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The expense on modifying an old Volvo can vary tremendously. Depending on what you do (and know how to do) yourself. Versus what you send out to be done. Anything beyond basic parts swapping involves a greater degree of knowledge, and it doesn't come cheap.
I'd agree with Phil that while there are numerous small tweaks you could make to a B20, your initial main bottleneck to creating more power is one of the most expensive to modify - the cylinder head. A B18/B20 head already has awkwardly shaped exhaust ports that make a very sharp bend exiting the valve, and the F heads in particular seem to choke down significantly just past the valve seat. Was this done for some arcane emissions reason (perhaps a sort of passive EGR through exhaust reversion?) or just to throttle the engine output down to increase longevity? Who knows.
You could get the local Chevy/Ford/Mopar head grinder to work on it. Don't work on the intakes at all (other than perhaps matching the port/intake manifold), have him spend most of the time/money on the exhaust, and put in hardened valve seats while they are at it. Using experience gleaned from working on domestic heads they can make a modest improvement. I think to get a major improvement you need to send it off to someone who has done a lot of Volvo heads (like john Parker, v-performance.com).
I happened to get really lucky and find a very rare Volvo R-sport head selling very cheaply on ebay. So instead of spending $1000+ for a well modified head I got one for $150, shipped. Really big valves, really roomy exhaust ports, double valve springs, hardened seats, one of my best eBay finds ever.
With the head out of the way, other things to increase the power don't necessarily cost more than a regular complete overhaul would cost. So if the engine is worn out, consider using non-standard replacement parts in the rebuild. Get the block bored out and use b21 pistons. Opinions vary on that move, but I figure a little extra displacement can't hurt. Won't increase peak HP with a stock head, but it will increase low end torque. And with a better head and cam, peak HP will rise too. A cam is another obvious area for improvement. With an otherwise stock motor, anything more radical than a 'D' or 'K' cam (very similar stock cams, the ES should have a 'K' in it) might make it feel zoomier, but probably won't help that much - shifting the power band higher in the rpm range doesn't help if the rest of the engine can't support the airflow.
I have a set of DCOE's (side draft Webers) in the possibly too small size of 40mm on my PV's motor. I used a wideband to set them uup, and am reasonably pleased with the results. However, in this day and age I think the time, effort, and money expended on aquiring, installing, and setting up Webers is probably best spent on aquiring, learning, and installing a modern programmable fuel injection system like Megasquirt. On an ES, you could re-use most of the existing D-Jet hardware. If your engine harness is in good shape (an iffy proposition on most 35 year old cars) you could even keep it in place for a very stock installation by getting the ECU plug from a dead D-jet computer and fashioning a short wiring harness adapter from it to the MS box. The D-Jet ECU already has most of the inputs and outputs MS would need. I did that on my '93 245 Turbo, although I didn't make the harness myself. It just plugs into the stock harness, the MS plugs into it, you can swap from stock computer to MS in about 15 minutes. then set it up with a loptop.
Although, if the engine is still in pretty much stock form, there is little to be gained by going to MS. While D-Jet isn't very tuneable, and it doesn't like physical changes in the engine, it does work reasonably well with a stock motor. MS could do closed loop O2 sensor feedback though.
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I'm JohnMc, and I approved this message.
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