I've got a couple of options for my next motor, and I'm wondering what direction to take.
Both are going to run a turbo, and I have E85 available locally. How do I figure out which is the "best" choice, and is there a calc. for this?
Use a dyno simulation software like desktop dyno. It won't be entirely accurate on peak numbers, but its excellent for comparisons like this.
How do the displacements differ? Is it with bore size or stroke? The bigger bore almost always does wonders for unshrouding valves and increasing flow. I would personally always choose the larger displacement. You can compensate for the lower compression by using a tad more boost.
Yep There is no substute for displacement.
What kind of engine? 4 cyl? Inline 6?
Which one has the better heads, manifolds, etc? 4 shots worth of displacement probably won't make all the difference.
All else equal...I like high compression. Much more fun to drive. You'll have a lower ultimate boost level, but you'll get faster spool and better response. Plus, using E85, you can run lots of boost even on high compression.
If you're building a drag car, go for the bigger displacement. If you're building a sports car, go for the high compression.
At the displacements you are dealing with if it was an NA motor I would go with the one that has the best flow characteristics and the one that has the most cost effective mode.
Also (again if ti is a NA application) if there were reasons to considder the larger motor (like it is in better shap, parts are cheeper, better fitment etc, you could deck the head and use a thinner head gasket and bring the compression up to close to the small higher comp motor. Then you have the best of both a larger comp motor with "high comp".
However if you are adding forced induction you negate a lot of the flow issues. Additional with forced induction and low compression and larger displacement you will have more HP easier (less stress on the motor that trying to make the equivalent hp with the small high comp motor). E 85 makes your job of tuning easier and will allow you to run more boost on either motor so I don't see this as a real deciding factor. I am obviously assuming that all other variable are equal like durability, ability to bolt to the trans you want cost of mods availability of parts etc.
So for your specific application with out knowing a lot more of the variables I would use the larger low comp motor.
shorter stroke, bigger bore. need revs for a 4 cylinder. on the low comp engine you can run more boost safely though, probably 15+ psi with comp that low if the internals are decent
then again, if its a toyota 1.8 (1zz) you can run 16psi on stock internals apparently lol (friend of mine made 243whp and 240ish w tq on an 07 corolla s stock everything except turbo kit running 14-16psi. on a mustang dyno. so figure 250 on a dynojet or the like.) and he ran it like that for a year or more. he ran it for a few years on 8psi. QUICK car
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