1679cc more power
- mikiega
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- Joined: Sun Nov 17, 2013 3:43 pm
1679cc more power
I am rebuilding my 1679cc with a 69 stroke if I go up to 79 stroke what all would I need to do besides the new crank?
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Re: 1679cc more power
clearance the case
set the deck height
set the deck height
- mikiega
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Re: 1679cc more power
So I don't need to buy new connector rods?
- Marc
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Re: 1679cc more power
Over the years there have been at least 5 variations on the 88mm bore; most are different cylinder outside dimensions...one thing that's been in short supply is "B" pistons with a smaller compression height (higher wrist pin) so they could run longer strokes without huge cylinder spacers. Gene Berg used to sell some (I have a set of those in the shop) but I don't think they're available anymore...they were spendy anyway.
Shorter rods would be another answer, if you didn't have a problem with running a totally f*cked-up rodlength-to-stroke ratio, but Porsche-length is about all that's available (without doing completely weird poop like modifying Ford rods to fit) and they're only ~1mm shorter than stock VW - still 4mm too long to get "normal" piston deck with such a stroke.
First question I have is WHAT flavor of 88s do you have? If they're the "slip-in" style that do not require any machine work to the case & heads they're crap at 1879cc due to the scary-thin cylinder walls, let alone at a higher displacement. I read elsewhere that yours is an old-school Berg motor, which would most likely have the early machine-in cylinders (about the same thickness as stock...and obsolete). Since you can't find replacement cylinders that size anymore you'd need to bore the case & heads larger to fit the current type, so you may as well cut them for 90.5 or AA thickwall 92s (even 94mm may be possible but there are conditions). "B" pistons are readily available in those bores should you still want to increase the stroke. What's the case and head-stud diameter?
Next, where do you plan to pick up a 79mm crank? Readily-available strokes are 74/76/78/82/84 (sometimes you come across an old-school 78.4) but 79 would be a custom deal.
Shorter rods would be another answer, if you didn't have a problem with running a totally f*cked-up rodlength-to-stroke ratio, but Porsche-length is about all that's available (without doing completely weird poop like modifying Ford rods to fit) and they're only ~1mm shorter than stock VW - still 4mm too long to get "normal" piston deck with such a stroke.
First question I have is WHAT flavor of 88s do you have? If they're the "slip-in" style that do not require any machine work to the case & heads they're crap at 1879cc due to the scary-thin cylinder walls, let alone at a higher displacement. I read elsewhere that yours is an old-school Berg motor, which would most likely have the early machine-in cylinders (about the same thickness as stock...and obsolete). Since you can't find replacement cylinders that size anymore you'd need to bore the case & heads larger to fit the current type, so you may as well cut them for 90.5 or AA thickwall 92s (even 94mm may be possible but there are conditions). "B" pistons are readily available in those bores should you still want to increase the stroke. What's the case and head-stud diameter?
Next, where do you plan to pick up a 79mm crank? Readily-available strokes are 74/76/78/82/84 (sometimes you come across an old-school 78.4) but 79 would be a custom deal.
- mikiega
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Re: 1679cc more power
The engine that I have is a old Gene Berg build I stand corrected on the crank is one of GB 78mm cranks.
The case bore is 94.4mm and the cylinder walls are 4.8mm.
The case bore is 94.4mm and the cylinder walls are 4.8mm.
- Marc
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Re: 1679cc more power
Actually the thickest they could be is 4.5mm - but that's still thicker than a stock 1600 so there's no problem there. Without those "B" pistons you'll have a package-width issue though. 9mm more stroke means it'd take 4.5mm in barrel spacer thickness to get nominal piston deck height, so the engine ends up ~⅜" wider. Your exhaust and sheetmetal can probably still fit, but you will be needing longer pushrods. And of course as Madmike said, the case is going to need some clearancing (perhaps the cam, too, a little).mikiega wrote:The engine that I have is a old Gene Berg build I stand corrected on the crank is one of GB 78mm cranks.
The case bore is 94.4mm and the cylinder walls are 4.8mm.
The increase in displacement also raises the compression ratio (assuming the same piston deck & chamber CCs)...in this case by roughly a full point (i.e. from the ~7½:1 that Berg probably would have built it to 8½:1) so your fuel octane requirement's going to go up. Enlarging the combustion chamber volume's the best way to compensate, but odds are the heads are already semi-hemi-cut and ≥ 56cc already. Alternatively you can have shallow dishes milled into the piston tops which'll have the same effect, and/or increase the piston deck height (making the engine even wider if the same rods are used). The latter method is frowned upon because it destroys the "quench" effect at TDC which promotes turbulence for better mixture uniformity, but if you have hemi-cut heads there isn't much left of the quench pads in them anyway, so I'd probably do it. The intake ports are probably small enough that there's good velocity & decent turbulence without quench.
