Shorter Rods to reduce detonation?
12-20-2005, 02:00 PM
#1
Racer
Thread Starter
Shorter Rods to reduce detonation?
A. Graham Bell in his "Forced Induction Performance Tunning" book writes the following statement: (Page 82)
"In naturally aspirated engines we ussually want to increase the rod length for superior performance, but forced induction engines are different. Here within reason (dont go to a rod length to stroke ratio of less that 1:5.1), a shorter rod can be use to open up the squish clearance and/or lower the compression ratio without loosing HP. In fact, because a short rod accelerate the piston more rapidly away from TDC they can help in keeping away from detonation"
It is interesting, his point of view differs from the majority of people such as Corky Bell. In his book "Maximum Boost", he said: (Page 157)
"Aproaches to lower the Compression Ratio that do not work are thicker head gaskets and shorter connecting rods"
Two different point of view from two knowledgeable and experimented persons.
My experience:
I installed shorter rods in my 968 Turbo. Have made like 40+ runs in a Dyno for tuning, from 11 to 21 PSI of boost, up to 7000 RPM on 93 octane pump gas. My compression is fairly high, about 9:1.1. Factory head gasket and Raceware studs. No problem up to date.
I'm leaning toward Mr. Graham Bell opinnion. It will be good to know if any one with shorter rods has experinced problems that support the contrary.
"In naturally aspirated engines we ussually want to increase the rod length for superior performance, but forced induction engines are different. Here within reason (dont go to a rod length to stroke ratio of less that 1:5.1), a shorter rod can be use to open up the squish clearance and/or lower the compression ratio without loosing HP. In fact, because a short rod accelerate the piston more rapidly away from TDC they can help in keeping away from detonation"
It is interesting, his point of view differs from the majority of people such as Corky Bell. In his book "Maximum Boost", he said: (Page 157)
"Aproaches to lower the Compression Ratio that do not work are thicker head gaskets and shorter connecting rods"
Two different point of view from two knowledgeable and experimented persons.
My experience:
I installed shorter rods in my 968 Turbo. Have made like 40+ runs in a Dyno for tuning, from 11 to 21 PSI of boost, up to 7000 RPM on 93 octane pump gas. My compression is fairly high, about 9:1.1. Factory head gasket and Raceware studs. No problem up to date.
I'm leaning toward Mr. Graham Bell opinnion. It will be good to know if any one with shorter rods has experinced problems that support the contrary.
Last edited by TurboCab; 12-20-2005 at 04:16 PM.
12-24-2005, 12:17 AM
#5
Two totally different books altogether. Corky Bell is old school, big V8 builder. Graham Bell is more new school, 4-valve kind of stuff. Squish area takes no effect on a hemispherical combusion chamber, i.e. almost all modern 4-valve heads, some 2-valve heads. Combustion theoretically would be equal across the board in the combustion chamber where as on an old Ford V8 with an oval combustion chamber and what not, changing the squish area totally changes the engine, as there would become hotspots and....just weird. Thats where Corky Bell is coming from.
In my research, here is what I have found. 968 rods aren't that terribly long, just low compression pistons are the Turbo S/RS pistons, and they cost an arm and leg. The solution for a non-scared, good health engine is to shorten the rods, leaving stock pistons. Some also feel that the silicon content is well matched, i.e. less blow by and change by conditions. Short rods are also about the same cost as a set of OEM Mahle Turbo S/RS pistons, on top of aftermarket rods.
Talking to several non-Porsche buiders and hope to speak to one Porsche builder (but non-951) has yeilded me different results than that of the average Porsche crowd. For one, on many engines, the Nikasil/Alusil coating is standard on sleeve/al. blocked engines. Most all of the builders I have spoken with have had no problems running aftermarket pistons on a coated block. One builder eventually had some wear, and the engine self-detonated. He blamed it on the block, but it could have just as easily been the build.
So for me, aftermarket pistons, aftermarket rods, same cost as OEM pistons. The rod length on the 968 isn't excessively long or anything, just its a different way to build an engine.
Wes
In my research, here is what I have found. 968 rods aren't that terribly long, just low compression pistons are the Turbo S/RS pistons, and they cost an arm and leg. The solution for a non-scared, good health engine is to shorten the rods, leaving stock pistons. Some also feel that the silicon content is well matched, i.e. less blow by and change by conditions. Short rods are also about the same cost as a set of OEM Mahle Turbo S/RS pistons, on top of aftermarket rods.
