pump gas/low compression/ high hp

[steelcomp]

You guys are so very sharp here, and I really appreciate the time taken to explain things. Having said that, can someone help me understand the fundamentals of what it takes to get good usable power out of a carb'd pump gas motor. I have heard of some BB Chev's runing as much as 11:1, pump gas, making well over 800 hp., and BMEP's in the 200 range. I understand a little about chamber design, but my main question is regarding camshaft profiles, and bulding dynamic pressure in the abscence of static pressure, without detonation. I also realize that there are countless variables here, and it's hard to "generalize" on one area but any insight would be appreciated.
Thanks in advance.

[SUPRSLO]

http://www.kb-silvolite.com/calc.php?action=comp

That is a good dynamic compression ratio calculator that i've used in the past to help you get started. It seems as if many indicate between 8.5 to 9.0 is the maximum thresh hold for pumpgas but things like the design of your combustion chamber, temperature of your motor, temperature of the incomming intake charge, etc. will come into effect here as well. It would also seem that if you just go by the dynamic compression ratio it would dictate how well the motor would work while outside of it's full throttle operating range as the full throttle operating ranger is where cylinder pressures (BMEP) will be the highest and where the motor will be most sensitve to detonation. A guestimated example would be a 23 degree headed 400" motor with a dynamic compression ratio of 9 to 1 and a SB2.2 headed 400" motor with a dynamic compression ratio of 9 to 1, rpms kept constant between the two. I would surely think the sb2.2 motor maybe more likely to detonate when considering only BMEP. Now while giving this example just thinking of how much more effecient the sb2.2 combustion chambers may be they may burn the mixture fast enough to offset the higher cylinder pressure. Anyone want to chime in here?

[SStrokerAce]

Little reading material for you

http://popularhotrodding.com/enginemast ... 603em_emc/

http://popularhotrodding.com/enginemast ... 604em_emc/

Bret

[highVE]

The biggest factors for this is Ignition timing, and cumbustion chamber efficiency!!
A fellow engine builder whom i do head work for builds low 600ci BBchevys for some marine applications. Pump Gas around 820Hp.

Mike Theroux

[steelcomp]

Thanks guys! This is great!

[beth]

I believe the secret to making low compression work is to make your low compression as high as possible. Compact and efficient combustion space, strong ignition that is timed correctly, good fuel vaporization, oil control, low temperature inlet air, efficient cylinder head cooling and proper gearing are the most important things IMHO.

Trapping high cylinder pressure is not hard, avoiding detonation with low octane is.

[jcd]

Good info given! Now I have a related question, that is, can you make more power with low compression and high octane (110) fuel? In my racing class we are required stock engines, but can "run any cam", "tubular headers with primaries not extending aft of flywheel" and "up to 350 cfm".
The fastest cars there are running 110 octane. With stock pistons, heads, no decking allowed, why do they need this? Can camshaft profile (late intake closing to increase dynamic CR) cause so much increase to need this much octane? I doubt it. Can ignition timing be advanced so much that they gain power with 9 to 1 compression? I wonder. Are they going so much faster because their engines have illegal parts or modification to increase CR and they really need 110 octane? I suspect so.
Any thoughts on this? I have posed this question at numerous sites and seem to have boggled everyone so far. Myself, I run a 1.9 engine, factory stock 9 to 1 flat top pistons, radical Isky cam, Holley 350, tube header at 37 degrees total timing on 92 octane pump gas. Three competitors at my track just run away from me and everyone else, two are Ford 2.3s and one Toyota 2.0, rumored but unproven (I'm wording this carefully) by many to be illegal...
Thank you!

[F1Fever]

if they run a tight lobe seperation cam, possibly advanced or ground with advance, itn is possible to raise the dynamic compression enough that higher octane could help. I have a 350 with dish pistons and after I installed a circle track cam with 105* ground on a 99 centerline it began to spark knock on pump gas.

I'm not saying that they aren't cheating, just that it's possible to need a higher octane gas with 9:1 and certain profile cams.

[SStrokerAce]

Yeah your cam profile could cause this but most race gas is going to make more power no matter what compression it's run on.

Bret

[F1Fever]

I would like to elaborate a little on the cam profile.
You mention late closing increasing the dynamic compression. Actually a later closing will decrease the dynamic compression if the amount overlap is the same.
You say you are running a "radical" cam. Can I get the specs on it?

[femtnmax]

F1 is correct. Late intake valve closing will reduce your DCR. Have you played with retarding or advancing the cam to change the intake valve closing event? A 'radical' cam probably has a late intake closing so you are loosing dynamic compression. Advancing the cam will raise the DCR, but reduce the high rpm power to some extent.

[SStrokerAce]

When ya'll going to explain to me how the overlap effects the DCR of the motor? It might change the VE but not the DCR......

[jcd]

According to my studies, later intake closing at high RPM will cause supercharging (yes, higher than 100%VE) because the incoming air at high velocity will want to keep coming in even though the piston has started coming back up. If the valve closes just as reversion begins to occur you have trapped more air than you would have at lower rpm, right? Thus your compression ratio is higher than the static compression ratio. Isn't this the theory of dynamic compression ratio?
Correct me if I'm wrong, going this heavy into the theory is new to me, but apparently this is what I need to learn to go faster...
My cam is Isky #OR-77. Intake is .020" from closed at 72* ABDC. It is .050" from closed at 54* ABDC. Compression is factory stock 9 to 1.
I turn about 7000 RPM.
femtnmax, where in MT are you?

[F1Fever]

After this thread I got to thining about how VE changes the DCR and wrote/modified a script to take that into account. With input from this board I also added duration @ .050 and advertised duration. You can play with it and see what you think.
http://www.hotroddersauctions.com/Dynam ... ulator.php

[MadBill]

According to my studies, later intake closing at high RPM will cause supercharging (yes, higher than 100%VE) because the incoming air at high velocity will want to keep coming in even though the piston has started coming back up. If the valve closes just as reversion begins to occur you have trapped more air than you would have at lower rpm, right? Thus your compression ratio is higher than the static compression ratio. Isn't this the theory of dynamic compression ratio?
Correct me if I'm wrong, going this heavy into the theory is new to me, but apparently this is what I need to learn to go faster...
My cam is Isky #OR-77. Intake is .020" from closed at 72* ABDC. It is .050" from closed at 54* ABDC. Compression is factory stock 9 to 1.
I turn about 7000 RPM.
femtnmax, where in MT are you?

The term 'DCR' is normally used only in regards to cylinder filling at low speeds, where reversion (and detonation) may be rampant. At high RPM, where V.E. has increased to 100% or more, the normal flame front consumes the end mixture before the critical 'lag time' (the interval between when the conditions for detonation are met and the actual moment when it occurs) runs out.