 Comression ratio advice, 6 cylinder motor build |
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| team914 |
 Apr 22 2023, 04:06 PM
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#1
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Newbie  Group: Members Posts: 3 Joined: 5-April 20 From: Ireland Member No.: 24,102 Region Association: Europe  |
Hi,
I am planning to build a 6 cylinder street motor for my 914/6 factory replica. My original 84mm barrels were worn, so I elected to machine the barrels to 84.5mm and have bought a set of 84.5mm Wossner pistons. With a 70.4mm crank, the compression ratio is stated at 10.11:1 with the pistons out of the box. I intend on using Weber carbs, E cams and an E distributor/123 Distributor. My machine shop engineer has recommended reducing the compression ratio from the 10.11:1 to a more suitable figure. The petrol quality in Ireland is poor, typically 95 octane, and we now have a compulsory addition of 10% Ethanol added (E10)! Any advice greatly appreciated |
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| gereed75 |
Apr 22 2023, 04:44 PM
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#2
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Senior Member Group: Members Posts: 1,260 Joined: 19-March 13 From: Pittsburgh PA Member No.: 15,674 Region Association: North East States 
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The general wisdom says 9.5:1 is the max you can do on pump gas with a single plug.
Not familiar with the Wosners but it is fairly easy to remove material from the top of the pistons to adjust compression. Measure the volumes, do the calculations and then adjust. |
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| porschetub |
Apr 24 2023, 05:02 PM
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#3
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Advanced Member Group: Members Posts: 4,706 Joined: 25-July 15 From: New Zealand Member No.: 18,995 Region Association: None |
QUOTE(gereed75 @ Apr 23 2023, 11:44 AM)  The general wisdom says 9.5:1 is the max you can do on pump gas with a single plug. Not familiar with the Wosners but it is fairly easy to remove material from the top of the pistons to adjust compression. Measure the volumes, do the calculations and then adjust. (IMG:style_emoticons/default/agree.gif) considering the fuel he is running this has to be taken into account,his motor will still be fairly mild so no point in introducing more heat into an iron bore engine ,these motors are known to be rather under stressed and last well so no point in risking that for a few horsepower. The cams will make the best single increase, and even better with an upgrade to better flowing 2.2 heads as Mr Dempsey suggests will be a good move,the larger valve sizes will benefit the better than stock cams which were very tame stock. Cam choice will be important ,appears to be the Solex cams or 'E" cams but don't know so hopefully someone will chime in here. End of the day this will be a rather strong short stroke and should make good power with well matched combo. Cheers. |
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Posts in this topic
team914 Comression ratio advice Apr 22 2023, 04:06 PM
mepstein You want to measure the actual compression ratio a... Apr 22 2023, 04:15 PM
nditiz1
You want to measure the actual compression ratio ... Apr 22 2023, 05:55 PM Retroracer Sounds like a nice build! I'm +1 on the ... Apr 22 2023, 05:11 PM rgalla9146 Summit racing has an easy to use compression ratio... Apr 23 2023, 06:24 AM Krieger You do need to cc your heads. Apr 23 2023, 08:57 AM rgalla9146
You do need to cc your heads.
OOPS. Yes, th... Apr 23 2023, 09:30 AM ogdougy I would also add that some piston manufactures wil... Apr 23 2023, 10:49 AM Krieger
I would also add that some piston manufactures wi... Apr 23 2023, 01:49 PM r_towle For a Daily Driver, I would suggest 9.5:1 to ensur... Apr 23 2023, 05:05 PM targa72e As others have said you need to CC the heads, figu... Apr 23 2023, 10:12 PM gereed75 The "9.5 compression" JE's i got wer... Apr 23 2023, 10:50 PM |
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