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Suggestions For A 998 Block Build

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#1 chocobochan

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Posted 03 October 2020 - 08:33 PM

I'm planning a holiday build for my old 998 block, my main goal is driveability but I do enjoy spirited driving and I do have a track in my city.
I found a 12G295 head with 80thou skimmed off and I was planning on going for a CR around 9.5 - 10.

 

I'm considering going as far as stripping and installing:

Cam: 250°-255° since I do enjoy some low end and a stable and silent idle, piper magnum or kent (maybe sw5 since I've heard marvels and read so much about them here!)

Pistons: Flat top circlip +60
Head: Port, angled valve seat cut

Rockers: 1.3
Carb: HIFF38

Lightening existing crank and rods.
Lightened flywheel.

Adequate intake and exhaust.

A good quality harmonic dampener (any recomendations?)

 

Although I more or less have a general idea of what I want I want to measure twice and cut once and make sure I have no oversights in my build, and this is where I have a few questions...

 

  1. Should I remove the 5-6cc's I need to achieve the CR from the chambers or the pistons? I don't think I'll find any good dished +60s for the 998 [Answered: Upping the CR to 10.5 means only a 2cc dish needs to be machined in the pistons]
  2. Are 1.5 rockers beneficial to a fast road car? I've seen them paired with the SW5 but I'm not sure they don't wear the cam and valve guides too much. [Answered: No]
  3. I've read about the 1100 crank and the exensive benefits it provides, but I've also heard it provides a very noisy, shakey and thirsty ride, balancing and lightening should take care of that, right? Since it's expensive and hard to come upon I was wondering if it was worth the effort.
  4. Vizzard goes wild about Hepolite 20950 KR pistons, I know this book isn't exactly modern so have modern developments left these pistons in the dust in comparison to what minispares offers or is it worth the hastle to hunt these down?
  5. The 12G295's valves are stock at 1 7/32" intake and 1" exhaust, will these be a limiting factor in my build? Is there any benefit to go bigger in a small bore?
  6. I was considering lightening my crank but then I can also pick up https://www.med-engi...econ-crankshaft since I'm not sure my machinist will be able to heat treat. On the other hand I can always pick up https://www.minispor...haft.html?qty=1 wich is wedged and I hope will run smoother, Thoughts?

Hopefully these aren't too many questions but I really want to get this build right and not simply throw pieces at a block and expect them to run smoothly.
Thanks for all your help


Edited by chocobochan, 10 October 2020 - 07:32 AM.


#2 Turbo Phil

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Posted 03 October 2020 - 08:54 PM

For a road going 998, don’t bother with 1.5 rockers and use a HIF38.
You’ll have to calculate the compression ratio by measuring to see what you have. With 80 thou off the 295 you’re probably looking at about 22cc in the chambers, but you need to measure this there’s no point guessing.
A rough calculation with flat tops +60, 22cc in the head, 0 piston/block face clearance and 2.8cc for the gasket puts your compression around 11.13-1.
So you may want to do a rethink on your piston/head choice.

Phil.

#3 chocobochan

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Posted 03 October 2020 - 11:46 PM

For a road going 998, don’t bother with 1.5 rockers and use a HIF38.
You’ll have to calculate the compression ratio by measuring to see what you have. With 80 thou off the 295 you’re probably looking at about 22cc in the chambers, but you need to measure this there’s no point guessing.
A rough calculation with flat tops +60, 22cc in the head, 0 piston/block face clearance and 2.8cc for the gasket puts your compression around 11.13-1.
So you may want to do a rethink on your piston/head choice.

Phil.

Sound advice, 1.5's are off the table, I though so.

 

Anyways you're absolutely right with the CR, I measured my head using a bourette and it came to exactly 22.46, wich sets my CR a little high, I'll either want to look into dished pistons or removing material from the chambers.

Capture.png

 

Is removing material without affecting the integrity of the head a hard job for a machine shop? I'd be adding aroung 4cc.

Or are there any decent 3 ring +60 pistons with dishes? All the ones I come upon are 4 ring and Vizzard recomends avoiding these.


Edited by chocobochan, 03 October 2020 - 11:49 PM.


