454 Repower
12-15-2014, 04:28 AM
#11
Gold Member
A cam with more duration / valve overlap needs more compression to maintain cylinder pressure as both valves are open at the same time for a longer time.
Guys with more intricate understanding of cam design can elaborate but, that's the "cheap and dirty" explanation.
Guys with more intricate understanding of cam design can elaborate but, that's the "cheap and dirty" explanation.
12-15-2014, 05:04 AM
#12
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As you move up in cam duration, you get into later intake valve closing points. When the intake valve closes later, some of the cylinder pressure is bled back into the intake manifold, mainly at low engine speeds, since there is simply more time for it to do so, when things are moving slowly.
For example. You can have a 8.0:1 engine, with a very short duration camshaft. That engine may build 150psi of cranking pressure. If you raised the static compression, to say 10:1, with that same cam, you may see 200psi of cranking pressure. Now, if you have a 10:1 engine, but go with a longer duration cam, you may see 175psi cranking pressure. With that same long duration cam, in the 8:1 engine, would drop the cranking pressure to say 125psi.
What happens when you have to much cam, and not enough compression, is the engine becomes a real turd, until the RPM (time lapse), increases to a point that the pumping losses arent as drastic. In other words, thats why some engine combos like that, cant get out of their own way until 4500RPM, idle like crap, etc. And vice versa, if you have too much compression, and not enough cam, your cylinder pressures can be very high, which could mean lots of power down low (but possible detonation), and a very limited upper rpm range.
Compression ratio and cam choice, go hand in hand. IMO, a high static compression, short duration cam, is a bad recipe for a marine engine. Like a 10:1 setup with a short cam like the OP has. IMO, that cam should be used with no more than 9:1 static at most.
The man to talk to on this stuff though is Bob Madara. IMO, if the builder spec'd that cam, with that compression for a pump gas pleasure marine engine, I'd look elsewhere for an opinion, and I definitly wouldnt want to lock the timing out on something like that.
For example. You can have a 8.0:1 engine, with a very short duration camshaft. That engine may build 150psi of cranking pressure. If you raised the static compression, to say 10:1, with that same cam, you may see 200psi of cranking pressure. Now, if you have a 10:1 engine, but go with a longer duration cam, you may see 175psi cranking pressure. With that same long duration cam, in the 8:1 engine, would drop the cranking pressure to say 125psi.
What happens when you have to much cam, and not enough compression, is the engine becomes a real turd, until the RPM (time lapse), increases to a point that the pumping losses arent as drastic. In other words, thats why some engine combos like that, cant get out of their own way until 4500RPM, idle like crap, etc. And vice versa, if you have too much compression, and not enough cam, your cylinder pressures can be very high, which could mean lots of power down low (but possible detonation), and a very limited upper rpm range.
Compression ratio and cam choice, go hand in hand. IMO, a high static compression, short duration cam, is a bad recipe for a marine engine. Like a 10:1 setup with a short cam like the OP has. IMO, that cam should be used with no more than 9:1 static at most.
The man to talk to on this stuff though is Bob Madara. IMO, if the builder spec'd that cam, with that compression for a pump gas pleasure marine engine, I'd look elsewhere for an opinion, and I definitly wouldnt want to lock the timing out on something like that.
12-15-2014, 06:37 PM
#13
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As you move up in cam duration, you get into later intake valve closing points. When the intake valve closes later, some of the cylinder pressure is bled back into the intake manifold, mainly at low engine speeds, since there is simply more time for it to do so, when things are moving slowly.
For example. You can have a 8.0:1 engine, with a very short duration camshaft. That engine may build 150psi of cranking pressure. If you raised the static compression, to say 10:1, with that same cam, you may see 200psi of cranking pressure. Now, if you have a 10:1 engine, but go with a longer duration cam, you may see 175psi cranking pressure. With that same long duration cam, in the 8:1 engine, would drop the cranking pressure to say 125psi.
What happens when you have to much cam, and not enough compression, is the engine becomes a real turd, until the RPM (time lapse), increases to a point that the pumping losses arent as drastic. In other words, thats why some engine combos like that, cant get out of their own way until 4500RPM, idle like crap, etc. And vice versa, if you have too much compression, and not enough cam, your cylinder pressures can be very high, which could mean lots of power down low (but possible detonation), and a very limited upper rpm range.
Compression ratio and cam choice, go hand in hand. IMO, a high static compression, short duration cam, is a bad recipe for a marine engine. Like a 10:1 setup with a short cam like the OP has. IMO, that cam should be used with no more than 9:1 static at most.
