Rebuilt Motor, What Compression?

[jackers]

I am getting a shortblock built up and am struggling with what compression to go with. My car will not see e85 since we don't have it around here. It will be driven on the street part time with a 93 octane tune and on the track with race gas.

It seems like everyone is going above 10:1 anymore. What should I shoot for?

 

[redfr03svtsnake]

9.0-9.5, turbo- 10.0 +

 

[cobracide]

^ yep, around there is good.

 

[jackers]

I'll be running a KB2.8LC or a TVS 2.3 with nitrous.

I have a bead on a built shortblock with 10:1 compression. They guy said I can lower the compression with cometic gaskets. Can I get it down to 9.0-9.5 with those?

 

[04torchred]

Big question is what do you plan to run for camshafts and what is there duration going to be.

This will bleed off cylinder pressure and help determine how high you can go.

Keep your quench in the 0.04-0.05" range don't **** with the head gasket thickness to drop compression. either get different pistons or longer cams to drop the dynamic compression. A wide quench is bad if you read all the engine build threads, 03steve's posts, Mihovetz's stuff as well.

 

[agpatrick12]

Big question is what do you plan to run for camshafts and what is there duration going to be.

This will bleed off cylinder pressure and help determine how high you can go.

Keep your quench in the 0.04-0.05" range don't **** with the head gasket thickness to drop compression. either get different pistons or longer cams to drop the dynamic compression. A wide quench is bad if you read all the engine build threads, 03steve's posts, Mihovetz's stuff as well.

Do you know if you can degree your cams a certain way to bleed off cyl pressure? (this may be a stupid question :shrug: ).

I'm in the same boat as op with -11cc pistons that I don't want to change with stock cams in a teksid. I *DON'T* want to use thicker head gaskets bc of what I've read about quench and such...

 

[dynobobstieg]

9.5 to 10
When adding 1 point of static compression to your OEM engine it will increase boost by 3#s. When selecting a stage 2 cam 220 degrees the lost boost associated with increased duration and cam overlap can be negated by adding 1 point of static compression. A stage 3 cam would need 1.75 increased static. A stage 1 cam like a Ford GT runs very well at OEM compression.

STIEGEMEIER:burnout:
Stiegemeier Porting Services, LLC - Supercharger Rebuild and Repair
636-949-2275
[email protected]

 

[04torchred]

Do you know if you can degree your cams a certain way to bleed off cyl pressure? (this may be a stupid question :shrug: ).

I'm in the same boat as op with -11cc pistons that I don't want to change with stock cams in a teksid. I *DON'T* want to use thicker head gaskets bc of what I've read about quench and such...

The only thing that really effects the dynamic compression from what I have read is when the intake valve is closed. So you could try to really retard the intake cam, but by doing so you are hurting performance bad as with a short duration stock cam(184-186* duration) the longer you push the valve opening the less ideal you get.

So really you want a longer duration intake cam that will have a later IVC event, but still open early to allow good cylinder fill.

I have no idea what is optimal.

I really wouldn't worry to much. If you compression is going to come in at 10:1 just knock the boost down a bit more then you planned and keep the timing low as well for pump gas. Don't get greedy :beer:

 

[ugotbit03]

Here is a good read 03Steve turned me on to-

Engine Compression Ratio - Tech - Popular Hot Rodding Magazine

 

[Quick Strike]

Here is a good read 03Steve turned me on to-

Even better read with a dowloadable DCR calculator here. I used this when setting up my engine for my custom cams. Although I have 9.8:1 static compression my dynamic compression is about 7.75:1. That dynamic compression is about the same as a stock 03-04 Cobra with 8.5:1 static compression. In the end, I have similar cranking compression as stock, but increased cylinder fill (charge) at that compression.

 

[ugotbit03]

Even better read with a dowloadable DCR calculator here. I used this when setting up my engine for my custom cams. Although I have 9.8:1 static compression my dynamic compression is about 7.75:1. That dynamic compression is about the same as a stock 03-04 Cobra with 8.5:1 static compression. In the end, I have similar cranking compression as stock, but increased cylinder fill (charge) at that compression.

I will see your DCR calculator and raise you-

CamChart

LOL-All good info in here! I have the same DCR read also.

 

[agpatrick12]

I will see your DCR calculator and raise you-

CamChart

LOL-All good info in here! I have the same DCR read also.

Wow great thread guys and thanks for the info! I hate having these sweet new -11cc diamond pistons and being worried its not gonna work with my ported eaton and 2.8 pulley... :uh oh:

Did you get your question answered, jackers (op)?

 

[jackers]

Wow great thread guys and thanks for the info! I hate having these sweet new -11cc diamond pistons and being worried its not gonna work with my ported eaton and 2.8 pulley... :uh oh:

Did you get your question answered, jackers (op)?

Well, I absolutley received reading information. I am using some of these tools to do my homework and dial in my setup.

Keep it coming guys!

 

[Posi]

9.1:1 is what we're doing on mine. You get the added security of the lower compression and nobody can tell you how much less power it will make. If any.

 

[Quick Strike]

Well, I absolutley received reading information. I am using some of these tools to do my homework and dial in my setup.

Keep it coming guys!

One of the better tools I have found online is Dyno2003. It predicts horsepower and torque curves for a given engine combination. It is getting a little older now, but it is free. It will take a while to get the hang of it, but there is a manual that dowloads with it. It accurately predicted several engines I have built within roughly 10 horsepower and torque over the entire curve. Once you have it going, you can change things like compression, cam specs and head flow to see the impact it will have on your intended build. You can critique just about any engine build you like between this and the DCR calculator in the link above. You can download it here (at least that is where I got it). it is part way down the page under software. Dyno2000 is also there, but has less features then Dyno2003. I have a sample of a completed run here if you are interested.

 

[jackers]

9.1:1 is what we're doing on mine. You get the added security of the lower compression and nobody can tell you how much less power it will make. If any.

I actually talked to Mike at L&M today. I'm really torn between him and Dave Guy at DGR Performance for my build. Both are wicked smart and have NASTY race programs! Good luck with your build. What block did you end up going with?

As for the thread, this thing keeps getting more and more great links in it. Thanks, definitely book marked.

 

[RacerXHG]

You can also lower static compression by polishing the chamber and unshrouding the valves. My chambers are all @ 55ccs with work and ss tulip faced valves. That 3cc is worth about a half point lower in static cr.

 

[Brutal Metal]

Wow great thread guys and thanks for the info! I hate having these sweet new -11cc diamond pistons and being worried its not gonna work with my ported eaton and 2.8 pulley... :uh oh:

You have absolutely nothing to worry about running those 11cc Diamonds, 6's would be even better but if you already have them no worries!:rockon: