I am preparing to change out my transmission and install my single turbo and new fuel system. So, I decided to check my compression before I pulled it out. Here is what I got.

Front - 65psi Hot @ 250rpm
Rear - 90psi Hot @ 250rpm

I am running 11.4 @ 117mph in the 1/4 on stock twins and a medium street port on slicks.
This tells me that for the most part, I have good power. I only have hot starting problems when I drag the car, get back to my pit, and try to start the car with AITs in the 70c+ area. Other then that, the car has no hot starting problems.

This has really got me stumped. My goal is 10 seconds in the quater mile, I have a GT35/74 that I will be running at 23psi and a new fuel system and dual fuel pumps. I am more then sure that this setup will drop .5 off my current time, which is all I need to make my goal.

My question is,

Rebuild it while its out (I would rather not do this because even though I will be doing the work myself, this is going to cost me $500+)?

OR

Forget about it and put in the new turbo and fuel system, and run the motor as is?

 

rebuild whil it`s out 100% ... does not make sense otherwise .. especially if you can do DIY ...
~400 for apex ... 100 for sides .... 60 for corner ... ~ 120 for springs ... and whatever esle + ~ 200 for a rotary aviation basic o ring kit ...
better rebuild while it`s out

 

Ill add this in. This motor has only about 5k miles on it. When I rebuilt it, it had about 90ish psi on both rotors, probably due to the fact of less then ideal rotor housings. So when you make your consideration, take into account that I will probably need to buy 2 housings. I might be able to reuse me RA Super Seals, but in the end, it will still cost me about 1k to rebuild it.

 

Apex seal springs might be weak for whatever reason.

thewird

 

New RA seal springs?

 

Do you have your pressure regulator control (PRC) system working? I know I had some similar symptoms, only to realize, after reinstalling some emissions equipment, that my PRC solenoid was removed (shop that did my motor before did this and I didn't know about it). I put it back in, and guess what? Hot start problems no more. It bumps up your fuel pressure when your motor is hot to help w/ hot start conditions.

I wonder about those comp #s, if they are really that low, I find it strange that you can even start the engine at all and that it idles OK... let's see what the gurus say

 

Those RA super seals will wear your housings out really quickly.

 

Thats what Ive heard, but Ive also heard that disproved many times over also. Anyway, thats kind of besides the point. I started with low compression and I still have low compression, its not the apex seals.

To Mdpalmer - I dont have hot starting problems. When the motor does heat up, if the idle is set below 1500rpm, it will idle hunt. That could also be due to a vacume leak somewhere though.

This really has me stumped, as it seems that I have a powerfull motor that has low compression. I cant figure this one out.

Im waiting for a reply from Rich, Ihor, Djseven, Howard, Jason, Arghx or any other engine builders out there, please explain this.

 

Vacuum leaks dont help compression test numbers.Fix your vacuum leak and re-check your compression.

 

Did you use the RA springs? I decided to use stock oem apex seal springs with the RA Super Seals when I built my engine recently. The builders I spoke with recommended against the RA Springs.

If those comp #s are correct (you're holding the throttle open when cranking?) then I'd rebuild it with fresh rotor housings.

 

your apex seals are probably straight since the rear has decent compression. chances are that your front rotor housing is not straight across giving you a lower comp number. the motor will tell you when you take it apart. just read the carbon on the rotor housing. lay a straightedge across the housing at the plugs and check w a 1.5 thou feeler gauge.

primary culprit... running hard w too hot heat range plugs. we should all run as cold a plug as possible. the area of the housing that degrades from heat is around the plug boss. colder plugs transfer this heat away from the housing and into the coolant.

also run lots of premix... 3/4 oz per gallon and more than 1 oz racing. lose the EOP.

finally, if you race the car consider two 500 CC/Min AI nozzles spraying methanol.

just my 2 cents...

BTW, it is no surprise to me that your motor makes great power w weak compression. rotaries do that. i personally watched Chris Dembs win the Runoffs at Road Atlanta w a motor that they had to pour oil down the Weber carb to hotstart.

good luck,

howard

 

Oh really...Ive been told that vacume does not effect your compression numbers.

 

Yeah, I used the RA Super Seal springs because that is what they recomended to use.

Yes I am holding the throttle wide open while cranking.

This is a wild theory, but is it possible that the springs would seal better while the motor is under load thus providing better compression and good power?

 

Im running heat range 9 plugs, which is what is recomended for 350rwhp.

I am running 6oz of premix per tank which comes out to almost 2oz per gallon. While that is not what you recomended here, most people agree that 1oz per gallon is enough.

I am running 315cc water injection. It provides me a 10% water to fuel mixture which is more though enough. I understand you are a meth guy, but that amount of water would do the same thing. I am a water guy.

And finally, what I really want to know is, would you guys spend the 1k to rebuild this again? Or just run it with the new single turbo?

 

I'd rebuild it since if you wait for it to blow for some other reason, the engine might sustain more damage and end up costing you more to replace than what your having to do now.

 

a full tank of fuel is 20 gallons, which is 2560 oz's (128 oz per gallon), so you're at an approx ratio of 427:1, a far cry from 2:1

I would absolutely build a strong engine with 120+ psi compression if I planned to run it at a high power level with a single turbo

 

70C are pretty toasty intake temps. What are you using for intercooling? Or is that after the car sits and heat soaks?

 

Correction on my part, I fill up 10-12 gallons, I dont run it till its dry.

Thanks for the opinion about the rebuild.

 

Large M2 SMIC.

That's sitting and heatsoaking. I dont boost it if its over 60c.

 

when u did the compression test which 1 did u test 1st the 65 or the 90? did you retest again after this reading? just wondering if valve in gauge maybe stuck & gave a false reading?

 

I did it both ways. I also used 2 seprate compression testers to rule out a bad tester.