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Fa20 boosted engine build

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I have fa20 that is going to be 10:8.1 compression instead of the stock 12:5.1 that it was. It is also going to be stoked from 86mm to 90mm. Increasing engine displacement by 100cc. I have a jackson racing c-30 supercharger that I made 355whp @14psi+E85 with the stock motor. I have been told that I’m going to loss a bunch of power cause of the lower compression and that if I upgrade to a c38 supercharger making 17psi @7500 rpm which is 4 psi more than my c30 . that I’m basically going to be back around my 355 whp mark making no real gains. Any engine builders or tuners out there have a way of calculating what will happen?? or any one that has built a fa20 with lower compression have first hand Experience?

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From my own experience at the power levels you're at and running E85 I'd have been inclined to retain the stock 12.5:1 CR. Our own development car makes around 385 whp on 13 psi on E85 and I while the engine can be made to knock, it's not really a limiting factor. Typically the supercharged engines happily take a lot more timing than a turbo engine too. If your engine is built then I wouldn't beat yourself up about it. Get it on the dyno and see where you're at. My best guess is that you'll probably find that the extra capacity will. likely cancel out any loss due to the lower CR. You'll probably need either more boost or the larger SC to go further at a best guess.

Thanks for your time to give me your input. I just sleeved the block I haven’t decide if I am going with a 86.05mm or 89mm pistons. Maybe I can send my pistons back for a higher compression pistons and get bigger pistons. I know for sure I’m going with 90mm crankshaft. I think the money I was going to spend on cams I may just bore out and go with the 89 pistons and with 90 stroke I’ll have 2,239.9cc engine instead of just a 2,093.6cc with stroke alone. In your opinion is the extra cc going to make more power than cams?? Also if and when everything is assembled if my supercharger is putting down enough power I may just do a 40-80 wet shot of Nitrous just when I’m drag racing on top of the supercharger. Just depends on funds!! I really want to swap out the supercharger kit for a turbo but NOS is way more affordable and easy to put on along with the supercharger. I just have no experience using nitrous on Subaru engines. I have tons of experience using nitrous on Honda B18 & K20. Is there any thing to watch out for or that is different from spraying on a Naturally aspirated Honda? What’s your thoughts on Nitrous express throttlebody plate fogger with the supercharger?My Internals will be 10:8.1 Mahle 2618 pistons,Manley turbo tuff 4340 con rods, forged crankshaft, ARP bolts,REI high flow oil pump and gear kit,LA Sleeve amphibious sleeves, supertech valves and complete valve train. Is there going to be any issues with tuning 14psi& e85 along with the Nitrous? I use ECUTek racerom and flex fuel with a 4 bar map sensor 770cc port injectors.

I can see that you are really concerned about the power that you are going to make/lose. Please take a look at the file attached to get a general idea of what you can expect with changinfg CR and boost. In general lowering CR helps with getting more power at high levels of boost whilst it is not helping to make power at low boost. So if you are not planning to go much more above 17 psi you better off keeping stock CR especially on e85 which is perfect. One thing to bear in mind is to check on engine cooling as higher CR quite often results in engine overheating when boosted. If you will introduce nitrous I would also recommend to change your exhaust camshaft for a bit longer duration. There are two reasons for that - first one is that higher CR engines get the most of the work done in the area closer to TDC so there is there little point to keep the exhaust valve closed as normal since the pressure in the combustion chamber is dropping much faster comparing to a lower CR engines- you would want to give the cylinder a little bit extra time to evacuate exhaust gases and free up some room for new air/fuel charge. The second reason is that nitrous generates a lot of exhaust gases which also requires additional time to evacuate combustion chamber. So combining these two factors would make sense to upgrade exhaust camshaft. The other thing which is consequence of those two is to go with larger intake valve - that should also help with getting more power but most of the people do not consider this option as it takes time and hassle with modifying the valve seat and head port as well...

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The thing is I was running 12.7@112mph in the 1/4 mile on my old engine With 355whp@ 7400 and 252wtq@7000rpm . My best 60ft was 2.0 with M&H Drag radials. I know that I could have gone low 12sec but Kept my launch at 4000 rpm to save my stock axles . I was at the point where I wasn’t bogging off the line but it took some time to get to my power band . So with that in mind I could have been running 12.1 with a 1.7 60ft . My whole goal is to run between 10.8 and an 11.2 with a 2800 pound street able Toyota 86. I believe that I can get there with 425-450whp! So I’m not sure what the safest route to that power level is? Spraying nitrous threw the nitrous express wet shot plate kit that goes in between my intake manifold and throttle body along with 14psi from my rotrex supercharger . Or take and sell my supercharger kit and Ace-350 equal length headers and by a turbo kit . I want to make the power but try and keep my engine together as long as possible and stay reliable since I drag race once a week from April till November in a ET racing series. I make around 5-9 1/4 mile passes each event . What’s the safer set up?? And what do you think my Minimum horsepower to run 10.9 will be with my car since I was running 12s with 355whp? I have attached a video of my 12.7 1/4 run below and time slip

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Well, the cheapest power adder is always nitrous especially when your supercharger is maxed out ( is it BTW?). In terms of safe way of increasing the power it is no different to anything else- if you know the limit of your short block and you tune your ignition timing and fuel accordingly it will serve its purpose just fine. It will also help with reducing intake air temperature which is quite important factor. My estimation is that you need additional 100-150hp shot of nitrous in order to achieve your quickest goal but a lot of it has to do with how well you launch - your 60ft is the most important area as you know and this is where i would be trying to shave off some time first.

Thanks for taking the time to talk engine’s!! My supercharger is at 95% max impeller speed at 7400 rpm at stock Redline and I rev my car out to 8000 rpm. The supercharger is rated to 100,000 rpm at 7400 it’s turning 95,000 . I’m not good with math so don’t know what it’s turning at 8000rpm! Do you know how to figure it out?

I'm just about to go to airport now - i'll get back to you soon.

95000/7400 = 12.837837 * 8000 = 102702.702

Thank you for the response so what rpm would my impeller speed be at 100,000 rpms? I guess I have been over reving my supercharger 2703rpms since 100,000 is the max . what type of damage can happen or will just create more heat than recommended since I only do it for very short periods while drag racing?

Regarding cams and compression ratio; when talking about CR consider that the figures quoted are static compression ratio, the actual or dynamic compression ratio depends on the volume of the cylinder at the point of the intake valve closes, in NA engines running sub 100%VE this is usually much less than the static CR and increasing the duration will further reduce the dynamic CR

Using longer duration cams in forced induction doesn't really net the same gain it would with an NA engine as in an NE application they allow the inertia of the on coming charge to increase VE - something that is overcome with boost on an FI engine. You might find that for FI its better to go for a higher lift and shorter duration increasing the dynamic compression ratio and providing better cylinder filling.

As for over speeding the compressor; find out what the hardware limits are at 100krpm i.e if the bearings can take it. In terms of performance consult the compressor map and see if 100kprm puts it into an area of low efficiency meaning an increase in heat and beware that although heat soak isn't generally an issue when drag racing if the temperature rise is rapid you could be in danger of pre-ignition towards the end of you run. Increasing capacity and air flow through porting and cams will allow the boost therefore compressor speed to be reduced for the same given power output taking the engine/compressor out of a potential danger area.

You may wish to review what you wrote, Scott, I know what you mean and have trouble following you - and he's a lay-person.

Thanks Gord, this might not help much though...

Essentially it sounds like you're at the limit of the supercharger already, the Jackson C30 is rated for around+50% - 300-350bhp. To achieve your 400-450bhp target you are going in the right direction with increasing capacity and go bigger if you have the option to do so. It seems like the engine isn't terribly knock limited and since you cannot really add anymore boost dropping the compression ratio isn't a benefit so going back to the stock CR will help.

As for cams I know HKS have done a lot of work in this area but it does require piston recess machining to get get the valve to piston clearance back so may not be an option for you.

I would also look into porting the cylinder heads as this would allow more airflow at the same boost level and increases of 8-12% are usually achievable (but there are additional considerations for solely GDI engines).

Based on your current numbers a really, really rough estimate would be around 390bhp with ported heads and 2.1L or 420bhp with 2.2L. Based on HKS data cams would add around 3% so get you up to 435bhp.

Fundamentally though with the C-30 the engine is airflow and rpm limited so the only way to increase the output further would be to use nitrous.

Again these numbers are a finger in the air estimate and dependant on many factors. The FA20 has has had a lot of aftermarket development already and have covered quite a lot of these aftermarket tuning options so you might find these useful.

Cheers, Scott.

As you say, the limiting factor is primarily going to be the mass airflow the supercharger can pass through to the engine, as that's all that can be used for oxiding the fuel to produce the power. There are dimishing returns on spinning it faster, even before running into the mechanical limits.

If it's not knock limited, dropping the CR is just going to compromise power, but on the other hand, for his power wants, he's going to need to run more air into the engine and this normally means more boost - going to have to bite the bullet and get the CC out...

Where we differ a little is in the mass-airflow/boost/VE area - but this may just be the way we look at them. "Boost" is just the measurement of the resistance to the airmass the supercharger (or turbocharger) is forcing into the engine. For me, because I'm 'old', true Volumetric Efficiency is how well the engine breathes internally - basically from the intake for N/A, the T/B or plenum for F/I. For the overall airflow with forced induction I strongly suggest the term Air Charge Ratio, or similar, be used - this would be the ambient air volume the 'charger forces into the engine to the nominal air consumption of the engine.

If the internal breathing of the engine is improved, boost will actually drop, because there is less resistance, and power will increase - even with constant displacement superchargers! The latter because there is less parasitic power loss used to compress the charge, and because the charge temperature is reduced potentially allowing more timing to be used - of course, it applies to ALL forms of forced induction.

Ultimately, though, I agree, he's either going to need more airflow capacity in his 'charger and/or chemical 'supercharging'.