Down on power, ideas and opinions needed

#1

At VIR this past weekend, I quickly realized I brought a pocket knife to a gun fight. My trap speed at the end of the back straight was at least 10mph off and am still trying to figure out why. My car doesn’t seem to pull very well and this could have been due to the contact I had with a TR at CMP in May. I was hit in the RR 1/4 but can’t seem to see how/where this could have had an impact on my power…unless something was knocked loose.

What I have tried thus far:

Two new DME’s
One AFM
Cleaned the Crank Position Sensor
Compression test 145-150-145-150-145-150

So, I am looking for input as to what to look at next but most importantly, what I need to look for.

TPS?
Coil?
Fuel Pump?
Injectors?

Something else?

Thanks guys.

#2

I hate to be the bearer of bad news, but your compression is low and it’s time for a rebuild. With numbers like that, your power is in the mid to lower 140s. Strong motors make ~180ish psi. Some will disagree with me, but I’ve found compression to be a very good indicator of engine performance. Your next step should be to execute a leak down test to determine if the problem lies in the head or lower end. With consistent numbers like yours, I’m suspect of your bottom end and bet you have leaky rings from normal wear. Valve leakage would likely be less consistent with a few cylinders much lower than others.

Minimum rebuild could be as simple as using a ball hone to deglaze the cylinders then re-assemble with new rings, rod bearings, lapping the valves, new seals, and a close inspection of the oil pump.

#3

Does wet vs. dry compression testing make a difference here? I assume Mike’s numbers were dry. Fish, is your 180 number wet or dry?

#4

Just to be disagreeable, let me take an angle contrary to Rich’s.

Your compression #'s don’t indicate a problem. All they indicate is that your cylinders “might” have similar compression depending on how rigorously the test was done.

Sure, it’s a good idea to investigate compression more by doing a wet compression test, and a dry and wet leakdown test. But everything has to be done over and over again until your results are repeatable.

Different test procedures, different batteries, different starters, different gauges, etc. all work together to make comparison of your compression #'s with Rich’s difficult. A perfectly reasonable scenario is that 180psi is “good” for him and 140psi is “good” for you.

It is very easy for a test to fool a person into thinking that they have a conclusive result. One has to carefully consider what the test result tells him and what it doesn’t. Most tests that a person can dream up are only “indicators” and you have to come up with alternate ways to test your suspicion. Otherwise a person can waste weeks and thousands of dollars on a wild goose chase fixing a problem that, “isn’t”.

I’m coming to a point, really. You can’t draw major conclusions based on only one kind of test and you can’t test solely the air pump. You also need to look for potential engine management problems. The engine is only an air pump. That is to say one part of a complex system that has to be working well to provide optimum power. For starters, you need good F/A info so you can see if anything funny is going on with your mixture. Either get on a dyno or get a F/A meter for your dash and connect it to your data logger.

#5

[quote=“Ranger” post=67462]Just to be disagreeable, let me take an angle contrary to Rich’s.[/quote] No surprise here!

[quote=“Ranger” post=67462] Your compression #'s don’t indicate a problem.[/quote] Wrong! They indicate low power like he is seeing on the track.

[quote=“Ranger” post=67462] Different test procedures, different batteries, different starters, different gauges, etc. all work together to make comparison of your compression #'s with Rich’s difficult. A perfectly reasonable scenario is that 180psi is “good” for him and 140psi is “good” for you. [/quote] Wrong again. In my experience testing compression on many different cars with different batteries, starters, gauges, etc, tired old engines made low compression 145ish and fresh/fast engines made 180ish. Dyno results of several cars with a wide range in compression showed large disparities in power on the dyno. The bottom line here is that compression is a good indicator for how much power your engine is making (assuming that electrical and fuel systems are well sorted). Scott, you are over thinking this and down playing a very simple test.

[quote=“Ranger” post=67462] You also need to look for potential engine management problems. [/quote] Agreed, but a proper functioning fuel and electrical system is not going to fix his low compression problem.

