Ian Patmore
Well-Known Forum User
Compression
Sean,
No problem about being asking such a question. Hopefully this explains, though I am not saying its the definative answer, just my impression.
My Rebello engine has a compression ratio of 10.5 to 1, and thus the previous figures. But the other Rebello engine I used as an example earlier produces slightly more than mine, but runs 9.5 to 1 compression ratio and a lower lift cam than mine. There maybe a slight difference between my Dellortos and the other engines Mukini (sp), but the biggest is the cam and compression. On paper mine should produce more, but its down to cylinder head work. Its easy to get the flow figures on the inlet port, but more difficult on the exhaust port. This is where only a small percentage of L series engine builders get right. I would conclude that a Rebello cylinder head probably has a slightly better flow than my Rusch cylinder head. Here is what Dave Rebello sent me in an email;
"Most heads we have seen only flow about 126 CFM on the exhaust with around 190 CFM on the intake. We need 75% of the intake flow on the exhaust which is about 142 at 25" . The math is simple 190 CFM on the intake X 1.5 for a six cyl. is 285 HP. Plus 8% VE is about 310 HP. If we only achieve 65% exhaust flow or less the engine will be exhaust limited and the power will suffer greatly. ".
I don't fully understand this, but I get the gist. Skiddell, maybe you can chime in and give us your imput on the above flow statement.
This question slightly ties in with a previous thread about engine builders in the UK, and the difficulty in getting 100bhp per litre, and what's required and can it be done on carbs. I keep banging on about it in this thread, but its cylinder head work first and foremost.
Zhead, a good idea, but I'm not into "mines bigger than yours", but could be interesting.
Cheers
Ian
Sean,
No problem about being asking such a question. Hopefully this explains, though I am not saying its the definative answer, just my impression.
My Rebello engine has a compression ratio of 10.5 to 1, and thus the previous figures. But the other Rebello engine I used as an example earlier produces slightly more than mine, but runs 9.5 to 1 compression ratio and a lower lift cam than mine. There maybe a slight difference between my Dellortos and the other engines Mukini (sp), but the biggest is the cam and compression. On paper mine should produce more, but its down to cylinder head work. Its easy to get the flow figures on the inlet port, but more difficult on the exhaust port. This is where only a small percentage of L series engine builders get right. I would conclude that a Rebello cylinder head probably has a slightly better flow than my Rusch cylinder head. Here is what Dave Rebello sent me in an email;
"Most heads we have seen only flow about 126 CFM on the exhaust with around 190 CFM on the intake. We need 75% of the intake flow on the exhaust which is about 142 at 25" . The math is simple 190 CFM on the intake X 1.5 for a six cyl. is 285 HP. Plus 8% VE is about 310 HP. If we only achieve 65% exhaust flow or less the engine will be exhaust limited and the power will suffer greatly. ".
I don't fully understand this, but I get the gist. Skiddell, maybe you can chime in and give us your imput on the above flow statement.
This question slightly ties in with a previous thread about engine builders in the UK, and the difficulty in getting 100bhp per litre, and what's required and can it be done on carbs. I keep banging on about it in this thread, but its cylinder head work first and foremost.
Zhead, a good idea, but I'm not into "mines bigger than yours", but could be interesting.
Cheers
Ian
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