Jump to content


Photo

Which Rods For A Blown 8 Port?


Best Answer mini13 , 12 June 2020 - 10:14 PM

As a little illustration of the issue heres a mishap i had a few years back see the attached...

 

as the journel is 1/4" smaller on the cooper S, this brings in the rest of the rod a similar amount, so it misses the cam lobe, the 1277 was oiginally designed like this as the 1275 S was the first of the erm... 1275s... when they lowered the material specs with the 1275GT they upped the big end size to compensate so all the original works 8 ports never had the issue as they were all S based engines.

 

on the modern 1275 1.75" journel H beam rods, they use a bolt, so that area is smaller and misses the lobe.

 

 

Go to the full post


  • Please log in to reply
12 replies to this topic

#1 Curley

Curley

    Speeding Along Now

  • TMF+ Member
  • PipPipPipPipPip
  • 466 posts
  • Location: Basingstoke, Hampshire
  • Local Club: Not MINI friendly

Posted 11 June 2020 - 10:35 PM

I'm looking at the cost vs gains of a NA KAD 16v vs a blown 8 port. I have pretty good info on what to expect from the 16V route but not so much for the 8 port. Fortunately I already have 90% of a bottom end. 4 speed dog box, unmolested block, billet 81.3mm A+ crank, flywheel etc, just need to decide direction and get the correct pistons for required compression ratio.

 

If I look at what is required for an 8 port, I understand that there's an issue with the rods clearing the cam lobs. Mini spares suggests using the AEG 625 rod as these clear without any modification. My first concern is if these standard rods are strong enough for sustained boost (not sure if supercharged or turbo yet).

 

The 8 port head specs list a 20.5cc chamber. Say we use a +40 piston, .20 deck hight, it looks like the MED 18cc pistons are the only real option that get the 8.something compression ratio I understand a blown A-series needs. The MED pistons are available in fix or floating pin.

 

Do I find a set of AEG625 and have the small end drilled for a floating pin? Is there a steel rod option available and if so is there any millage in playing with rod length? What sort of MAX boost could reliably be achieved with steel rods and crank?

 

J



#2 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,794 posts

Posted 12 June 2020 - 07:28 AM

its the rpm that stresses the rods not the "power" you'll be fine with the 623 rods,

 

generally with a boosted motor you'll be running a more docile cam, and have a lower max rpm anyway. so the rods will be less stressed, also theres a theory that higher cylinder pressures counter the rpm loading on the rod ( on the power stroke anyway).

 

one point you may have dificultty with, the A+ crank has 1.75" big ends and the 625 rods are for the 1.625" big ends on the S crank, so you'll either need to get them resized, or the alternative is H beam rods, you'll need to weigh up the cost of each route.



#3 mitch19

mitch19

    Learner Driver

  • Noobies
  • PipPip
  • 16 posts

Posted 12 June 2020 - 12:41 PM

Your bottom end would suit a kad 16v rather than a turbo as lots of guys are running serious turbo power on normal balanced cranks. If your weighing up between the two I’ve gone down the kad route so if you’d like a perspective on experience/costs etc feel free to pm me.

#4 Curley

Curley

    Speeding Along Now

  • TMF+ Member
  • PipPipPipPipPip
  • 466 posts
  • Location: Basingstoke, Hampshire
  • Local Club: Not MINI friendly

Posted 12 June 2020 - 06:15 PM

Thanks guys! What's so special about the 625 rods then that makes them suitable for 8 port conversions?



#5 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,794 posts

Posted 12 June 2020 - 06:35 PM

the only reason the 625's are used is that they miss the cam lobes.



#6 Curley

Curley

    Speeding Along Now

  • TMF+ Member
  • PipPipPipPipPip
  • 466 posts
  • Location: Basingstoke, Hampshire
  • Local Club: Not MINI friendly

Posted 12 June 2020 - 07:04 PM

the only reason the 625's are used is that they miss the cam lobes.

 LOL - thanks! I assume this is because they have a thinner waist or something? 



#7 imack

imack

    Camshaft & Stage Two Head

  • TMF+ Member
  • PipPipPipPipPipPipPipPip
  • 1,859 posts
  • Location: Orpington, Kent

Posted 12 June 2020 - 07:41 PM

I think it's the physically smaller big end size of the Cooper S aeg625 rod compared with a standard 1275 rod that gives the required clearance with the cam lobe.

#8 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,794 posts

Posted 12 June 2020 - 10:14 PM   Best Answer

As a little illustration of the issue heres a mishap i had a few years back see the attached...

 

as the journel is 1/4" smaller on the cooper S, this brings in the rest of the rod a similar amount, so it misses the cam lobe, the 1277 was oiginally designed like this as the 1275 S was the first of the erm... 1275s... when they lowered the material specs with the 1275GT they upped the big end size to compensate so all the original works 8 ports never had the issue as they were all S based engines.

 

on the modern 1275 1.75" journel H beam rods, they use a bolt, so that area is smaller and misses the lobe.

 

 

Attached Files



#9 Curley

Curley

    Speeding Along Now

  • TMF+ Member
  • PipPipPipPipPip
  • 466 posts
  • Location: Basingstoke, Hampshire
  • Local Club: Not MINI friendly

Posted 13 June 2020 - 01:47 AM

Thanks mini13! I was struggling to picture the problem but those photos are really helpful.



#10 Richspec

Richspec

    Learner Driver

  • TMF+ Member
  • PipPip
  • 22 posts

Posted 07 July 2020 - 09:24 AM

You can use  the bigger crank and rods but it does mean modding the nuts on the rod bolts. For that reason I'd say don't do it, although i have several times.

I've just had a block failure on my supercharged arden so I think as I need rods and stuff I might just go straight for the S parts this time round



#11 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,794 posts

Posted 07 July 2020 - 01:58 PM

how do you mod the nuts?



#12 Richspec

Richspec

    Learner Driver

  • TMF+ Member
  • PipPip
  • 22 posts

Posted 07 July 2020 - 07:31 PM

how do you mod the nuts?

With a file, just file till theres clearance on the rod bolt, can you see why i dont recommend it!! LOL!



#13 mini13

mini13

    Up Into Fourth

  • Members
  • PipPipPipPipPipPipPipPipPip
  • 3,794 posts

Posted 07 July 2020 - 07:39 PM

Ah ok, when you said nut i what thinking  eh???

I wouldnt trust clearanced ones after seeing how much my cam cleanece my rods in the pic! :lol:






0 user(s) are reading this topic

0 members, 0 guests, 0 anonymous users