#6

Does wet vs. dry compression testing make a difference here? I assume Mike’s numbers were dry. Fish, is your 180 number wet or dry?[/quote]

There should be very little difference between wet and dry test numbers on a sorted engine. If your numbers go up on the wet test, you have leaky rings.

I’ve also found that compression tests are consistent when testing a cold and hot engine. Leakdown testing, on the other hand, should be done with the engine at temperature to get accurate results.

If your car is slow, the first thing I would do is check your compression…

#7

[quote=“FishMan” post=67468]
If your car is slow, the first thing I would do is check your compression…[/quote]

Which is what I did. Dry test, cold motor.
Sigh :frowning:

#8

I still maintain that compression #'s around 140 are “interesting” but not “conclusive”. No single test is conclusive unless you can isolate all the variables. Before a bunch of time and energy is spent attacking a problem, you have to find different ways to test in order to get some kind of confirmation that the problem is correctly identified.

#9

Have you checked the rear toe? Chuck

#10

[quote=“Ranger” post=67471]I still maintain that compression #'s around 140 are “interesting” but not “conclusive”. [/quote] Yes, they interestingly low… Properly functioning fuel/engine management systems can’t compensate for an engine’s inability to pump air and the engine is therefore down on power regardless of the condition of the other systems. I’ll take it all back if you show me one front running engine with compression less than 174ish…

#11

The disagreement here is that you see the test as being fairly rigorous and I’m talking up all the things that could distort the numbers. You give me an engine that you test consistently at ~180psi, we’ll put it in my car and I’ll find a way to make get test results in the 140’s. Then, ah, I’ll go do a couple events in it.

I can screw up anything. Me and entropy are fierce opponents and doom on the person who stands against us. No test result can be trusted in isolation. There’s plenty of ways for a healthy engine to turn in 140’s and all I have to do is suggest one way and the test becomes suspect. That’s why problems have to be tested from different angles so you can confirm the indications of one test with another. It’s up to the individual to dream up different ways to do a test.

#12

This is a fair point and my previous comments are only valid when applied to properly performed compression tests. Screwing the test up is something entirely different. That said, a compression test is fairly simple and beyond stuffing dirty socks in the intake boot, I just don’t see how someone could screw the test up enough to have much of a measurable effect on the results. I’ve forgotten to test with the throttle body open before and then immediately retested; the results didn’t change much.

Maybe I’m missing something, but please explain to me what would cause a healthy engine to test in the 140 range. I’m convinced that battery voltage, starter speed, barometric pressure, and engine temperature have negligible impacts.

I agree that multiple angle testing is a good idea. In my mind, improvement in compression numbers with a wet test is enough to confirm the need for rebuild if you want more power. Or, you could just run your engine understanding you are a bit down on power and have a blast racing the myriad of other racers in the exact same situation.

#13

Do a leakdown test. If over 3-4% look and a motor rebuild. Chuck

#14

If engine is cold maybe 10%?

#15

Yes, but I didn’t spend all the money on this build to not have a shot at winning or more importantly, be able to back up all the trash talking I have been doing the last two years. :unsure:

#16

Yea, max 10% on cold motor. Mine last checked at 2-3% cold after 13 race weekends. Do the leak down and see where the loss is. You may be able to just freshen the head. In addition, if you have missed a shift, you probably have 6 bent exhaust valves and that is where your power has gone. Chuck

#17

Yes, but I didn’t spend all the money on this build to not have a shot at winning or more importantly, be able to back up all the trash talking I have been doing the last two years. :unsure:[/quote]
You’re in SpecE30 now and the competition is fierce here. As long as winning isn’t all that important, you’re going to have a blast. But if winning is important, well, there’s cheaper ways to feel inadequate.

#18

[quote=“Ranger” post=67506]
You’re in SpecE30 now and the competition is fierce here. As long as winning isn’t all that important, you’re going to have a blast. But if winning is important, well, there’s cheaper ways to feel inadequate.[/quote]

Winning isn’t everything…it’s the ONLY thing.

#19

Throttle plate open during the compression test?

#20

No and I realized that was in the Bentley manual just a short while ago. I can re-test mañana if the result could vary large enough.