Post by: ron.h on May 29, 2020, 01:05:57 PM
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Post by: angermgmt on May 29, 2020, 05:01:45 PM
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Post by: TrollerDave on May 29, 2020, 05:08:21 PM
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Post by: ron.h on May 29, 2020, 05:48:31 PM
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Post by: ron.h on May 29, 2020, 06:01:23 PM
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Post by: FordLover on May 29, 2020, 08:49:08 PM
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Post by: K-034 on May 31, 2020, 03:24:46 PM
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Post by: eliminatorbuild on May 31, 2020, 03:46:36 PM
No way bud. I am no expert, but I had a 460 ford stroked out to a 521 with aluminum heads at slightly above 11 to 1 and I felt sometimes 91 wasn't enough. But that is just fordlovers thoughts.What would a good CR be for a 521 running 91 octane? I’m a newbie here and was thinking of going 12 to 1 on a 521 build I’m going to put together at some point.
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Post by: mobboss on May 31, 2020, 04:30:05 PM
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Post by: eliminatorbuild on May 31, 2020, 04:55:07 PM
have you ever seen a engine that has been detonating???? it will put a hole in the top of the piston, as well as various of other damage. the rule of thumb is 91 for anything over 9.5 to 1 and you want 12 to 1 ???? that is race gas .No I haven’t. I really don’t know much about engines that’s why I’m asking. I know the cam I’m looking at needs more then 10 to 1. Or at least that’s what the guy at Comp Cams told me. Is 10 to 1 to much for 91?
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Post by: eliminatorbuild on May 31, 2020, 05:01:30 PM
No I haven’t. I really don’t know much about engines that’s why I’m asking. I know the cam I’m looking at needs more then 10 to 1. Or at least that’s what the guy at Comp Cams told me. Is 10 to 1 to much for 91?Sorry. Not more then 10 to 1. At least 10 to 1.
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Post by: mobboss on May 31, 2020, 06:13:01 PM
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Post by: ron.h on May 31, 2020, 06:21:48 PM
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Post by: ron.h on May 31, 2020, 06:24:53 PM
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Post by: angermgmt on May 31, 2020, 07:09:56 PM
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Post by: TrollerDave on May 31, 2020, 07:10:00 PM
Sorry. Not more then 10 to 1. At least 10 to 1.10 to 1 would probably be the max to run on pump gas in a jet boat. Jet boat motors run at higher rpm for a longer duration than a Regular car motor. You can mix in an octane boost or some higher octane gas just to be safe.
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Definitely talk to some engine builders that know jet boat motors.
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Post by: Flusher on May 31, 2020, 11:21:35 PM
Dynamic compression ratio is the similar to static compression ratio EXCEPT the piston location at the intake valve closing point is substituted for BDC. Intake valve closure occurs after BDC. The theory is that the air fuel mixture can not compress until the valves are closed. The effective stroke in the dynamic compression ratio is less than the actual stroke, therefore the dynamic compression ratio will be significantly less than the static compression ratio, the later the intake valve closes.
The affective compression ratio is what the engine actually sees during running conditions. Atmospheric conditions, volumetric efficiency, etc. influence the affective compression ratio. The more well refined the engine combination, the better the VE, the higher the affective compression ratio will be.
All of this relates to how much cylinder pressure and therefore temperature is created during the compression stroke.
There are other factors that influence detonation resistance, such as chamber shape, quench distance, and mixture motion.
Taking these factors into consideration during parts selection and assembly is the difference between a real engine builder and someone who just slaps part together.
You mentioned the tech support said the cam in question requires at least 10:1 static compression ratio. The most important thing in selecting a cam is making sure that it will produce power in the RPM range that the engine will operate. You need to tayler this RPM range to something compatible with a jet boat impeller. Peak horsepower RPM should cross the impeller curve, that will define your cut size. Peak torque should occur at approximately 80% of peak power RPM.
You will also want power to carry beyond peak HP and not quickly fail off.
Jet boats do not need "bottom end torque!" You are also not going to want to build a high-RPM screamer. Build your engine for your application. A bigger boat is going to require a larger impeller. Surprisingly, most 18'-19' drag boats utilize a BC cut impeller despite a HP range of 750-1200+.
With all that being said, really need to know a lot more about your overall combination. Hopefully you got a grind number for the cam that Comp recommended. It would be nice to have the cam specs for advertised duration opening and closing points for intake and exhaust.
That will be a good starting point.
Cheers
Post by: eliminatorbuild on June 01, 2020, 05:54:42 AM
Static compression ratio is what everyone is talking about. That is the actual volume above the piston at BDC divided by the actual volume above the piston at TDC. This is the number that you manipulate once you have your cam selection.
Dynamic compression ratio is the similar to static compression ratio EXCEPT the piston location at the intake valve closing point is substituted for BDC. Intake valve closure occurs after BDC. The theory is that the air fuel mixture can not compress until the valves are closed. The effective stroke in the dynamic compression ratio is less than the actual stroke, therefore the dynamic compression ratio will be significantly less than the static compression ratio, the later the intake valve closes.
The affective compression ratio is what the engine actually sees during running conditions. Atmospheric conditions, volumetric efficiency, etc. influence the affective compression ratio. The more well refined the engine combination, the better the VE, the higher the affective compression ratio will be.
