Just finished up mods in sig. (Stock bottom end, no changes to bore/pistons) The car is having difficulty idleing, tends to surge. Mechanic has checked car from top to bottom (IAC/TPS were okay). After hearing that those AFR 190 heads have 74cc combustion chambers, he believes the CR is ~ 8:1!!! This low CR combined with the LPE 219/219 cam is causing the idle problems. :banghead:
How can I determine the CR??
any suggestions to correct this??
TIA

 

theres TWO ways to measure compression ratio, static and dynamic.. your engine only sees and uses DYNAMIC COMPRESSION
here read this http://members.uia.net/pkelley2/DynamicCR.html

http://members.uia.net/pkelley2/Overlap.html
http://cochise.uia.net/pkelley2/solidcams.html

BTW WITH THAT CAM YOUR CLOSER TO 7.4:1 dcr


if your sure everthing about your engines tuning and valve adjustments are correct, the cheapest and easiest way to improve things would be to swap to 1.5 roller rockers which would effectively pull a few degrees off the cams effective duration and overlap..... the correct cure would be to get 11:1 pistons or a lower duration cam, but get a good tune up and use a fuel/air meter for tuning http://www.rbracing-rsr.com/rsrgauge.htm




[Modified by grumpyvette, 3:22 PM 3/20/2003]


[Modified by grumpyvette, 3:27 PM 3/20/2003]

 

Your idle problem could be in the the tuning. Mail order chips usually get close but sometimes don't always hit the nail on the head. My 90 had troubles idling after I put it together and it was a fuel mixture problem matched with an IAC problem. The car was getting so much fuel that the tb blades had to be opened up quite a bit, which it turn upset the IAC. After some dyno sessions with a wide band and some different chips from Ed Wright, I now have almost all my bugs worked out. The first chip had the car idling at 10.0:1 A/F

 

Don, WOW you only have that much compression??? That is not very useful for power. You *should* have had them mill the heads to 58cc or even 56cc as mine were, for a little extra squeeze.

Gus B has the same tune down low as you, and his car idles great from what feedback I have got from him. Intuitively, you most likely could use a little more timing down low, to help things along. But the CR being so low is not helping anything, and most likely hurting everything.

You can do a compression check, and get a PSI reading, from that you can guessimate your actual compression ratio. You PSI readings *should* be in the neighborhood of 180PSI. I am betting your down around 150 or less.

Sorry to be the one to have to tell you this, but I would pull the heads back off and have them milled to the correct spec. You will be MUCH happier in the long run. Any type of shortcuts are going to lead to less than adequate results. You have that much invested already, most of the work, you can do to correct the problem. Its really not that hard. A long weekend and some weektime to allow for a Reputable shop to mill them would be more than enough time to complete the job.

I hope things work out for you. If there is anything you need, let me know. IF you are willing to drive your car down to PA. i would be more than happy to assist you in the teardown, and rebuild. Email me if you need anything at all.

Jesse

 

Thank you all for you posts. However, as we just found out after a little panic session, the heads were milled to 58cc. So, our compression ratio is ~9.5.
:party:
Regardless, thank's for you insight!!

btw, the mechanic seems to think our stock torque converter, having such a low stall speed, could be part of the reason we're getting stalling at idle, - the engine is so loaded down it wants to stall when in gear, stumbles/searches a bit, then stalls.

 

I doubt that it's in the convertor. Does your mechanic have access to some sort of data aquisition tool, such as a snap on scanner or autoXray. If he does check the BLM's (my short term counts at idle were at 100, which is the richest it will read and now they are at 118 which is still a little rich) Check the TPS ( Mine sits at .57) and check the IAC counts (ideal is around 30 at idle and mine sits at 20 or so....kinda fluctuates) I've noticed on a speed density setup that these 3 things can greatly effect driveability and idle quality.

 

My idle was all over the place...Stumbling, surging, it really sucked.
Jesse set me up with a custom chip to help the computer compensate for my mods. It now idles smoooooth @ 700 rpms. No surging, no stumbling, no idle hunting. I would have to say your problem is in the tune.

Thanks again Jesse (AKA, Ski_dwn_it) for the tune. :cheers: