2 similar engines both make the same horsepower and Torque.
One has 7.8-1 compression but flow potential to make 700+ horsepower.
The other has 11.5-1 compression but max power is limited to about 450.
ignition timing on the 7.8-1 compression is 38 BTDC
while timing on the 11.5-1 is 17 BTDC
What's your goal, response or power?
In reply to Keith Tanner :
Power.
In reply to Keith Tanner :
It's pretty obvious to me that if you pull back timing you can get away with some boost even at high compression. Exactly how much is probably only found on a dyno .
Some, yes. But if you want bulk power, you go for lower compression - you'll be able to run more boost that way. Add in the higher flow heads and it's not much of a question. I am assuming that ignition timing is completely under your control.
That makes a lot of sense. In addition there is more timing room with 38 degrees instead of 17.
Is there any formula to give me guidance as to how much boost I can get by with?
It's a 250 cu in Roots type blower feeding 326 inches.
Keith Tanner said:Some, yes. But if you want bulk power, you go for lower compression - you'll be able to run more boost that way. Add in the higher flow heads and it's not much of a question.
This. Compression makes power, but boost makes compression. Say you can get 100% VE on a 5L engine. Squeeze it down to 1/11 and ignite. Boom. Or boost your way to 200% VE (10L of air/fuel!) on a 5L engine and "only" squeeze it down to 1/8. Bigger boom. Want more? Crank the snail and push 300%.
Im no scientist, but more volume is more volume. More air/fuel is more power. Toss in some of your favorite corn juice and go crazy.
Another way to look at it is energy stored in a given fuel is static, constant. Adding compression is going to give more efficient combustion, to a point. But even if you can find the perfect combination of compression and timing to burn 100% of the fue in a cylinder (is that even possible?) you'll still be better off jamming twice as much fuel in that same cylinder. If x is your fuel, which is gonna make more power? 100% of x, or 60% of 2x?
Ideally, you'd find out what the best efficiency range of that blower is. Spin it too fast and you'll spend your time just heating air and dragging on the crank - a Roots blower will hit a plateau at a certain point. For example, a 45ci Roots on a 110ci engine peaks out at 8 psi and all more boost does it make more heat. There's also the question of strength of other components but I think you're confident in that regard :)
Superchargers can usually run some extra ignition timing over turbos, which helps make up for the parasitic loss. And boost and corn go together like chocolate and peanut butter.
In reply to barefootskater :
I agree, except the faster you spin the blower the more heat is created. At some point heat generated can no longer be handled by gas and corn juice is called for. But at some other point even corn juice isn't enough.
Can I trade less boost for higher compression and come out ahead?
Keith Tanner said:Ideally, you'd find out what the best efficiency range of that blower is. Spin it too fast and you'll spend your time just heating air and dragging on the crank - a Roots blower will hit a plateau at a certain point. For example, a 45ci Roots on a 110ci engine peaks out at 8 psi and all more boost does it make more heat. There's also the question of strength of other components but I think you're confident in that regard :)
Superchargers can usually run some extra ignition timing over turbos, which helps make up for the parasitic loss. And boost and corn go together like chocolate and peanut butter.
That's exactly what I was thinking. At some point static compression and slower blower speed is going to offset better flow and lower compression.
Then there are the intangibles. Starting ease, idle quality, peakyness?( is that really a word?) Fuel tolerance etc.
I mean I'm ready to accept any of those issues but if one combination has no issues and there is no real power difference I guess I know which way to go.
Right now I have pulley's that yield a 15% overdrive or a 15 % underdrive So I could drive it to the event underdriven and then swap the pulleys for the event and return them to underdriven for the trip home.
For what it's worth, the Proline Racing engines making 2500+ hp are also over 13:1 compression.
In reply to frenchyd :
It's all about finding the best compromise for your situation. Compression is about efficiency. Turbos are the best way to add efficiency. if your only goal is power/displacement, a turbo is king. If your boost comes from a roots blower, that's a bit different. As to where the best balance of boost and compression occurs, that's some math I'm not super familiar with. I'd still imagine you'd be better off boosting 7.5:1.
It sounds like you really really want to build the high compression engine with the bad heads. It's the wrong choice, but it's a choice :) It's pretty well known that moar boost and less compression wins over less boost and more compression when sheer horsepower is concerned. Especially with a blower.
I'd love to know more about those Proline engines. Their website does talk about methanol, but doesn't give compression ratios.
In reply to barefootskater :
Thanks, yes that is the formula I'm looking for.
Here's my thinking so far. 250 cu in roots type spinning the same RPM as the engine has to feed 3 cylinders ( of 12) per revolution that's 81.5 cu inches that should result in about 7 psi per rpm.
I'm not familiar with roots blowers, so maybe this question will help everyone learn?...
1- for a given pulley size, you get a given ratio blower/engine speed. More blower speed=more air (boost)
2- power is a function of engine speed. Kind of. But usually more engine speed=more power
if a certain displacement blower maxes out at 8psi or so, is it possible to change the pulleys so that you're getting 8psi @ 6000rpm instead of @ 4000? Or to do it @7000?
it may be a silly question, but a better flowing head is better at high speed, so use the better headed engine and only add boost up high? Idk. Just thinking.
In reply to Keith Tanner :
I don't know, that's why I'm asking. Since I have both engines. I'm trying my best to pick a winner. My gut wants the better heads /lower compression but I see modern boosted engines with pretty high compression making serious power and torque. Hence my uncertainty.
Can it be a function of better programming ? I'm sure it is but how close can I duplicate that? I've got my own distributor machine so I should be able to get the timing pretty close to working. Plus I'm willing to spray WW fluid of whatever blend of methanol and water achieves my goals. Right now WW fluid is about the price of gas, so cost won't be a factor.
