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3.9 Engine Build
- Thread starter Harv
- Start date
Still working away here.
Doing compression ratio check with the engine partially built (Engine on TDC for piston 1, old head gasket and head fitted) using the chamber fill method to get an acurate figure.
I have all the data to calc the CR but some of that data is manufacturer supplied, for example the piston data is -13.8cc which added to my heads would yield 63.8cc.
12:1 will be my absolute maximum limit and equally 10.5:1 would be too low so either a removal of material from Pistons or head maybe required to dial this in, there's the option also to add an additional shim into head the gaskets (stacking).
One thing I have struggled to find is the piston to head min clearance figure. At the top of the engines stroke you don't want the piston getting too close the the cylinder head face, a figure of around 0.037 - 0.040" seems acceptable (around the 1mm mark)
The printed plenum design is finalised and I have managed to get a consistent 3mm material wall thickness for the unit which should prove to be strong. The material is also good for 200degC though not expecting those kind of running temps even with heat soak. There will be some hand finishing required.
Doing compression ratio check with the engine partially built (Engine on TDC for piston 1, old head gasket and head fitted) using the chamber fill method to get an acurate figure.
I have all the data to calc the CR but some of that data is manufacturer supplied, for example the piston data is -13.8cc which added to my heads would yield 63.8cc.
12:1 will be my absolute maximum limit and equally 10.5:1 would be too low so either a removal of material from Pistons or head maybe required to dial this in, there's the option also to add an additional shim into head the gaskets (stacking).
One thing I have struggled to find is the piston to head min clearance figure. At the top of the engines stroke you don't want the piston getting too close the the cylinder head face, a figure of around 0.037 - 0.040" seems acceptable (around the 1mm mark)
The printed plenum design is finalised and I have managed to get a consistent 3mm material wall thickness for the unit which should prove to be strong. The material is also good for 200degC though not expecting those kind of running temps even with heat soak. There will be some hand finishing required.
From your post, I'm assuming your doing a dry build with Plasticine on the pistons to get those measurements? Good squish will promote turbulence and a quicker burn of the mixture. Though 1mm sounds the minimum you would want to go considering heat and material expansion at 7,000 rpm?
Not in the same league, but I built a Fiat SOHC engine with that sort of tolerance. The pistons actually came above the block deck into the gasket space by 0.5mm as the gasket was 1.8mm compressed thickness. It only needed 28.5 degrees advance even at 7,500 rpm.
Do you know who did the design and development work on the Wossner pistons, they might have some data?
Don't forget the volume of the ring land to piston/cylinder walls. You've probably got that in hand anyway......
Cheers
Not in the same league, but I built a Fiat SOHC engine with that sort of tolerance. The pistons actually came above the block deck into the gasket space by 0.5mm as the gasket was 1.8mm compressed thickness. It only needed 28.5 degrees advance even at 7,500 rpm.
Do you know who did the design and development work on the Wossner pistons, they might have some data?
Don't forget the volume of the ring land to piston/cylinder walls. You've probably got that in hand anyway......
Cheers
Some back of an envelope maths....
Cylinder volume = 643.5 cc
Chamber vol = 50 cc
Piston is dommed? = -13.8 cc
Gasket volume at 1.8mm (nominal compressed) thickness = 14.13 cc
Ring land volume approx = 0.8 cc?
Static Compression ratio = 13.6 : 1
Variables are gasket thickness and volume and piston dommed or dished. ie intruder or not?
Cylinder volume = 643.5 cc
Chamber vol = 50 cc
Piston is dommed? = -13.8 cc
Gasket volume at 1.8mm (nominal compressed) thickness = 14.13 cc
Ring land volume approx = 0.8 cc?
Static Compression ratio = 13.6 : 1
Variables are gasket thickness and volume and piston dommed or dished. ie intruder or not?
Harv, best to use a squish of 1.2-1.3 mm to allow for any carbon build up in the cylinder. If you have the squish too tight, the carbon will start to impact the head, which is not a good idea. In fact, on a large cylinder capacity, we have found more power by increasing not decreasing squish.
Measuring the volumes the way you are doing it is not accurate. It is very difficult to ensure all air is out of the chamber, it does some very strange things as fluid is added! Our preference is to use a plexiglass plate to measure the head volume, where the trapped air can be observed clinging for dear life to the surfaces.
To measure the piston crown and deck volume, move the piston down from TDC, lightly smear a little grease in the bore to seal the ring end gap, then return the piston to TDC again. We then make spacer rings of known volume (calculated from diameter and depth), with the same plexiglass plate greased on top to measure the nett volume. The simple calculation reveals your total intruder/dish volume volumes.
Head gasket wise, measure the compressed thickness and diameter to calculate the HG volume. This is okay as long as the gasket is circular, for those with cutouts, plexiglass with clamps to a surface plate works well.
A very quick calc from your figures above gives; 3900/6 = 650cc per cylinder. 650+63.8 total volume (if that's what you measured?)/ 63.8 gives 11.18:1
To find a theoretical compression volume from a known or desired CR, divide the swept volume by the CR-1. Example, for 12:1 desired CR, take 650/11 (12-1) = 59.09 unswept volume required. Feeding this back in gives 650+59.09= 709.09/59.09 which gives 12:1.
