The rule of thumb is, if you want to increase boost you lower engine compression. I understand the mechanics of forced induction and why this is done. But what I dont understand is doesnt retarding ignition and lowering compression lower the output of your engine? And wouldnt this just negate the power the increased boost gave you? in effect having to boost MORE to get the same hp.

low compression (-hp) + retarded ignition (-hp) + boost (+hp)

Why not just build up the engine.. couldnt you leave the compression high and not have to boost as much to get the same hp?

either way your still getting high compression in your engine. maybe there is more engine dynamics than i am aware?

 

you gotta consider how much hp compression and timing advance gives you compared to boost, cuz boost gives more hp that compression and timing . you can also run a drag turbo kit at 7 pounds safely with stock timing and compression on a b18c or b16. you want to keep compression stock or 9:1 to promote drivability, opposed to the raceprepped car that just wants high end hp. it all depends on fuel delivery. lowering compression and retarding timing gives you extra insurance for tuning errors


[Modified by 01geeser, 9:28 PM 11/2/2001]


[Modified by 01geeser, 9:30 PM 11/2/2001]

 

so technically if you get all the tuning considerations perfect. Fuel/Air mixture, Ignition timing and strength, then you could concievably keep high engine compression and safely run high boost?

damn youd think with all the turbo going around for years that someone would have this stuff down... and sell it in a nice, tuned, expensive kit.

 

I agree with what was said, but lets put a little scaling on it.

Your equation:

low compression (-hp) + retarded ignition (-hp) + boost (+hp)
and with scaling
low compression (small-hp) + retarded igntion (small-hp only at highest boost) + boost (huge jump in hp)
with fake numbers (no correlation)
-5-2+100=93

The compression ratio just allows higher boost numbers without resorting to exotic fuel.

BTW-what is low? small? many domestics run 8.5:1 stock. the retard iginition can be as little as 0.2 deg per psi, can you actually put a horsepower reduction number on that? All these things are used because of the available fuels we have. You run into this problem NA or FI no matter what. Its not HP limiting, its using the most out of the fuel we have.

Your question about not boosting as high is well taken. But with boost comes heat, unfortunately. So a 12:1 enigine with 3 psi is not going to be as fast and will predetonante like crazy than a 8.5:1 running 12psi both on 97 octane.

Does that make sense?

Randy

 

thats all it has to do with... higher or stock comp is good... u just have to tune it right

 

Here's what I understand: The air/ fuel mixture takes time to burn, so you advance the timing such that by the time the mixture burns the piston is in a good position to recieve the resultant pressure. I guess this idea has to be balanced with trying to keep the mixture from detonating.

When you boost, the mixture is more densely packed and therefore will burn faster. You need to retard the timing.

A higher compression ratio raises the efficiency of the engine because the mixture is more densely packed, raising power output. This must also be balanced with keeping the mixture from detonating. Since the turbo compresses the air, you need to lower the engine's compression ratio so that the final compression is not too high.

So basically a turbo achieves what advancing the timing and raising the compression do.

Someone correct me if I'm wrong there. I actually just pulled this all out of my ***, but it kind of makes sense.



[Modified by Lsos, 3:50 AM 11/4/2001]

 

ok these are good points you guys brought up. this was a good topic...

My original question was brought up because it always seemed to me like the best charateristics of a VTEC car were destroyed (such as lowering the compression). But obviously VTEC makes good NA cars not FI. It makes sense now.

Thanks for the replys guys.

 

My car has 10:1 in CR.Im going to run 4psi with stock injection in the beginning and when i can afford it get an adjustable rising rate FPR and 1 or 2 extra injectors controlled by a pressure sensor and rise the boost to 7,5psi

An intercooler are also going in when the budget allows it