High compression keep or change?
11-03-2020, 04:50 PM
#1
High compression keep or change?
Hello I have an engine installed that has a 85 bottom and S4 cylinder heads. From what I read on this forum I would need to use high octane fuel. Will using racing fuel or airplane fuel hurt the fuel tank? I don't drive it daily.
11-03-2020, 05:27 PM
#2
Rennlist Member
Dont use airplane fuel, its the worst...your engine wont like lead salts deposited in it, and you DONT want to run dispersant aviation oils to manage that, at all.
What is your engine management, did they add knock sensors?
IMHO, 85 timing is so far behind best timing, its not an issue on good pump gas. Dont run E85 either.
What is your engine management, did they add knock sensors?
IMHO, 85 timing is so far behind best timing, its not an issue on good pump gas. Dont run E85 either.
11-03-2020, 10:59 PM
#4
Former Sponsor
You might want to calculate the compression ratio, before you just use premium fuel.
As I recall, it's high enough that I decided, long ago, that this was not practical.
As I recall, it's high enough that I decided, long ago, that this was not practical.
Last edited by GregBBRD; 11-03-2020 at 11:00 PM.
11-04-2020, 12:06 AM
#5
Rennlist Member
^^ IIRC it's around 12:1. Way too high for a street engine. Try to keep it under 10.5:1 if you want to use pump gas in it.
Otherwise pure unleaded race gas will work if you can stomach the added cost and lack of availability.
Otherwise pure unleaded race gas will work if you can stomach the added cost and lack of availability.
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11-06-2020, 03:41 PM
#8
Rennlist Member
11-07-2020, 08:42 PM
#9
Rainman
Rennlist Member
Rennlist Member
Slightly off topic but I have a 12.5:1 SOHC 944 engine running E85.
I did replace all original hoses from tank to rail with alcohol-compatible material. Stock pump, filter, fuel rail, but larger fuel injectors.
It is good fuel if you have it easily available at the pump locally AND the ability to tune your ECU - it is harder to start on cold days than regular gasoline and requires a lot of cranking enrichment.
That said....the 944S/S2/968 DOHC engines came with 10.9 compression and ran on 91 octane gas just fine, but they didn't have super-aggressive timing maps.
An 85 bottom end with S4 top end should be about 12:1 compression but the 85 still has fairly mild ignition timing, so I would think you could likely run it on 93 octane if that's all you can get.
If you install a factory 1.4mm service head gasket (not the standard 1mm gasket) that would drop the compression to about 11.3:1, even better.
Keeping the engine cool, keeping inlet air cool, and letting the exhaust flow freely will help this along.
I did replace all original hoses from tank to rail with alcohol-compatible material. Stock pump, filter, fuel rail, but larger fuel injectors.
It is good fuel if you have it easily available at the pump locally AND the ability to tune your ECU - it is harder to start on cold days than regular gasoline and requires a lot of cranking enrichment.
That said....the 944S/S2/968 DOHC engines came with 10.9 compression and ran on 91 octane gas just fine, but they didn't have super-aggressive timing maps.
An 85 bottom end with S4 top end should be about 12:1 compression but the 85 still has fairly mild ignition timing, so I would think you could likely run it on 93 octane if that's all you can get.
If you install a factory 1.4mm service head gasket (not the standard 1mm gasket) that would drop the compression to about 11.3:1, even better.
Keeping the engine cool, keeping inlet air cool, and letting the exhaust flow freely will help this along.