raise or lower compression???

Vader's Fury · Nov 30, 2015

Planning to put together a motor. Debating if I should raise or lower the compression.

Pro's and con's of each?

I have a few different ideas for each way but wanna hear other opinions first.

Groomzybanshee · Nov 30, 2015

What are you doing with the truck??

Vader's Fury · Dec 1, 2015

Sorry should have specified. It will be a strictly 1320 truck.

Stripped down, race only chassis.

gwunter · Dec 1, 2015

Nitrous??

Vader's Fury · Dec 1, 2015

I am sure it will get sprayed. Plan to start small and fuel only but then either go compounds/trips. It will have a nitrous setup on it from the get go though. I am sure it will get sprayed though. If I manage to clean up the fuel without the nitrous, will just add more fuel LOL.

Magnum PD · Dec 1, 2015

The lower the compression, the more likely hood you will need either to start it with. I think.

Vader's Fury · Dec 1, 2015

Ya when cold it will be harder to start. Especially without a glow plug system. Not worried about starting it though.

02BigD · Dec 1, 2015

If you're going to go to a stage 2 cam, valve reliefs requirements may make most of the decision for you, no?

Vader's Fury · Dec 1, 2015

I am going with a cam change. I have actually looked into a couple different cams and have talked to one company about a 1 off cam grind. It will need valve reliefs and I know that will cause a drop in compression, but not that much if my math is correct. I am thinking if I drop compression, going down around 14:1 or so.

My thought is that this will allow for more air to be crammed into the cylinders so therefore more oxygen to burn.

My concern is though that it will not burn very efficiently and will outweigh the added benefit of the extra oxygen. Although the Mazda diesel dropped compression down into the 14 range and was able to be emmisions compliant without the use of urea.

Arisley · Dec 1, 2015

Don't go crazy on lowering it. It's not one of those things that if a little is good, more must be better. Just a little bit. If you lower to much, you will have to cram more air in there just to get to the same place as stock compression. You just want to lower enough to where you will not lift the heads with the added compression. The same effects can mostly be controlled with wastegates that you can adjust without an engine teardown.

Vader's Fury · Dec 1, 2015

I agree on the if some is good, more is not always better. Drop compression to low and you will start causing issues with ignition due to the lack of compression.

I just think that there could be a benefit of dropping compression in a race only application. I don't believe getting back close to stock compression numbers will be that hard with compound turbos.

Stock compression(17.5:1) and 18# of boost verses say 14.5:1 compression and 80+ lbs of boost.