The man to talk to on this stuff though is Bob Madara. IMO, if the builder spec'd that cam, with that compression for a pump gas pleasure marine engine, I'd look elsewhere for an opinion, and I definitly wouldnt want to lock the timing out on something like that.
For example. You can have a 8.0:1 engine, with a very short duration camshaft. That engine may build 150psi of cranking pressure. If you raised the static compression, to say 10:1, with that same cam, you may see 200psi of cranking pressure. Now, if you have a 10:1 engine, but go with a longer duration cam, you may see 175psi cranking pressure. With that same long duration cam, in the 8:1 engine, would drop the cranking pressure to say 125psi.
What happens when you have to much cam, and not enough compression, is the engine becomes a real turd, until the RPM (time lapse), increases to a point that the pumping losses arent as drastic. In other words, thats why some engine combos like that, cant get out of their own way until 4500RPM, idle like crap, etc. And vice versa, if you have too much compression, and not enough cam, your cylinder pressures can be very high, which could mean lots of power down low (but possible detonation), and a very limited upper rpm range.
Compression ratio and cam choice, go hand in hand. IMO, a high static compression, short duration cam, is a bad recipe for a marine engine. Like a 10:1 setup with a short cam like the OP has. IMO, that cam should be used with no more than 9:1 static at most.
The man to talk to on this stuff though is Bob Madara. IMO, if the builder spec'd that cam, with that compression for a pump gas pleasure marine engine, I'd look elsewhere for an opinion, and I definitly wouldnt want to lock the timing out on something like that.
Sent Bob a message, also got the cam card from erson cams that was spec for my build. I have to agree cam is too small. Idk what the heck they are thinking. My biggest question is will a cam in the 224/230range on 112lsa revert water thru Imco SA risers? From what I'm reading it should be ok but anyone running similar cam specs with short or stock exhaust?
12-15-2014, 07:11 PM
#14
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What heads are they?
I'd discuss the cam selection and reversion concerns with Bob.
I'm not a big fan of the edelbrock carbs, but thats just me. "20cc bigger on the intake ports" flow better statement scares me.
It sounds like you need to hook up with the right people to help you out with this engine build. You goal is a very realistic one, but parts combo needs to be a #1 concern.
I'd discuss the cam selection and reversion concerns with Bob.
I'm not a big fan of the edelbrock carbs, but thats just me. "20cc bigger on the intake ports" flow better statement scares me.
It sounds like you need to hook up with the right people to help you out with this engine build. You goal is a very realistic one, but parts combo needs to be a #1 concern.
12-15-2014, 07:19 PM
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Sent Bob a message, also got the cam card from erson cams that was spec for my build. I have to agree cam is too small. Idk what the heck they are thinking. My biggest question is will a cam in the 224/230range on 112lsa revert water thru Imco SA risers? From what I'm reading it should be ok but anyone running similar cam specs with short or stock exhaust?
12-15-2014, 07:30 PM
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What heads are they?
I'd discuss the cam selection and reversion concerns with Bob.
I'm not a big fan of the edelbrock carbs, but thats just me. "20cc bigger on the intake ports" flow better statement scares me.
It sounds like you need to hook up with the right people to help you out with this engine build. You goal is a very realistic one, but parts combo needs to be a #1 concern.
I'd discuss the cam selection and reversion concerns with Bob.
I'm not a big fan of the edelbrock carbs, but thats just me. "20cc bigger on the intake ports" flow better statement scares me.
It sounds like you need to hook up with the right people to help you out with this engine build. You goal is a very realistic one, but parts combo needs to be a #1 concern.
12-15-2014, 07:47 PM
#18
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I'll say it again...Dave's Radiator Shop ?
12-15-2014, 07:53 PM
#19
Why trash the guy over an engine the money has already been spent on .If Op asked the builder to up the cam roughly 10 degrees intake and exhaust ,It will probably be fine ...I don't know crap about pro comp except what I read ....OP already owns them .I'm assuming he doesn't want to start over ...Idle rambling
12-15-2014, 07:58 PM
#20
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He said repowering and also is questioning the cam and eluding that he may want to change it, therefore I did not take it as a done deal.
He put it up to the public....so one must be prepared for less than pleasing answers, even if it is mostly truth.
'Trashing' is more harsh than what is going on here. Plenty examples of that in this forum.
He put it up to the public....so one must be prepared for less than pleasing answers, even if it is mostly truth.
'Trashing' is more harsh than what is going on here. Plenty examples of that in this forum.