Shout if you need any help Martin.
Measuring the volumes the way you are doing it is not accurate. It is very difficult to ensure all air is out of the chamber, it does some very strange things as fluid is added! Our preference is to use a plexiglass plate to measure the head volume, where the trapped air can be observed clinging for dear life to the surfaces.
To measure the piston crown and deck volume, move the piston down from TDC, lightly smear a little grease in the bore to seal the ring end gap, then return the piston to TDC again. We then make spacer rings of known volume (calculated from diameter and depth), with the same plexiglass plate greased on top to measure the nett volume. The simple calculation reveals your total intruder/dish volume volumes.
Head gasket wise, measure the compressed thickness and diameter to calculate the HG volume. This is okay as long as the gasket is circular, for those with cutouts, plexiglass with clamps to a surface plate works well.
A very quick calc from your figures above gives; 3900/6 = 650cc per cylinder. 650+63.8 total volume (if that's what you measured?)/ 63.8 gives 11.18:1
To find a theoretical compression volume from a known or desired CR, divide the swept volume by the CR-1. Example, for 12:1 desired CR, take 650/11 (12-1) = 59.09 unswept volume required. Feeding this back in gives 650+59.09= 709.09/59.09 which gives 12:1.
Shout if you need any help Martin.
Thanks for you're replies.
As you can see, many ways of skinning a cat!
The pistons are not domed but have pockets and the manufacturer quotes the cc of them as - 13.8 but effectively this add's cc's to our total chamber volume.
Thanks Mike for the detail on measuring the cc of the piston crowns installed, I like that suggestion as I have done the heads with valves and plugs installed.
The cylinder heads gaskets for the 3.8 997 are 0.7mm compressed so quite thin.
I need to measure piston crown protrustion above the liners yet but believe its around 0.15mm
More once I have proper data :thumb:
As you can see, many ways of skinning a cat!
The pistons are not domed but have pockets and the manufacturer quotes the cc of them as - 13.8 but effectively this add's cc's to our total chamber volume.
Thanks Mike for the detail on measuring the cc of the piston crowns installed, I like that suggestion as I have done the heads with valves and plugs installed.
The cylinder heads gaskets for the 3.8 997 are 0.7mm compressed so quite thin.
I need to measure piston crown protrustion above the liners yet but believe its around 0.15mm
More once I have proper data :thumb:
Great work. I have a 3.4 that I was intending to build to 3.7 to go in my boxster. I see the picture of the under drive pulley.... I'm the guy that made those....or at least had them made!
Would be interested in buying a 3d plenum if you arwant thinking of making more.
Would be interested in buying a 3d plenum if you arwant thinking of making more.
Not to hijack anything, but .... my worry would be that the under drive pulley would exacerbate the low speed running cooling issues. Especially water flow to number 6 cylinder. Is this unfounded? I appreciate things are different in a track only car, so the question relates really to running this pulley on a road car.
I believe that the pulleys do not adversely effect the cooling.
Many racers in the US use them in much higher ambient temperatures than us. Supposedly they help with cavitation at higher engine speeds.
With regards to street driving. ..there seems to be a good amount of folk use these with seemingly no ill effects.. I have no hard data for any of this but likewise have heard no bad news against the use of them.
I have heard plenty of advice against lwfw though. Particularly on larger displacement engines. But if all balanced then that is the best approach. .....I use a lwfw on my 3.3.2 986. Stock engine.
Many racers in the US use them in much higher ambient temperatures than us. Supposedly they help with cavitation at higher engine speeds.
With regards to street driving. ..there seems to be a good amount of folk use these with seemingly no ill effects.. I have no hard data for any of this but likewise have heard no bad news against the use of them.
I have heard plenty of advice against lwfw though. Particularly on larger displacement engines. But if all balanced then that is the best approach. .....I use a lwfw on my 3.3.2 986. Stock engine.
MisterCorn
Fuji
- Joined
- 8 Jan 2011
- Messages
- 9,410
The only time I have seen coolant issues on my C2 has been at high ambient/ low vehicle speed conditions. I don't have the data to say if this is a lack of flow or a lack of airflow though. I haven't seen it on other cars I own. I have spent a week driving my 560 SEC in 30+ degree ambient conditions with the tempersture rarely going over 80. My C2 would be up close to 100 at the first sniff of traffic. If non of the other learned menbers have an answer I will hit mine with a bunch of thermocouples and see what is going on.
MC
MC
The plenum is here to allow fitment of the 82mm gt3 throttle body.
Quite strange to draw something on CAD and it arrives in the flesh via Pat the postman :thumb:
This will require some hand finishing, jig drilling and fitting of inserts to allow bolting on of the TB and a small amount of plastic clean up. The min wall thickness at any point is 3mm so it's strong and surprisingly light.
Based on this version I will tweak the design but this unit will run on the engine.
Quite strange to draw something on CAD and it arrives in the flesh via Pat the postman :thumb:
This will require some hand finishing, jig drilling and fitting of inserts to allow bolting on of the TB and a small amount of plastic clean up. The min wall thickness at any point is 3mm so it's strong and surprisingly light.
Based on this version I will tweak the design but this unit will run on the engine.
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