Talking to several non-Porsche buiders and hope to speak to one Porsche builder (but non-951) has yeilded me different results than that of the average Porsche crowd. For one, on many engines, the Nikasil/Alusil coating is standard on sleeve/al. blocked engines. Most all of the builders I have spoken with have had no problems running aftermarket pistons on a coated block. One builder eventually had some wear, and the engine self-detonated. He blamed it on the block, but it could have just as easily been the build.
So for me, aftermarket pistons, aftermarket rods, same cost as OEM pistons. The rod length on the 968 isn't excessively long or anything, just its a different way to build an engine.
Wes
12-24-2005, 06:38 AM
#6
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I know it's just me, but if
i'm dropping 5k+ in a new rebuild, especially a 968T type pwoer plant, I'll need a HELL of of a lot better than,:
"Talking to several non-Porsche buiders and hope to speak to one Porsche builder (but non-951) has yeilded me different results than that of the average Porsche crowd. For one, on many engines, the Nikasil/Alusil coating is standard on sleeve/al. blocked engines. Most all of the builders I have spoken with have had no problems running aftermarket pistons on a coated block. One builder eventually had some wear, and the engine self-detonated. He blamed it on the block, but it could have just as easily been the build.
So for me, aftermarket pistons, aftermarket rods, same cost as OEM pistons. The rod length on the 968 isn't excessively long or anything, just its a different way to build an engine."
No offense intended at all. After 20 YEARS of these engines, If it hasn't been proven to work, there is a HUGE chance that there is a VERY good reason!
Seriously TWENTY YEARS! Who are these builders of which you speak?
i'm dropping 5k+ in a new rebuild, especially a 968T type pwoer plant, I'll need a HELL of of a lot better than,:
"Talking to several non-Porsche buiders and hope to speak to one Porsche builder (but non-951) has yeilded me different results than that of the average Porsche crowd. For one, on many engines, the Nikasil/Alusil coating is standard on sleeve/al. blocked engines. Most all of the builders I have spoken with have had no problems running aftermarket pistons on a coated block. One builder eventually had some wear, and the engine self-detonated. He blamed it on the block, but it could have just as easily been the build.
So for me, aftermarket pistons, aftermarket rods, same cost as OEM pistons. The rod length on the 968 isn't excessively long or anything, just its a different way to build an engine."
No offense intended at all. After 20 YEARS of these engines, If it hasn't been proven to work, there is a HUGE chance that there is a VERY good reason!
Seriously TWENTY YEARS! Who are these builders of which you speak?
12-24-2005, 09:34 AM
#7
I clearly suggested that they were not 951 builders. They are all BMW builders. I also never said I was going to drop $5k into an engine build for the 968. I am not a follow the pack kind of guy...not one to put 20k in to a Powerhaus kit. The 968 engine is nothing special, its an engine. Porsche didn't put special magic cream in it to get 3.0L, and 236hp, in which the 236hp is on the upper side of the spectrum. Hundreds of engines in the world have run Nikasil and Alusil blocks for many years, and 951 builders are about the only ones that seem to feel you CAN NOT run anything else. Seems to me that if Vitesse Racing, Lindsey Racing, or whatever told you to jump, 99% of the people on this board would. I don't find the dramatic difference between a BMW turbo build, Audi turbo build, Nissan turbo build, etc... I am still looking for the Porsche magic, and the 1000's of documents, papers, webpages, forums, etc...that I have read have no clear magic.
Wes
No offense taken or intended, but sometimes if you don't think outside the box nothing will ever progress.
Wes
No offense taken or intended, but sometimes if you don't think outside the box nothing will ever progress.
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12-24-2005, 10:15 AM
#8
Racer
Thread Starter
I have to agree with Wes. Some times you have to reason things by yourself. I take the risk, and against the majority of the opinions on this forum and experts like Corky, installed shorter rods with excellent results. I figured out that with a pent roof chamber design as in the 968 (not 951), the flame propagation will be good, no matter if the piston crown at TDC is not at deck level. Well, up to date my reasoning prove to be right. Regarding the pistons, if installing aftermarket pistons in other alusil/nikasil bored blocks worked fine for others, why not for us? Some times people just repeat what they hear without any type of real world testing to sustain their arguments. Just my opinnion.
