5.7 Vortec TBI pistons, computer air/fuel?

[duststang]

I have a 1998 5.7 Vortec roller cam motor. long story short I need to rebuild and bore cylinder due to scratches. I was wondering if anyone has experience with replacing factory dished pistons with flat tops valve releif. I run premium gas anyways so not too worried about that. My concern is more will the stock computer, TBI work or will I require some type of tuning? The vortec heads are shaved .015. I have heard that you can do flat tops and CR will be close to 10.4:1. I have also heard that I would need to use a LT1 knock module. I have also hear that I can use everything stock TBI, Knock, etc with no issues. I was planning on using factory everything cam included just adding flat tops. Has anyone put flat top pistons in a vortec 5.7 if so any advice?

Additionally without O2 sensors how does the computer control Air/Fuel mixture? I understand that we have a map, knock etc but I don't understand how the computer knows if it is rich or lean and how to adjust?

All help is greatly appreciated
thanks

 

[Bondo]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

I have a 1998 5.7 Vortec roller cam motor. long story short I need to rebuild and bore cylinder due to scratches. I was wondering if anyone has experience with replacing factory dished pistons with flat tops valve releif. I run premium gas anyways so not too worried about that. My concern is more will the stock computer, TBI work or will I require some type of tuning? The vortec heads are shaved .015. I have heard that you can do flat tops and CR will be close to 10.4:1. I have also heard that I would need to use a LT1 knock module. I have also hear that I can use everything stock TBI, Knock, etc with no issues. I was planning on using factory everything cam included just adding flat tops. Has anyone put flat top pistons in a vortec 5.7 if so any advice?

Additionally without O2 sensors how does the computer control Air/Fuel mixture? I understand that we have a map, knock etc but I don't understand how the computer knows if it is rich or lean and how to adjust?

All help is greatly appreciated
thanks

Ayuh,...... That changes the game, 'n the math has to be done,... Not just internet rumor,...

Vortec heads really make use of the D-dish, or inverted dome pistons,....
If yer buyin' pistons, 'n are willin' to Do the Math,...

 

[Tail_Gunner]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

As bondo says however a 4.3 marine motor does not have dished piston's from the factory..relief's howver. So going to a dished now would make sense....However if your CR takes you to 10 and above that will take forged piston's and those are expensive.

As to your EFI system what are you using....mefi1..2...3 the all those use a knock module why dont you have one now... or are you converting something that's a big job marine injection used speed density

http://www.pleasurecraft.com/manuals/L510004.pdf

 

[duststang]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

update my engine is a 5.7 Vortec TBI and does utilize a knock sensor. I decided to use the stock style dish pistons for a few reasons. The motor is now .030 and the heads are milled .015 and flattops will be over 10.5:1 CR with the combo. I did not want to invest in a tune or worry about detonation. I did find a few people that are using flattops in a Vortec motor with CR over 10:1. Some had a issue with the knock sensor acting up preventing the motor from making power so they swapped the knock module for a LT1 which is less sensitive. Oh well it should run good and be reliable hopefully. I still do not understand how the computer knows how to adjust rich or lean without O2 sensor?

 

[QC]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

I still do not understand how the computer knows how to adjust rich or lean without O2 sensor?

MAP sensor and then calced. MAP, plus throttle position, equals air. They basically assume the calced air is close.

 

[duststang]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

MAP sensor and then calced. MAP, plus throttle position, equals air. They basically assume the calced air is close.

I was thinking this was likely the case. so it really does not know if it is rich/lean and doesn't have the ability to adjust. the computer is programmed to assume at a certain throttle with a vacuum this should be the air/fuel.

 

[QC]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

Correct. It is not "closed loop" on fuel/air ratio, but it is electronically controlled. There are two reasons. O2 sensors don't like the wet exhaust, and without a 3 way Catalyst (then) there was no risk of damaging anything by being off a point or two from time to time.

 

[Volphin]

Re: 5.7 Vortec TBI pistons, computer air/fuel?

Great choice on the piston. Those should serve you just fine.