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Originally Posted by SB
(Post 4087989)
This is basics, so if someone wants more technical at this point don't get all rowdy.LOL.
SC/TC don't add velocity, even though you would think cause' more air in the same space should move faster right ? Not really. Here's why. They are compressors. They take more air volume and compress it down to a smaller space then if it wasn't compressed. The tighter intake runner is going to need more pressure to move the same mass into the cylinder. In my mind, you'd want the intake runner sized for the best efficiency, which will be a mix of boost vs. port velocity. These comments/ramblings are worth what you paid for then. I'm not an engine builder, just an engineer with a little flow experience. |
I'm just asking so I understand
More air tighter in the same space would be under pressure wouldn't it? I get most of what you are saying. I just can't see how an 8-71 spinning efficiently at 3500 rpm and not making manifold pressure isn't increasing the air speed. |
Ever try Dart for a good combo? They have to have some numbers for what's best at what rpm. I used to get a ton of info from comp cams until some jackass sued them.
Let me throw a curve ball here. Are you going to longer rods? On my blower apps I've always run higher wrist pins to accommodate a longer rod. Slower piston speed works well with blowers and makes better torque. |
Originally Posted by abmotorman
(Post 4088039)
Ever try Dart for a good combo? They have to have some numbers for what's best at what rpm. I used to get a ton of info from comp cams until some jackass sued them.
Let me throw a curve ball here. Are you going to longer rods? On my blower apps I've always run higher wrist pins to accommodate a longer rod. Slower piston speed works well with blowers and makes better torque. |
Originally Posted by Cole2534
(Post 4088008)
But it increases your mass flow rate.
The tighter intake runner is going to need more pressure to move the same mass into the cylinder. In my mind, you'd want the intake runner sized for the best efficiency, which will be a mix of boost vs. port velocity. These comments/ramblings are worth what you paid for then. I'm not an engine builder, just an engineer with a little flow experience. If you have a big innefficient roots without intercooler, than yah, physical volume will be. Heated air is expanded air. We are not dealing with just how much oxygen (other gases too obviously) is going into the motor, but the size of the oxygen going in the motor. 200+F air is going to take up more volume than 30F air. That's for sure. The volume correction factor of air at higher or lower temperatures than Normal Air can be found in the table below: Air Temperature Volume Correction Factor (oC) (oF) Correction Factor -46 -50 0.77 -40 -40 0.79 -34 -30 0.81 -28 -20 0.83 -23 -10 0.85 -18 0 0.87 -9 10 0.89 -5 20 0.91 -1 30 0.93 4 40 0.94 10 50 0.96 18 60 0.98 22 70 1.00 27 80 1.02 32 90 1.04 38 100 1.06 43 110 1.08 49 120 1.10 |
Originally Posted by SB
(Post 4087997)
Problem now is, that all SC and TC are not created equal.
Roots / Screw are positive displacement. Screw compresses air in it's 'screws' Roots compresses air in the intake manifold =================================== Extreme example: A 7.5:1 big headed 8-71 bown BBC can work pretty good. How about a 7.5:1 big headed BBC with turbocharger or ATI/Vortec ? Dog for a while until the mojo starts to flow. Not so much fun. ATI/Vortec/etc are centrifugal with gear ratios. IE: step up gears. Turbochargers are exhaust (rpm/load/etc) driven. Thus Roots/Screw are very popular in the I/O world because from idle to where ever, they respond 'right now.' Always full boost when the throttles are wide open now matter what rpm. Centrifugals increase with rpm, again they have gearboxes. TC needs rpm and load to make them to do more. |
Originally Posted by check300
(Post 4088079)
We have used a compression ratio of less than 7.5:1. Our heads flowed just under 400cfm. They started out as dart 360's. We made boost at 1500 rpm's and made over 1000 ft lbs of torque at 2300 rpm's. Turbos are 75mm.
LOL. See. This is why I didn't want to get too into it because there are a quazzillion avenues to take with this subject. How light to you talk about it ? LOL. Try to make a general point and bang, someone has different info. Engine + turbo sizing / load / use / boat design / camshaft / etc,etc,etc This is why a difficult subject to talk about without having an exact goal + vehicle to discuss. BTW: I've followed your boats performance info (if you are the one I remember with checkmate and turbo motor) and it's impressive ! |
It is twin turbo in a 30'. It's been fun for sure.:)
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What SB said.
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its like a river flowing with NA.. You have water going down a river , it has to go up a slope then back down. To much going in would make a swirling mess as the right amount would flow faster.So the same size ports to exhaust matching is gonna work best. You can't cram too much air a a pocket that wont take it because it wil interfear with flow.. So yea NA maybe not good to go to big.
But Blown set up. You can never be too big because it is based on Pressure Not air flow. |
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