quick compression ratio question
Thread Starter
Intermediate
Joined: Aug 2007
Posts: 27
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From: Springfield Or
quick compression ratio question
Is it possible and/or reliable to run a 11.7 to 1 compression ratio. I have been told several different thing and don't know what to believe. I have aluminum heads infact they are the stock 58 cc and woud like to save alittle money if possible and run a higher compression but don't wont to cause problems either. Also if it make a difference I have a superam setup with the LPE 219 cam and the shortblock in a 383. I will of course port and polish the heads if I go this route but I haven't yet because I don't know if it will function.
Team Owner
Joined: Sep 2001
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From: Athens AL
C7 of the Year - Unmodified Finalist 2021
C4 of Year Finalist (performance mods) 2019
That'd be too high for me to be comfortable, I'd lower it to 11ish at least.
Whats the car used for though?
Whats the car used for though?
Race Director
Joined: Jan 2006
Posts: 18,396
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Depends on the engine's tune. My LS2 runs an 8 psi blower, resulting in a CR of approximately 16:1, and the car runs at WOT without pulling any timing. As long as you have an AF ratio of, say 12:1 for a na car, and the car runs cool (180*), I think you can safely run your 11.7:1 wuth 93 octane fuel. Good luck.
Melting Slicks
Joined: Jun 2002
Posts: 2,705
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From: Chesapeake Beach Maryland
2015 C5 of Year Finalist
I know that ratios around 11.1 to 11.7 are doable. When I asked Lloyd Elliot about head gaskets this is one of the responses I got back regarding head gasket thickness and compression. I was initially concerned about the using the thinner Impala SS gasket, but on my setup he recomended and this raises factory compression a bit.
"Don
The .029 thickness is what we want.
We are wanting all the compression we can get.
No worries about detonation.
The heads and cam is designed for that extra compression.
We should end up about 11.3 to 1
Lloyd"
"Don
The .029 thickness is what we want.
We are wanting all the compression we can get.
No worries about detonation.
The heads and cam is designed for that extra compression.
We should end up about 11.3 to 1
Lloyd"
Safety Car
Joined: Sep 2001
Posts: 4,157
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From: Glenbeulah Wi
11.7 to 1 is too high for pump gas with the 219 cam. You need to caculate your dynamic compression ratio to know for sure. The dynamic compression ration needs to be 8.5 to 1 or less for reliable operation on pump gas. My guess is that you will have a dynamic compression ratio of more then 9 to 1 with your set-up.
8.5 to 1 dynamic compression is the max you can run on 93 pump gas and I would try to keep it down around 8 to 1 for trouble free operation. My 10.6 to 1 compression engine uses the 219 cam and the dynamic compression is ratio is 8 to 1.
8.5 to 1 dynamic compression is the max you can run on 93 pump gas and I would try to keep it down around 8 to 1 for trouble free operation. My 10.6 to 1 compression engine uses the 219 cam and the dynamic compression is ratio is 8 to 1.
Melting Slicks
Joined: Jun 2002
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From: Chesapeake Beach Maryland
2015 C5 of Year Finalist
Melting Slicks
Joined: May 2007
Posts: 2,382
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From: Kathleen FL
I had 10:1 static compression ratio on my old iron head 383. Yes the compression ratio was very high for iron heads. But because the cam (CC306) had lowered the dynamic compression ratio to 8.2:1, I was able to run it on pump gas without any detonation. I'll be building another 383 shortly and plan on using the 306 cam again.
Le Mans Master
Joined: Aug 2000
Posts: 8,838
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From: Australia
http://www.wallaceracing.com/dynamic-cr.php
Be aware you have to use the "advertised duration; not the @50 duration.
0.050 is used to compare cams as it is generally agreed that there is not useible flow below .050.
However the valve is still open and you get no compression until the valve is actually on the seat.
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Pro
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From: Rochester PA
Dynamic Cr
rodj,
It doesn't ask for duration, just where the int.valve closes BTDC. I have an Extreme Energy hyd flat tappet cam (224*/230*, .477"/.480") in a 353, w/puny World Products S/R Torquer heads (170cc int. runner, 67 cc comb. chamber) that I bowl blended, smoothed out the int. and ex. areas by the valve stems, and gasket matched them to a Holley Street Dominator dual plenum manifold. Then it is topped off with a 750 AFB style Edelbrock, vac secondaries carb. It was in a 72 Nova SS that I totalled in 2001 (that hurts). Anyway, the dynamic cranking pressure is 190 PSI + or _ 1 or 2 PSI in all 8 cyls. If I put the cam #'s into that formula (int. closes 60 ABDC), my static CR works out to be 11.4:1. That seems awful high, but that motor is a beast. This spring it will find a new home in a '70 Nova that I gave to my son, and finally we are going to see what it will do in the 1/4.
Sorry this is so long winded. I wanted to find out if I used the formula right. My dynamic CR works out to be 9.16:1. I had to run racing gas mixed with Sunoco 93 octane, so I suppose it is close.
Your thoughts? Thanks.
It doesn't ask for duration, just where the int.valve closes BTDC. I have an Extreme Energy hyd flat tappet cam (224*/230*, .477"/.480") in a 353, w/puny World Products S/R Torquer heads (170cc int. runner, 67 cc comb. chamber) that I bowl blended, smoothed out the int. and ex. areas by the valve stems, and gasket matched them to a Holley Street Dominator dual plenum manifold. Then it is topped off with a 750 AFB style Edelbrock, vac secondaries carb. It was in a 72 Nova SS that I totalled in 2001 (that hurts). Anyway, the dynamic cranking pressure is 190 PSI + or _ 1 or 2 PSI in all 8 cyls. If I put the cam #'s into that formula (int. closes 60 ABDC), my static CR works out to be 11.4:1. That seems awful high, but that motor is a beast. This spring it will find a new home in a '70 Nova that I gave to my son, and finally we are going to see what it will do in the 1/4.
Sorry this is so long winded. I wanted to find out if I used the formula right. My dynamic CR works out to be 9.16:1. I had to run racing gas mixed with Sunoco 93 octane, so I suppose it is close.
Your thoughts? Thanks.
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
Is it possible and/or reliable to run a 11.7 to 1 compression ratio. I have been told several different thing and don't know what to believe. I have aluminum heads infact they are the stock 58 cc and woud like to save alittle money if possible and run a higher compression but don't wont to cause problems either. Also if it make a difference I have a superam setup with the LPE 219 cam and the shortblock in a 383. I will of course port and polish the heads if I go this route but I haven't yet because I don't know if it will function.
Would probably run the nuts on E85 though.
-- Joe
Safety Car
Joined: May 2006
Posts: 4,290
Likes: 140
From: Salem NH
Melting Slicks
Joined: May 2007
Posts: 2,382
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From: Kathleen FL
I'd just say they wouldn't need so much timing because the heads are super efficient compared to most SBC heads. My friend, who is into LSx motors runs his 381 cid stroker LS1 Camaro around town and that is the timing he uses. Nasty street car indeed!!!
Safety Car
Joined: Jan 2004
Posts: 3,556
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