You could fit early 90.5 cylinders into that case/heads with no additional machinework, if you could find a good set (they've been out of production for years). No problem finding 90.5B pistons, but those cylinders are only ~3¼mm thick, barely thicker than classic 92s, so I wouldn't recommend going that route. "B" pistons' compression height is theoretically perfect for an 80.2mm stroke, so at 78mm you may run into the opposite problem and need to shorten the cylinders to set the piston deck.
Another thought would be to peddle or trade off the 78mm crank and go for something less ambitious. 74mm would give you 1800cc and need lighter modifications. With your heads & 44s that should still have no problem making an honest 100HP or so.
- mikiega
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- Joined: Sun Nov 17, 2013 3:43 pm
Re: 1679cc more power
Here are the specs on the Pistons and cylinders 88mm Cima/Mahle piston and cylinder set standard 69mm stroke (39.6mm pin height). These are thick wall spigots with cylinder sizes the same as standard 90.5mm, case 94.4mm, head 97mm, 6mm thick crown. Approximately 8.0:1 CR. (POTL) RW: 1.5/1.5/5. See Notes A, B, D, E. SW 30#
- Marc
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Re: 1679cc more power
Yes, those are the "normal" late machine-88s, with "A" pin height for stock stroke. Implied when you said they had 94.4mm spigots - the only others that do are the early 90.5s.
With 78 stroke I would not be surprised to find that the piston skirts need slight trimming. Case openings (and possibly also the cylinder spigots) may need to be notched a little for rod clearance...all par for the course with a stroker, more to deal with than there is when adding displacement with bigger pistons.
I know it's not what you asked, but if you were to leave the 69mm crank in and remachine the case & heads for the thickwall 92s now available (case 96, head ~101mm) you'd have >96% of the displacement of the 88x78 combination with far fewer issues...wouldn't be able to call it a "Berg" motor anymore though
With 78 stroke I would not be surprised to find that the piston skirts need slight trimming. Case openings (and possibly also the cylinder spigots) may need to be notched a little for rod clearance...all par for the course with a stroker, more to deal with than there is when adding displacement with bigger pistons.
I know it's not what you asked, but if you were to leave the 69mm crank in and remachine the case & heads for the thickwall 92s now available (case 96, head ~101mm) you'd have >96% of the displacement of the 88x78 combination with far fewer issues...wouldn't be able to call it a "Berg" motor anymore though
- Piledriver
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Re: 1679cc more power
There is a way to "save" semi-hemi cut heads, restoring squish area, and give yourself an effectively shorter piston...
...If the pistons have sufficient meat. (they probably do)
Pic is obviously T4 heads/pistons, but same idea, the semi-hemi cut and the "dome" match.
Plenty of squish area. (more that stock, actually, and it makes the effective chamber very shallow, and all the squish is aimed ~at the plug.)
Even if you can only get 10-13mm around the edge as squish area, it works.
The "dome" here was only ~3mm at the center of the piston IIRC, could be simple lathe work.
Got the idea from Harley builds, they do it to give their actual hemi chambers squish area.
(These are custom 4.060" JEs via LN Engineering)
You do need to be particularly careful with piston<>valve clearance.
...If the pistons have sufficient meat. (they probably do)
Pic is obviously T4 heads/pistons, but same idea, the semi-hemi cut and the "dome" match.
Plenty of squish area. (more that stock, actually, and it makes the effective chamber very shallow, and all the squish is aimed ~at the plug.)
Even if you can only get 10-13mm around the edge as squish area, it works.
The "dome" here was only ~3mm at the center of the piston IIRC, could be simple lathe work.
Got the idea from Harley builds, they do it to give their actual hemi chambers squish area.
(These are custom 4.060" JEs via LN Engineering)
You do need to be particularly careful with piston<>valve clearance.
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Addendum to Newtons first law:
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
zero vehicles on jackstands, square gets a fresh 090 and 1911, cabby gets a blower.
EZ3.6 Vanagon after that.(mounted, needs everything finished) then Creamsicle.
- mikiega
- Posts: 6
- Joined: Sun Nov 17, 2013 3:43 pm
Re: 1679cc more power
Thanks for all the info and advice. With this info I think the best thing isto repair the Berg motor and build another motor.