Stock LT4 Cam - HP Limit?

[LSChev]

Hey all,

I am curious at what level everyone finds the stock LT4 cam seems to become a limiting factor.

I have done about 10 Magnuson 2650 cars with stock cams so far. I am able to get 800-830whp running around 16psi of peak boost. This equates to 140-144mph trap speeds. I am at altitude so my dynojet regularly corrects 11-15%.

I have also done 3 cars with the same mods plus BTR stage 3 cams and they can make 1000whp+ quite easily (i haven't pushed much over this but all indications point to more not being a problem.) This is at 18psi. unfortunately none of these cars have seen a track yet so I cant comment on mph.

However, I am wanting to push the blower a bit harder with the stock cam so I am working on a car atm that is a manual, has meth injection, low side aux pump and c16. In addition to the regular LTH, cat delete, Magnuson 2650R, 103mm TB, upgraded icebox and heat exchanger that the cars listed above have. So far I am making 20psi peak but not making any extra power.. This car originally made 801whp, with 4psi extra boost and C16 I am unable to get over 820whp. Something does not seem right to me here. I have played with cam timing and injector timing and nothing seems to make a significant difference. I am not running out of low side pressure, it is 70+psi the whole run. I am able to maintain my target high side pressure of 20MPA as well.

So I am wondering, have I simply hit the airflow limit of the stock cam? Is it unable to ingest anymore air hence why my boost numbers go up but no corresponding power increase?

Thanks in advance!

[HeavyChevy305]

You're flooding the blower with meth. Air has no where to go.

[LSChev]

You're flooding the blower with meth. Air has no where to go.

Hey thanks for your reply.

This is definitely a hypothesis I can test. With C16 I have some room to reduce the amount of meth being sprayed. I will give that a go and report back.

For arguments sake though, the cars with the BTR Stage 3 are spraying the same amount of meth. If the blower is being overloaded with meth, wouldn't this be a problem for them as well?

[Ttmerc]

In my opinion from experience in this same boat, the c16 isnt the best fuel for this and shortens life on sensors. Try a mix with Q16. I've also shimmed the high side pump a bit for some added high side pressure.

[HeavyChevy305]

In my opinion from experience in this same boat, the c16 isnt the best fuel for this and shortens life on sensors. Try a mix with Q16. I've also shimmed the high side pump a bit for some added high side pressure.

You can't shim a stock lt4 cam with a stock lt4 HPFP.

[LSChev]

In my opinion from experience in this same boat, the c16 isnt the best fuel for this and shortens life on sensors. Try a mix with Q16. I've also shimmed the high side pump a bit for some added high side pressure.

Hmm yea I don't think C16 is the best fuel for max power either but I chose it because it has a high stoich value (need less of it) and it prefers to run leaner than a lot of other race fuels. Which obviously helps when you're fuel system limited.

I am not dropping high or low side fuel pressure so that's all good. Simply not able to make more power, even with more boost.

Just had the car on the dyno again, I gapped the plugs down to .020 to see if it could be spark blowout (its not) it made the same 805whp it has been making.

I also retuned the car on straight c16, no meth. Just to see. My low side is starting to drop now but I still maintained high side fueling. The car did not pickup anymore power. In fact it lost 40whp with the same timing and everything as the c16/meth combo. I even swung the A/F around a bit to see if that makes a difference and it did not. I figure the power loss with no meth can be attributed to the loss of cooling pre-blower. I think we can put the meth loading up the blower theory to rest.

Any other ideas?

[Ttmerc]

Post a tune and log

[LSChev]

Ok I did some more playing around.. went to a larger upper to see if it was belt slip. Verdict: No, it doesn't seem to slip on every run but once in a while it will lose some up top.

I am picking up a lot of torque with the extra boost. But not as much HP as I hoped. I am starting to think this is simply the limit of the cam on this particular combination of parts. These stock LT4 cams really do no like rpm, they're even worse on the stock blower.

Heres some data:

**All tests completed at 4000ft altitude, DA changed but dyno is using SAE correction.
This is my final run for all 3 configurations:
Green: 90mm upper, stock lower: 14.5psi at 6500rpm, 24deg. 8psi at 3000rpm
Red: 85mm upper, stock lower: 16.75psi at 6500, 23deg. 10psi at 3000
Blue 85mm upper, 18% lower: 19psi at 6500, 23deg. 12psi at 3000
bret stages.png


I went back and looked at a simlar combo: it was a corvette so not apples to apples but both are manual and have identical mods; EXCEPT the vette had a BTR stage 3 cam. It made 210whp more (1031whp total) at 1psi less boost. I think I can safely assume the cam is affecting the 5000rpm+ power substantially on these mods.

Thanks for everyones help!

[HeavyChevy305]

Did you mess with the cam timing on the stock cam? I found gains by doing so.

[LSChev]

Did you mess with the cam timing on the stock cam? I found gains by doing so.

Yes, definitely power to be had up top by advancing it.

[HNK]

So you managed 23degs with c16 and 19psi of boost?? What sort of afr did u run with the C16??

[HeavyChevy305]

So you managed 23degs with c16 and 19psi of boost?? What sort of afr did u run with the C16??

.85 is safe.

[LSChev]

So you managed 23degs with c16 and 19psi of boost?? What sort of afr did u run with the C16??

It also has meth. TBH we usually make this power with pump gas and meth. I don't push it to 23deg on those though. This was for experimentation to see if I could make more power on stock cam.

I swung the A/F from 11.0 to 13.0, no significant power gain above 12.0. So all these tests would be 12.0-12.2ish.

also.. 23deg is of course the peak timing. Don't try to run that at peak torque.

[Corvette4ever]

Are you are running in open loop & disconnecting the stock O2 sensors? From your experience, how long do wideband O2 last with leaded fuel and is there any issue running leaded fuels with DI injectors?

[Eng.LSX]

Yes, definitely power to be had up top by advancing it.

you mean retarding it ?

[veee8]

We have seen north of 1100 rwhp om the stock LT4 cam with a proper valve spring upgrade for the higher boost levels.

[06MAX]

Sounds like valve spring issues

[LSChev]

We have seen north of 1100 rwhp om the stock LT4 cam with a proper valve spring upgrade for the higher boost levels.

Aha that is interesting! I was wondering if it could be valve float. Would have the same symptoms that I am seeing. Thanks for the insight I will do some more investigating into this.

[LSChev]

Are you are running in open loop & disconnecting the stock O2 sensors? From your experience, how long do wideband O2 last with leaded fuel and is there any issue running leaded fuels with DI injectors?

No sir, o2s are functional. I have ran quite a few LS setups on C16 with lots of miles. Haven't seen an issue as long as the tune isn't pouring raw fuel into the exhaust. May fail slightly more often but the narrative that people like to push online where the o2s fail instantly, I have not seen.

I have not ran as many setups with DI injectors on C16 but so far no issues as well.. I would imagine the lead is probably better for the injectors since it is intended to lubricate.