Higher comp. pistons with stock exhaust?

[Space Boy Matt]

Hi,

I'm getting prepared to do a rebuild on my KA24DE and I'm wondering if this is okay to do. (I did a search and couldn't find anything about my question.)

1) Rebulid my engine and put in 10.5 to 1 pistions while the engine is apart.

I'm looking at 10.5 to 1 to increase my HP, still be able to use 91 octane fuel, not piston tap the stock valves, and have it still be useful as a daily driver.

By increasing the compression I increase exhaust pressure right?

So my question is will there be a problem if I run the "improved" engine with a stock exhaust until I can get some $$$ for a higher flow exhaust system?

Thanks for your help.

[RedShred]

I ran 10:1 on my SOHC for awhile until I bought the Header... I didnt have any problems.

Save your pennies for some Cams too. That is where you'll get your greatest improvement. If you can help it... save for the Headers before the head comes off. That way, while the engine is getting redone, you can portmatch the head to the headers and intake ! Since you'll probably be getting the valve seats reground, and the Valves have to come out... that would be another good time to put in some Cams. Once it's apart, it doesnt really cost anything more to put in other shiz, really. But it sure does when you have to rip apart a good engine to install ****.

[C-Kwik]

It shouldn't. You are still moving the same amount of air. And with higher compression, you are putting more energy to the pistons, which means the exhaust temp might be slightly cooloer. I would still opt for a freer flowing exhaust though. With higher compression, you are closer to getting detonation. Restricted flow may add some heat to the exhaust valves. Not to mention, a good exhaust on a KA tends to free up the very restrictive exhaust. Many have acheived 10-15 HP peak gains on the KA with just an exhaust. Pretty good considering how much an exhaust system costs. With all the engine work you are doing, I would think you already had an exhaust.

[Turbo_Nismo_EC]

I thought that when you increased pressure it increases heat. How can an increase in compression decrease exhaust heat? Just something I read somewhere.

[Dattebayo]

yeah sure thats one of the things the turbo guys talk about all the time... one of the reasons they buy intercoolers.

[C-Kwik]

I should be more specific. I was referring to the same peak temp in the cumbustion chamber. The higher compression puts more of the heat in the combustion chamber into pushing the piston down. If you take the same amount of heat(energy) and convert it into mechanical energy, there will be less heat left. I wasn't very clear and my first post is very contradictory when I reread it.

[Dattebayo]

OK tell me if im wrong, but displacement doesn't change at all and you are only decreasing the total (adjusted) cylinder volume. Since you have the same quantity of heat in a smaller area, are you not going to have higher pressure and thus more heat coming out of the exhaust as well as into all the surrounding metal? Heat doesn't push down only, you know.

[p00t]

OK tell me if im wrong, but displacement doesn't change at all and you are only decreasing the total (adjusted) cylinder volume. Since you have the same quantity of heat in a smaller area, are you not going to have higher pressure and thus more heat coming out of the exhaust as well as into all the surrounding metal? Heat doesn't push down only, you know.

heat is because of wasted excess energy, its like a seesaw, more energy spent pushing... less left for heat. can be the other way around too. It wont be dramatic like 300 degrees cooler, but there is a difference.

im behind you on this one C-Kwik!@!@!!!!!! *puts fists up*

[Dattebayo]

thats odd. but if you say so, IT MUST BE TRUE. whatever. So heat already is kinetic energy so transfer to mechanical at a higher rate equals less leftover heat. Makes sense now. What about your exhaust? Is there more exhaust gas pressure now for some reason?