#4 chocobochan

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Posted 04 October 2020 - 08:37 AM

turbophil just watched your videos and tight stuff!
I'm hoping a few cc's can be removed from the chambers without affecting the head's integrity, I saw your other posts and how most people have the "beak" removed.

 

On the other hand I've found +60 dished pistons that may have 3 rings and not 4 if I'm to trust the photo, so not everything is lost.
https://bmc.minispor...pe-060-set-of-4

With a piston dish of 5cc my CR is exactly 9.55 but I'm expecting the dish to be 6cc wich would put my CR at 9.28. I've messaged and asked for specifincs on the number of rings and cc's in the dish.

If I where to remove cc's from the head I could easily source son flat tops but I think that poor skimmed head has gone through enough.

 

Edit: I've also read some people machine the pistons and make a custom cc dish http://www.turbomini...p=vt&tid=298281

 


Edited by chocobochan, 04 October 2020 - 01:09 PM.


#5 Echan42

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Posted 04 October 2020 - 07:46 PM

I skimmed my 12G295 80 thou too, but my pistons where dished so my CR was 9.4:1, so I'm guessing you either have to cut 6cc from the chambers or have your pistons dished by a machinist.

 

No idea if any of those two operations are safe.



#6 blacktulip

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Posted 04 October 2020 - 09:10 PM

I had to make my old 12g295 chamber volume 25cc on +60 flat tops with the crowns flush to the deck to achieve 9.5cr

#7 Echan42

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Posted 04 October 2020 - 10:36 PM

Compression issues aside I think you have a solid build there.



#8 chocobochan

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Posted 05 October 2020 - 07:26 AM

I've updated the first question in the first post.



#9 chocobochan

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Posted 09 October 2020 - 03:17 PM

Updates! After careful consideration and some consulting I've upped the CR to 10.5, 95 RON on tap where I live.

This means my pistons need to be machined to include a 2cc dish.

Capture.png

 

I've upaded my questions on my first post.



#10 chocobochan

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Posted 12 October 2020 - 03:12 PM

Kinda set on the sw5-07 after running some numbers.

https://swiftune.com...-kit-2-955.html This seems like the best option, is the steel/alloy timing gear a sensible option for a road car?

 

Still seeking thoughts on the original post though, especially since I can pickup a 1100 crank for the same price as a wedged 998.


Edited by chocobochan, 12 October 2020 - 04:59 PM.


#11 cal844

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Posted 12 October 2020 - 03:53 PM

1100 engines are good but the 998 is better as it can take more of a thrashing

#12 chocobochan

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Posted 12 October 2020 - 05:01 PM

1100 engines are good but the 998 is better as it can take more of a thrashing

That's exactly what I was thinking, the crank will be wider and more prone to stress on the crank and rods and the travel distance of the pistons will be longer resulting in more friction and wear, right?



#13 ACDodd

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Posted 12 October 2020 - 05:38 PM

With that cam a 1100 crank means more capacity, that means torque. Road cars need that!

Ac

#14 Minigman

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Posted 12 October 2020 - 06:47 PM

As revvy and sweet as the 998 engines are you can’t beat the torque of a 1098. More grunt to pull you out the corners and back up to speed with!

#15 chocobochan

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Posted 12 October 2020 - 07:41 PM

As revvy and sweet as the 998 engines are you can’t beat the torque of a 1098. More grunt to pull you out the corners and back up to speed with!

 

 

With that cam a 1100 crank means more capacity, that means torque. Road cars need that!

Ac

I hear you, a 998 can definetly use a crank that brings more torque to the table, stroke is definetly more important than bore in this aplication, does the 1098 make the ride less smooth or the engine more noisy? I've read some opinions on this same forum that make me think the 998's crank is more sensible and roadworthy.

I've also read that most 1098 users attest for the strenth of the crank even though it's got a bad reputation for cranking.

 

Another limiting factor to take into account is that adding a 1098 crank would require I fit 6cc dish pistons to get my compression ratio down to sensible levels (~10:1) and I'n not finding any dished +060, 3 ringed, circlip pistons anywhere.


Edited by chocobochan, 12 October 2020 - 08:07 PM.






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