PM.

[MRA]

AE pistons are now made in Germany by Nural, so the quality will drop as they only see it a s piston for 1275 63bhp engines....... so it was over engineered (