In reply to barefootskater :
1 yes but faster blower speed equals more heat. One reason to use a big blower on a small engine is the slow spinning compressor doesn't generate much heat.
super chargers don't accept intercoolers very well. They just kill the flow numbers even if they can cool the charge.
In reply to Keith Tanner :
This article is several years old but it gives a pretty good breakdown of what's in these engines. This says 11-11.5:1 but it was written in 2014. I know for fact they are running more than that now. My best friends' brother owns Proline.
Faster blower speed means more heat - but not fast enough means low efficiency. This is less of a problem with a Roots than a twin screw, but still. You want to size the blower correctly for your intended use. So you either start with a given blower and figure out where it's most efficient or you start with a power goal and you pick a blower that's going to be best suited for that goal. Do you have any information on this particular supercharger?
You have to watch out for blower redline as well as efficiency. We used to have a twin-screw setup for the Miata that was an absolute screamer and super-fun to drive, but in order to get the supercharger into the best efficiency range (and make real power) we had to spin it reeeeally close to the compressor's redline. Lost a few superchargers.
There are a few ways to intercooler superchargers. You don't really lose flow, but on a positive displacement supercharger the intercooler is throttled volume. This leads to some real driveability problems. I'm also assuming EFI, otherwise things can get quite interesting as I'm not sure you can do a blow through carb on a super. Can you? Anyhow, if you don't want that high throttled volume you can use an air/water intercooler. That's exactly what both OEs and good aftermarket setups do. Any percieved loss of flow is more than compensated by the increased air density.
Watch out when using water injection and superchargers. If something happens to interrupt that injection - clogged nozzle, empty tank, pump wiring, blown hose - things can go very wrong in a hurry. On a turbo, you can monitor flow and cut the boost in case of a failure. Can't easily cut boost on a supercharger.
Basically asking is it better to boost the v12 with them early heads VS an HE?
Keith Tanner said:Faster blower speed means more heat - but not fast enough means low efficiency. This is less of a problem with a Roots than a twin screw, but still. You want to size the blower correctly for your intended use. So you either start with a given blower and figure out where it's most efficient or you start with a power goal and you pick a blower that's going to be best suited for that goal. Do you have any information on this particular supercharger?
You have to watch out for blower redline as well as efficiency. We used to have a twin-screw setup for the Miata that was an absolute screamer and super-fun to drive, but in order to get the supercharger into the best efficiency range (and make real power) we had to spin it reeeeally close to the compressor's redline. Lost a few superchargers.
There are a few ways to intercooler superchargers. You don't really lose flow, but on a positive displacement supercharger the intercooler is throttled volume. This leads to some real driveability problems. I'm also assuming EFI, otherwise things can get quite interesting as I'm not sure you can do a blow through carb on a super. Can you? Anyhow, if you don't want that high throttled volume you can use an air/water intercooler. That's exactly what both OEs and good aftermarket setups do. Any percieved loss of flow is more than compensated by the increased air density.
Watch out when using water injection and superchargers. If something happens to interrupt that injection - clogged nozzle, empty tank, pump wiring, blown hose - things can go very wrong in a hurry. On a turbo, you can monitor flow and cut the boost in case of a failure. Can't easily cut boost on a supercharger.
Thank you Kieth. Yes it is a Roots type. With a big Holley on top. As far as WW fluid goes, my program will be simple. A big plastic tank in plain sight with a on/off toggle switch right next to the boost gauge.
As a practicing Luddite I believe in keeping it simple. Boost goes up fluid should go down. If for any reason it doesn't the foot comes out and the boost drops.
yupididit said:Basically asking is it better to boost the v12 with them early heads VS an HE?
The real question is what are the parameters allowing the use of boost with high compression. I project that blower to efficiently produce 7 psi at normal operating speed. ( now I freely admit my calculations may be off) I think asking for input is clearly indicated.
Kieth has been very gracious helping me to understand supercharging. I know I'm in unexplored territory.
Unless you have a very small reservoir, you won't be able to monitor if the injection is working right now, and things can go sideways pretty quickly. I'd recommend a light triggered by a flow sensor so you can tell if you're trying to inject water but nothing is happening. It's not an expensive thing to do, really: https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/114991172/1597-1519-ND/7387423
Requiring the driver to turn on the water injection is great until that one time you forget. It's not difficult to trigger the pump with a pressure switch instead and it eliminates the human aspect.
Keith Tanner said:Unless you have a very small I reservoir, you won't be able to monitor if the injection is working right now, and things can go sideways pretty quickly. I'd recommend a light triggered by a flow sensor so you can tell if you're trying to inject water but nothing is happening. It's not an expensive thing to do, really: https://www.digikey.com/product-detail/en/seeed-technology-co-ltd/114991172/1597-1519-ND/7387423
Requiring the driver to turn on the water injection is great until that one time you forget. It's not difficult to trigger the pump with a pressure switch instead and it eliminates the human aspect.
As a card carrying Luddite I want a great big gated lever at least 3 feet long made out of brass. ( I actually have such a thing stored someplace in my shop) to turn on the water injection.
If the pump is below the dash and the tube runs next to the boost guage it would be clear when empty and blue or purple ( depending on the brand ) as it goes through the line.
What does the tube look like when the nozzle is clogged and there's fluid in the line and not moving? This is a legit failure mode. I'm not suggesting anything fundamentally more complex than your switch and tube, merely more failure-proof. Simple doesn't mean incomplete.
Of course, Luddites were active saboteurs of progress who protested by breaking machinery :) So maybe I've misinterpreted your intentions!
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