Unfortunately I have seen this situation and sentiments hundreds of times (as an entrepreneur, advisor, mentor, educator ... and developer). Most of the time this is due to the developer or engineer overvaluing their contribution or value to the new venture. Yes. This probably means you, too. OK. Let it sink in for a moment, and just consider my advice before waving it off as "he doesn't know the details of this situation" or "I'm different".
There's a famous quote: "Any fool can paint a picture, but it takes a wise man to be able to sell it." ... Of course I am not calling you or any developer a fool. But the point is there are a LOT of important facets to making a startup company - and everyone thinks their contribution is the critical force deserving 50% or more. I'm am sure the founders - FOUNDERS - (I will shout it again FOUNDERS) believe that they should get nearly 100% and giving away equity to a "mere techie" is an annoyance and tradeoff they have to suffer because they don't have the cash. And rest assured, right now, you are (to them) both an annoyance and a huge risk. Feel better? Of course not. It's a bitter pill.
My advice. Do some soul searching. But most important - understand you are definitely not in a position to see things in the best or fairest perspective. Probably you are best working either for a funded startup as a well paid employee, or founding your own startup and running the show (then you will understand things from the founder's perspective). More specific advice - TALK with the founders. and NEVER do anything vindictive or destructive to the company. This behavior will follow you for a long time - and brand you as being "difficult" (who want's to work with a 'difficult' person - much less bet their future on them)?
Here's a little extra perspective. 20% equity is HUGE - for someone who is not a cofounder and not slated to be CTO or senior management. Understand that yes, while RIGHT NOW - your contribution seems like an overwhelming part of the company - EQUITY IS Forever (or at leats for the long term). To build and run that company from startup to $billions will take a huge marketing, sales, management effort and fundraising, a team of people that will ALSO put in 20 hours a day, sacrificing family, finances etc. Not just you. Don't you think the CEO who is also putting in 20 hours a day (and using his/her rolodex and reputation) to raise, say $5million in venture capital thinks that they deserve 80% of the company?
More practical advice:
1) Do some soul searching. decide if you think the company's idea, market, and founders really have have a chance of major success. If not - Talk with them and arrange a friendly, graceful exit.
2) If you think the venture is something that has a chance of major success - TALK to the founders. You will find out quickly if they value your contribution (for the long term - not just short term panic). If they DO - then talk to them about some kind of Equity split or OTHER compensation (future bonuses if they get funded; a guaranteed salary/CTO position?) - and you can suggest that the equity split - while "large" be vested shares or options - actually they would be stupid to offer otherwise - but this way you can get your shares if your contribution is indeed as important as you think it is, yet they don't have to risk a huge part of their company if you turn out to be a problem.
Again ... perspective - a little humility. All lead developers think they should own 50% or more of a startup even if they weren't the founder. And the majority are wrong. If the founder's came up with some cash - do you think they could hire a few guys to get the job done with 0% equity?
I know this is some bitter advice - but it is also some independent advice from someone who has been part of many new ventures - as a developer, founder, investor and advisor.
Best of luck!