All of this relates to how much cylinder pressure and therefore temperature is created during the compression stroke.
There are other factors that influence detonation resistance, such as chamber shape, quench distance, and mixture motion.
Taking these factors into consideration during parts selection and assembly is the difference between a real engine builder and someone who just slaps part together.
You mentioned the tech support said the cam in question requires at least 10:1 static compression ratio. The most important thing in selecting a cam is making sure that it will produce power in the RPM range that the engine will operate. You need to tayler this RPM range to something compatible with a jet boat impeller. Peak horsepower RPM should cross the impeller curve, that will define your cut size. Peak torque should occur at approximately 80% of peak power RPM.
You will also want power to carry beyond peak HP and not quickly fail off.
Jet boats do not need "bottom end torque!" You are also not going to want to build a high-RPM screamer. Build your engine for your application. A bigger boat is going to require a larger impeller. Surprisingly, most 18'-19' drag boats utilize a BC cut impeller despite a HP range of 750-1200+.
With all that being said, really need to know a lot more about your overall combination. Hopefully you got a grind number for the cam that Comp recommended. It would be nice to have the cam specs for advertised duration opening and closing points for intake and exhaust.
That will be a good starting point.
Cheers
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Post by: eliminatorbuild on June 01, 2020, 05:55:45 AM
This is the cam I’m looking at.
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Post by: Flusher on June 02, 2020, 06:48:36 AM
This is the cam I’m looking at.My initial reaction after seeing your cam selection, everyone is right, there is no fracking way you are going to run that on 87 octane.
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No matter, I will go through the motions with you. I'm not a Ford guy, so you are going to have to do all the leg work, getting all the measurements. Need the head CCs, head gasket CCs, deck height, pin height, bore, stroke, and rod length. Summitracing.com is good about providing head gasket crushed thickness and CC volume as well as piston information. Be advised that most head gaskets are not round, so it's difficult to calculate volume. You can find the CompCams Master Lobe Profile Catalog at https://www.compcams.com/lobe-catalog.
There are a number of online calculators, some of which are terrible. I prefer to calculate in Excel and to actually measure the volumes in CCs rather than using printed specs. Wallace Racing has some cool calculations if you want to play around (http://www.wallaceracing.com/Calculators.htm). Anyway, his sit is a good read, go spend some time reading his page, this will make a lot more sense.
Wallace Racing Compression Ratio
http://www.wallaceracing.com/cr_test2.php
Wallace Racing Dynamic Compression Ratio
http://www.wallaceracing.com/dynamic-cr.php
Wallace Racing Effective Compression Ratio
http://www.wallaceracing.com/eff-cr.php
A blanket statement of, "Use 9:1 compression ratio for pump gas" is incomplete at best.
Lastly, if you are going to try to maximize your compression ratio, make sure that your quench distance is less than .055". I'm sure Lakes Only (http://www.highflowdynamics.com) here will have the best Ford specific information for you.
Post by: eliminatorbuild on June 02, 2020, 07:00:34 AM
My initial reaction after seeing your cam selection, everyone is right, there is no fracking way you are going to run that on 87 octane.Thank you. I’ll read it all.
No matter, I will go through the motions with you. I'm not a Ford guy, so you are going to have to do all the leg work, getting all the measurements. Need the head CCs, head gasket CCs, deck height, pin height, bore, stroke, and rod length. Summitracing.com is good about providing head gasket crushed thickness and CC volume as well as piston information. Be advised that most head gaskets are not round, so it's difficult to calculate volume. You can find the CompCams Master Lobe Profile Catalog at https://www.compcams.com/lobe-catalog.
There are a number of online calculators, some of which are terrible. I prefer to calculate in Excel and to actually measure the volumes in CCs rather than using printed specs. Wallace Racing has some cool calculations if you want to play around (http://www.wallaceracing.com/Calculators.htm). Anyway, his sit is a good read, go spend some time reading his page, this will make a lot more sense.
Wallace Racing Compression Ratio
http://www.wallaceracing.com/cr_test2.php
Wallace Racing Dynamic Compression Ratio
http://www.wallaceracing.com/dynamic-cr.php
Wallace Racing Effective Compression Ratio
http://www.wallaceracing.com/eff-cr.php
A blanket statement of, "Use 9:1 compression ratio for pump gas" is incomplete at best.
Lastly, if you are going to try to maximize your compression ratio, make sure that your quench distance is less than .055". I'm sure Lakes Only (http://www.highflowdynamics.com) here will have the best Ford specific information for you.
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Post by: FordLover on June 02, 2020, 03:14:14 PM
Funny story when I bought my crate engine I figured I would just stick my old aluminum cobra jet heads on the engine. What a rookie mistake! Didn't realize with the pistons I got plus the tiny ass combustion chambers that my compression went through the roof. Actually talked with Lakes Only about it. That dude is crazy knowledgable. So had to sell the CJ heads because I didn't want to go full on race fuel.
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Post by: FordLover on June 02, 2020, 03:20:32 PM
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Post by: Flusher on June 03, 2020, 11:58:05 AM
Here is a good basic introduction to compression ratio.
https://youtu.be/5qH_ThYLD6Q
Great quote regarding 12.5:1 compression ratio (4:07 - 4:27).
Post by: FordLover on June 04, 2020, 08:55:40 AM
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